the investigation and management of heterotropia

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The investigation and
management of heterotropia
Module 14 Part 4: Binocular Vision
Course code: C-14568 O/D
Bruce Evans
The previous article in this series described the investigation and
management of heterophoria, which is a normal finding but which can
lead to visual problems if the heterophoria is no longer compensated.
This article is about heterotropia, which occurs when a person is unable
to compensate for the deviation and develops a strabismus. It is written
from the perspective of the community optometrist. When the community
optometrist has a patient with heterotropia sitting in their chair, what tests
do they need to do and what management options are available to them?
The answers to these questions are dependent on the age of the patient,
and so this article will be divided into two sections for different age groups,
although emphasis is on paediatric cases. The article begins though, by
describing the main types of heterotropia that may be encountered.
Heterotropia in
pre-school children
Neonatal misalignments
Brief neonatal misalignments of the
visual axes commonly occur in the
first month of life and should become
less frequent in the second month.1
Referral should be arranged for any
infant whose neonatal misalignment
behaviour worsens after two months or
who has an intermittent deviation after
four months. These cases should be
monitored closely for amblyopia, even
if the deviation seems to be improving.
Infantile esotropia syndrome
Constant strabismus with an age of
onset before one year is most commonly
infantile esotropia syndrome in which
case it will require referral. This is also
known as early acquired esotropia and
used to be called congenital strabismus,
although it is not usually present at birth.
Infantile esotropia may be caused by
an innate defect of fusion and is fairly
easy to recognise since the following
clinical characteristics are present:
• onset in first 6 months
• large (>30∆) stable angle
• initial alternation with crossed
fixation.
These cases may be further subdivided as
essential infantile esotropia, nystagmus
blocking syndrome, or a sixth nerve
palsy. None of these types of strabismus
with a very early onset are likely to
respond to optometric treatment. When
these types of strabismus are found
in young children, they should be
referred urgently to the hospital eye
service (HES), for possible surgery. The
prognosis for sensory and motor fusion
is poor, but is significantly improved by
early surgical intervention, if possible
at about three months of age. There is
no justification for waiting until the
child is old enough for sensory testing.2
An early interruption to binocularity,
typically from infantile esotropia
syndrome, often results in three clinical
signs which persist throughout life,
even if the visual axes are surgically
straightened. These three conditions are
latent nystagmus, dissociated (vertical)
deviation (DVD), and inferior oblique
overaction. Latent nystagmus is a
horizontal nystagmus that is either only
present or greatly increased on monocular
occlusion and always beats towards the
uncovered eye. In DVD, each eye deviates
upwards when it is covered, giving the
appearance of alternating hyperphoria.
Often, the eye starts to deviate before
the occluder actually covers the eye.
Infantile accommodative
esotropia
As many as 15% of patients with
infantile esotropia may have infantile
accommodative esotropia, nearly half
of whom can be fully straightened with
spectacles. The earlier correction begins,
the better the chances of success, and if
more than +2.25D is detected in an infant
with esotropia then spectacles should
be tried before surgery.3 Surgery is only
indicated on the portion of the deviation
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that spectacles do not control after a trial
of two to three months, and spectacle
wear should be continued after surgery.
Esotropia after the first year
of life
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Esotropia with an onset after the first
year of life will not be infantile esotropia
syndrome and the chances of there being
a significant accommodative element
increase. Cycloplegic refraction and,
if significantly hypermetropic, full
plus prescription for constant wear
are required as soon as possible. If the
esotropia is eliminated by spectacle
wear, then usually any amblyopia also
resolves, although some cases might
require occlusion. Constant occlusion
should be avoided in these cases because
this could jeopardise the establishment of
binocularity. The treatment of these fully
accommodative cases is within the scope
of practice of the community optometrist.
Cases that cannot be straightened
fully with spectacles will require
referral to the HES for two reasons.
First, they will require referral to
investigate the aetiology of the
strabismus, and second to investigate
whether surgery would be helpful.
