University of Edinburgh
 

Disorders of vision in children: a guide for teachers and carers

CHAPTER Three: Disorders of the visual system and their implications

 

It is important to work with a qualified teacher of visually impaired children to establish the day to day educational implications of a child's visual impairment. The following educational implications described should be used as indicators - all readers should also become familiar with the practical approaches contained in Chapter 4. Where appropriate, children should always be asked what works best for them.

The front of the eye

a) Congenital abnormalities of the cornea, eg; Peter's anomaly, sclerocornea
b) Xerophthalmia
c) Keratoconus
d) Corneal dystrophies
e) Metabolic diseases, eg; Hunter's. Hurler's
f) Cataract

a) Congenital abnormalities of the cornea, eg; Peter's anomaly (Plate 21), sclerocornea

Plate 21 Congenital abnormality of the cornea

Definition: Rare disorders of the size or consistency of the cornea which the child is born with.

Cause: The development of the eye structures takes place in early pregnancy and any disturbance which takes place at this time such as maternal illness or the unwitting taking of harmful drugs may cause this and other congenital anomalies. Genetic causes are also possible. Involvement of both eyes is common.

Eye structures affected: The cornea is normally transparent because it is made of very regularly arranged fibres. In sclerocornea. as the name suggests, the fibres of the cornea have taken on the random pattern shown by those of the normal sclera (as opposed to the highly regular lattice structure present in the normal cornea and responsible for its transparency). Hence the whole cornea is white and non-transparent, as though the white of the eye covers the front of the eye. In Peter's anomaly there is usually a central corneal opacity due to a defect in the back layer of the cornea.

Effect on vision: In both cases light cannot get through to the retina and hence there is a profound reduction in visual acuity. It is possible that some useful visual field might remain in an eye where the corneal clouding is only at the centre and some light can get round it to the outer retina. Because this corneal clouding causes severe reduction in vision right from birth, this will tend to cause severe deprivational amblyopia.

Other associations: Associated glaucoma is common in such conditions because the angle between the cornea and iris where aqueous fluid drains out of the eye is also affected. General, non-ocular associations such as learning difficulties are uncommon.

How it is detected: The appearance of the cornea will be abnormal at birth and other causes of corneal opacity (such as those outlined below) must be eliminated by the doctors looking after the baby. This may involve examining the eyes under an anaesthetic as well as general medical investigation.

Medical treatment: The intraocular pressures must be measured (requiring a general anaesthetic in younger children) so that any associated glaucoma can be treated. Surgical management would involve a corneal graft and sometimes in Peter's anomaly removal of the lens and vitreous as well. Corneal grafts are very difficult to manage in young children and even if it were possible, grafting produces a lot of astigmatism which itself produces amblyopia.

One operation which is occasionally helpful involves making a new artificial pupil by cutting a hole in the iris if the normal pupil is covered by a corneal scar - this will at least allow some light into the eye for possible navigational vision. This operation is called an optical iridectomy.

Progression: The corneal opacities are not progressive and indeed in sclerocornea a degree of clearing may occur between the centre and the edge of the cornea. Severe visual impairment is almost inevitable if, as is usual, both eyes are involved.

Educational implications: Both near and distance acuities are severely reduced to variable degrees. The extent of this reduction will determine whether Low Vision Aids (LVAs) will enable print reading or whether braille will be required. Clouding of the cornea can cause uncomfortable glare because of the way light is scattered as it enters the eye. Positioning of light sources and windows behind the child may be both more comfortable for him or her and prevent worsening of visual function (due to scattering of the light by the corneal opacities) if glare is a factor.

b) Vitamin A deficiency and measles (xerophthalmia) (Plate 1)

Plate 1 Cosmetically noticeable and visual impairing corneal scars of both eyes caused by Vitamin A deficiency

Definition: Scarring of the cornea due to poor wetting by the tear film caused by vitamin A deficiency.

Cause: In developing countries the diet of children is often deficient in this vitamin. Red meat, liver, green vegetables and carrots are all rich in vitamin A but rice, wheat and maize, often the staple diet in developing countries, are not. Vitamin A is essential for the health of a number of different types of cell in the body including the cells of the conjunctiva, which produce the mucus of the tear film. The health and transparency of the cornea depend on wetting from tears and chronic severe deficiency of this vitamin results in loss of corneal transparency often with blood vessels growing into it. Vitamin A is also a main ingredient for the rod visual pigment, rhodopsin and hence deficiency can also produce night blindness. Another group of cells for which this vitamin is important are the skin cells. After an attack of measles with its accompanying rash, the body's supply of vitamin A is used up as the skin is regenerated. Hence this virus can severely worsen any existing vitamin A deficiency and accelerate the corneal scarring.

From the above explanation it will be clear that this will be a very uncommon cause of visual impairment in developed countries but it remains a very important factor worldwide.

Eye structures affected: Conjunctiva, tear film, cornea and the rod receptors of the retina as above.

Effect on vision: This will depend on the size and position of the corneal scarring, central scars having a more severe effect on visual acuity.

Other associations: These children are often suffering from severe malnutrition and about a quarter will not survive.

How it is detected: Characteristic appearance of the dried-out conjunctiva and cornea in a situation where malnourishment is likely.

Medical treatment: Prevention is by far the most important medical intervention for this condition. This involves education to mothers about diet and, where necessary, supplementary vitamin A given in single large doses. Once the damage has been done medicine or surgery offer little. Corneal grafting is notoriously difficult and gives poor results. An optical iridectomy as explained in the previous section may be helpful.

Progression: If severe vitamin A deficiency persists the cornea may thin out and eventually perforate which will cause the eye to collapse and results in permanent and complete blindness in one or both eyes.

Educational implications: These are as explained for the congenital anomalies but in the countries where this condition tends to arise, facilities for education of visually impaired children are usually limited if indeed they exist at all. Therefore the outcome may unfortunately be that the child receives no school education.

c) Keratoconus (Plate 22)

Plate 22 Keratoconus

Definition: A progressive thinning of the cornea causing it to become cone-shaped.

Cause: There is an association with allergic conjunctivitis which causes itching and it may be that repeated eye rubbing plays a part in causing this condition, but in most cases, no cause is found.

Eye structures affected: The central cornea becomes thinner as the child gets older and becomes the tip of the cone shape

Effect on vision: The distorted shape of the cornea produces astigmatism (page 37). The cornea may lose its transparency in the centre in some cases. Hence visual acuity can be reduced due to distortion and scarring. Educationally significant visual impairment in childhood is rare in the UK.

Other associations: In addition to allergic eye disease, keratoconus is more common in children with Down's syndrome, Marfan's syndrome, retinal dystrophies, aniridia and congenital rubella.

How it is detected: An optician may pick this up by noticing increasing astigmatism, and it may subsequently be confirmed by microscopic examination by the ophthalmologist. In severe cases the conical shape of the cornea can be seen with the naked eye when it is viewed from the side (Plate 22).

Medical treatment: Initially the astigmatism may be managed by spectacles. As the astigmatism progresses, contact lenses may be required. Eventually in the severest cases the cornea becomes so distorted that contact lenses can neither correct it nor stay in the eye. Central corneal scarring (loss of transparency) may also occur. In such cases corneal grafting (see Glossary) will usually be recommended and this is a condition where the outcome of graft surgery is usually very good. Few of these children should therefore nowadays be left with significant visual impairment.

Progression: Mild cases may remain stable and continue to manage with spectacles or contact lenses. In a significant proportion however, as the child progresses through the teens, the distortion and hence the astigmatism worsen and corneal grafting may be considered. There may be short episodes during which the vision worsens severely because the cornea becomes waterlogged - these episodes almost always recover with time and appropriate eyedrop treatment.

Educational implications: This condition is now successfully treated by corneal grafting and few if any affected children require specialist educational input. However keratoconus does sometimes accompany other conditions such as Leber's amaurosis and Down's syndrome and it is therefore worth noting. Children at risk of developing keratoconus should be discouraged from persistent eye rubbing. Visual acuity for near and distance may be reduced by corneal scars or severe astigmatism. Children may require help with contact lens care before or after surgery. Children who do undergo surgery require many hospital visits and the operation should be timed with the child's educational programme in mind. Any child who has undergone a corneal graft should be referred back to the hospital urgently if he or she develops a red eye since there is a risk of the graft rejecting and if a rejection is treated quickly the graft can often be saved.

Footnote: Keratoglobus is a related but much rarer disease involving general thinning of the cornea so that it protrudes in a globular rather than a conical shape. The condition is more difficult to treat than keratoconus and the eyes may perforate
after minimal trauma.

d) Corneal Dystrophies

Definition: These are a group of diseases passed on through families and affecting both corneas.

Cause: Genetic

Eye structures affected: Different dystrophies affect different layers of the cornea. For instance congenital hereditary stromal dystrophy (CHSD) affects the middle layer of the cornea, whereas in congenital hereditary endothelial dystrophy (CHED) it is the back layer of the cornea (which usually acts as a pump to keep the cornea dry and hence transparent) which is affected, and so the cornea becomes waterlogged, losing its transparency. Another rare dystrophy called Posterior polymorphous dystrophy (or PPMD) also affects the back layer.

Effect on vision: Although the disease is present from birth, loss of corneal transparency usually progresses with age and only those mentioned above are likely to cause visual impairment in childhood. As for other causes of corneal opacity, visual acuity is reduced and glare may be a problem. Amblyopia may be an additional problem particularly with CHED and PPMD where the cornea is cloudy from birth.

Other associations: None

How it is detected: There may be a family history (though often not in severe cases). Cloudiness of the cornea is noted and microscopic examination of the cornea in the clinic will reveal which layer of the cornea is affected and hence which dystrophy is present.

Medical treatment: Corneal grafting may be required in severe cases though recurrence of the disease in the graft is possible.

Progression: The cornea tends to become more cloudy with time.

