University of Edinburgh
 

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

CHAPTER Two: Assessment and measurement of vision

This chapter is concerned with how various aspects of vision can be measured and assessed. It aims to clarify how the measurements are made and what they mean in the context of the child's understanding and education. When a doctor, optician or orthoptist tests vision, it is usually to find out how well each eye is functioning. This is either to make a diagnosis or to follow up the vision after treatment. For this reason the test is usually carried out at a distance (which detects people who are short sighted) and for each eye in turn.

It is essential that teachers and parents working with a child who is visually impaired are able to understand what the child can and cannot see. There is no point in using educational material which has images which are too small to be seen and are therefore 'not there'. (Look at woven cloth through a magnifying glass. The fine detail of the weave which was not there before suddenly becomes apparent and has significance.) For this reason we are not interested in what each eye sees independently and the clarity of vision in the distance is only one of many factors. It is therefore important to be able to 'test' vision with both eyes open, and to assess near and distance vision for acuity, colour contrast, visual fields, optimum lighting conditions and movement perception. Educational material is of limited or no value if parts or all of it cannot be seen by the child. The child 'knows' that his or her vision is 'normal'. Just as you accept as normal your inability to see detail, the further away you are, children with reduced vision are unaware of what they are missing and accept this as normal. This is why it is so important for teachers to be aware of what various assessments of vision mean in terms of what is accessible to a child and what is not, and to be aware of which components of educational material can and cannot be seen by children.

History taking

Before looking at the various techniques for measuring vision it must be remembered that a lot of information can be gained about various aspects of the child's functional vision from talking to the child and parents. Indeed the parents may well have picked up much more useful information about educationally relevant aspects of the child's visual impairment from years of living with him or her, than a doctor can elicit from a relatively short consultation in the somewhat artificial environment of the clinic.

It is our opinion that before a child comes into a school or class for the first time, the teacher should spend a considerable time talking to the parents, asking questions and learning from their vast experience of the child's visual functions and problems. The art of 'history taking' (the name doctors give to the process of eliciting useful information from talking to patients and families) is learned by the practice of doing it throughout a career. An important element is the use of the specialist information that the interviewer, in this case the teacher, has gained from training and from experience with other children with similar problems. This should enable the interviewer to ask specific questions with the aim of identifying or eliminating specific visual problems or strengths which the child might be exhibiting. The parents may well have an impression of these problems or strengths but may have difficulty articulating them accurately without prompting from specific expert questioning. Another reason that this detailed interview is important is that although we may be very familiar with the sort of problems to expect in a child with albinism for instance, both from our reading and our experience, we know that no two children with albinism will have exactly the same visual problems to the same degree. Thus the interview with the parents is a good source of information about the individual capacities and requirements of that particular child.

This sort of interview should be repeated regularly because the visual abilities and disabilities of any child may not be static. Repeating the interviews may therefore elicit new and useful information from the parents while also conveying the message to them that change and progress are generally to be sought and expected.

We propose a possible structure for questions covering the principal aspects of functional vision but a teacher may want to develop her own set of questions to be modified in the light of experience.

1. Introductory questions

Opening questions should be very general so as not to bias the parents' responses. For example 'What do you think your child can and cannot see?' or 'How well do you think he or she can see?' or 'Do you have any particular concerns about your child's vision?' The answers to these general questions may affect the further direction of the interview with more detailed and directed questions being addressed to particular problems or strengths identified by the parents.

2. Peripheral vision

As explained in Chapter 1, this is particularly important for navigation and therefore the sort of questions to ask would be 'How does he manage to get around the room or house?' 'Does she bump into things much?' Children with cortical visual impairment often see better on one side (hemianopia). 'Does she see better on the right or left?' 'Does she bump into things more on the right or left?' 'Does he have to move his head to the left or right to look there?' Answers to such questions may have obvious implications for positioning in the classroom. It should be realised that unusual head movements or postures may not be indicative of a peripheral visual deficit but can occur in eye movement disorders. For instance, wobble of the eyes (nystagmus) can be reduced, hence improving vision, by the adoption of a particular head posture in some children. Other children may be able to eliminate double vision by holding their head in a certain way. Adoption of such head postures should not be discouraged.

