binocular vision anomalies part 1

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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 inheterophoria 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 childmay 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 theirheterophoria 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

Heterophoria

OverviewIf 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 distanceof 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

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Bruce Evans

This article wil l 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 areparticularly 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 wil l 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.

Binocular vision anomalies: Part 1Symptomatic heterophoria

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information in Figure 2 to determine

what factor(s) have caused a

heterophoria to decompensate, is not

just academic. When an optometristencounters 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 isdecompensating 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

SymptomsThere is no single method which is

perfect at diagnosing decompensatedheterophoria, 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,

Mallett unit fixation disparity testIt 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, non-

fusible, 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 fusionlock (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 showsthat 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

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, typicallyyoung 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.

Cover t estThe cover test can provide a great deal

of information. It can be used todifferentially 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.

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

< Figure 2A simple model of binocular vision.Reproduced with permission from Evans,B.J.W. (2002) Pickwells Binocular VisionAnomalies, 4th edition, Elsevier

< Table 1A grading system for cover test recovery

Grade Description

1 Rapid and smooth

2 Slightly slow / jerky

3 Definitely slow / jerky but notbreaking down

4 Slow / jerky and breaks dow n w ithrepeat covering, or only recovers aftera blink

5 Break s d ow n read ily after 1- 3 covers


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aligning prism), starting in 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 thedecompensated 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

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 patientwith 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 patients eyes

should be watched to confirm the

break point, when the vergence

movement should cease.

In exophoria, Sheards criterion is auseful way of interpreting the fusional

reserves. Sheards criterion says that

the fusional reserve that opposes the

heterophoria should be at least twice

the heterophoria. In esophoria,

Percivals criterion is more useful,

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 fusionA 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 particularlyimportant 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 testsDissociation 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 isan essential part of every primary care

eye examination and also can be used

to monitor the size of deviation (Evans,

2005).

ManagementThe 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 poorsensory 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

< Figure 3The Mallett near fixation disparity test. The left hand picture is for testing horizontal and the right forvertical heterophoria.

< Figure 4

Graph of mean symptom score v. aligning prism atnear. 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)


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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 theinvestigation 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 inemmetropic 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, hencereducing 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, theamplitude 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

deteriorates, especially i f 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

OverviewConvergence insufficiency occurs

when the patient has a remote nearpoint 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 decompensatedexophoria 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

< Figure 5

Measuring the fusional reserves with a prism bar

< Table 2 Main approaches to treating decompensated heterophoria

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

Intervention Most suitable for(in descending order)

Comments

Eye exercises Exophoria at nearExophoria at distance

Esophoria at nearEsophoria at distance(rarely useful for hyperphoria)

Various methods are available, and acombination of approaches is often helpful

Refractive modification Esophoria at distance & near inlatent hypermetropesEsophoria at near (multifocals)Exophoria at distance or near(negative add)

In esophoria, latent hypermetropia shouldalways be suspected and a cycloplegicrefraction is required for young patients.Even in cases without a refractiveaetiology, refractive modification is oftensuccessful

Pr isma ti c cor re ct io n Hyp erph or iaEsophoria

Near exophoria

Prismatic correction is occasionally used inexophoria, typically in reading glasses for

older patients

Surgery Cyclophoria & hyperphoriaVery large esophoria or exophoria

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


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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.

InvestigationA 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 patientswho 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. Thepatient 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 outtasks 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

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.

ManagementThere 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 notrequire 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 patients nose

whilst the patient tries to keep it

single. If the accommodation is

adequate, or needs training as well (seenext 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

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

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 childs 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 patientsuppresses 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

OverviewThis article wil l 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 weaknessexophoria 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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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 childs eye

examination.

InvestigationThe 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 ofaccommodative function is the push-

up 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 mountedon 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 minuteis 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

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 isthought 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

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.

ManagementThere are two options for the

management of accommodativeanomalies: 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

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 accommodativelag. This is a form of dynamic

retinoscopy which is carried out at the

patients 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

< Table 3 Clinical characteristics of the four m ain types of accomm odative anomalies

Symptoms/test

results

Accommodative

insufficiency

Accommodative

infacility

Accommodative

fatigue

Accommodative

spasm (excess)

Symptoms Near blur Difficulty changingfocus (e.g. copyingfrom board)

Near blur towardsend of day

Transient blur ofdistance or nearvision

Accommodativeamplitude

Low Normal Declines with repeattesting

Normal

Accommodativefacility

May be slow withminus lenses

Poor Declines withrepeat testing

May be slow withplus lenses

Accommodativelag

Need high plus(>+0.75)

Normal Initially OK,increasing plus aftermuch near vision

Need negativelenses

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 13.2540 5.00 20.00

50 2.50 40.00

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


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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 learningdifficulties (dyslexia)

OverviewDyslexia 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; forexample, 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

diffi culty. This is almost always

associated with specific spelling

difficulty and is often called dyslexia.

Dyslexia attracts more attention than

other specific learning di fficulties

because reading is a skill that is

central to so many academicactivities.

Dyslexia describes a problem that

can have many causes. There is very

good scientific evidence indicating

that most people with dyslexia have a

diffi culty wi th 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 optometristshould not expect to cure the

dyslexia, but if they treat a visual

problem that is contributing to the

persons diffi culties 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.