Educational implications: The implications of corneal grafting are as for keratoconus. One of the parents may be affected by the disease in CHED which may help the family understand better the visual difficulties which the child may encounter. As for other conditions with corneal opacity, glare may be a factor. Positioning of the child in the classroom with respect to windows and lights can therefore be important.

e) Metabolic diseases, eg; Hunter's syndrome, Hurler's syndrome

Definition: A group of diseases where various compounds are deposited in certain parts of the body (including the cornea) where they should not be.

Cause: The body lacks the ability to digest these particular
compounds.

Eye structures affected: Cornea, retina and optic nerve to varying degrees. Glaucoma may also be present.

Effect on vision: This depends on which of the above parts of the eye are worst affected. If the cornea is the main problem the effect on vision will be as for the corneal dystrophies; if it is the retina it will be as for retinitis pigmentosa, and if it is the optic nerve then the effects will be as for optic atrophy.

Other associations: These may include learning difficulties, deafness and abnormalities of facial appearance and skeletal structure. Life expectancy may be significantly shortened.

How it is detected: From the combinations of the above features and biochemical tests.

Medical treatment: Paediatricians will be involved in the general medical care of the child. Corneal grafting is sometimes performed but often the additional retinal or optic nerve damage or severe learning difficulties make it unhelpful.

Progression: The retinopathy and glaucoma may be progressive.

Educational implications: The child will be facing multiple problems. The educational implications of corneal disease, retinal dystrophy and optic atrophy may all need to be considered in addition to possible hearing impairment and learning difficulties.

f) Cataract (Plates 23-24)

Plate 23 Cataract

Definition: Loss of transparency of the lens as shown in Plate 23.

Cause: A variety of causes include genetic, infections affecting the mother during pregnancy, general biochemical disorders of childhood which affect the eye (such as defects in the way certain sugars or calcium salts are handled). Sometimes no cause is found.

Eye structures affected: By definition it is the lens of the eye which is affected. Sometimes cataract may be part of a syndrome affecting other parts of the eye as well: for example, aniridia and microphthalmos.

Effect on vision: It has recently been discovered that good vision during the first three months of life is critical for the growth and development of the visual brain. When cataracts are discovered at birth, cataract surgery is ideally carried out during the first few weeks of life (provided the cataracts are severe enough to be significantly affecting vision) and replacement lenses given straight away. In cases where surgery has been delayed there may be permanent significant visual impairment (called deprivational amblyopia) despite later surgery, because the initial programming of the visual brain has not taken place. Children born with cataract in one eye rarely regain useful vision following surgery in the affected eye since deprivation amblyopia is exaggerated when the other eye sees well, although such children will not be visually impaired.

Some opacities of the lens are of no consequence and may not even be noticed. The effect on vision depends on:

  1. The age of onset - cataracts present at birth or in infancy can cause severe amblyopia;
  2. The density of the opacity - that is, how much light can pass through it and;
  3. Position - opacities in the centre of the lens interrupt light passing to the macula and therefore tend to cause a more severe loss of acuity than opacities situated near the edge of the lens.

In addition to reduction in vision, cataracts can cause light entering the eye to be scattered therefore causing the child to experience glare. This is particularly common if the opacity is situated towards the back of the lens, and this type of cataract is often associated with retinitis pigmentosa and uveitis.

Other associations: Cataract may be associated with a variety of disorders affecting the eye such as the rubella syndrome, retinitis pigmentosa, chronic uveitis, aniridia and many others. It may also be associated with disorders affecting the rest of the body such as Down's syndrome, or the biochemical disorders mentioned above.

How it is detected: Since early treatment is very important for successful treatment of cataracts present at birth, it is essential that they are detected very soon after birth. Doctors in charge of babies make a routine examination of the whole baby soon after birth and this should include shining an ophthalmoscope at the eye and looking at the pupil which normally has a reddish glow (as seen in flash photography). Significant cataract will block this 'red reflex' showing as a black shadow (Plate 23). Thus all newborn babies are 'screened' for cataract. Suspicious cases are then referred to an ophthalmologist for examination with the microscope.

Plate 24 Aphakic spectacles

Medical treatment: Children with cataracts significantly affecting the vision of both eyes require cataract surgery which involves removing the lenses. Absence of the lens is termed aphakia. The lens accounts for about a third of the eye's refractive power and its absence results in reduced focusing power. All aphakic people to have to wear thick, convex spectacle lenses (Plate 24) after cataract surgery to correct this, but now almost all adults undergoing cataract surgery have an artificial lens implant inserted in the same site as the natural lens. This removes the need for such thick glasses and gives much less distortion of images than the glasses used to. There is an increasing trend towards the insertion of such implants in infants and children as well, but there are a number of technical problems associated with this so it has not been popular in the recent past. There are a large number of school-age children at present who have had cataracts removed and have not had artificial lenses implanted. They require either contact lenses or thick glasses (called aphakic glasses) to achieve a clear image. Contact lenses give a better quality of image and may therefore be preferable if they can be tolerated by the child and managed by the family. Indeed, aphakic spectacles enlarge the image so much (a side effect of their thickness) that they cannot be used unless both eyes are aphakic, since if one eye is normal, the sizes of the images created by the two eyes would be so different that the brain could not combine them resulting in double vision. Whichever form of optical correction is used (including the intra-ocular lens implant), extra reading glasses are essential for near work since, as described in Chapter 1, it is the elasticity of the natural lens which allows the eye to focus on near objects (accommodation) and this variable focus is absent in aphakia.

The trend is towards early surgery if it is required. The other trend is towards implanting artificial lenses wherever possible even in very young babies. It is of course essential that aphakic children (especially under the age of eight) wear their aphakic optical correction all the time, be it glasses or contact lenses, because, as we have said, blurred images in young children can lead to permanent amblyopia. Even if a lens implant is present, glasses or contact lenses may also be necessary since as the eye grows, its focusing power changes. Reading glasses will certainly be required before school is started because the lens implants in current use do not have the facility to change shape as the natural 'elastic' lens does to bring the normal eye into focus (Chapter 1). Usually further surgical or laser procedures are required if the lens capsule (left in place at the initial operation to support the lens implant) thickens up causing a similar blurring of vision to the initial cataract.

Progression: This is variable. Some cataracts may not interfere with vision when first detected, but the child needs to be watched carefully over time in case vision gets worse and requires surgery at a later stage. Other cataracts may never progress to a stage requiring surgery.

Educational implications: Children should be encouraged to wear their glasses or contact lenses at all times at home and school. Separate reading glasses will usually be required for close work. Bright diffuse lighting without glare will improve visual performance in many cases. This is because it causes the pupil to become smaller, resulting in greater depth of focus, allowing the child to see more clearly for near, distance and intermediate distances. The teacher should look out for any apparent deterioration in vision in the child with unoperated cataract (where the cataract may be getting worse) or with operated cataract (where capsule thickening may occur as explained above). Frequent hospital visits and sometimes multiple operations may be necessary which may interfere with school, and should ideally be planned during school holidays if practicable and appropriate.

The whole eye

a) Infections passed from mother to child during pregnancy
b) Absent or very small eyes
c) Coloboma (plural -colobomata) (Plate 25)
d) Albinism (Plate 26)
e) Aniridia (Plate 28)
f) Childhood glaucoma (Plate 29)

a) Infections passed from mother to child during pregnancy

Definition: Most infections acquired during pregnancy do not damage the developing foetus but there are some notable exceptions, including those listed.

Cause: During pregnancy, virus infections such as rubella, cytomegalovirus (CMV), and the AIDS virus (HIV) as well as other types of infection such as toxoplasma and syphilis may damage the foetus and the developing visual system. The incidence of rubella has decreased dramatically with the widespread introduction of effective vaccination, but the incidence of HIV infection is on the increase.

Toxoplasma infection is acquired from eating undercooked meat or by eating unwashed vegetables contaminated by infected cat faeces. It is the most common infective cause of childhood visual impairment in developed countries, but fortunately only affects one eye in most cases.

Eye structures affected: Almost any part of the eye can be affected but there are certain characteristic features. Rubella infection causes greatest damage to the foetus during the first three months of pregnancy when the organs such as the eye, ear and heart are being formed. Because of this the eye may be underdeveloped and smaller than it should be (microphthalmos). Other ocular features of the rubella syndrome include congenital cataract (the clouding is usually in the centre of the lens and will significantly reduce acuity and require early surgery to prevent amblyopia); a mottling of the retina (pigmentary retinopathy) which may progress with age but usually only reduces the acuity to around 6/12 if no other parts of the eye are affected; corneal clouding and glaucoma (usually associated with microphthalmos).

Foetal damage in general, and ocular involvement in particular, are less common after CMV infection but can include, in addition to the above, inflammation of the choroid and retina (chorioretinitis) and damage to the optic nerve (optic atrophy).

Life expectancy for children infected with HIV has been poor but as treatment improves these children may live long enough for visual impairment to become a concern - certainly HIV infected adults may become severely visually impaired usually fairly near the end of the course of this fatal disease. The cause is a viral infection of the retina (usually CMV) and this is an example of the reduced immune defences of the body allowing a virus, which rarely has serious consequences in a person with normal immune defences, to cause serious damage to the AIDS patient.

The damage from toxoplasma infection usually also involves inflammation in the retina and choroid. This may be present at birth but there are examples of infected infants being apparently normal at initial examination who develop severe visual impairment even years later. Microphthalmos, cataract and optic atrophy can also occur. Syphilis is, like rubella, now uncommon in the UK, though it is seen more frequently in some other countries. Corneal clouding is the best known manifestation of congenital syphilis but all the other defects mentioned above with other infections can occur.

Effect on vision: Microphthalmos is usually associated with poor vision making associated cataract and glaucoma difficult to treat. Cataract and corneal clouding tend to cause reduction in visual acuity as does chorioretinitis affecting the macula. Peripheral retinal inflammation, optic atrophy and glaucoma may all cause additional loss of visual field.