3. Central, detailed vision (visual acuity)

This can be ascertained, perhaps, by asking the parents to bring in books or pictures which they know that the child responds to. The smaller and more detailed a text or picture which the child favours, the better is the functional acuity. It will usually be found that the material favoured by a child will be larger than might be suggested by the 'official' acuity, which by definition is the smallest detail which the child can possibly see (not what he or she can comfortably and easily see in practice).

4. Near and distance vision

'How far does he sit from the TV?' 'How close does she hold books or toys to her eyes?' 'Does she seem to see better for near or for distance?' Certain conditions are associated with better near than distance vision (eg myopia, albinism) and others with better distant vision than near (eg macular disorders). Answers to such questions again may help positioning in the classroom or may give clues about which if any Low Vision Aids (LVAs) might be helpful for a particular child (eg; text may be held very close, not because a child is short sighted, but to magnify it, see Figure 2.2).

5. Lighting conditions

'How does he see in the dark?' 'Does bright sunlight bother her" 'Does he prefer general lighting or lighting for a specific task?' Different children with different conditions will give different answers to these questions.

6. Three dimensional vision

'How does she cope with steps and kerbs?' This is a problem which we have encountered in a number of children with cortical visual impairment who see steps and kerbs just as lines on the ground. This is relevant not only to classroom set-up (eg; marking steps clearly) but also to the type of educational material used (since 3D objects may be seen as flat).

7. Colour vision

'Are some colours seen better than others?' 'Does she have favourite colours?' As explained later in this chapter, the objective tests used for assessing colour vision are often not designed with visually impaired children in mind and direct questioning can therefore be quite useful. Again, educational materials can then be tailored to the individual child. Children learn to match colours before they can name them. If naming is a problem, questions about how well colours are matched are appropriate.

8. Specific disorders

A whole range of specific visual challenges occur in a variety of conditions, some of which are outlined in Chapter 3 and which the teacher might specifically enquire about if the diagnosis is known. The best examples probably occur in cortical visual impairment where a number of different functional deficits are now beginning to be recognised. These can sometimes be quite subtle and not elicited unless you specifically ask about them. Examples include difficulties knowing how to 'see' where one is and how to move to a new location, difficulty recognising faces and failure to see fast-moving objects. More is said about these problems in Chapter 3.

Refraction

The measurement of the correct lenses required to bring the images seen by the eyes into best focus is known as refraction. All children with reduced vision should be refracted, and testing of vision carried out with the child wearing the correct spectacles, unless of course this is impracticable.

Refraction is the specialist field of optometrists, though it is also commonly performed for children by ophthalmologists. It can be carried out subjectively in children who are old and able enough to co-operate. This is the procedure familiar to any of us who have our eyes tested at high street optometrists where the person being tested informs the tester whether particular lenses improve or worsen the clarity of vision until the best strength of lens is found. If the child is too young or is unable to co-operate, the refraction can be performed objectively, a process involving the observation of a moving reflected light on the retina. This technique is known as retinoscopy.

Retinoscopy

In young children, eye drops which paralyse the ability of the eye to focus are used. A technique known as retinoscopy (objective refraction) Is then used to find out which lenses are needed. Figure 2.1 shows that a short sighted eye brings light to a focus in front of the retina and that a lower ray hits the upper retina, but in a long sighted eye this ray hits the lower retina. The view through a retinoscope is the same as for a flash photograph. A red glow is seen in the pupil. When the retinoscope moves, the red glow moves. In short sighted eyes the red glow moves in the opposite direction to the movement of the retinoscope (because the light rays cross inside the eye) but in long sighted eyes the red glow moves in the same direction as the retinoscope is moving. So all the refractionist (optician or ophthalmologist) has to do is to find out which lens in front of the eye is accompanied by no movement of the red glow. From this knowledge the lens required is calculated. Older children and adults are asked which lens suits them best in a structured process of trial and error known as subjective refraction.

Figure 2.1-Normal sight, short sight (myopia), long sight (hypermetropia)

Early diagnosis and correction of refractive errors is very important in young children because a blurred image on the developing retina causes amblyopia as already described in Chapter 1.

