Disorders of vision in children: a guide for teachers and carers
Blindness and Visual Impairment Definitions
World Health Organization guidelines for classifying levels of vision in childhood are based on three types of measurement.
1 Distance visual acuity (with both eyes open)
Visual impairment - visual acuity less than 6/18 but better than or equal
Severe visual impairment - visual acuity of less than 6/60 but better or equal to 3/60.
Blind - visual acuity of less than 3/60.
2 Functional vision
- Can see to walk around. Tested by getting the child to walk between two
chair, placed 1 metre apart.
- Can recognise faces. The child is able to identify a person known to them
by visual recognition of face alone at a distance of 3 metres.
- Can see print. A cross, square or circle about 2 cm in size is drawn and
the child is asked to describe or copy it.
- Believed to have useful residual vision (where formal testing not possible).
- Defined as sufficient vision for at least one of the following:
- Independent mobility
- Making social contacts
- Near vision
3 Visual fields
Some assessment of visual field should be made for all children, though categories are not specified for this (see Chapter 2). This classification is designed to be possible to perform in any situation, in any country, however limited facilities are. It recognises that visual acuity is not the only measure of visual function. Some children will have an acuity of poorer than 6/18 for distance but still function remarkably well educationally, whereas others with acuities better than 6/18 may have significant impairment of visual function due to cognitive dysfunction or gross field defects. In practice the best definition for visual impairment in children is any impairment of visual function sufficient to require special educational assistance.
Glossary of Terms
Accommodation: The name given to the shape change of the lens, (which becomes thicker and rounder) to allow focusing on near objects. This flexibility is lost with age (presbyopia). The average age of presbyopia is 45 years, when reading glasses are usually required.
Agnosia: Disorder of processing sensory information. There are many different types and agnosia forms a suffix for words such as prosopagnosia and simltanagnosia.
Akinetopsia: This is a condition in which there is an inability or difficulty in seeing moving objects. This is associated with damage on both sides of the brain, just in front of the area responsible for vision. In this condition, children and adults are able to see and analyse material that does not move. However when it does move it is less easily seen. The quicker an object is moving the harder it becomes to see it, until fast moving objects are no longer seen at all.
Albinism: Genetic disorder involving lack of pigment (colour) in the eyes and/or skin.
Amblyopia: This is a type of visual impairment resulting from inadequate development of the visual pathways during young childhood. It may occur when images formed by an eye are degraded during the early years of life due to for example, uncorrected refractive error or untreated cataract or corneal opacity. This is called deprivational amblyopia. It can also occur if one eye is squinting and the brain suppresses the vision from it to avoid double vision (strabismic amblyopia).
Aniridia: Genetic disorder in which the iris is almost completely absent.
Antibodies: Proteins produced in the blood to fight infections. They can be measured by looking at a blood sample in the laboratory and may therefore help to diagnose the presence of a particular infection such as rubella or toxoplasma.
Aphakia: Absence of the lens of the eye, usually due to surgical removal of a cataract. This condition produces marked long sightedness and requires thick spectacles, contact lenses or a surgically implanted intraocular lens to correct it.
Aqueous humour: Transparent fluid which fills the front of the eye, giving the eye its pressure (which is called intraocular pressure).
Astigmatism: Sometimes illustrated by considering the front of the eye to be shaped like a rugby ball rather than a football (as it should be). The curvature and hence the focusing power of the eye is different in different directions. This is corrected by using cylindrical optical lenses (spectacle or contact) instead of spherical.
Blindsight: This is a condition in which there is absent vision at a conscious level due to damage to the visual cortex. However such children may be able to see to walk about without bumping into anything, and may well have fairly good vision for moving targets. This vision is primarily to the side and probably does not involve central vision. It may be due to intact visual function in the part of the brain responsible for perception of movement, which is absent in those people with akinetopsia.
Buphthalmos: From the Greek meaning ‘ox-eye’ - glaucoma in children’s eyes can produce enlargement of the eyes because of their elastic walls.
Cataract: The name given to loss of transparency of the lens. Commonly age-related but can also be due to trauma, or can be present from birth (congenital). It is the commonest cause of blindness worldwide (adults and children), preventable if enough surgery were carried out.
Choroid: Coloured layer behind the retina at the back of the eye containing many blood vessels.