Other associations: The following features are particularly associated but the list is not exhaustive. Neither are these features always present:

  • Rubella: growth retardation, congenital heart defects (such as a hole in the heart), auditory impairment.
  • CMV: malformations of the brain with associated learning and motor disorders.
  • HIV: prone to a number of disabling and dangerous infections and tumours; reduced life expectancy.
  • Toxoplasma: calcium deposition in the brain, epilepsy, hydrocephalus.
  • Syphilis: auditory impairment, teeth malformations, bone malformations.

How it is detected: The mother may be aware of an infection (eg rubella) during pregnancy but quite often there may be no such history. Toxoplasma and CMV infections, for instance, often pass unnoticed in adults. Blood tests can be performed (in mother and child) to measure antibody levels to specific infections if they are suspected - if antibody levels are raised, this indicates recent infection and helps make the diagnosis.

Medical treatment: Cataract surgery may be indicated and corneal grafting is occasionally helpful. Inflammation may recur in the older child (eg toxoplasma in the retina) and this may require treatment with antibiotic and steroid tablets. Therefore if a pupil with a known diagnosis of toxoplasmosis complains of a recent deterioration or distortion of vision then an urgent referral to the ophthalmology clinic is appropriate since drugs may damp down the inflammation and help preserve remaining vision.

Progression: Although the damage in these conditions is present at birth, sometimes the infecting organism stays in the body and the eye of the child for years and can cause progressive damage or recurrent attacks of inflammation. If glaucoma is present this can cause progressive visual field loss. As is always the case with significant cataract or corneal clouding present at birth, the potential for severe deprivational amblyopia exists if the vision is not surgically cleared quickly.

Educational implications: These are very variable depending on which parts of the eyes are affected in a particular child and which non-ocular abnormalities are also present.

b) Absent or very small eyes

Definition: A range of abnormalities of development of the eye including complete failure of development with no eye present (anophthalmos); incomplete development and a small eye (microphthalmos); small eyes with varying degrees of skin covering them (cryptophthalmos).

Cause: These are usually 'one-off' anomalies rather than being passed on in families. A number of environmental factors have been implicated including maternal infection during pregnancy, radiation, and chemical exposure. There are also some genetic syndromes which include anophthalmos or microphthalmos as part of a wide range of abnormalities. In most cases no specific cause is identified.

Eye structures affected: In microphthalmos the cornea is usually but not always small. The pupil may be an odd shape due to coloboma of the iris. This condition is dealt with in more detail in section c but represents a failure of a cleft in the developing eye of the embryo to close properly.

Effect on vision: In bilateral anophthalmos blindness is inevitable. In microphthalmos the degree of visual impairment depends on the severity of the abnormality in each eye.

Other associations: There are a number of syndromes which include microphthalmos for example the CHARGE syndrome which comprises heart defects, nasal abnormalities, growth retardation, genital and ear anomalies.

How it is detected: The defect is often obvious on examination though careful examination or even ultrasound or X-ray scanning may be required to detect buried remnants of poorly developed eyes.

Medical treatment: The level of vision must be ascertained and any refractive error corrected (small eyes are likely to be long sighted (hypermetropic)). Long sighted spectacle lenses will also make the eye appear bigger to others and may therefore provide a cosmetic improvement (long sighted lenses always have a magnifying effect). Cosmetic contact lenses or shells may be appropriate for non-seeing eyes. Glaucoma may be associated and this must be treated with surgery and or eye drops.

Progression: These developmental abnormalities are likely to be non-progressive. Associated glaucoma may however cause progressive visual field loss.

Educational implications: Visual impairment may be severe, microphthalmos accounting for 10% of blind children in one study. Braille usage may be required.

c) Coloboma (plural colobomata) (Plate 25)

Plate 25 Coloboma

Definition: A notch-like defect in any part or parts of the eye from the eyelid at the front to the optic nerve or retina at the back which is present at birth and is non-progressive.

Cause: Any interference with the development of the eye in the embryo can cause a coloboma. The notch-like cleft is present in the developing eye and the normal process is for the two edges of the cleft to grow towards each other and then join. It is failure of this joining which results in the coloboma. Rarely the cause can be genetic. Often, as for many other congenital anomalies, no cause is found.

Eye structures affected: Any eye structure can be affected, the eyelid, iris, lens, optic disc and retina being among the more common. The size of the defect varies from being barely noticeable to an almost complete absence of the structure involved. Optic disc and retinal colobomata may result in retinal detachment later in life.

Effect on vision: This will depend on the size and the position of the defect. In general, larger defects and those nearer the back of the eye (optic disc and retina) are more likely to cause visual impairment. There may be other associated abnormalities within the eyes, or occasionally within the visual pathways of the brain, which may affect vision.

Other associations: A large number of associated conditions can occur both within and outside the eye. Colobomata affecting both eyes may be associated with some abnormalities of the central part of the brain which is responsible both for control of hormones (for example growth and sexual development) and for the connections between the right and left sides of the brain.

Microphthalmos or anophthalmos can be thought of as the most extreme type of colobomata and in some genetic cases both microphthalmos and colobomata may be present in different eyes of different family members.

One relatively common syndrome associated with optic nerve colobomata may result in dual sensory impairment (other causes including congenital rubella and Usher's disease) and has acquired the mnemonic CHARGE syndrome, representing Coloboma, Heart defect, Atresia (narrowing) of the nostrils, Retardation of growth (and sometimes intellectual development), Genital anomalies and Ear abnormalities and deafness.

How it is detected: Colobomata affecting the front structures of the eyes (eg the lid or iris) are easily noticeable. Smaller, more subtle defects and those at the back of the eyes are detected by ophthalmological examination.

Medical treatment: Surgery is rarely useful but may be so if there is a large defect in the eyelid resulting in exposure of the front of the eye or jf there is an associated retinal detachment. Any associated ocular or non-ocular conditions must be treated appropriately.

Progression: The defect itself is non-progressive although, as explained, complications may develop progressively such as damage to the cornea because it is not adequately covered, or retinal detachment.

Educational implications: These are as variable as the condition itself. Only children with both eyes affected may be visually impaired. Severe anomalies may produce significant visual and cosmetic impairment. Large iris colobomata may result in photophobia and reduced vision in bright focal lighting conditions similar to that experienced by children with aniridia. The importance of specialised training for the teachers of children with dual sensory impairment (whether due to CHARGE syndrome, Usher's syndrome or other causes) is well·recognised.

d) Albinism (Plate 26)

Plate 26 Albinism

Definition: Any congenital condition in which the colouration (pigmentation) is reduced involving the skin and the eyes (oculocutaneous albinism), or the eyes alone (ocular albinism).

Cause: This is a genetic condition, though the way it is inherited varies. Some forms are passed down through the generations (dominant), some occur with no warning when both parents are coincidental carriers without the disease (recessive) and some forms affect only male children but are passed on only through female carriers (X or sex linked). The more severe forms tend to have recessive inheritance and the milder forms dominant. Albinism affecting only the eyes (ocular albinism) tends to be inherited as X-linked. (See Glossary for more details of these inheritance types.)

Eye structures affected: The iris and choroid lack coloration and in obvious cases the light reflected from the retina through the iris may give the eye a reddish appearance. The fovea is not properly developed and there is some miswiring of nerve fibres where they cross.

Effect on vision: The abnormal fovea means that visual acuity is reduced and this is associated with nystagmus (or wobbly eyes). (Other diseases which cause severe visual impairment from birth, such as untreated cataract or cone dystrophy, may also be associated with nystagmus.) Distance vision tends to be more affected than near, probably because the nystagmus is less for near vision (Plate 8) and hence the image is stabilised. The degree of visual impairment can vary with visual acuities between 6/9 and 6/60 in the majority of cases. The lack of coloration in the iris and choroid means that too much light enters the eye and is not absorbed properly. Therefore bright light causes discomfort which is called photophobia. In focal bright light conditions the image can be degraded (Plate 27). Refractive errors and squint are common.

Plate 27 Photophobia

Other associations: Children with oculocutaneous albinism lack colouration in the skin as well as the eye. This coloration is protective against the harmful effects of sunlight and hence these children are more prone to skin cancers. Appropriate skin care is therefore required. This is a more significant risk in hot countries where the normal population have darker skin for greater protection from the sun. In such countries light skinned people with albinism will also look more noticeably different from the normal population and may suffer correspondingly more social stigma. Rarer associations include repeated infections (Chediak Higashi syndrome) and easy bruisability (Hermansky-Pudlak syndrome). There is some evidence that people with oculocutaneous albinism have a tendency to have a high IQ.

How it is detected: Ophthalmic examination may reveal subtle thinning of the iris in mild cases as well as light coloration of the choroid. Nystagmus may be the first indication, and knowledge that other family members are affected, or examination of family members to detect subtle disease or gene carriers may be helpful.

Medical treatment: Any refractive errors must be corrected and low vision aids are often helpful. Tinted lenses are helpful in reducing photophobia.

Progression: The disease is non-progressive.

Educational implications: Positioning in the classroom is important both in relation to lighting (having windows and lights behind the child rather than in front will reduce photophobia as will the use of translucent material to diffuse the sunlight) and in relation to distance from the blackboard, remembering that distance vision is usually poorer than near.

e) Aniridia (Plate 28)

Plate 28 Aniridia

Definition: This is a group of disorders involving under-development of the iris.

Cause: This is caused by an error in the genes (mutation), sometimes passed on from parents and sometimes arising as a new mutation.

Eye structures affected: Almost all the eye structures can be affected. Although the iris may appear to be completely absent to the naked eye there is usually a microscopically small stump of iris present. Corneal opacities, cataract, lens dislocation, glaucoma and optic nerve hypoplasia can be present in varying combinations in some cases.