Refractive errors

Figure 2.1 is a simple ray diagram of the way the eye focuses light onto the retina. The curvature and consistency of the cornea and the lens determine their 'power' as optical lenses which means the degree to which they bend light rays. Figure 2.1 shows that for light to be focused into a sharp point image on the retina, the power of the cornea and lens need to be matched exactly with the length of the eyeball It is a mismatch between these two factors, (the optical power and the length of the eye) which gives rise to the two commonest refractive errors, short sightedness (myopia) and long sightedness (hypermetropia). Figure 2.1 also shows that if the focusing power of the eye is too powerful (ie the eyeball is too long and short-sighted) then the image is formed in front of the retina and the image at the retina will be blurred because it is out of focus. Increased focusing power is needed to see near targets (see accommodation) so that a short-sighted eye is in focus for near, hence the term. The nearer an object has to be brought towards the eyes for it to be seen clearly, the more short-sighted the person is. The closer something is to the eye, the bigger it appears (Figure 2.2). This means that a short-sighted child with an additional cause of poor vision may choose to remove his glasses and view what he is looking at from close up, which makes it look bigger. This has the same effect as using a magnifying glass. giving higher magnification with a narrower field of view.

Figure 2.2  The closer an object is to the eye. the larger it appears

In long sightedness the opposite occurs (see Figure 2.1). The focusing power is too weak and the eyeball is relatively short, so objects further away may be seen more clearly than near ones. Often, glasses are not prescribed for small amounts of long sightedness because the focusing power of the eye compensates for the long sight by bringing the image into focus.

Refractive error and squint

We can focus using accommodation to overcome long sightedness and children, in particular, have good focusing power. So mild to moderate degrees of long sightedness do not blur vision in children. However, whenever we look at something which is close to us we also turn our eyes in (they converge). The two reflexes of focusing (or accommodation) and turning in (or converging) are closely linked. A long-sighted child has to accommodate without turning the eyes in. If he can't do this then the long sightedness causes the eye to turn in, resulting in a convergent squint. Convergent squint due to long sight is common. Children with brain damage may have difficulty in focusing to overcome long sightedness without at the same time turning their eyes in. Relatively minor degrees of long sightedness may be the cause of convergent squint in such children. The provision of glasses may therefore be required to straighten the eyes rather than to make vision clearer.

When an eye turns in, the brain may choose to neglect the image from that eye, with resultant poorer vision or amblyopia. Patching the 'good' eye may be required to reverse this process and to restore vision in the squinting eye.

Astigmatism

Another common refractive error is called astigmatism. For accurate focusing of light the optical surfaces should be smoothly and regularly curved like the surface of a perfect sphere. Astigmatism simply means that the optical surface (usually the cornea) is not spherical but has different curvatures in different orientations (rather like the curvature of a rugby ball compared to a football). It is easy to imagine how the image of a three dimensional object is distorted by this situation. The lens required to correct astigmatism has a reciprocal rugby ball shaped curve aligned at 90 degrees to the rugby ball curve alignment of the surface of the cornea.

Refractive errors can usually be compensated for by placing optical lenses in front of the eyes in the form of spectacles or contact lenses (see Figure 2.1).

Visual acuity

Simply, this means the ability to see fine detail and is one of the most important and commonly recorded measurements of visual function. This ability to see detail depends on a number of different factors which must be taken into account when measuring acuity. Such factors include the level of background illumination, the contrast of the target against its background (eg it is easier to see a black letter on a white background than a dark grey letter on a lighter grey background), the distance between eye and the target, and whether a refractive error is present (and if so whether it has been accurately corrected with lenses).

Figure 2.3a  Distance visual acuity: Snellen charts - the visual acuity measure is normally based on the line of letters/symbols which can be read at 6m. The small numbers (enlarged on left) refer to the distance at which a normal eye can read the letter. Subjects who cannot read the top letter at 6m will be tested closer to the chart.

The commonest way of measuring visual acuity is to use the Snellen Chart (Figure 2.3a) at six metres distance away (Figure 2.3b). This is based on the principle that an object of fixed size gives a smaller image on the retina the further away it is viewed. Conversely objects of different sizes can give the same size of image on the retina if viewed from different distances (Figure 2.2). The chart, shown on the left in Figure 2.3a, consists of rows of black letters on a white background. The upper row of letters is the largest and is said to be seen by a normal eye at a distance of sixty metres away. The smallest sized letters are on the lowest line and are said to be readable by a normal eye at five metres away. The letters of the top row when viewed from sixty metres give an image on the retina which is the same size as the image of the letters on the bottom line when they are viewed from five metres.