Ciliary body: The pump which supplies aqueous humour to the eye (making it from blood).
Cognitive visual disorders: These usually result form damage to parts of the visual cortex andmay involve specific problems such as inability to recognise faces of difficulty in simultaneously processing multiple pieces of visual information.
Colour blindness: This is usually used to describe genetically inherited anomalies of cone receptors which would better be termed ‘colour confusion’. Such conditions do now have significant educational implications. Some visually impaired children such as those with cone dystrophies, optic nerve disease or cortical visual impairment, may have abnormal or reduced colour perception.
Cone cells: The less numerous photoreceptors used for central, detailed vision and colour vision.
Congenital: Any condition with which a child is born.
Contrast sensitivity: The ability to distinguish different shades of grey from each other - an important aspect of functional vision. Formal assessments such as the Peli Robson chart are available. Colour contrast sensitivity entails the ability to distinguish varying shades of colour, for example a light blue from a darker blue.
Cornea: The transparent front surface of th eye which covers the coloured part and focuses incoming light.
Corneal graft: An operation in which a scarred or damaged cornea which has lost its transparency is replaced by a donor cornea removed from someone who has died recently. Very fine nylon thread is used to suture the new cornea into place. There is a risk of rejection and many hospital follow-up visits are required in the first two years after the operation.
Crowding: A phenomenon seen in disorders of the visual brain such as amblyopia or cortical visual impairment. If too much visual information is presented to the child at once then visual performance is reduced, whereas good responses may occur when one target is presented at a time.
Emmetropia: Normal ocular optics: that is, no correcting lens is necessary to produce optimum vision from the eye. Visual impairment may or may not be present.
ERG: Stands for electroretinography, a method of assessing vision in children unable to cooperate with subjective methods and also helpful for making diagnoses such as retinitis pigmentosa. The technique involves measuring the electrical signals generated in the retina in response to visual targets.
Fovea: Centre of the macula (2 mm across), where the cones are densely packed. The fovia is the most important point in the retina for detailed vision.
Functional vision: The ability to use eyesight in everyday conditions.
Genetic inheritance: A
condition passed on from generation to generation through an abnormality
in the genes. Some people may have the abnormal gene without having the
disease (carrier). There four main types of such inheritance.
- Autosomal dominant inheritance
We all have two copies of each gene. In these conditions only one of the copies has to be abnormal for the disease to occur so the disease may be passed on from just one parent. As a result, a family tree may show several affected generations. In general when there is such a family history, there will be an approximately 40% chance of any child of the affected parent inheriting the disease. The number of affected or unaffected children already born to the parent does not affect the risk to any future children; it will always be roughly 50%.
- Autosomal recessive inheritance
In these conditions, both copies of the gene have to be abnormal (one normal copy of the gene is sufficient to protect against the disease) and therefore the disease has to be inherited from both parents. The diseases therefore tend to be rarer because they depend on the coincidental reproduction from two unaffected carriers (without the disease because each has only one gene affected). The parents will therefore not know that they are both coincidentally carriers of the disease until their first affected child is born. Any child born to such parents has a one in four chance of being affected and unaffected children have a 67% chance of being carriers. Family trees with affected patients in multiple generations are uncommon in recessive conditions.
- Sex-linked recessive
In these conditions, the gene abnormality is carried on one of the sex chromosomes (a chromosome is a strand of genetic material) called the X chromosome. Females have two of these whereas males have one X and one Y chromosome. As for other recessive conditions (see above) one normal copy of the gene is sufficient to protect against it and therefore in general, females do not have the disease because it is very unusual for both their X chromosomes to be affected, but males do get the disease because there is no 'spare' normal X chromosome to protect against it. Females with the abnormality in one of their X chromosomes are therefore unaffected carriers for the disease and their sons will have a 50% chance of having the disease and their daughters will have a 50% chance of being carriers.
- Maternal inheritance
An unusual inheritance pattern, where both sexes can be affected by the disease but only the mother can pass it on to her children for example with Leber's optic neuropathy.
Glaucoma: Group of eye disorders relating to raised pressure inside the eye.
Homonymous hemianopia: This literally means ‘same-sided half-blindness’. A left homonymous hemianopia means that the left half of vision from both eyes is absent and that a person can only see things on the right side. It can be produced in a variety of conditions affecting the visual brain and pathways after they have crossed the optic chiasm.