Effect on vision: Glare and photophobia are problems due to the lack of protection of the eye from light by the iris. The acuity may be reduced due to any of the above features or combinations of them.

Other associations: This condition can be associated with the kidney tumour called Wilm's tumour though this is uncommon in the inherited form. Other abnormalities of the kidneys, learning difficulties and growth retardation have also all been associated.

How it is detected: The characteristic reddish appearance of the eye results from the absence of iris tissue.

Medical treatment: This involves correction of any refractive errors which might be present and provision of tinted glasses to reduce the amount of light getting into the eye. Occluder contact lenses with an artificial iris painted on have also been tried for this purpose. Cataract and glaucoma may require surgical treatment.

Progression: Corneal problems and glaucoma may follow in later life.

Educational implications: Positioning of the child in the classroom needs consideration with light sources behind him or her. As for albinism, diffuse lighting is preferable to focal lighting. LVAs may be appropriate when acuities are reduced. The child may be concerned about the unusual appearance of the eyes in which case cosmetic contact lenses may be helpful. Regular hospital visits may be necessary if glaucoma or cataract are present.

f) Childhood glaucoma (Plate 29)

Plate 29 Childhood Glaucoma (Buphthalmos)

Definition: A range of conditions all involving the pressure inside the eye being too high and causing progressive damage to the optic nerve and impairing vision.

Cause: The exact cause of straightforward childhood glaucoma is not known though hereditary factors are thought to playa part.

Eye structures affected: The wall of the young eye (sclera and cornea) is more elastic than that of the adult eye and it will stretch when the pressure inside it goes up, enlarging the eye (buphthalmos) mentioned earlier. The stretching of the cornea may damage its transparency and leave characteristic stretch marks, visible under high magnification. As explained in Chapter 1, the optic disc where the optic nerve leaves the eye is vulnerable to damage by raised pressure and the nerve fibres can progressively die and disappear as they pass through the disc.

Effect on vision: In many cases early detection and surgical treatment controls the pressure and prevents loss of vision, but blindness can occur in a minority of cases. Corneal stretching, damage to the optic nerve and amblyopia due to visual deprivation at an early age all contribute to the visual impairment. Optic nerve damage at the optic disc usually affects parts of the peripheral field of vision before central visual acuity is lost because nerve fibres from the peripheral retina are the most susceptible to raised pressure.

Other associations: Glaucoma can also occur as a part of or as a result of several other conditions affecting the eye such as aniridia, rubella syndrome, Reiger's anomaly (a group of conditions running in families in which the pupil may be an irregular shape and the iris is abnormal), uveitis (particularly the type of uveitis associated with childhood arthritis) and retinopathy of prematurity.

How it is detected: Apart from enlargement of the eye and corneal clouding which may be obvious, affected infants characteristically rub their eyes and may dislike bright light. The eyes will often water a lot (though glaucoma is an extremely rare cause of watery eyes in infancy).

Medical treatment: Surgery is usually required to achieve a lasting reduction in pressure though even if this is achieved, a significant proportion of eyes may remain visually impaired. Because the eye may be enlarged, a refractive error is likely to be present (enlargement of the eye tends to cause myopia as explained in Chapter 2) and prescription of appropriate glasses may help the child make full use of available vision. Eye drop treatment to maintain a low pressure in the eye may be required in the long term.

Progression: Once the pressure in the eye is controlled the visual impairment should not progress. Uncontrolled pressure will result in progressive damage to the optic nerve and with it the visual field.

Educational implications: Visual impairment may involve acuity (corneal clouding and amblyopia) and/or peripheral visual field (optic disc damage). Both factors must be considered for each child, remembering that children with limited acuity may benefit from sitting close to the front of a class and possibly from LVAs, and that children with limited peripheral vision may have only partial visual appreciation of the classroom environment around them and may need help with mobility. Classroom lighting needs to be considered since photophobia can be a feature, in which case diffuse lighting is therefore preferable. The wearing of glasses if appropriate should be encouraged. The child may be self-conscious about the unusual appearance of the eyes which may be different sizes from each other or obviously enlarged. Regular hospital visits and repeat operations are sometimes necessary which may interfere with schooling.

The back of the eye

a) Retinoblastoma (Plate 30)
b) Retinopathy of prematurity
c) Stationary night blindness
d) Stationary cone disorders
e) Leber's amaurosis
f) Retinitis pigmentosa (Plate 31)
g) Macular dystrophies
h) Norrie's disease and Incontinenta pigmenta
i) Batten's disease

a) Retinoblastoma (Plate 30)

Plate 30 Retinoblastoma

Definition: This is the commonest cancer of the eye which occurs in childhood. It consists of a tumour or tumours of retinal cells and usually develops before the age of four.

Cause: The cause of this disease is an abnormality in a particular gene responsible for normal cell growth in the retina. This mutation predisposes to tumour formation and may be passed down the generations of a family in the inherited type of the disease. Isolated cases can also occur in which the mutation has taken place in a single cell in the retina. Genetic testing can reveal whether a child who survives retinoblastoma but has not inherited it from the family (an isolated case) is at risk of passing the disease on to his or her children.

Eye structures affected: The tumour(s) arise(s) in the retina but can spread forward into the rest of the eye or backward along the optic nerve. Inherited retinoblastoma is more likely to produce multiple tumours and to affect both eyes than the non-hereditary form. The inherited disease tends to arise in younger children than the non-hereditary form.

Effect on vision: Children with only one eye affected may not be visually impaired, but if both eyes are involved, one eye may have been removed surgically and the vision of the surviving eye depends on the size and position of the original retinal tumour. Small tumours well away from the macula may have little effect on vision, whereas those involving the macula may significantly impair vision even if the tumour itself responds well to treatment.

Other associations: This is a potentially fatal condition and if untreated death is almost inevitable. With modern treatment, however, more than 90% of children survive.

How it is detected: The tumour is usually picked up because the pupil appears white rather than the normal black appearance. A squint or a red painful eye are alternative presentations. All children with a family history should be examined regularly and the disorder detected at an early stage. Examination by an ophthalmologist reveals white tumours in the retina in one or both eyes. Scanning of the brain or other organs such as the liver may be necessary to see if the cancer has spread, so that appropriate treatment can be planned.

Medical treatment: The medical care in retinoblastoma involves careful examination and investigation of the child for any sign that the cancer might be spreading. Treatment of the tumour itself may involve removal of the whole eye (enucleation) when the other eye is healthy and unaffected. If both eyes are affected the worse affected eye may be removed or treated with radiation and the tumour in the better eye may be treated with radiation, freezing or laser. Anti-cancer drugs may also form part of the treatment. Only occasionally is removal of both eyes necessary. Genetic counselling is offered to the family members, particularly about the complicated genetics and the risks to other and future family members.

Progression: New retinal tumours or recurrences of treated ones are most likely in the first year after diagnosis.

Educational implications: The type and severity of visual impairment depends on whether both eyes are involved, where the tumours are situated in the retina and how big they are. For example, tumours involving the macula are likely to have a severe effect on acuity and central vision. Radiotherapy may sometimes cause cataract development which poses a further threat to vision. In addition to visual impairment there are the psychological implications of a child having to cope with cancer and possible removal of an eye. These psychological effects may be influenced in hereditary cases by the previous experiences of other family members. Because of the complexity of medical treatment and the risks of recurrence, regular hospital visits, some prolonged, are inevitable; such visits may be sight and life-saving but may severely interfere with education and, if practicable, should be timed as far as is possible in conjunction with educational needs and programmes in mind.

b) Retinopathy of prematurity

Definition: A scarring disease of the retina developing in premature and low birth-weight infants. The disease used to be known as retrolental fibroplasia.

Cause: This disease has developed only relatively recently (1942) as the care and therefore survival of severely premature babies has dramatically improved. Before this, babies at risk of the condition would not have survived. Retinopathy of prematurity is becoming an increasingly common cause of visual impairment in developing countries as the quality and availability of neonatal care improves.

The exact cause of the condition is not known, but relates to the fact that the blood vessels which supply the retina are not fully developed until a baby reaches full term, and so when babies are born very prematurely, parts of the retina do not have adequate blood supply. In the early cases, excessive oxygen therapy was thought to be the principal cause, but now this is administered to babies at risk more cautiously. This has resulted in a reduction in the frequency of the disease but not eradication, and it is likely that other factors relating to birth weight and genetics also playa part.

Eye structures affected: The retina is the main site of the problem and there are various stages of severity of involvement. The milder degrees of the disorder usually resolve spontaneously and only the more severe stages require any form of treatment. These severe stages involve scarring of the retina and most severe of all, the retina being pulled away from its normal position against the choroid (retinal detachment). As explained in Chapter 1, the retina depends on the choroid for most of its oxygen supply, and therefore such detachment from the choroid produces permanent damage to the retina.

Other effects on the eye seen in this condition include myopic refractive errors, squint (misalignment of the eyes so that they are not looking in the same direction), amblyopia and optic atrophy.

Effect on vision: As understanding of the condition and its prevention and treatment has increased, the outlook for vision has improved significantly, so that serious eye disease is now largely confined to infants of less than 1000g birthweight. Even among these children, only a minority become blind in both eyes. In less affected children, acuity may be reduced by macular scarring, amblyopia or optic atrophy, and the peripheral visual field may be affected by peripheral retinal scarring or optic atrophy.

Other associations: Because these children are usually born prematurely there is a relatively high incidence of associated brain damage, which may cause learning difficulties and cerebral palsy.

How it is detected: Currently all premature and low birthweight babies at risk should be examined by an ophthalmologist so that any disease requiring treatment can be detected.

Medical treatment: This is performed either with a freezing probe applied to the outside of the eye which freezes right through the wall of the eye onto the affected retina, or with a laser beam directed at the affected retina through the pupil. If severe scarring and detachment develop, complex surgery is occasionally carried out, but the results of this are often very disappointing in terms of the vision achieved. Older children may require patching therapy for amblyopia, and refraction and correction of the short sightedness which commonly accompanies the disease.

