Binocular Vision Part 1

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MODULE 10:3 • COURSE CODE: C-5196

Binocular vision anomalies:

Part 1

Symptomatic heterophoria Bruce Evans This article will concentrate on ways in which optometrists can enhance visual function in patients who may have symptomatic yet non-strabismic binocular vision anomalies. The most common ocular motor status is heterophoria, and the largest section of this article will deal with this condition. Heterophoria is normal, and only infrequently requires treatment. Patients who require treatment will usually have symptoms, and so are particularly likely to consult optometrists. Convergence insufficiency is a fairly common cause of symptoms in primary care optometric practice. The treatment of this condition is usually straightforward and will be described. The diagnosis and treatment of accommodative problems also will be discussed. Dyslexia is the most common specific learning difficulty and affects about 5% of the population. Although dyslexia is not usually caused by visual anomalies, certain visual problems are more likely to be present in dyslexia than in good readers and the diagnosis and treatment of these visual correlates will also be discussed.

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Heterophoria Overview If a person is placed in a completely dark environment, then the visual system has no feedback that can be used to control ocular alignment. The eyes are free to remain aligned or to deviate, and in most cases they deviate. In terms of vergence, the eyes move to their resting position in which the vergence angle is aligned for a distance of about one metre. Conceptually, if the resting position of the vergence system is with the eyes aligned for a distance of about one metre, then distance vision can be thought of as divergence away from this resting position and near vision as convergence away from this resting position (Figure 1). Vergence is influenced by several factors, including an awareness of the distance of the object (proximal vergence), cross-linking with the accommodative system (accommodative vergence) and the fine tuning of ocular alignment during the

fusion of each monocular image into a single percept (fusional vergence). When an eye is covered, for example during a cover test, there is no fusional vergence and the eye behind the cover is likely to revert towards the resting position. This is why, on average, the normal heterophoria is a small degree of esophoria for distance vision and exophoria for near vision (Figure 1). A normal, healthy, visual system is usually able to overcome these heterophorias without any difficulty: the heterophoria is compensated. Optometrists become interested in heterophoria in cases where the patient is not able to fully compensate for the heterophoria: it becomes decompensated. Figure 2 schematically illustrates the factors which normally cause a heterophoria to be compensated, and there is therefore usually one (or more) of three reasons for a heterophoria becoming decompensated. First, there may be an inadequacy of the vergence system. The vergence system manifests as the

fusional reserves, which bring about motor fusion. For example, a child may have a fever, or sometimes even stress or tiredness, which can cause the fusional reserves to be reduced. Second, there may be a problem with sensory fusion. The process of sensory fusion requires each monocular image to be clear and similar to one another. Problems that can interfere with sensory fusion include anisometropia, cataract, or metamorphopsia from a macular lesion. The third reason why a patient may be unable to compensate for their heterophoria is if the heterophoria is unusually large. For example, there may be an anatomical reason why the resting position of the eyes is very different to the average described above, where the eyes are approximately aligned at a distance of 1m. Another reason for an atypical heterophoria is the effect of accommodative vergence, for example in uncorrected high hypermetropia. This approach, of using the

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< Figure 1 Schematic illustration of resting position of vergence system, divergence, and convergence

information in Figure 2 to determine what factor(s) have caused a heterophoria to decompensate, is not just academic. When an optometrist encounters a patient whose heterophoria is decompensating then it is important for the practitioner to determine why this is happening. If there is a non-pathological explanation then it is appropriate for the optometrist to treat the condition. For example, the optometrist may cure a decompensating esophoria by correcting the underlying hypermetropia. As another example, they may help an older patient whose long-standing near exophoria is decompensating due to poor sensory fusion from untreatable macular degeneration by prescribing base in prism. If there is a large change in the heterophoria for no apparent reason then this could be a sign of pathology and the patient requires referral.

Investigation Symptoms There is no single method which is perfect at diagnosing decompensated heterophoria, although most cases will have symptoms. The symptoms can be classified as visual problems (blur, diplopia, distortion); binocular problems (difficulty with stereopsis, a tendency to close or cover one eye,

Cover test The cover test can provide a great deal of information. It can be used to differentially diagnose heterophoria from strabismus, can reveal the type and size of the heterophoria (Evans, 2005), and the cover test recovery movement can be used to assess whether the heterophoria is compensated (Table 1). In some cases (eg, young, uncooperative patients or patients who are intellectually impaired) the cover test recovery may be the only indication as to whether the heterophoria is compensated. Grade Description

