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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
(www.optometry.co.uk/pages/articles)
Evans, B. (2004c).
The role of the optometrist in
dyslexia. Part 3: Coloured fi lters.
Optometry Today26 March, 29-35
(www.optometry.co.uk/pages/articles)
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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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.
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
ratio.
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.