Progression: The disease develops and progresses only during the first weeks of life and it is during this time that careful examination is required. Once the child reaches school age, any damage will have been done and further progression would not be expected.

Educational implications: These of course vary with the severity of the visual impairment which varies from mild to blindness, indicating possible braille usage. The macula is often involved and Low Vision Aids (LVAs) may therefore be helpful in addition to spectacles for myopia. Additional learning or movement control disorders (cerebral palsy) may be present, posing additional challenges to the child, family and teachers.

c) Stationary night blindness

Definition: This is a non-progressive inherited retinal dystrophy in which difficulty seeing in the dark is the principal feature.

Cause: This is genetic and it may be inherited in a dominant, recessive or sex-linked form.

Eye structures affected: The rods and cones of the retina are abnormal, although the retina looks normal when it is examined.

Other features which may be associated are marked short sightedness (high myopia), nystagmus or squint.

Effect on vision: Difficulty seeing in the dark is the main effect but some children, particularly with the recessive or sex-linked forms of the disease may have reduced acuity (these children are more likely to have nystagmus).

Other associations: Two other forms of stationary night blindness are called Oguchi's disease and fundus albipunctatus, but these do not normally cause a reduction in visual acuity and there may not therefore be special educational needs.

How it is detected: The complaints of difficulty seeing in the dark associated with poor vision and a normal looking retina will lead to electrical tests being performed as explained in Chapter 2 and these have a characteristic appearance in this disorder.

Medical treatment: No treatment is available for the actual disorder of the retinal receptors, but associated problems such as myopia and squint can be treated.

Progression: By definition this disease is not progressive.

Educational implications: Good levels of lighting will help the child maximise vision. For children with reduced acuities, LVAs may be helpful.

d) Stationary cone disorders (achromatopsia)

Definition: These are non-progressive retinal disorders affecting visual acuity and colour vision.

Cause: Genetic, inherited in recessive or sex-linked forms.

Eye structures affected: As the name suggests it is the cone receptors which are affected and may be completely absent (known as rod monochromatism) or in less severe types only the blue cones are present (blue cone monochromatism), the red and green cones being absent. High hypermetropia, nystagmus and photophobia may be present.

Effect on vision: The vision is often better in dim illumination. The visual acuity is about 6/60 with absent colour vision in the most severe form in which the cones are completely absent (called rod monochromatism). Less severe forms may have better acuities. Peripheral visual fields are usually intact, though there may be a defect (or scotoma) in the central field. This is because peripheral vision is mediated by rods and cones whereas central vision and acuity are mediated by cones only (Chapter 1).

Other associations: None is known.

How it is detected: A combination of the above features and abnormal electrical tests give the diagnosis. As explained in Chapter 2, the electrical tests can be used to distinguish between faulty cone function such as in this condition, and faulty rod function which is a major feature of retinitis pigmentosa.

Medical treatment: As for other retinal dystrophies, there is no specific medical treatment available but associated refractive errors must be looked for and treated.

Progression: By definition these conditions are non-progressive.

Educational implications: The main limitations to be borne in mind in the provision of material are reduced acuity and reduced or absent colour vision. LVAs and or large print may therefore be beneficial. Dim lighting conditions will often help the child. Tinted glasses and sometimes dark goggles may also be used to produce darkened conditions with associated improved vision.

NB A related condition called cone-rod dystrophy or cone dystrophy tends to present later in adults or older children with decreased vision and photophobia. Visual acuity gradually deteriorates to 6/60 and later involvement of rods is also common, giving similar symptoms to retinitis pigmentosa

e) Leber's amaurosis

Definition: An inherited retinal dystrophy (dystrophy means an inherited weakness or disorder) involving both rod and cone receptors and causing problems from birth or the first few months of life. Amaurosis comes from the Latin meaning blindness.

Cause: This is genetic and is inherited in a recessive way. This means that both parents are coincidentally carriers of the disease without suffering from the disease and are unaware of the fact that they are carriers until one of their children is affected -each child that they have has a one in four risk of being affected by the disease, and children without the disease have a two in three risk of being a carrier.

Eye structures affected: Although the retina may look normal to the examining ophthalmologist in the early stages, most children develop the characteristic features of a retinal dystrophy as they get older. These include a black speckling on the retina, narrowing of the retinal arteries and the optic disc becoming pale. The eyes do not lock onto targets in the normal way because vision is poor from birth or a very young age, and roving eye movements and nystagmus result. The child may poke his or her eyes since mechanical stimulation of the retina can send signals up the visual pathway which are perceived as flashes of light. Children often have high hypermetropic or occasionally high myopic refractive errors. Cataract and keratoconus may develop in older children and in some cases may be due to habitual eye rubbing.

Effect on vision: Both peripheral visual field and visual acuity are severely affected in this condition because both rods and cones are affected. Visual acuities tend to range from 3/60 to perception of light. It is a relatively common cause of blindness in children.

Other associations: Most cases of Leber's amaurosis occur in otherwise normal children. However a variety of associated problems including learning difficulties, and a range of brain disorders, as well as kidney and heart problems can occur. The frequency with which brain disorders are reported has decreased in recent years and this is due in part to the recognition of specific syndromes involving the retina and brain (see next section) which might have previously been wrongly ascribed a diagnosis of Leber's amaurosis. Moreover, the recognition that blind children may reach some developmental milestones later than sighted children and that this is not due to additional brain damage but simply to blindness has led to fewer cases of 'brain damage' being reported. Hearing difficulties occur in about 5% of children with leber's amaurosis.

How it is detected: The combination of apparently poor vision, nystagmus, eye rubbing, poor pupil light responses and a relatively normal retinal appearance will usually lead the ophthalmologist to investigate by means of the electrical tests explained in Chapter 2 . The ERG is severely affected in Leber's amaurosis and performing the test in the light and dark helps distinguish the condition from congenital stationary night blindness or achromatopsia (see previous section) which may otherwise look similar.

Medical treatment: As for all retinal dystrophies there is no treatment available at present for the basic underlying condition of the rods and cones and so medical care is limited to treating any associated refractive errors or cataract (if appropriate).

Progression: The disorder is progressive in some cases. In other cases however, an unexpected improvement of vision can take place during the first few years of life.

Educational implications: These children have at best severe visual impairment involving acuity and field and often are completely blind, and need to use braille. The fact that most of these children will never have experienced useful vision will have an effect on their concept of the world and the way they learn compared to those who have become progressively blind at an older age as, for example, in some cases of retinitis pigmentosa.

f) Retinitis Pigmentosa (RP) (Plate 31)

Plate 31 Retinitis pigmentosa

Definition: A group of genetic conditions initially involving progressive night blindness and peripheral visual field loss and in some cases progressing to loss of vision. The condition may be confined to the eye or part of a more widespread disorder affecting other parts of the body.

Cause: This is genetic and it can be inherited in a dominant, recessive or sex-linked way. The recessive and sex-linked types tend to come on at an earlier age and to affect vision more severely than the dominant types.

Eye structures affected: It is the rods of the retina which are primarily affected, though cones may be affected later. This accounts for the poor night vision and Joss of peripheral vision as early symptoms, and the later involvement of central vision. The appearance of the retina when examined by the ophthalmologist gives a black speckled pattern, paleness of the optic disc (since the nerve fibres of the ganglion cells which make up the optic disc gradually die off as a result of the retinal damage) and narrowing of the blood vessels which supply the retina.

Apart from the primary retinal problem, cataract may develop and also the macula of the retina may become waterlogged (called macular oedema).

Effect on vision: The early problems comprise reduced vision in poor light and reduced peripheral visual field. Early visual field defects tend to be in the upper field but these gradually grow to give a ring shaped field defect (ring scotoma). Further progression in the more severe cases may lead to only a tunnel of central vision remaining (Plate 12). Central vision may be affected by progressive involvement of the cones in the disease or by cataract or macular oedema (see above).

Other associations: There are a number of syndromes associated with RP-like retinal dystrophies and they are outlined here:

How it is detected: The condition may be diagnosed at various ages depending on its severity. The child will usually describe difficulty seeing in the dark. Older patients may present when they notice their field of vision becoming increasingly constricted (by which time they often have severe visual field loss). Examination reveals the signs mentioned above and the diagnosis is confirmed by performing an ERG (see Chapter 2) which has a characteristically reduced result especially in the dark.

Medical treatment: There is no specific treatment for retinitis pigmentosa except in the case of Refsum's disease and abetalipoproteinaemia. Medical care may initially be limited to full and sympathetic explanations together with genetic counselling. Macular oedema is sometimes treated with medication such as steroids and cataract surgery is sometimes required, though not usually in childhood.

Progression: This varies according to the type of the disease. Sex-linked cases of retinitis pigmentosa are night blind in early childhood. and tend to show extensive field loss by the early teens and central visual loss in the twenties. By the fourth decade most patients have vision reduced to less than the ability to count fingers. The recessive form is extremely variable but is usually early in onset and severe. The outlook is much better in the dominant form in which night blindness and field loss may develop in childhood, but in the long term most patients retain reasonable visual acuity until the age of forty or fifty or even throughout life.

Educational implications: It must be remembered that most children with RP will have quite restricted visual fields despite reasonable acuity, and that this has implications for the way in which the learning environment in the classroom should be set up, and for the child's ability to navigate through the environment as he or she moves around. Obviously, lighting levels must be adequate since vision is poor in dim lighting. Ironically many RP children have poor vision in bright sunlight and have problems in adapting from bright to dim illumination. Tinted lenses can be helpful in this respect, especially in summer. The most severe cases will have reduction in acuity even at school age and Low Vision Aids (LVAs) may be required.

  1. Abetalipoproteinaemia
    This is a rare, recessively inherited condition affecting the way fat is absorbed into the bloodstream from the diet and can also produce a disorder of balance because the balance part of the brain (the cerebellum) is affected in addition to the retinal dystrophy.