1

Rapid and smooth

2

Slightly slow / jerky

3

Definitely slow / jerky but not breaking down

4

Slow / jerky and breaks down with repeat covering, or only recovers after a blink Breaks down readily after 1-3 covers

5

< Table 1 A grading system for cover test recovery

< Figure 2 A simple model of binocular vision. Reproduced with permission from Evans, B.J.W. (2002) Pickwell’s Binocular Vision Anomalies, 4th edition, Elsevier

Mallett unit fixation disparity test It is probably true to say that the Mallett unit fixation disparity test has revolutionised the diagnosis of decompensated heterophoria in primary eyecare in the UK. The test detects fixation disparity and measures the aligning prism or aligning sphere: the prism or sphere that eliminates the fixation disparity. It is important to stress that the test is very different to dissociation tests that measure the magnitude of the heterophoria whilst the eyes are dissociated: in dissociation tests, the eyes typically view different, nonfusible, stimuli (eg, the Maddox rod test). In the Mallett fixation disparity test (Figure 3) the eyes are associated: they view very similar images which aid sensory fusion. In particular, there is a peripheral fusion lock (the text around the test) and a central fusion lock (the O X O). The design of the fusion lock is probably an important feature of the test, and one reason why it is better to use genuine Mallett units rather than copies. Whilst in dissociation tests, it is normal for the eyes to be misaligned, in the associated Mallett test, the eyes do not usually misalign. Indeed, any misalignment that is reported in this test is potentially abnormal and might be a sign of decompensated heterophoria. Recent research shows that the instructions that are given to the patient with this test are important: patients should be asked to say whether the lines ever move, even by a very small amount. This is then investigated by adding prism (the

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difficulty changing focus); asthenopia (headaches, aching eyes, sore eyes); or referred problems (general irritation). The difficulty is that most of these symptoms are non-specific: they could be caused by problems other than decompensated heterophoria. This means that there is a need for clinical testing of patients with these symptoms: the practitioner must detect signs as well as symptoms. There are also two occasions when patients with a decompensated heterophoria might not report symptoms. Some patients, typically young ones, may not recognise their symptoms until they have been corrected: a child may have always had blurred vision when reading and so feels that this is normal. A second reason is that occasionally patients with decompensated heterophoria may develop a compensatory strategy to avoid symptoms: foveal suppression.

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which says that the two fusional reserves should not be markedly different: the divergent fusional reserve should be more than half the convergent reserve.

Tests of sensory fusion

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< Figure 3 The Mallett near fixation disparity test. The left hand picture is for testing horizontal and the right for vertical heterophoria. aligning prism), starting in ½ prism dioptre steps, until the lines maintain perfect alignment. A recent study suggests that, when used this way, the test is quite good at detecting symptomatic heterophoria and the higher the aligning prism the worse the symptoms are likely to be (Figure 4). The aligning prism or aligning sphere is also a useful indication of the prismatic or refractive correction that might eliminate symptoms, if it is felt appropriate to correct the decompensated heterophoria in this way (see later section on Management). Although the Mallett fixation disparity test is a good indicator of decompensated heterophoria at near, research suggests that the distance

< Figure 4 Graph of mean symptom score v. aligning prism at near. The error bars represent the standard error of the mean (SEM). The number of participants (shown above scale for horizontal axis) is small for higher degrees of aligning prism and this may explain why the SEM increases. Reproduced with permission from Karania and Evans (2006)

version of the test is not so good at discriminating patients with symptoms. This may be because of the different nature of distance heterophoric deviations. Although the Mallett fixation disparity test is very helpful in diagnosing decompensated heterophoria, it is not infallible. In some cases, patients will have a fixation disparity, yet no symptoms and no need for treatment or correction. Less commonly, a patient with no fixation disparity may require treatment. The other tests in this section can be used to detect these cases.

Fusional reserves (Figure 5) The fusional reserves are a measure of how much vergence the person has ‘in reserve’, that can be used to overcome their heterophoria. The fusional reserves can be measured with rotary prisms, but they are most commonly measured these days using a prism bar. The fusional reserve that opposes the heterophoria should be measured first: base out to force convergence in exophoria. The patient should fixate a detailed target, and the prism is introduced until the patient reports (i) blur (if this occurs), (ii) diplopia; and then (iii) the prism reduced until they report single vision. The patient’s eyes should be watched to confirm the break point, when the vergence movement should cease. In exophoria, Sheard’s criterion is a useful way of interpreting the fusional reserves. Sheard’s criterion says that the fusional reserve that opposes the heterophoria should be at least twice the heterophoria. In esophoria, Percival’s criterion is more useful,

A well-compensated heterophoria requires good sensory as well as motor fusion (Figure 2), and testing of the sensory aspects of binocular vision can be useful in assessing compensation. The Mallett unit foveal suppression test is useful for detecting foveal suppression. This is particularly important in cases where the cover test and/or fixation disparity test indicate that the heterophoria may be decompensated, but the patient does not report any symptoms. It is possible that the patient has foveal suppression as a compensatory mechanism to avoid symptoms. The use of the foveal suppression test was described by Tang and Evans (2005). Stereoacuity tests can also be a useful method of assessing sensory fusion.