The visual acuity is recorded as a fraction, the numerator or first number of the fraction being the distance that the chart is placed from the eye in metres (most commonly six) and the denominator or second part of the fraction denoting the smallest (lowest) line which can be read by the eye at six metres (in other words the distance from which a normal eye could see that line). The large top letter on the chart can just be made out at a distance of sixty metres. So, for instance, an eye which could only read the top letter of the chart at six metres would be said to have an acuity of six over sixty (or six sixty), meaning that what a normal eye could read from sixty metres away this eye can only see from six metres away. This can be thought of as six sixtieths or ten percent of normal visual acuity.

In theory the smallest line a normal eye would be able to read at six metres would be the six metre line and would be recorded as having an acuity of six six (In the USA the chart is situated twenty feet away and hence the term 'twenty twenty' for perfect acuity.) In practice however, the standards of the chart err on the generous side and most normal eyes can read the smaller five metre line at six metres. Hence a perfect visual acuity is closer to six five than to six six.

If a child cannot read even the top letter at six metres, that is, the acuity is worse than six sixty, then the child can be moved closer to the chart until the top letter can be seen. If the top letter can be seen at three metres, the acuity would be recorded as three over sixty; if it is not seen until the child is one metre away then the acuity would be one over sixty. If the top letter cannot be seen at all then poor visual acuities are conventionally classified into the ability to count fingers (this can be subdivided into the distance at which the child can count fingers again from one to six metres, often abbreviated to eg CF at 1m); the ability to detect movement of a hand (HM); the ability to perceive light (PL); and no ability to perceive light (NPL).

Figure 2.3b Distance visual acuity: Snellen charts -the child is reading the Snellen chart (back to front as above) with his left eye via a mirror facing him 3 metres away so that the total distance is 6 metres.

The two eyes are normally tested separately by ophthalmologists and opticians, but functional vision is best tested with both eyes open and may be one line better than either of the two monocular acuities.

Obviously, to use the Snellen chart requires that a child can read, recognise and verbalise the letters on the chart. Variations of the Snellen chart have therefore been devised for younger, preverbal children or for those with learning difficulties. The Landolt ring (Figure 2.3a) Illiterate Echart and Lea heart-shape chart require the child to detect the orientation of a particular figure and to match it with a similarly orientated figure from a selection of figures of various orientations contained on a smaller chart held in their hand. Pictures instead of figures (Figure 2.3a), can also be identified or matched as in the Kay's picture test.

Even these adaptations require a measure of co-operation from the child and for very young children or those with severe learning difficulties an attempt to estimate their acuity can be made either with electrical measurements, as explained in a later section of this chapter, or with a technique called the acuity card procedure which relies on the principle of preferential looking. This latter technique relies on the finding that if a child is presented with two visual targets, one plain and one patterned, then the child will reflexly look towards the patterned target. This is thought to arise from the visual system's built-in preference for contrast and edges (see Contrast and movement, Chapter 1). Thus the test involves the child being presented with a series of large cards each containing two target areas, one plain grey and one with black and white stripes. The assessor observes the child's eye movements through a small hole in the card and decides which way the child is looking. The size of the stripes is gradually reduced and below a certain size corresponding to the child's limit of resolution or acuity, the black and white stripes will just appear as a grey blur and the fixation preference will be lost. A variation of this test for slightly older children involves Cardiff cards (Plate 4). These are rectangular cards which are presented vertically with a picture at the top or bottom. The picture is bounded by a zebra like line which is made up of black and white stripes. The card is presented in front of the child and his eyes are observed to see if they move to look at the picture. Successive cards are shown in which the boundary line gets finer and finer until it can no longer be seen and the black and white images merge into the central grey and the picture effectively disappears. This is one of the best tests of acuity for young school age children and pre-school children.

Plate 4 Cardiff acuity cards

A common misconception is to confuse the concepts of refractive error and acuity and hence to equate six six or twenty twenty vision with someone who does not need glasses. In fact, visual acuity should always be recorded with glasses on if they are needed; most of the spectacle wearing population do have acuities of six six or six five with their glasses on and are obviously not considered to be visually impaired.