  2. Refsum's disease
    This is another rare, recessively inherited condition due to a biochemical abnormality and producing an RP-like condition as well as a cerebellar disorder causing balance problems and damage to the nerves running throughout the body to and from the brain. Other features can include an absent or reduced sense of smell, deafness and heart problems. Night blindness is a common early symptom but the condition is not usually diagnosed until early adult life. Special dietary restrictions (for example, avoiding meat from grazing animals) may prevent many of the above features but the effect of diet on the progression of the retinal disease and deafness is not certain. A rare form of the disease comes on in infancy.

  3. Usher's syndrome
    Again, this is usually a recessively inherited disease featuring severe hearing loss from birth as well as RP. It is the most common of the various syndromes associated with RP. Because the severe deafness can be present from birth, normal speech rarely develops. Night blindness is usually reported in late childhood or early teens and often reasonable acuity is maintained throughout the school years but may deteriorate to 6/60 or worse by middle age. It is now clear that there are different types of Usher's disease. Type 1 involves profound deafness from birth with no intelligible speech. Balance problems and learning difficulties may be additional features. Type 2 Usher's disease involves less severe hearing and retinal problems and speech may be present. It should be noted that although Usher's is the best known cause of visual and hearing impairment, a number of the syndromes outlined in this section can produce hearing loss in conjunction with retinitis pigmentosa. Congenital rubella can also cause these problems together though the retinal condition is unlikely to be progressive in rubella.

  4. Kearns-Sayre syndrome
    This is a disease affecting different parts of the body in different ways. It is inherited in an unusual pattern through the mother who is usually an unaffected carrier of the disorder(maternal inheritance, see glossary). Heart rhythm problems can sometimes occur and other features can include brain, muscle and kidney disorders. The eye problems include an RP like dystrophy and progressive limitation of the movements of the eyes which is due to damage of the fibres of the six muscles which move each eye.

  5. Cockayne's syndrome
    Again rare and recessively inherited, this condition involves growth retardation, deafness, a physical appearance of being older than the child's actual age, learning difficulties and RP. In most cases visual and general development is normal during the first year of life, following which there is slow physical and intellectual deterioration with a life expectancy of between ten and thirty years. Additional abnormalities of the eye can include corneal opacity, cataract and nystagmus.

  6. Joubert's syndrome
    This recessively inherited condition involves:
    a) underdevelopment of a part of the brain used for control of movement called the cerebellum;
    b) a retinal dystrophy;
    c) breathing difficulties as a baby;
    d) disturbance of eye movement.

  7. The retinal dystrophy is similar to Leber's amaurosis in infancy but the visual outlook is better with the acuity sometimes reaching 6/18. The cerebellar abnormality will affect the child's ability to learn to perform tasks involving movement and control of the muscles.

  8. Mucopolysaccharidoses
    An RP like dystrophy can develop in most of these conditions (described in the section on 'biochemical disorders'.)

g) Macular dystrophies

Definition: These are a group of inherited disorders where the central part of the retina responsible for central vision and visual acuity (the macula) is predominantly affected.

Cause: The cause is genetic, and both dominant and recessive forms of inheritance are seen.

Eye structures affected: By definition it is the macula which is primarily affected.

Effect on vision: The typical picture is of gradual onset loss of visual acuity in both eyes during the first two decades of life. The degree of acuity loss is variable. In one type of macular dystrophy, Best's disease (which is dominantly inherited), most people keep reasonable reading vision until adult life. Another type, called Stargardt's disease (usually recessively inherited) involves gradual deterioration of acuity to the 6/60 or 'count fingers' level in most patients though acuity may only be mildly reduced at the outset. Colour vision is usually normal early in the disease but becomes abnormal as the macular damage progresses. Peripheral visual fields are usually well maintained in these conditions since the peripheral retina is unaffected. It is appropriate to mention another condition called 'juvenile retinoschisis' in this section. It is not strictly a macular dystrophy but the main effect on vision is from damage to the macula which involves splitting of the layers of the retina in the macular region (hence the name schisis). It is inherited in an X-linked pattern and only boys are affected. It is often picked up between the ages of 5 and 10 causing reading difficulties or failure of the school eye test. Visual acuities are usually in the range of 6/12 to 6/36 at presentation and the outlook for vision is quite reasonable, progression being slow.

Other associations: These disorders are not associated with abnormalities in other parts of the body.

How it is detected: There is often a characteristic appearance of the macula when it is viewed with an ophthalmoscope. For instance, there is a yellow discoloration of the macula in Best's disease which is said to resemble egg yolk (hence its alternative name of vitelliform dystrophy from the Latin for 'yolk'). Electrical tests of the retina as outlined in Chapter 2 (page 51) can be helpful in making the diagnosis.

Medical treatment: No specific medical treatments are yet available. Genetic counselling is important for the families of these children as in all cases of inherited disorders of vision.

Progression: Slow deterioration through life is the general rule, though the speed and degree of this deterioration is variable.

Educational implications: Those children with more severe macular damage may benefit from the use of Low Vision Aids (LVAs). As the condition tends to progress gradually during school-age years, during which time educational material becomes progressively more detailed, a time can be reached when the child starts to experience visual difficulties. It is important to watch out for this problem. Initially, it remains possible to read but it takes longer. This can be associated with frustration, falling behind, and on occasion, challenging behaviour. Enlargement of educational material, where appropriate, and other strategies such as LVAs will help to address these problems.

h) Norrie's disease and incontinentia pigmenti

Definition: Both are rare genetic disorders involving incorrect development of the vitreous jelly and retina.

Cause: Both are inherited in an X-linked pattern, but Norrie's disease only affects boys and incontinentia pigmenti only affects girls (since it is fatal in boys).

Eye structures affected: The eyes are affected in all cases of Norrie's disease, but only in about 35% of cases of incontinentia pigmenti. Because of the abnormalities of the vitreous and retina the pupil may appear white.

Effect on vision: Children with Norrie's disease are blind from birth or early infancy, whereas children with incontinentia pigmenti may retain varying degrees of vision.

Other associations:
Norrie's disease: About a quarter of children have learning difficulties and about a third have hearing problems which may develop at any time from infancy to adult life. lncontinentia pigmenti: The skin (with unusual linear patterns of discolouration), bones, teeth, and brain can all be affected.

How it is detected: The unusual combinations of symptoms usually lead to diagnosis in early infancy.

Medical treatment: Children with incontinentia pigmenti should be seen at regular intervals by ophthalmologists since it is possible that early treatment of the retina with a laser or a freezing probe may prevent progression of the disease.

Progression: Although some progression may be seen in incontinentia pigmenti, which may be preventable by treatment, progression is not a major feature of either of these conditions.

Educational implications: Children with Norrie's disease may need to have access to learning braille or a tactile alphabetsuch as Moon where appropriate, although additional learning and hearing difficulties may present further challenges in their education.

i) Batten's disease

Definition: A progressive condition involving blindness, epileptic seizures and mental deterioration.

Cause: This is a genetic disorder inherited in a recessive pattern. If certain blood cells are examined under a microscope they are seen to contain packets of an abnormal substance. This is because the abnormal gene which causes the disease affects the ability of the body to eliminate a particular fat-like substance. Brain and retinal cells therefore become loaded with this substance causing damage. The exact biochemical nature of this substance has yet to be identified.

Eye structures affected: Initially the macula and soon the whole retina is affected. Ophthalmoscopic examination reveals a characteristic 'bull's eye' appearance of the macula early on in the disease, and later on generalised pigment speckling, pallor of the optic disc and thinning of the retinal arteries (as in retinitis pigmentosa) may all be seen. Although it is the retina which is primarily affected and responsible for the poor vision, keratoconus and cataract may be additional features.

Effect on vision: Children usually present with failing vision between the ages of 4 and 10. One characteristic finding at presentation is eccentric viewing, with the child tending to look above the target, which may be due to relative preservation of the upper retina (if the child fixes above the target, light rays from the target will strike the upper retina). Children usually become functionally blind within three years of diagnosis.

Other associations: Mental deterioration and behavioural disturbance occur early, often predating visual deterioration. The epileptic seizures usually begin between the ages of 7 and 16.

How it is detected: The combination of the bull's eye macular appearance, the fits and the mental deterioration are suggestive of the diagnosis. Electrical tests of vision show characteristic abnormalities and a brain scan shows some thinning of the brain. The microscopic appearance of certain blood cells is also abnormal as described above.

Medical treatment: Anti-convulsant medication is usually successful in controlling the seizures. No other specific treatments are presently available.

Progression: The disease follows a slow downward path with dementia occurring in the teens and death sometimes well into the second or third decade.

Educational implications: Blindness is invariable and therefore access to braille may be appropriate. Additional challenges are presented by the progressive learning difficulties and the seizures. Chronic anti-convulsant medication will be required and side effects such as occasional drowsiness may result. The adverse prognosis will obviously have an effect on the family and their attitude to the child's education.

NB Other rare biochemical storage diseases with similar features to Batten's disease (visual impairment, learning difficulty and seizures) include Tay-Sachs disease and Nieman-Pick disease. A slightly different but related group of biochemical storage disorders (the leucodystrophies) primarily affect a different type of brain cell. In these conditions, difficulties in movement and clumsiness may be more of a feature than seizures or learning difficulties. Optic atrophy and cortical visual impairment can occur in the leucodystrophies.

The seeing nerves

a) Optic nerve hypoplasia
b) Optic atrophy

a) Optic nerve hypoplasia

Definition: Congenital underdevelopment of one or both optic nerves, so that the optic nerve head appears small on ophthalmoscopic examination.

Cause: In the majority of cases, no causeis identified. Maternal diabetes or alcohol abuse, generalised brain damage, and genetic causes can contribute to the development of this disorder. The common link is that something disturbs the development of the optic nerve at an early stage of the embryo in the uterus.