Quite often, the optometrist is presented
with cases of suspected pseudo-esotropia:
where a large epicanthus leads the
parent to suspect a strabismus but where
none can be found on cover testing.
Differential diagnosis of epicanthus can
be quite difficult, especially since an
intermittent esotropia or microtropia also
might be present. Even if the optometrist
is confident that there is no esotropia
present at the time of the appointment, the
child still needs to be monitored closely
and a cycloplegic refraction is advisable.
Exotropia
Infrequent intermittent exo-deviations
(divergent drifts) are quite common up
to the age of six months and should only
be considered abnormal if they become
more constant or persist beyond six
months.4 Some authors argue that early
onset exotropia is likely to be associated
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with nervous system pathology, although
others disagree. In any event, constant
exotropia in a pre-school child is unlikely
to respond to optometric management and
will require prompt referral to the HES to
investigate the aetiology of the deviation.
Investigation
When pre-school children present to
the optometrist with heterotropia, it
is likely to be of recent onset. In such
cases, there are three main goals for the
clinical investigation of the strabismus.
General goal
Detail
Tests
Detect
pathology
Is retinal pathology present?
Ophthalmoscopy, usually with pupil dilation and
rarely in hospital under anaesthesia
Is central nervous system (CNS)
pathology present?
• are there any other systemic signs of CNS
pathology (eg, epileptic fits)
• test pupil reactions
• examine optic discs
• check for incomitancy
• look for nystagmus
Is other systemic
pathology present?
• Is the child dysmorphic?
• Are there other health problems (eg, not
developing or growing normally)?
• Is the child’s behaviour or development unusual
for their age?
Is there a relevant
family history?
• Strabismus
• High hypermetropia
• Nystagmus
• Amblyopia
• Eye surgery in young children
Is there a relevant personal
history?
• Complications during mother’s pregnancy
• Premature and / or low birth weight
• Complications during birth (eg, forceps, hypoxia)
• Complications after birth (eg, jaundiced)
• Complications in first year (eg, operations, severe
illnesses)
• Illness (eg, causing fever) around the time of onset
of heterotropia
• Trauma (including non-accidental injury)
Is there an incomitant deviation
that may have been present since
birth?
• Duane’s syndrome
• Fourth nerve palsy
• Sixth nerve palsy
Is there a significant refractive
error?
• Cycloplegic refraction
Is it comitant?
• Motility testing (see below)
Type of deviation?
• Cover testing at distance & near
Is it accommodative?
• Effect of refractive correction
Is amblyopia present?
• Visual acuity testing
Find the
cause of the
heterotropia
(assuming that
no pathological
cause is
apparent)
Determine the
characteristics
of the
heterotropia
Table 1
Summary of the investigation of heterotropia in pre-school children
to be comfortable and experienced at
testing pre-school children and have
the appropriate tests. In the first year of
life, acuity assessment usually requires
preferential looking grating cards. Over
the age of one year, many children can be
tested with Cardiff acuity cards, which
are more widely available. But these
methods of assessing acuity are not as
good at detecting strabismic amblyopia
as crowded optotype tests and, as soon
as possible, this type of test should be
used. The computerised Test Chart 2000
is extremely useful for this because a
variety of optotypes can be presented,
including lower case letters, numbers,
Lea symbols, and Kay pictures. These
can be presented in isolation, which
makes the task easier for children,
yet with crowding present for better
detection of amblyopia (Figure 1). The
optotypes can also be randomised,
which
prevents
young
children
from memorising the letter chart!
If amblyopia is detected then it is
widely believed that treatment should
be started as soon as possible. Even
if the optometrist is going to refer the
child to a hospital eye clinic, it is still
often helpful for the optometrist to start
amblyopia treatment if there is likely to
be a wait of a few months until the child
is seen at the hospital. The treatment
of strabismic amblyopia is described
in more detail later in this article.