Another misconception to be aware of is equating visual acuity with vision. The ability to see fine detail is a very important aspect of vision but is not the only one - it involves the central macular area of the retina and the central part of the visual field (see next section). It is therefore quite possible for a child with retinitis pigmentosa for example, to have acuities of six six in each eye but to have severe tunnel vision and difficulty seeing in poor light and so to be significantly visually impaired. Conversely, a child with a disease of both maculae (for example cone dystrophy) may have a poor acuity but minimal impairment of mobility owing to intact peripheral vision. Plates 5-7 attempt to simulate the perception of blurred focus due to an uncorrected refractive error
and reduced visual acuity.

Plate 5 View with a visual acuity of 6/18

Plate 6 Blurred focus due to uncorrected refractive error

Plate 7 Reduced visual acuity (3/60)

Functional acuity

It is important to realize that the official Snellen acuity fraction that is recorded in the clinic may not correlate simply with the functional acuity in the home or the classroom. The level of illumination in a room in fact has surprisingly little effect on acuity (as long as there is enough light for cone stimulation) - visual acuity remains constant over a wide range of lighting levels extending from the level of full moonlight to that of a bright sky on a sunny day. However, although clarity may not be reduced by lower light levels, the time taken to perceive fine detail is prolonged as background lighting is reduced. This is very important for schools where the time taken to accomplish a task is important. (See section on Lighting in Chapter 4.)

Other factors such as the spacing of visual targets or the distance from which they are viewed may have more significant effects on whether they can be clearly seen. These factors can be regulated in the clinic but not always in the real-life situation of home or classroom.

Visual acuity can be reduced in children when targets are presented too close together. This phenomenon is known as crowding and is particularly noticeable in children with amblyopia or cortical visual impairment. Indeed, the Snellen chart suffers from a degree of inaccuracy because the spacing between the letters varies on different lines. An eye with amblyopia in particular will score significantly better on modified Snellen charts with only one letter on each line.

Contrast is another factor to be considered. This will be described in greater detail in the section on contrast sensitivity but it is worth emphasising that visual acuity is a measure of the ability to discriminate detail in targets of maximum contrast such as black letters on a white background. The visual targets in the child's everyday environment may not be seen in such ideal black and white contrast conditions. Finally, it must be remembered that Snellen acuities are measured at six metres and therefore represent an assessment of distance vision. It is often, however, objects at close range which require detailed visual discrimination. There is a strong correlation between near and distance visual acuity but there are conditions where one is significantly better than the other. Disorders of the macula, for instance, although adversely affecting both near and distant acuities sometimes have more of an adverse effect on the near acuity. Conversely, nystagmus (wobbling eyes) is reduced and the eyes wobble less when they turn in or converge to look at a near target, giving better near vision. Plate 8 attempts to simulate the effect of nystagmus on visual perception.

Plate 8 Possible effect of nystagmus on vision

Because of these differences it is often helpful to measure formally the near visual acuity with a near reading chart containing print of different sizes and record both at the normal comfortable reading distance, and in the position that the child normally chooses to adopt. The tests available are poorly standardized and include reduced Snellen charts, Maclure charts (Figure 2.4) and the Point and Jaeger charts. These may use letters, unrelated words or meaningful text. Near acuity is a better predictor of the optimum magnification for visually impaired readers than distance acuity and is a better predictor of educational performance than distance acuity. A better predictive test still is to observe the ease and speed of periods of sustained reading of different sized prints in the school environment. The near acuity as measured in the eye clinic involves only reading a few words and the very smallest that can be read (often with difficulty) will be recorded. Obviously this should on no account be taken as the optimum print size for educational material, which should be well within rather than at the child's visual limits.