Eye structures affected: Primarily it is the optic nerve. Secondary features may include nystagmus and squint.

Effect on vision: This is variable and depends on the severity of the condition and whether one or both eyes are involved. Severe hypoplasia causes blindness and this condition is now a significant and possibly increasing cause of childhood visual impairment. Some children with lesser degrees of hypoplasia may have reasonable acuities but visual field defects, while others may appear to have normal vision.

Other associations: This condition may sometimes be associated with more generalised brain damage or underdevelopment. One specific association with bilateral optic nerve hypoplasia is called septo-optic dysplasia and can involve underdevelopment of some of the middle parts of the brain including the central connections between the left and right sides of the brain and hormonal centres responsible for control of growth and sexual development.

How it is detected: It is detected by ophthalmoscopic examination which reveals that the optic nerve head is smaller than usual. This is sometimes a difficult sign to pick up and the diagnosis is sometimes missed initially.

Medical treatment: Associated abnormalities such as a lack of hormones in septo-optic dysplasia may require specific medical treatment.

Progression: This is a non-progressive condition.

Educational implications: These depend on the severity of the visual impairment and on the presence of any associated developmental delay. Patients with both nerves affected may have some degree of associated brain damage, particularly the central connections between the left and right brain, which may produce subtle or marked learning difficulties. This condition may also be associated with more profound disorders of brain growth and development which may predispose to a wide range of special needs.

b) Optic atrophy

Definition: A group of diseases involving damage to the nerve fibres of the optic nerve which normally transmit signals from the retina to the visual brain.

Cause: The cause is usually genetic and the type of inheritance varies. Dominant inheritance is the most common and tends to be associated with milder visual impairment than the rarer recessively inherited optic atrophy. Leber's optic neuropathy (completely different from Leber's amaurosis) has an unusual genetic inheritance whereby it is only transmitted through the mother and more commonly affects males, usually as teenagers or young adults. Other causes for such damage to the optic nerves include poisonous side effects of certain rarely used drugs or exposure to heavy metals such as lead. Pressure from a tumour or invasion from a cancer such as leukaemia can damage the optic nerves as can a severe head injury, particularly if the skull is fractured near the small openings at the back of the orbits through which the nerves pass towards the brain.

Eye structures affected: The optic nerves are the only structures usually affected.

Effect on vision: Children with dominant optic atrophy usually present before the age of ten often at the school eye test with symmetrical mildly reduced acuities in the range of 6/9 to 6/24. Visual field defects may affect both fixation and enlargement of the normal blind spot (centrocaecal scotoma). Blue-yellow colour vision defects may also be present. Leber's optic neuropathy tends not to present until the late teens with fairly sudden and profound loss of central vision in one eye, followed shortly thereafter by the other eye. Limited but useful recovery may occur over the following two years. DIDMOAD syndrome (see below) is a rare condition which produces severe but not total loss of central vision and colour blindness. One specific type of visual field loss occurs when the optic atrophy is caused by a tumour pressing at the crossover of the optic nerves (optic chiasm). It is the nerve fibres from the nasal side of each retina which cross over and it is these fibres which are damaged. The nasal part of the retina corresponds to the outer (temporal) part of visual space and hence it is the outer part of the visual field which is damaged in both eyes (called a bitemporal hemianopia), leaving only a narrow strip of vision in the centre. A child with such a bitemporal hemianopia may complain that objects disappear because there is a segment of blindness arising from overlap of the temporal field defects beyond the object of interest, and may show behaviour consistent with this observation.

Other associations: Wasting of the optic nerves (optic atrophy) may occur in association with a number of other illnesses. DIDMOAD syndrome (Diabetes Insipidus, Diabetes Mellitus, Optic Atrophy, Deafness) consists of two types of diabetes as well as both visual and hearing impairment. There is a group of inherited diseases involving the brain and spinal cord and which cause learning difficulties, and difficulties in controlling posture and movement, which may also be accompanied by optic atrophy. Such diseases include Behr's disease, Friedreich's ataxia and others.

How it is detected: Instead of having a normal healthy pink appearance, the optic nerve heads appear pale and white when examined with the ophthalmoscope. Electrical tests may also help make the diagnosis.

Medical treatment: Any cause for the wasting of the nerves such as a tumour, malnutrition or poisoning must be removed if possible. Any additional abnormalities such as diabetes must also be appropriately treated. There is no successful medical treatment for the optic atrophy itself.

Progression: As already mentioned this is variable. Dominant optic atrophy is only slowly progressive and usually mild, whereas Leber's optic neuropathy is characterised by rapid onset severe visual loss with only limited if any recovery.

Educational implications: The type of educational material appropriate will depend on the acuity and contrast sensitivity. Teachers should also be aware of the visual field defects (peripheral vision is often reasonably good) and of the colourvision defects (more often blue-yellow). Good lighting increases reading speed in most forms of optic atrophy.

The seeing brain

a) Cortical visual impairment
b) Hydrocephalus

a) Cortical visual impairment

Definition: Impairment of vision due to brain damage affecting the visual cortex or the pathways within the brain.

Cause: The most common cause of this condition is inadequate oxygen reaching the brain around the time of birth. Premature babies are particularly vulnerable to this type of injury (since the blood vessels supplying parts of the brain are fragile). Since more infants are now surviving due to improved medical care, the incidence of cortical blindness is increasing, and probably now represents the most common cause of visual impairment in children in developed countries. Other causes of damage to the visual brain include accidental or non-accidental head injury, infection such as meningitis and, as described in the following section, hydrocephalus.

Eye structures affected: In general the eyes themselves are unaffected, though there may be disordered control of eyemovement (with difficulty in tracking moving objects), or squint present.

Effect on vision: This can range from the subtle defects of complex visual processing to complete absence of functional vision, depending upon the site and extent of the brain damage. One­sided field defects (hemianopias) may occur if the damage is asymmetrical. Damage to the left visual brain results in absent or disordered vision on the right side and vice versa. If the damage has been caused by an interruption of the blood supply to the brain there may be sparing of central vision despite severe damage to the peripheral fields on one side or the other. This phenomenon is called macular sparing and is thought to arise because the part of the visual cortex corresponding to the maculae (and hence central vision), which is situated right at the back of the brain, has a spare blood supply to keep it going if the main supply is damaged. Cortically blind children may navigate comfortably round objects of furniture and show awareness of moving objects surprisingly accurately even though they appear blind when their vision is tested by normal methods. This form of subconscious vision is called blindsight and is thought to arise from the alternative visual pathway which never reaches the visual cortex and hence never reaches consciousness. This pathway is faster than the cortical pathway and is specially adapted for navigational tasks involving perception of movement (stationary furniture is moving visually when a child moves past it). This form of vision is present in human beings and most other species. It is more highly developed in animals such as birds which have to fly through trees at high speeds without crashing and where fast visual reflexes are essential. This type of vision may not be apparent at birth or even during the first two or three years of life. However, gradual development of navigational vision may take place during the ensuing years. Different aspects of visual processing such as the recognition of motion and recognition of geometric shapes or faces, occur at different sites in the visual cortex, some predominantly in one side of the brain (eg face recognition on the right side, and reading vision on the left) and other aspects of vision on both sides (eg colour and movement perception). The specific weakness of one such aspect of visual processing is called a visual agnosia. For example, inability to recognise faces is called prosopagnosia. Many of these agnosias are still being discovered and defined but they clearly have a profound effect on the way a child handles visual educational material. Figure 1.6 illustrates some of the agnosias which are known to be associated with damage to particular areas of the brain. This information has come from carefully assessing patients' visual capabilities, identifying a particular weakness and looking at brain scans to locate the site of the brain damage. For example it has been found that patients with prosopagnosia commonly have damage to one particular site in the right side of the brain near the back, and so we conclude that the processing of face recognition occurs in this site in the brain.

Other associations: Learning difficulties, posture and movement difficulties (cerebral palsy), seizures and hydrocephalus may all accompany the visual problems in these cases.

How it is detected: The child has visual problems but examination of the eyes themselves reveals no abnormality. Scans of the brain are good at showing up areas of damage and can enable doctors to localise the areas of damage which may give clues as to what type of visual disorders to look for. Children with cognitive visual impairment tend not to be aware that they have a visual problem. Damage to a part of the brain for a specific visual function is associated with damage to the part of the brain responsible for knowing and understanding that function, so that the affected person is not aware of her deficiency. The diagnosis is therefore made by watching the child's visual behaviour and identifying specific inconsistencies of this behaviour.

Medical treatment: No specific treatment is available once the damage is sustained and medical care of vulnerable neonates and infants is directed at trying to reduce the risk and severity of the brain damage occurring in the first place.

Progression: Episodes of cortical visual impairment may be temporary, though complete restoration of vision is unusual. Gradual improvement of visual function may be slow, taking months to years. One study found that cortically blind children often had no light perception initially and with time gained colour vision, form perception and finally, improved visual acuity. Some degree of visual agnosia often remains, however.

Educational implications: These children often have a short visual attention span and their visual skills often seem to vary from minute to minute which can cause confusion to carers -tiredness and overly complex visual information can impede visual perception. Colour vision sometimes appears to be better preserved than vision for shapes and form, and children may be attracted to red and yellow objects in particular. This may be because colour vision is processed on both sides of the brain and so even if damage is sustained to one side, there may be some remaining 'spare' brain on the other side for processing colour. It will therefore be helpful to represent shapes and letters being taught to the child in colour against a highly contrasting and uncluttered background. Children with hemianopias may turn their head to the side when reaching for objects in order to use the good side of their visual field.

Cortically visually impaired children may find it easier to view educational material at a closer range than would be expected. In addition to magnifying the target (the closer you move your eyes to an object, the bigger it becomes) this also reduces the amount of visual information to be processed (the closer you move your eyes to an object, the fewer additional objects there are round the edges of the picture). The reduction in visual performance due to too much visual information is called 'crowding' and teachers should be aware of the limits of the amount of visual information that can be taken in by the child at one time.