Heterotropia in
school children
Overview
Optometrists are the main providers of
eye care to school children. Indeed, the
most common visual disorder in the
age-group is refractive error, and the
prescribing of refractive corrections is
almost exclusively the province of the
optometrist in the UK. It is therefore
not surprising that optometrists play a
key role in the detection of heterotropia
in this age-group. Unfortunately, it is
still much less common for children
to have routine eye care than, for
example, routine dental care. Many of
35
Figure 1
A Lea symbol presented in a ‘crowded box’ using the
computerised Test Chart 2000. Reproduced with
permission from Thomson Software Solutions.
the children who need eye care most (eg,
those from deprived backgrounds, with
learning disabilities, or with specific
learning difficulties) still do not receive
professional eye care. It is also a cause for
concern that many parents assume that
their children will have vision routinely
screened in schools. It is important for
the profession to educate the public that
vision screening at school has become
less frequent and in many areas is
conducted only once, on school entry.
This means that any refractive error or
binocular vision anomaly that is either
not detected at school entry, or develops
after school entry, may be missed. Many
parents and nearly all grandparents will
be under the care of an optometrist and
this is an opportunity for the profession
to educate these people about the
need for routine professional eye care
in their children and grandchildren.
For younger children, who are in the first
few years at school, the management of
strabismus is similar to that outlined
for pre-school children. However,
the testing becomes much easier as
the children understand optotypes
and communicate better, and the
possibilities for optometric management
improve since children become more
likely to cooperate with spectacle wear
or eye exercises. In older teenagers, the
management of heterotropia is more
akin to that for adults. This section
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First, the practitioner must take
reasonable steps to detect pathology.
In pre-school children, ‘reasonable
steps’ usually means assessing pupil
reactions and ophthalmoscopy. Further
investigation (eg, electrophysiology or
neuro-imaging) are not generally felt to
be appropriate unless these initial steps
reveal an abnormality (eg, suspicious
discs) or the heterotropia is atypical
(eg, increasing in angle, incomitant,
associated with nystagmus, amblyopia
not responding to treatment). This
means that every case of heterotropia
should be investigated by either an
optometrist or an ophthalmologist; since
these are the eye care professionals
who are skilled at ophthalmoscopy.
To obtain a good view of the fundus in
heterotropic pre-school children, pupil
dilation is usually required. In some very
young or very uncooperative cases, a good
view of the fundus can only be obtained
during general anaesthesia. Although
the risk of serious pathology, such as
retinoblastoma, is only slight, the serious
nature of this and other conditions means
that ophthalmoscopy under anaesthesia
may be appropriate in suspicious cases.
The second goal of the clinical
investigation is to find the cause of
the strabismus. For the primary care
optometrist the only aetiology that
they may be able to manage in this age
group is a purely refractive esotropia,
so a cycloplegic refraction is important.
An investigation of the personal and
family history is also relevant (Table 1).
The third goal is to determine the
characteristics of the strabismus. This is
important for future monitoring and to
determine the priority for the referral.
If there is a sudden onset incomitant
deviation then referral should be urgent.
Table 1 summarises the investigation
of strabismus using the three goals
outlined above. It is easy to see why
some community optometrists do not
feel happy managing these cases in the
primary care setting. The only cases
for which optometric management is
appropriate are the fully accommodative
cases, and the practitioner will need
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Figure 2
A case of fully accommodative esotropia without (left) and with (right) refractive correction.
Reproduced with permission from Evans.5
of the article concentrates on the age
range from 5-12 years, and there are two
reasons for this emphasis. First, it is
quite common for strabismus to develop
during this interval, because children are
starting to carry out a lot more detailed
visual tasks and for longer periods.
Second, amblyopia treatment becomes
less successful and more risky after the
age of about 7-8 years, and is not usually
treated beyond the age of 12 years.