Other tests Dissociation tests such as the Maddox rod and Maddox wing, which measure the size of the heterophoria, are not described in detail in this article because the size of the heterophoria is a poor predictor of whether it is compensated. However, these tests can be useful for monitoring the size of the deviation, particularly in cases where the practitioner is concerned that the angle may be changing, which could be a sign of pathology. The cover test is an essential part of every primary care eye examination and also can be used to monitor the size of deviation (Evans, 2005).

Management The first stage in the management of decompensated heterophoria is to remove the cause of the decompensation. For example, if a patient has a decompensated heterophoria resulting from poor sensory fusion owing to unilateral cataract, then cataract surgery may render the heterophoria compensated once more. Similarly, a refractive correction for anisometropia may be an effective treatment. If there is a

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deteriorates, especially if the heterophoria angle increases for no apparent reason, then investigation for incomitancy and referral is required. Indeed, ocular motility testing is an important part of the investigation of any binocular vision anomaly, although incomitancy is rare in heterophoria.

Convergence insufficiency < Figure 5

Overview

Measuring the fusional reserves with a prism bar

goal is to reduce the refractive modification over time, usually checking every 3-4 months. Decompensated exophoria at near is easiest to treat with exercises, such as the Dinosaur cards or aperture rule trainer, and the IFS exercises developed at the Institute of Optometry (IOO) have been found to be successful as a system of exercises that can be dispensed by the practitioner for the patient to use at home (Figure 6). With any form of treatment, the patient needs to be carefully monitored to ensure that the treatment plan is successful. If not, then a new plan is needed, or referral to a colleague for a second opinion. If the situation

Convergence insufficiency occurs when the patient has a remote near point of convergence. Confusingly, in some literature a convergence weakness exophoria, or decompensated exophoria at near, is often described as a convergence insufficiency. But the two are separate conditions which often, but not always, occur together. For example, some patients may be orthophoric at their reading distance (eg, 40cm), or even esophoric, and yet not be able to converge to 10cm. Conversely, many patients with a decompensated exophoria at near can converge to a very close distance, until the target reaches their nose. The distinction between the two conditions is not just academic. From the perspective of treatment, if a patient has a remote

Intervention

Most suitable for (in descending order)

Comments

Eye exercises

Exophoria at near Exophoria at distance Esophoria at near Esophoria at distance (rarely useful for hyperphoria)

Various methods are available, and a combination of approaches is often helpful

Refractive modification Esophoria at distance & near in latent hypermetropes Esophoria at near (multifocals) Exophoria at distance or near (negative add)

In esophoria, latent hypermetropia should always be suspected and a cycloplegic refraction is required for young patients. Even in cases without a refractive aetiology, refractive modification is often successful

Prismatic correction

Hyperphoria Esophoria Near exophoria

Prismatic correction is occasionally used in exophoria, typically in reading glasses for older patients

Surgery

Cyclophoria & hyperphoria Very large esophoria or exophoria

Surgery is a last resort for any case of heterophoria, and is only rarely required

< Table 2 Main approaches to treating decompensated heterophoria

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decompensated esophoria owing to uncorrected hypermetropia, then the hypermetropia needs to be corrected. If the heterophoria is decompensated because the fusional reserves are low, then eye exercises to increase the fusional reserves are likely to be helpful. This simple approach, of finding out why the heterophoria is decompensating and eliminating or treating the cause, is often all that is required to treat or correct the condition. This is why much of this article has been devoted to the investigation of heterophoria: a thorough investigation usually reveals the solution. The main approaches to treating decompensated heterophoria are summarised in Table 2. In any case of esophoria, hypermetropia should be suspected and in young patients a cycloplegic refraction is usually required. When decompensated esophoria is caused by hypermetropia, then refractive correction is clearly the appropriate management. But even in emmetropic patients, refractive modification can often be a very useful management strategy. Most practitioners would consider multifocal spectacles as an option for treating decompensated esophoria at near. Many cases of decompensated exophoria can also be treated refractively, using a ‘negative add’. This is when a patient who may not have a significant refractive error is given negative lenses to induce accommodative convergence, hence reducing an exophoric deviation. In cases that are managed refractively, the Mallett fixation disparity test is generally useful for determining the aligning sphere: the minimum spherical correction that eliminates the fixation disparity. This is usually the refractive correction that is required, but this should be checked with a cover test. The potential for correction by refractive modification is dependent on the size of the heterophoria, the amplitude of accommodation, the effect of any pre-existing uncorrected refractive error, and the amount of vergence that is induced by a change in accommodation (the AC/A ratio). In any case of refractive management, the