Figure 2.4 Near visual acuity: Maclure chart test

Older children with milder forms of visual impairment may be considering learning to drive. There are strict criteria issued by the medical advisers to the DVLC on minimum acuities and visual fields for safe driving. The visual acuity criteria require the ability to read a car number plate at a distance of 20.5 metres (with glasses if needed) and this is included in the driving test itself. This cannot simply be translated into a Snellen acuity recorded in the clinic for the reasons outlined above (conditions outside are not idealized as in the clinic for example. glare may be a problem and the number plate may not be black against white), but the nearest estimate is in the region of six over twelve in the better eye. The visual field criteria which must be met are not included in the driving test and are described in the next section. Another aspect of functional vision of relevance to education is the concept of dynamic acuity, which is simply a measure of the visual acuity when either the target or the person is moving. This has been the subject of small studies mainly in sportsmen and women; eye muscle coordination is thought to be important in the ability to localise accurately and quickly, and to resolve the detail of moving targets. Children with eye movement disorders, which cause difficulty in tracking moving objects, can have reduced clarity of vision for moving targets, and may, for example, have impaired ability in extracting visual information from films and videos - particularly fast moving cartoons. Such children are typically those with cerebral palsy who may also have cortical visual impairment.

Rarely, children with cortical visual impairment may have damage on both sides of the part of the brain responsible for seeing movement. These children appear not to see moving targets unless they are moving very slowly, yet their clarity of vision may even be normal. It is as if the whole moving world blurs out. This condition is called akinetopsia.

Visual fields

This concept was introduced in Chapter 1. The visual field is the proportion of space around us which is visible at anyone moment. Figure 2.5a and Plate 9 show the normal boundaries of the visual fields. Assessing visual fields involves measuring these boundaries (that is; finding out how far right, left, up or down an eye can see while looking straight ahead) and also measuring how sensitive the vision is (eg how small a target or dim a light can be seen) at various places within these boundaries. Formal mapping of the visual field involves the child looking at a central target on a screen (Plate 9). Points of light of various size and brightness are then momentarily flashed up on the screen at different positions on the screen, and the subject indicates if they have been seen. In this way a map of the visual field is built up as shown in Plate 9.

Plate 9 Topcon perimeter

FIGURE 2.5a The visual field of each eye is cut off by the nose. The extreme right field of vision is seen by the right eye only and the extreme left field of vision by the left eye only (Diagram shows view from above.)

There are various computerised machines available for performing visual field tests and analysing the results, but these sophisticated tests usually take a long time to perform and are also quite difficult even for adult patients. They are therefore of limited use in children. though some child-friendly versions are being developed using the type of graphics used in computer games.

Simpler methods can give an estimate of visual field boundaries; eg one adult engages the child's visual attention while another introduces a toy into the edge of the child's vision noting when the child turns his head to look at it. This can easily be done at home or in the classroom and provides a very useful idea of the true functional field of vision (Plate 10). However, a negative test must be treated with caution and repeated because the central target can be so interesting to the child that everything else is ignored. Sensitivity decreases in all directions away from the point of fixation and hence the visual field can be imagined as a meringue shaped island or hill of vision in a sea of blindness. As mentioned in Chapter 1 and shown in Figure 2.5b the normal visual field of each eye contains a blind spot corresponding to the optic nerve head where there are no photoreceptors (see Figure 2.6 to demonstrate your own blind spot). With both eyes open there is no blind spot since the field of one eye covers the blind spot of the other eye. This normal blind spot is an example of what is termed a scotoma, an area of reduced light sensitivity. Scotomata other than the normal blind spot are due to disorders of the eye or visual pathway, and indeed the pattern of the scotoma may indicate where in the eye or visual pathway the disorder is located. Scotomata may be absolute (no light perception in that area) or relative (reduced, but not absent, light sensitivity). Scotomata can also be classified as 'positive' when they are noticed as a shadow obscuring part of the visual field <eg a retinal detachment is often initially seen as a grey curtain-like shadow over part of the vision), or 'negative' when they are not noticed as a shadow (such as the blind spot or the various scotomata which result from damage to areas of the visual brain) but in which vision is reduced or absent.

Plate 10 Confrontational visual field testing

Figure 2.5b 3D map of visual field of right eye

Figure 2.6 -The normal blind spot. Close your left eye and look at the cross with the right. Hold the page close to your eye and slowly move it away keeping your eye fixed on the cross. The dot will disappear when it reaches the point in space corresponding to your right eye's blind spot. If you open the left eye it will appear again - with both eyes open we do not have a blind spot.