A typical example in a younger child is the inability to find a toy on a patterned carpet but the ability to find it on a plain one. A cortically visually impaired child may need to be presented with visual targets one at a time whereas children with other forms of visual impairment may be able to perceive a number of targets simultaneously. Another strategy which may help the child overcome this crowding phenomenon is to trace or follow objects with a finger.

An analogy which illustrates the impairment of processing would be to consider the effect on supermarket queues if, instead of having ten checkout counters working simultaneously, nine of them went out of order and all the customers had to pass through the same counter, one after the other. This lack of ability to parallel process visual information has a profound effect on the way in which visual tasks and processes are learned.

If, for instance, a simple task such as dressing a doll is considered, a child will typically learn to do this by seeing the doll and the dress and imagining simultaneously in his or her mind's eye the doll with the dress on, so that he or she knows what the next stage of the task involves. A cortically visually impaired child may not be able simultaneously to perceive the doll, the dress and the imagined dressed doll and hence will need to be taught to perform the task in a stepwise manner by imitation, with repetition and gradually more of the task being performed by the child him/herself. This will be quite time consuming, especially when more visually complex educational tasks are taught and learned. It is important not to expect the same speed of performance from children with cortical visual impairment as those children in the same class with visual impairment resulting from eye disease.

Some children like to gaze at bright lights, even to the point of gazing into the sun which should be discouraged by the carer because of the risk of burns to the retina. Paradoxically, other children with this condition are distressed by bright light (photophobia).

Some educational strategies which have been found to be helpful in dealing with some of the specific visual problems experienced by many of these children are outlined here:

  1. Problems with recognition
    In order to recognise somebody or something we have to keep a store of the pictures we have seen in the past. Recognition is the result of an image which we see matching an image in our memory. If there is a match, the image is recognised. If there is no match, the image is explored (using all the senses in harmony) ready for future recognition. This is a complex task and different parts of the brain are responsible for different recognition processes. As has already been mentioned, the right part of the brain at the back (also responsible for seeing on the left hand side) is responsible for recognising faces, and damage to this area can cause difficulty in recognising faces (prosopagnosia). This is usually compensated for by voice recognition, but training in memorising certain dressing features or other characteristics can prove helpful.

    The back of the brain on the left (also responsible for seeing on the right hand side) has a major role for the recognition of shape and form. Children with difficulty in this territory often have difficulties with jigsaws, and in understanding and interpreting three-dimensional images.

    Difficulty in reading (dyslexia) or inability to read (alexia) in children with cortical visual impairment may occur in those who have difficulty seeing on the right side. For these children the principal problem is one of converting the written word into a real (or imagined) sound otherwise known as a phoneme. The basis of the strategies required to overcome this problem is to use visual pattern recognition in the same way as Chinese is taught. For example, the child can be encouraged to build up a dictionary of his or her own words upon which they have imposed their own pictures. Rapid 'look and see' reading in which the meaning is extracted but without reading out loud is a useful strategy to attempt in this unusual situation.

  2. Problems with orientation
    This is more common in individuals with damage to the back of the brain on the right. In order to find your way around you need to have a route map in your memory which can be matched to the world outside. Some children with cortical visual impairment have great difficulty in finding their way around, both within buildings and outside. They also have difficulty in remembering where they have put things. A wide range of strategies can be used to get round these difficulties. In the home, colour coding for different doors in the house can be used and can be gradually faded out in order to use a colour based memory system. Outside, by constantly talking about where things are as you move past them, a language based memory can be developed. On a smaller scale, problems with losing things are best circumvented by making sure that 'everything has its place'.

  3. Difficulty in seeing parts of an image against a complex background
    This seems to be most common in children with cerebral palsy and impairment of movement of all four limbs. Such children appear to see a single picture fairly easily against a plain background or can find a toy on a plain carpet. but are unable to extract any meaning from the same picture set against a complex background or to find a toy on a patterned carpet. In education, the term 'figure ground' has been used to apply to this problem.

    From an educational point of view it is logical to provide small amounts of information sequentially against a plain background. Some children can be helped by masking off other words in the text. Reading, of course can be difficult and is very slow. When the purpose of reading is to learn to read, the amount of text is reduced to the amount that the child can cope with. When the purpose of reading is however to access information, use of the spoken word is more efficient.

  4. Impaired depth perception
    As we look at the world around us we create an immediate mental pictorial image of what we see which, of course, we take for granted. This is, however, a very complicated computing task and it is not surprising that difficulties can arise due to the incomplete formation of such 3-dimensional mental images. A particular problem which such children experience is one of going down stairs or steps. On occasion, they find it very difficult to know whether a line on the floor represents a step or not. They also may have a problem in mentally constructing a three-dimensional tactile map of the world. A mobility training programme needs to take these problems, when they are present, into account.

  5. Impaired perception of movement
    This is usually seen in children with damage to the back of the brain on both sides. Children with this problem appear to have difficulty in seeing and interpreting fast movement, whether it be movement of objects past them, or their own movement through the visual world. Such children may prefer cartoon images which do not move much, such as Spot the Dog, or Thomas the Tank Engine.

It is important for teachers of visually impaired children to recognise that no two children with the same diagnostic label will experience exactly the same type of impairment of vision. This is particularly true of cortical visual impairment because of the complexity of processing which takes place in the visual brain. One child with cortical visual impairment may only see stationary objects, another may see only moving objects, Such children need to be assessed as individuals and to have their educational material presented to them in a way that matches their particular visual abilities. It is
also important to recognise that the reason so much brain tissue (40%) is devoted to visual function is that our understanding, interpretation and movement through the visual world is a very complex process. A child who has sustained damage to this part of the brain at or around birth is inevitably unaware that there are any visual problems and it is only by careful observation of behaviour that it is possible to recognise that such problems exist. Moreover, because a child's brain is developing and growing, a child may well learn a range of subconscious strategies in order to either circumvent or overcome the problems. With assistance and training the child may well be able to deal with some of his or her visual problems to such an extent that they are no longer manifest a few years later.

b) Hydrocephalus

Definition: The brain contains cavities (called ventricles) into which clear fluid is pumped from the blood, The fluid passes through the cavities and then out to surround the brain like a water bath and then is absorbed back into the bloodstream, If the fluid is not drained adequately, the pressure in the ventricles builds up (similar to inadequate drainage of aqueous fluid in the eye leading to a build-up of intraocular pressure in glaucoma), the brain tissue is compressed and the size of the cavities increases. This is called 'hydrocephalus' which literally means 'water on the brain'.

Cause: A number of different problems can cause inadequate drainage of brain fluid back into the blood. The child may be born with a malformation of the brain or spinal cord such as spina bifida which causes a block to the normal passage of the brain fluid. Other causes may be acquired in infancy or childhood such as meningitis (which may leave scarring in the brain), bleeding into the brain (infants born prematurely are particularly prone to this problem) or tumours in the brain which may compress the outflow channels.

Eye structures affected: The optic nerves may show wasting or atrophy (see optic atrophy).

Effect on vision: Optic atrophy and cortical visual impairment are both common, and the effects on vision are as explained in the sections devoted to these topics. The visual parts of the brain or visual cortex may be damaged by direct pressure or by interruption to the blood supply, and in some cases by the surgery which involves insertion of a plastic bypass tube into the ventricles (possibly through areas near the visual cortex) to let the water drain out and which may be necessary to save the child's life. Control of eye position and movement is often affected in hydrocephalus. The child may be unable to move his eyes upwards and therefore have to move his or her whole head back in order to achieve this. The nerves to the muscles which move the eyes out to the side are also vulnerable to raised pressure in the brain and if they are damaged the eye may be temporarily or permanently turned in. In this instance, an older child may experience double vision, especially if both eyes see well.

Other associations: In infants the skull bones are relatively elastic and the head may be enlarged (analogous to enlargement of the eye in infantile glaucoma). In older children that elasticity is not there to accommodate the increased pressure and hence features such as headache, drowsiness, nausea and vomiting may occur in addition to any of the visual features mentioned.

How it is detected: The features already mentioned lead to suspicion of hydrocephalus and the diagnosis is confirmed with a brain scan, which clearly shows the enlarged ventricles in the brain.

Medical treatment: Surgeons can perform operations to bypass the blockage to the absorption of the brain fluid. This involves inserting a tube (called a shunt) with one end in a brain ventricle and the other end draining either directly into the heart or into the abdominal cavity from where the fluid then drains naturally back into the bloodstream. These shunts are life saving but are prone to complications such as infection or blockage. All carers and professionals involved with the child should be aware of the signs that a shunt may not be functioning properly since urgent surgical correction is often required. The signs which are indicative of raised pressure include headache, drowsiness, nausea, vomiting, visual or behavioural disturbances, or new abnormalities of eye position or movement. The shunt also has to be inserted through brain tissue which in itself may cause visual field defects or deficits of specific visual function localised to the area of cortex through which the shunt was inserted.

If eye movement or position is a problem then prisms stuck onto the surface of spectacle lenses (or incorporated into them) or even simple occlusion of one eye may alleviate double vision. Squint surgery is sometimes helpful in correcting abnormal eye position. This simply involves repositioning some of the muscles which move the eyes.

Progression: Progression of this condition can usually be avoided by appropriate surgery and careful follow up for any signs of shunt dysfunction.

Educational implications: Cortical visual impairment and optic atrophy are common, and the reader is referred to the educational implications outlined in these sections. The teacher must be aware of the signs of shunt dysfunction as explained above. Repeated hospital visits and or admissions are likely to be necessary which may potentially interfere with schooling.

Richard Bowman, Ruth Bowman & Gordon Dutton
First published by RNIB in 2001
ISBN: 1858782139