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near point of convergence but no decompensated exophoria at their reading distance, then eye exercises should concentrate on improving the near point of convergence. If the patient has decompensated exophoria caused by low fusional reserves at the reading distance, but can converge to their nose, then treatment should concentrate on increasing the fusional reserves. If the patient has both a remote near point of convergence and low convergent fusional reserves then treatment should address both deficiencies.

Investigation A measurement of the near point of convergence should be a part of every routine eye examination. Classically, a push-up test is carried out where the target is slowly brought towards the patient until diplopia occurs. The eyes should also be watched since often a break point (when the eyes stop converging) can be observed. This should confirm the subjective diplopia point, or this may be the only available measure of the end point in patients who suppress at the break point. There are various quoted values for the normal near point of convergence: some say a break point of 10cm, others 8cm. The key thing is the closest distance at which the patient ever works. Small children might hold a book very close, so need better convergence than an adult who works on a computer screen at 50cm. Another method of measuring convergence, which is underused, is to assess the jump convergence. The patient is asked to alternate their fixation between a distant target and one at 15cm. A prompt and smooth convergence movement should be seen between distance and near and a failure of this can indicate abnormal convergence (Pickwell and Stephens, 1975). The symptoms of convergence insufficiency are similar to those of decompensated heterophoria. Of course, the condition will only cause symptoms if the patient carries out tasks at a distance at which the convergence insufficiency will cause problems, such as threading a needle. The diagnosis of convergence insufficiency is often helped by carrying out tests for decompensated

< Figure 6 Institute free-space stereogram (IFS) exercises. Reproduced with permission of I.O.O. Sales

heterophoria at an unusually close working distance. Rarely, pathology can result in a paralysis of convergence. An unexpected sudden loss of convergence would therefore require referral.

Management There are only three reasons for treating a binocular vision anomaly: (i) if it is causing symptoms or impaired performance, (ii) if it is likely to deteriorate if not treated, and (iii) if it might one day need treatment and would be more effectively treated now than in the future. So if a patient has a slightly remote near point of convergence (eg, 12cm) but does not work at or near this distance and does not have symptoms, then they may not require treatment unless the situation deteriorates. An exception may be patients, often children, who do not appreciate symptoms until these have been corrected. Convergence insufficiency can nearly always be treated successfully with eye exercises. The simplest are push-up exercises, where the target is slowly brought towards the patient’s nose whilst the patient tries to keep it single. If the accommodation is adequate, or needs training as well (see next section) then the target should have fine detail and the patient should try to keep the target clear as well as single. There is some evidence that more sophisticated exercise regimens are

more successful (Scheiman et al., 2005). At the very least, push up exercises can be combined with jump convergence, when a distance target is introduced and the patient alternates fixation between the near and the distance target. As they do this, the near target is brought closer in towards the eyes. With children, it helps if a parent can watch the child’s eyes to ensure that the appropriate convergence and divergence movements are occurring. A parent watching the eyes will help to detect cases in which the patient suppresses at the break point. This is important, because these patients may be unaware of the break point and need some form of feedback to inform them of when their convergence breaks. Another very useful form of feedback can be gained from physiological diplopia. Here, another target is introduced and the patient is taught to appreciate this in physiological diplopia. This approach can be very successful and is described in more detail in Evans (2002). Methods based purely on physiological diplopia (eg, the three cats card and the dinosaur card) are also often successful. Another approach is to give the patient a self-contained system of exercises, that train convergence in a variety of ways. The IFS exercises are such a system (Figure 6) and can be dispensed to the parent to do at home with the child. This system includes self-test questions to ensure that the exercises are being done properly.