InvestigationThe main visual problems that are

correlated with dyslexia are Meares-

Irlen 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 adetailed 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 visualperceptual 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 includesa 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 unstableheterophoria (eg, unstable green

strip(s) on the Mallett fixation

disparity test).

Accommodative insufficiency is

infrequently found in dyslexia. The

investigation of this condition i s

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.

ManagementWhen people with dyslexia consult an

eyecare practitioner they need adetailed 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 breakingdown 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 willmake 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 stil l

present once their sensory perception

has been improved.

If binocular instability does require

treatment then fusional reserveexercises 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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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 mostwidely 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 ofHealth 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 persons

diffi culties, and wi ll 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.

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, Specifi c learnin g

diff i culties. Optometry Today

January 30th, 29-34

(www.optometry.co.uk/pages/articles)

Evans, B. (2004b).The role of the

optom etrist in dyslexia. Part 2:

Optom etric correlates of dyslexia.

Optometry TodayFebruary 27, 35-39


Evans, B. (2004c).

The role of the optometrist in

dyslexia. Part 3: Coloured fi lters.

Optometry Today26 March, 29-35


Evans, B. J. W. (2005).

' Eye Essential s: Bin ocular Vision.'

(Elsevier: Oxford.)

Marran, L. F., De Land, P. N., andNguyen, A. L. (2006).

Accommodative insufficiency is the

primary source of symptoms in

children diagnosed with convergence

insufficiency. Optom Vis.Sci83,

281-289.

Pickwell, L. D. and Stephens,

L. C. (1975). Inadequate convergence.

Briti sh Journ al 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

Un it Foveal Sup pression 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 A ssociation55,

31-74.

www.optometry.co.uk/pages/articles

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6

Module questions Course code: c-51 96Please note, there is only one correct answer. Enter online or by form providedAn 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.

1. Which one of the following is the strongest indication of a need for acycloplegic refraction in a young patient?

a. A decompensated hyperphoriab. A decompensated exophoriac. A decompensated esophoriad. Accommodative lag on MEM retinoscopy of +0.50

2. Which one of the following is least likely to be a symptom ofdecompensated heterophoria?

a . Me ta morphopsiab. Blurred v is ionc. Sore and tired eyesd. Hea da che s

3. Which one of the following is least likely to contribute to a heterophoriabecoming decompensated?

a. Low fusional reservesb. An impairment to sensory fusionc. Dyslexiad. 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 disparityb. It detects the presence of an aligning sphere and measures fixation disparityc. It detects the presence of fixation disparity and measures the size of

the heterophoriad. 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 thefollowing information:

a. Differentially diagnose strabismus from heterophoriab. Indicate whether a heterophoria is compensatedc. Estimate the size of the deviationd. All of the above

6. Which of the following is the correct description of Sheards criterion?a. The fusional reserve that opposes the heterophoria should be at least twice

the heterophoriab. The fusional reserve that opposes the heterophoria should be at least half

the heterophoriac. The fusional reserve that opposes the heterophoria should be at least one

third of the heterophoriad. The fusional reserve that opposes the heterophoria should be at least twice

the size of the other fusional reserve

7. The following statements refer to fusional reserves. Which one is incorrect?a. During the test, children should fixate an accommodative (detailed) targetb. The fusional reserve that opposes the heterophoria should be measured firstc. In exophoria, the fusional reserve that opposes the heterophoria is

measured with base in prismsd. The prism should be reduced until the patient reports single vision after diplopia

8. In determining whether to treat a case of decompensated exophoria byrefractive modification, which of the following is the least im portant factorto consider?

a. The amplitude of accommodationb. Whether the patient prefers spectacles or contact lensesc. The effect of any pre-existing uncorrected refractive error

d . The AC/A ra tio

9. Which one of the following statements about accommodativeanomalies is incorrect?

a. Patients with accommodative insufficiency will, on testing with flippers, beslower to clear plus lenses than they are to clear minus lenses

b. Patients with accommodative fatigue are likely to report near blur towardsthe end of the day

c. Patients with accommodative infacility are likely to have problems copyingfrom the board

d. Patients with accommodative spasm are likely to need negative lenseswhen their accomm odative lag is tested

10. Which of the following would be easiest to treat with fusionalreserve exercises?

a . Hypermet rop iab. Accommodative insufficiencyc. Decompensated esophoria at neard. Decompensated exophoria at near

11. Dyslexia affects what proportion of the population?a. 5%b. 10%c. 15%d. 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 insufficiencyb. Meares-Irlen syndrome causes unstable visual perception which may

contribute to binocular instabilityc. Meares-Irlen syndrome is easily corrected with blue lensesd. 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 a fter this date answ ers to the module will be published in our April 6 issue


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CET answers Course code: c-51 93

These are the correct answers to M odule 10 Part 2, which a ppeared in our February 9th, 2007 issue

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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.

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 achild 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.


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.


Base out prism stimulates motor fusion. When a base out prism lens is placed infront 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 accomm odation is reduced in children with Downs

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.


According to the work by Mutti et al (2000) m yopes have the highest AC/A rat io, while

emmetropes have a lower AC/A ratio with hypermetropes having the lowest AC/A

ratio.


Whilst adequate cycloplegia is achieved 20 m inutes after instillation, more completecycloplegia is found after 30 40 minutes.


The Mohindra technique does not relax the accomm odation and as a result, dilation

will elicit m ore 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 w hile 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 BChildren 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.

binocular vision anomalies part 1

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