Hypermetropia (hyperopia): Long sightedness. Things seen more clearly in the distance than near.
Iris: The coloured part of the eye which is also joined to other coloured parts of the eye, the ciliary body and the choroid, which are not visible without special instruments. Together they make up the uvea. Because the iris is coloured, light cannot pass through it, but instead mjst pass through the central hole in the iris called the pupil.
Ishihara test: A test for colour confusion.
Lens: Transparent structure (similar in shape to a Smartie) situated behind the pupil. Flexible shape allows variable focusing (accommodation) in young people. Loss of transparency is called cataract.
Low Vision Aids (LVA): The term can apply to anything that improves a person’s functional vision, but is normally used when referring to optical LVA, that is, those that use lenses. Examples include hand-held magnifiers and Closed Circuit TV (CCTV).
Maclure Reading test: A standard reading card designed to measure near vision.
Macula: Central (4 mm across) area of the retina which contains most of the cones and is responsible for central and colour vision (it includes the fovea).
Myopia: Short sightedness. Things seen more clearly when near than when in the distance.
Nystagmus: Unstable or 'wobbly' eyes. Usually due to certain types of visual impairment eg; cataract, albinism or cone dystrophy, but sometimes due to abnormalities in the parts of the brain which control eye movement such as the cerebellum.
Ophthalmologist: A doctor who has special qualifications and experience with disorders of the eye and treating them with appropriate medicine and surgery. Also known as eye doctors or eye surgeons.
Ophthalmoscope: A special instrument used by optometrists and ophthalmologists to view the retina. It consists of a torch and lens combination.
Optic chiasm: The (partial) crossover of the optic nerves whereby behind the crossover, all signals from the left of our vision (of both eyes) are processed in the right side of the brain and vice versa.
Optic disc: The exit point of the optic nerve from the eye, seen with an ophthalmoscope, and susceptible to damage in glaucoma.
Optic nerves: The nerves which carry visual signals from the retina to the brain. The cross over in part at the optic chiasm.
Optometrist: A person who specialises in measuring and prescribing corrections for refractive errors. Also known as ophthalmic opticians, they may work in a high streetshop or a hospital department or both. They are also trained to screen for common medical and surgical problems of the eye such as glaucoma and cataract.
Orthoptist: A person who specialises in assessing certain aspects of children’s vision, usually with a particular interest in the management of squint and amblyopia.
Paediatrician: Doctor who specialises in child health.
Peripheral vision: Although our detailed vision is confined to the thing we are looking at any given time,we are also aware of shapes and movements in the edge of our vision. This is very important in our awareness of where we are in space and in helping us to move about.
Photoreceptors: Rod and cone cells in the retina which convert light energy into electrical nerve signals to the brain.
Phthisis: Collapse of the eye due to failure of production of aqueous humour - indicates serious damage to the eye, which will be blind.
Presbyopia: The normal loss of accommodation which is seen with age.
Prosopagnosia: This is a condition in which there is a disability in recognising faces. It is particularly associated with left hemianopia, as it is related to problems of the right visual cortex. In order to recognise people we have to have a store of faces in our minds. If we are unable to build up or access such a store, then the ability to recognise faces is impaired.
Pupil: The hope (aperture) in the iris which appears as a black circles in a normal eye. The size alters according to the amount of light shining on the eye to regulate how much light enters the eye and to increase depth of focus.
Refraction: This is the process by which the eyes are measured for long or short sightedness and/or astigmatism.
Refractive error (emmetropia): The absence of emmetropia, that is, the presence of hypermetropia, myopia, astigmatism or any combination thereof. Visual impairment may or may not be present.
Retina: The screen of the eye onto which the picture is projected (like the film of a camera). It is made up of rods, cones and nerve cells which link to the visual brain.
Retinoscopy: An objective method of refraction suitable for use with babies or those with learning difficulties. It is performed by ophthalmologists and optometrists.
Rod cells: The more numerous photoreceptors used for peripheral vision and for low illumination levels.
Sclera: The white of the eye; a strong coat for the eyeball to protect the delicate structures within.
Scotoma: An area of the visual field where visual sensitivity is reduced (relative scotoma) or completely absent (absolute scotoma).