Some of the most common characteristic field losses associated with various visual impairments are represented in Plates 11-16. The best known example is probably the progressive closing in of the visual fields seen in retinitis pigmentosa. In severe cases this can lead to very narrow tunnel-like visual fields or even complete extinction of the fields and ultimately, blindness. The child with such severe field restriction may still have a reasonable visual acuity and may therefore be able to read, but perception of the visual world is severely limited to this narrow tunnel of vision with profoundly impaired appreciation of or navigation through the surroundings of the home or classroom, and inability to detect visual events that occur anywhere outside the tiny intact central visual field. Damage to the central retina (for example a macular dystrophy) gives almost a mirror image of this type of tunnel vision field defect, with a centrally located scotoma affecting the central fixation point (as explained in Chapter 1, on the retinal map of visual space, the macula represents the fixation target). Because of its central location such a scotoma is likely to have a profound effect on acuity and the ability to read, while the child's general awareness of her environment and ability to navigate through it will be relatively unimpaired.

Plate11 Worksheet: Normal visual field

Plate 12 Worksheet: Peripheral visual field loss

Plate 13 Hall: Normal visual field

Plate 14 Hall: Central visual field loss

Another common form of visual field loss involves one complete half (left or right) of the visual field of both eyes. This is called a homonymous (meaning it is the same side of the field that is affected in each eye) hemianopia (meaning half blind) and is illustrated in Plate 15. Behind the optic chiasm or crossover, all the visual information from the left side of the visual field of both eyes travels together in the visual pathways in the right side of the brain, and likewise all the visual information from the right side of the visual field of both eyes travels together in the visual pathways in the left side of the brain. Hence any abnormality (whatever the cause) which affects the right visual brain will tend to cause a left homonymous hemianopia, and any abnormality affecting the left visual brain will tend to cause a right homonymous hemianopia.

Plate 15 Left-sided visual loss (homonymous hemianopia)

It is very important for parents, teachers and mobility specialists to be aware of this type of field defect; only one side of the child's visual world is functioning and obviously all visual interaction should be directed to take place from that side. If you imagine reading print such as this with a hemianopia, the print would suddenly appear out of nowhere for a right hemianopia and suddenly disappear into nowhere for a left hemianopia as the eyes scan across the text. Children with hemianopias may therefore find it easier and eventually faster to tilt the page and read in a direction nearer to the vertical so that the print does not keep appearing or disappearing like this. This technique of reading however requires practice, and acceptance by parents and teachers as being a reasonable and appropriate reading strategy. This provides an example of where the need to conform with what is 'normal' comes into conflict with what may ultimately prove a much faster and more efficient way of accessing information. Chinese is much easier to read!

Because homonymous hemianopia commonly impairs one side of the vision completely it can be tested by a method termed confrontation. The child and examiner face each other and the child is asked to look at the examiner's nose. The examiner then holds up two or three fingers of each hand momentarily so that the examiner can see them in his left and right peripheral vision simultaneously -the child is simply asked how many fingers were held up each side and a significant visual weakness on one side will soon be revealed. The test may be converted into a game where the child is encouraged to look at or grasp the finger which moves. In younger or less cooperative children, lack of head or eye movement to the side of the moving peripheral target suggests that a hemianopia may be present. Children with impaired movement on one side (hemiplegia) often have homonymous hemianopia on the same side.

Children with optic nerve disease may have interrupted visual fields; as though they are looking through a colander as shown in Plate 16.

Plate 16 Interrupted visual field

Returning to the question of driving, it is obviously important for drivers to have reasonably good peripheral vision so that they can see a child running out from the pavement, or a cyclist. Therefore people with significant hemianopia or tunnel vision are not allowed to drive even if they have perfect visual acuities. There are strict guidelines issued by the British Licensing Authorities on the minimum degrees of peripheral vision allowable for driving. Any visually impairing condition must be declared to the licensing authority and if there is a question over adequate peripheral vision this can be formally tested in an ophthalmology clinic. The test is performed with both eyes open and therefore someone who is completely blind in one eye but has a full field of vision in the other eye should meet the standard required for a private licence. There are obviously stricter criteria in terms of both acuity and field for issuing licences for professional drivers of heavy goods vehicles or public transport vehicles.