Accommodative anomalies Overview This article will not cover accommodative anomalies in great detail, since the emphasis of the article is on heterophoria. However, no assessment of near heterophoria in a pre-presbyopic patient is complete without an investigation of accommodation. This is particularly true for convergence weakness exophoria and convergence insufficiency. Indeed, it has been argued that accommodative insufficiency is the primary cause of symptoms in patients with convergence insufficiency (Marran et

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Investigation The four main types of accommodative anomalies are summarised in Table 3. Rarely, pathology can result in a paralysis of accommodation. An unexpected sudden loss of accommodation would therefore require referral. It is essential in any child with presumed accommodative dysfunction to know the full refractive error. An apparent accommodative problem could result from latent hypermetropia, so a cycloplegic refraction is usually required. The simplest measurement of accommodative function is the pushup test: typically, the child is asked to read detailed text as it is slowly brought towards the eye. The text should be random words or letters, so that words cannot be guessed from context e.g. the I.O.O. fixation stick. Norms for accommodative amplitude are given in Table 4. The rate of change of accommodation, or accommodative facility, can be tested with flippers. These are two pairs of lenses mounted on a stick so as to form a binocular twirl. Typically, +2.00DS and –2.00DS lenses are used. The patient views a detailed target, ideally with suppression checks, at their usual reading distance. The practitioner holds up the pair of +2.00D lenses and the patient reports when the target becomes clear. The lenses are then ‘flipped’ to the pair of –2.00D lenses. When the text is clear, the practitioner ‘flips’ again, and so on. The number of flips that can be completed in a minute is counted and halved to give the number of cycles per minute (cpm). The binocular test norms are that about 90% of the population perform better than 2.7 cpm and about 50% of the population perform better than 7.7

Symptoms/test Accommodative Accommodative results insufficiency infacility

Accommodative fatigue

Difficulty changing Near blur towards focus (e.g. copying end of day from board) Normal Declines with repeat testing

Accommodative spasm (excess)

Symptoms

Near blur

Accommodative amplitude

Low

Accommodative facility

May be slow with Poor minus lenses

Declines with repeat testing

Accommodative lag

Need high plus (>+0.75)

Need negative Initially OK, increasing plus after lenses much near vision

Normal

Transient blur of distance or near vision Normal

May be slow with plus lenses

< Table 3 Clinical characteristics of the four main types of accommodative anomalies cpm (Zellers et al., 1984). If there is an abnormal test result binocularly, the test can be repeated monocularly. These norms for the accommodative facility test show that the normal range of responses is very wide, no doubt reflecting the highly subjective nature of the test. An extremely useful objective test of accommodative function is to measure accommodative lag. This is a form of dynamic retinoscopy which is carried out at the patient’s usual reading distance, whilst the patient wears any refractive correction that they usually use for reading. The patient fixates a target on the retinoscope. Because the target is in the plane of the retinoscope, no correction needs to be made for working distance. The target is viewed binocularly, although the retinoscopy

Age (yrs)

Minimum (D) Minimum (cm)

4

14.00

7.00

6

13.50

7.50

8

13.00

7.75

10

12.50

8.00

12

12.00

8.25

14

11.50

8.75

20

10.00

10.00

30

7.50

13.25

40

5.00

20.00

50

2.50

40.00

< Table 4 Norms for accommodative amplitude measured by the push-up test

is of course only carried out on one eye at a time, usually only in the horizontal meridian. Typically a “with” movement is seen indicating that the accommodation is lagging behind the target (plus lenses need to be added). An “against” movement suggests accommodative spasm (see Table 3). Spherical lenses are introduced of a power that it is thought will neutralise the reflex. For a typical “with” movement, the first lens might be +0.50. The lens is introduced monocularly and is rapidly interposed: it should be present for no more than ½ a second. This should be just long enough for a “sweep” of the retinoscope to see if the reflex is now neutralised, and the procedure is repeated using different lenses until the reflex is neutralised. The process is then repeated for the other eye. The normal range of response (mean ± 1.00D) is plano to +0.75D. This test is particularly useful for cases who report blur during accommodative testing, or indeed at any time during the eye examination which suggests accommodative dysfunction, but where the practitioner is suspicious that there may be a visual conversion (hysterical) reaction.

Management There are two options for the management of accommodative anomalies: eye exercises or spectacles. The main types of eye exercises are push up (like push up convergence exercises but with the emphasis on keeping the target clear) and flippers. With flipper exercises, the patient is

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al., 2006). Recent GOC disciplinary cases concerning children reveal that assessments of accommodative function are often missing from practitioners’ record cards, which is a cause for concern. Accommodation can be measured in several different ways, but at least an assessment of accommodative amplitude should be included in any child’s eye examination.

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given flip lenses of a power that they can cope with (e.g., ±1.00) and they try to improve their speed with these, and then build up the power. If accommodative insufficiency or fatigue (Table 3) does not respond to eye exercises, or if the patient is not willing to do eye exercises, then the condition can be corrected with spectacles. These might take the form of reading spectacles, but more often bifocal or progressive addition lenses will be required.