Simultanagnosia: This is a condition in which complex pictures cannot be simultaneously processed. This most commonly occurs in children who have quadriplegia and who have damage to both sides of the brain in the central region. Such children may only be able to access part of a picture. For example, they may look at a picture of a bicycle and say they see a wheel, but are unable to see the whole. One strategy for the management of this condition is to build up pictures sequentially using simple, large images one at a time.
Snellen chart: The most common method of testing visual acuity. It consists of black letters of different sizes on a white background, held at six metres from the person being tested. The smallest letter that can be read is taken as a measure of acuity and the result is expressed as a fraction, eg; six over sixty, which indicates that an eye can read at six meters away what a normal eye could read at sixty metres away.
Strabismus (squint): Failure of alignment of the two eyes, such that they are looking at two different objects simultaneously instead of at one. Often in children the second image is suppressed by the brain so that they do not see double.
Topographic agnosia: This condition is a disability in route finding, both on large and a small scale, and commonly accompanies prosopagnosia.
Uvea: The iris, ciliary body and choroid together make up the uvea.
VEP: Stands for ‘visual evoked potential’, an electrical technique for assessing vision, measuring electrical signals generated in the visual cortex.
Visual acuity: A measure of the finest detail that can be seen by an eye under optimum conditions.
Visual cortex: The cortex refers to parts of the surface areas of brain responsible for conscious perceptions, thoughts and actions. The visual cortex consists of a primary visual (striate) cortex which receives the signals directly from the eyes and secondary (visual association) areas responsible for various aspects of complex analysis of vision such as shape or face recognition or detection of movement.
Visual field: This is simply everything that can be seen by an eye in any one position, and includes the detailed central vision as well as the peripheral vision.
Vitreous gel: Transparent gel filling the back of the eye.
1. Sitting close to the television can harm your eyes
There is no evidence that sitting close to a TV causes any harm to the eyes. It is just considered antisocial because it blocks the view for others. Children with visual impairment choose to sit at the distance they can see the pictures, so by telling them to sit back from the TV you are impairing their enjoyment and education because they can’t see the programme any more!
2. Children should be barred from activities because of poor vision
Automatic bars on children’s activities because they might hurt themselves probably do more harm than good because educational opportunities are lost. With appropriate support most children with impaired vision can and do take part in a wide range of sports and physical activities. If there are safety concerns, appropriate assessment under supervision will allow any risks to be addressed in a positive manner.
3. The assistant should escort and look after the child
Children included in mainstream education often have an adult who escorts them to school. A learning support or teaching assistant may also accompany them to varying degrees in the day. It is a difficult balancing act to ensure that the assistant’s presence aids rather than impairs social inclusion and the development of mobility. It is essential that assistants receive training for this important role.
4. You can strain your eyes
Eyes are unstrainable! It doesn't matter how close reading material is, or how long you read for, the eyes cannot be harmed.
5. VDU harm your eyes
They don't! There is no scientific evidence showing that VDU and computer screens can harm the eyes. They may cause headaches but they do not cause eye damage.
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'Teaching children with visual impairments' Anthony B Best, Open University Press, 1995
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'AFB practice report: show me how' M Brennan, American Foundation for the Blind, 1982
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'Can't your child see?' J and S Freeman, Pro,ed Publications, 1995
'The world in our hands' (set of five videos and booklets for parents of visually impaired children) Royal National Institute for the Blind, 1993
'Children with visual impairments in mainstream settings -a guide for teachers' Christine Arter, Heather L Mason, Steve McCall, Mike Mclinden, Juliet Stone, David Fulton Publishers, 1999
'One of the class' (folder for all those involved in the education of visually impaired pupils in mainstream secondary schools) Royal National Institute for the Blind, 2000
'Towards excellence -effective education for students with visual impairments' Pat Keeley and Gillian Gale, Royal Institute for Deaf and Blind Children, Australia, 199B
'A vision shared' (video which promotes and identifies aspects of good practices and strategies for the successful inclusion of visually impaired children and young people into mainstream education) Royal National Institute for the Blind, 1999 The above can be ordered via the RNIB Book Sales Service do Customer Services Centre, PO Box, 173, Peterborough PE2 6WS Tel: O845 7023153 Fax: 01733 371555 Email: email@example.com Website: www.rnib.org.uk
Richard Bowman, Ruth Bowman & Gordon Dutton
First published by RNIB in 2001