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


    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

    MODULE 10:3 COURSE CODE: C-5196




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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.


    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


    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


    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,


    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


    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


    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)


    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,


    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


    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


    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


    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










    spasm (excess)

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

    Near blur towardsend of day

    Transient blur ofdistance or nearvision


    Low Normal Declines with repeattesting



    May be slow withminus lenses

    Poor Declines withrepeat testing

    May be slow withplus lenses


    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


    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


    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.


    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


    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,


    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,






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    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





    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.

    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 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.

    8. Correct answer is B

    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


    9. Correct answer is D

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

    10. Correct answer is D

    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.