Specific learning difficulties (dyslexia) Overview Dyslexia affects 5% of the population and can have many causes. We are not all equally good at everything that we do. When children have specific difficulties with some academic skills then they are sometimes described as having specific learning difficulties. Usually, this term will only be used for people with a marked problem; for example, people of average intelligence whose performance in the specific subject falls in the bottom 5% of the population. The most commonly diagnosed form of specific learning difficulty is specific reading difficulty. This is almost always associated with specific spelling difficulty and is often called dyslexia. Dyslexia attracts more attention than other specific learning difficulties because reading is a skill that is central to so many academic activities. Dyslexia describes a problem that can have many causes. There is very good scientific evidence indicating that most people with dyslexia have a difficulty with phonological decoding: they have trouble translating text into the sound units that are needed to pronounce and understand what they are reading. In some cases of dyslexia, there is also a visual component to the problem. In these cases, the optometrist can help. The optometrist should not expect to cure the dyslexia, but if they treat a visual problem that is contributing to the person’s difficulties then they are likely to help that person to read for longer with greater clarity and

comfort. This does not replace the need for specialist teaching, but means that the person will be more likely to benefit from this extra teaching.

Investigation The main visual problems that are correlated with dyslexia are MearesIrlen syndrome/visual stress (MISVIS), binocular instability, and accommodative insufficiency. It is helpful if a person with dyslexia can see an optometrist who has specialised in this subject and can carry out a detailed special investigation to look for the symptoms and signs of these problems. Typically, this requires an additional appointment for tests that would not normally be included in a normal eye examination. This subject can only be summarised in the present article (for more information, see Evans, 2004a-c). The most common visual correlate of dyslexia seems to be MISVIS. This condition is characterised by symptoms, on viewing text, of visual perceptual distortions (text moves, blurs, doubles, and shapes and patterns are seen on the page) and ‘1eyestrain’ and headaches. There is accumulating evidence suggesting that the cause of the condition is hyperexcitability of the visual cortex: a sort of ‘overload’ occurs from viewing high contrast striped patterns such as text. The intervention that seems to be most helpful is individually prescribed coloured filters (see below). The investigation of the condition includes a detailed analysis of symptoms, testing with coloured overlays, the Wilkins rate of reading test, the pattern glare test, and the MRC intuitive colorimeter and precision tinted lenses. Binocular instability is sometimes found in dyslexia. The condition is related to decompensated heterophoria and is characterised by symptoms of blur, diplopia, and eyestrain and headaches. Clinically, there will be low fusional reserves and an unstable heterophoria (eg, unstable green strip(s) on the Mallett fixation disparity test). Accommodative insufficiency is infrequently found in dyslexia. The investigation of this condition is

described above. Other visual anomalies (eg, significant refractive error, strabismus) are not specifically correlated with dyslexia, but can, of course, occur in a dyslexic child just as they can in any other child. Although not causes of dyslexia, these problems would represent an added burden for a dyslexic child and should therefore be detected and treated.

Management When people with dyslexia consult an eyecare practitioner they need a detailed visual assessment to determine whether any of the above factors are present. It is not uncommon for the practitioner to find signs of MISVIS and also subtle signs of binocular instability, and this leads to a dilemma: which should be treated first? If there is a clear motor problem (eg, a marked deficit of convergence or very low fusional reserves) then the treatment of this condition is a priority. This is particularly important if the heterophoria is at risk of breaking down into a strabismus. It is more common to find that, when binocular instability coexists with MISVIS and dyslexia, the binocular instability is very subtle. Typically, the reported benefit from coloured filters is very marked compared with, for example, the effect of a prismatic correction or occlusion on the binocular anomaly. MISVIS is an anomaly of sensory processing and this condition will impair the clarity of the monocular percepts, which will make sensory fusion more difficult (Figure 2). In cases where any binocular vision anomaly is subtle (borderline), then it is often best to start by correcting the MISVIS. The patient can be seen again a few months after collection of their precision tinted lenses to investigate whether the binocular vision anomaly is still present once their sensory perception has been improved. If binocular instability does require treatment then fusional reserve exercises usually are the most appropriate treatment. MISVIS is usually diagnosed on the basis of symptoms and an improvement with coloured overlays, either over time or via an immediate