Contrast sensitivity

Contrast sensitivity is the ability to discriminate shades of grey, one from the other, while colour contrast sensitivity is the ability to distinguish varying shades of colour -for example light blue from a darker blue. Good contrast sensitivity is required, for example, to be able to make out people's facial features.

Contrast sensitivity can be tested in a number of different ways. In the laboratory, stripes of light and dark grey are made progressively lighter and darker (ie more contrasting) until they can be seen. However there are useful, more practical tests available (Plate 17).

Plate 17 Contrast sensitivity test

Any condition which impairs central vision, whether it be in the eye, the optic nerve or the brain can impair the ability to detect low contrast. The ability to discriminate contrast is as important as the ability to tell one colour from the other.

Elecctrical tests of vision

Tests of visual function require a degree of cooperation by the child and the results are therefore affected by the child's ability and inclination to perform set tasks. With very young children or those with severe learning difficulties, such tests may be impossible. A more objective test of visual capability involves measurement of the electrical signals being sent along the visual pathways. Electrodes which pick up these electrical signals can be placed on the lower eyelids or on the head over the visual brain areas. The two types of such electrical test which are most often of clinical use are the electroretinogram (ERG) and the visual evoked potential (VEP). The ERG, as its name suggests, measures the electrical signals generated by the retina. These signals are picked up at the front of the eye from an electrode made of silver or gold which is attached to a contact lens or is tucked inside the lid, or is attached to the lower eyelid (Plate 18). The retinal signals are measured in response to a number of different light stimuli to give different information. When the test is performed in bright lighting, the cones of the retina are generating the electrical signal. After the subject has been in the dark for twenty minutes, it is the rods (responsible for night vision) which are being tested. An ERG picked up from someone who is watching a moving black and white chequer board is testing the cells (ganglion cells) which conduct the electrical signals to the brain.

Plate 18 Electroretinography (ERG) testing using skin electrodes

The conditions in which the ERG is most commonly useful are the rod and cone disorders. For instance retinitis pigmentosa predominantly affects the rods and hence the dark-adapted ERG is more affected than the light-adapted ERG.

The VEP involves sticking skin electrodes on to the scalp at the back of the head over the seeing brain or primary visual cortex. Flashes of light or patterned stimuli are again used and the size of the electrical signal generated by the primary visual cortex in response to the pattern is measured and, importantly, any prolongation of the normal one tenth of a second delay between the stimulus and the signal formation in the brain is sought. (We all live one tenth of a second in the past!)

Any disorder at any point along the visual pathway from cornea to visual cortex itself can cause a reduction or a delay in the cortical signal and hence the VEP is a much less specific diagnostic tool than the ERG. VEPs can be useful in telling the difference between a child with a physical disorder in the visual pathway and a child who appears not to be seeing well but who has no physical disease present. The VEP can be particularly useful to estimate the approximate visual acuity in young and multi-disabled children for whom the behavioural methods of assessing acuity may be difficult to perform or interpret. As the pattern of black and white stripes or squares which the child is being shown is gradually reduced in size, the brain signal which is picked up becomes smaller until it becomes undetectable. From this, the approximate visual acuity at a cerebral level can be estimated.

Colour vision and colour contrast

The most commonly used test for colour vision is the Ishihara colour plate test (Plate 19). Patterns of coloured and grey dots are designed to reveal one pattern to the child with normal colour vision and another to the child with colour confusion. They can be adapted for younger children or those with learning difficulties by getting the child to trace out the pattern with his or her finger rather than naming the figure, or by using plates with pictures instead of numbers. Ishihara plates test only for red-green confusion but other test plates are available for testing blue deficiencies. As mentioned in Chapter 1 this colour vision test was designed to test genetically inherited colour confusions (dichromacy and anomalous colour vision) and may not detect colour deficiencies acquired as a result of retinal or optic nerve disease.

Plate 19 Ishihara test

More complex tests include the 100 Hue Test. where the child arranges 84 pastel chips in a colour order which is quite obvious to the child with normal colour vision, but which produces characteristic errors in children with colour confusion. It is quite a difficult test for children to perform and the panel D15 test for children recently developed by Lea Hyvarinen (Plate 20), which involves a smaller number of chips, may be more useful in practice.

Plate 20 paster coloured chips to be arranged by child as developed by Lea Hyvärinen

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