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References Evans, B. J. W. (2001). 'Dyslexia and Vision.' (Whurr: London.) Evans, B. J. W. (2002). 'Pickwell's Binocular Vision Anomalies.' 4th edition (Elsevier: Oxford.) Evans, B. (2004a). The role of the optometrist in dyslexia. Part 1, Specific learning difficulties. Optometry Today January 30th, 29-34 (www.optometry.co.uk/pages/articles) Evans, B. (2004b). The role of the optometrist in dyslexia. Part 2: Optometric correlates of dyslexia. Optometry Today February 27, 35-39 (www.optometry.co.uk/pages/articles) Evans, B. (2004c). The role of the optometrist in dyslexia. Part 3: Coloured filters. Optometry Today 26 March, 29-35 (www.optometry.co.uk/pages/articles) Evans, B. J. W. (2005). 'Eye Essentials: Binocular Vision.' (Elsevier: Oxford.) Marran, L. F., De Land, P. N., and Nguyen, A. L. (2006). Accommodative insufficiency is the primary source of symptoms in children diagnosed with convergence insufficiency. Optom Vis.Sci 83, 281-289. Pickwell, L. D. and Stephens, L. C. (1975). Inadequate convergence. British Journal of Physiological Optics 30, 34-37. Scheiman, M., Mitchell, G. L., Cotter, S., Cooper, J., Kulp, M., Rouse, M., Borsting, E., London, R., and Wensveen, J. (2005). A randomized clinical trial of treatments for convergence insufficiency in children. Arch Ophthalmol 123, 14-24. Tang, S. T. W. and Evans, B. J. W. (2005). The Near Mallett Unit Foveal Suppression Test. Optometry Today 45, 36-39 Zellers, J.A., Alpert, T.L., Rouse, and M.W. (1984). A review of the literature and a normative study of accommodative facility. Journal of the American Optometric Association 55, 31-74.

www.optometry.co.uk/ pages/articles

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increase in speed of reading. Randomised controlled trials show that the optimum treatment for MISVIS is individually prescribed coloured lenses: different people need different colours and the colour needs to be prescribed with some precision. It is a cause of concern that some approaches prescribe colours without precision (eg, using a range of only a few colours) since most research suggests that this is not an appropriate way of correcting MISVIS. In the UK, the MRC Intuitive Colorimeter system seems to be most widely used and the research support for this system is now considerable. When people are prescribed coloured filters, the required colour should be monitored, usually yearly. The optimum colour sometimes changes over time. The NHS optical voucher can be used to make a contribution towards the cost of these tinted lenses if the patient requires correction of a refractive error, but cannot be used if there is no refractive error. The Department of Health is aware of the inconsistencies inherent in this provision, and it is hoped that proper NHS funding of the testing and prescribing of these interventions will one day be available. It is important to emphasise that any optometric intervention for people with specific learning difficulties will only address the visual component of the person’s difficulties, and will not take away the need for specialist teaching. But there is some evidence that MISVIS, which can also occur in good readers, is not only more prevalent in people with dyslexia but is also more of a problem for people with dyslexia than for people who are good readers.

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Module questions

Course code: c-5196

Please note, there is only one correct answer. Enter online or by form provided

An answer return form is included in this issue. It should be completed and returned to CET initiatives (c-5196) OT, Ten Alps plc, 9 Savoy Street, London WC2E 7HR by April 4 2007.

9/03/07 CET

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1. Which one of the following is the strongest indication of a need for a cycloplegic refraction in a young patient? a. A decompensated hyperphoria b. A decompensated exophoria c. A decompensated esophoria d. Accommodative lag on MEM retinoscopy of +0.50

7. a. b. c.

2. Which one of the following is least likely to be a symptom of decompensated heterophoria? a. Metamorphopsia b. Blurred vision c. Sore and tired eyes d. Headaches

8. In determining whether to treat a case of decompensated exophoria by refractive modification, which of the following is the least important factor to consider? a. The amplitude of accommodation b. Whether the patient prefers spectacles or contact lenses c. The effect of any pre-existing uncorrected refractive error d. The AC/A ratio

3. Which one of the following is least likely to contribute to a heterophoria becoming decompensated? a. Low fusional reserves b. An impairment to sensory fusion c. Dyslexia d. An increase in the size of the heterophoria 4. The following statements refer to the Mallett fixation disparity test. Which one is correct? a. It detects the presence of an aligning prism and measures fixation disparity b. It detects the presence of an aligning sphere and measures fixation disparity c. It detects the presence of fixation disparity and measures the size of the heterophoria d. It detects the presence of fixation disparity and measures aligning prism or sphere 5. Which of the following is correct? The cover test can provide the following information: a. Differentially diagnose strabismus from heterophoria b. Indicate whether a heterophoria is compensated c. Estimate the size of the deviation d. All of the above 6. Which of the following is the correct description of Sheard’s criterion? a. The fusional reserve that opposes the heterophoria should be at least twice the heterophoria b. The fusional reserve that opposes the heterophoria should be at least half the heterophoria c. The fusional reserve that opposes the heterophoria should be at least one third of the heterophoria d. The fusional reserve that opposes the heterophoria should be at least twice the size of the other fusional reserve

The following statements refer to fusional reserves. Which one is incorrect? During the test, children should fixate an accommodative (detailed) target The fusional reserve that opposes the heterophoria should be measured first In exophoria, the fusional reserve that opposes the heterophoria is measured with base in prisms d. The prism should be reduced until the patient reports single vision after diplopia

9. Which one of the following statements about accommodative anomalies is incorrect? a. Patients with accommodative insufficiency will, on testing with flippers, be slower to clear plus lenses than they are to clear minus lenses b. Patients with accommodative fatigue are likely to report near blur towards the end of the day c. Patients with accommodative infacility are likely to have problems copying from the board d. Patients with accommodative spasm are likely to need negative lenses when their accommodative lag is tested 10. Which of the following would be easiest to treat with fusional reserve exercises? a. Hypermetropia b. Accommodative insufficiency c. Decompensated esophoria at near d. Decompensated exophoria at near 11. a. b. c. d.

Dyslexia affects what proportion of the population? 5% 10% 15% 20%

12. Which one of the following is incorrect? a. The main visual correlates of dyslexia are Mearles-Irlen syndrome / visual stress (MISVIS), binocular instability, and accommodative insufficiency b. Meares-Irlen syndrome causes unstable visual perception which may contribute to binocular instability c. Meares-Irlen syndrome is easily corrected with blue lenses d. Binocular instability may be corrected with eye exercises

Please complete on-line by midnight on April 4 2007 - You will be unable to submit exams after this date – answers to the module will be published in our April 6 issue

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CET answers

Course code: c-5193

These are the correct answers to Module 10 Part 2, which appeared in our February 9th, 2007 issue

2. Correct answer is C A child of 18 months is not usually interested in Keeler cards and is too young to reliably name picture cards. The Cardiff cards are ideal to use between the two stages as those children who can name the pictures, will do so, and otherwise they can act as preferential looking cards. 3. Correct answer is D Print should be made available that is 3 times the size of the smallest print that a child can just manage in order to make it easy for the child to be able to see the print clearly. 4. Correct answer is A By the age of 5 years, child with normal vision should be able to see N5 print but should not be given this size to work with. 5. Correct answer is B Young infants are likely to exhibit abnormal ocular alignment but Sondhi et al (1988) suggested that intermittent exotropia should stop by the age of 6 months. 6. Correct answer is D Base out prism stimulates motor fusion. When a base out prism lens is placed in front of one eye, both eyes should move in the direction of the prism apex. If there is binocular fusion, the eye not being covered by the prism should be seen to refixate to the centre in order to overcome diplopia.

7. Correct answer is A Woodhouse (1998) found that accommodation is reduced in children with Down’s syndrome and Stewart et al (2005) found that because of their reduced accommodation, these children benefit from wearing bifocal spectacles. They advised giving a +2.50D addition with the segment top in line with the pupil. 8. Correct answer is B According to the work by Mutti et al (2000) myopes have the highest AC/A ratio, while emmetropes have a lower AC/A ratio with hypermetropes having the lowest AC/A ratio. 9. Correct answer is D Whilst adequate cycloplegia is achieved 20 minutes after instillation, more complete cycloplegia is found after 30 – 40 minutes. 10. Correct answer is D The Mohindra technique does not relax the accommodation and as a result, dilation will elicit more plus prescription than Mohindra. Borghi and Rouse (1985) found that dilated retinoscopy produced 0.50 – 0.75 more plus than the Mohindra technique. 11. Correct answer is C Ciner et al (1991) found the near stereo acuity should be 60 seconds of arc by age 5 years and Kulp & Mitchell (2005) suggested that most 4-year-olds should have a stereoacuity of at least 70 seconds of arc while most young school-aged children should have at least 50 seconds of stereoacuity. Adult levels of contrast sensitivity are reached by the age of 10 years. 12. Correct answer is B Children with learning difficulties may not be able to concentrate for long and the most important tests should therefore be carried out first. Parts of the routine eye examination will not be possible to carry out at all.

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1. Correct answer is B The Hall Report (2003) recommended that in the UK, orthoptists should undertake vision screening once the child is in education. This should be carried out at between 4-5 years of age.