Investigation
Aetiology
One way of thinking about the
investigation of heterotropia, which
was introduced in Table 1, is to look for
a negative sign and a positive sign. The
negative sign is the absence of pathology,
and the tests for this in Table 1 should
be easier to carry out in school children
than in pre-school children. Indeed, for
most school children visual field testing
can be added to the tests listed in the
first section of Table 1. The positive
sign is the detection of a cause for the
strabismus, and if a non-pathological
cause is found (eg, high hypermetropia)
then this greatly reduces the likelihood
of a pathological cause being present.
The detection of positive signs (nonpathological causes) in strabismus have
already been discussed for younger
children (Table 1) and is only slightly
different in older children. In esotropia,
the optometrist should look for
hypermetropia, including a cycloplegic
refraction. Before the cycloplegic is
instilled, the effect on the heterotropia of
correcting the refractive error revealed by
‘dry’ refraction should be investigated.
If the proposed refractive correction
does not fully straighten the patient
at distance, then investigate the effect
of a reading addition at near. Usually,
it is enough to carry out the cover test
with the proposed refractive correction.
Type of strabismus
Tests need to be carried out to determine
the type of strabismus which will in turn
determine the aetiology and indicate the
prognosis for optometric management.
The key tests are a cover test at
distance and near, with and without
any significant change in refractive
correction that has been found. It is
useful to record a quantification of the
angle of deviation (ie, ‘15∆ esotropia’
rather than ‘medium esotropia’) and
to note the method that is used to
obtain this.5 The two main methods
are estimation by observation (Table 2)
and measurement with the prism bar.
The cover test is good at detecting
most types of strabismus, except for
some intermittent cases and for some
types of microtropia. Intermittent cases
usually have a marked heterophoria on
cover testing and are also recognisable
from the symptoms: the patient may
report episodes of diplopia and/or
their family and friends may report
episodes when one eye deviates.
Microtropia is usually defined as a
strabismus where the angle of deviation
is less than 10∆. Most cases are unilateral
esotropia and there are often deeply
engrained sensory adaptations that have
caused some authors to describe this as
a ‘fully adapted squint’. It is certainly
true that most patients with microtropia
are asymptomatic: they usually have no
diplopia, coarse levels of stereoacuity
on contoured tests (eg, the Titmus Fly,
Wirt circles), and do not appear to
have any ocular deviation to the casual
observer. Indeed, some cases do not
have any movement on cover testing:
the angle of deviation is the same as
the angle of eccentric fixation. These
cases can be very difficult to diagnose,
but typically have amblyopia and
eccentric fixation, as described below.
Strabismic amblyopia and
binocular sensory adaptations
If a child in the first few years of life
develops a constant unilateral strabismus
then they will probably develop strabismic
amblyopia. This is characterised by
reduced visual acuity in the strabismic
eye which does not immediately improve
with refractive correction. It is almost
invariably associated with eccentric
fixation. The easiest way to detect this in
community practice is to use the fixation
star in the direct ophthalmoscope. It
helps to carry this test out on the nonamblyopic eye first to train the patient.
If an adult develops heterotropia then they
are likely to have diplopia and confusion.
In young children (up to the teen years),
the onset of strabismus usually leads to
binocular sensory adaptations to prevent
diplopia. In small-angle deviations, the
patient usually develops harmonious
abnormal
retinal
correspondence
(HARC) and in large-angle deviations
there is usually suppression of the
binocular field of the strabismic eye.
From the perspective of the community
optometrist, if a patient has a strabismus
and they do not have diplopia then they
must have a binocular sensory adaptation.
If the strabismus has been there for some
time, then the sensory adaptation is
likely to have developed during early
Table 2
Estimating the angle in cover testing.
Reproduced with permission from Evans.5
childhood, when the visual system
was most adaptable. In these cases,
the sensory adaptation is likely to be
quite deeply ingrained and treatment to
straighten the visual axes in these cases
is likely to be difficult and probably
undesirable. This is because an attempt
to treat or correct the angle without
eliminating the sensory adaptation
could cause intractable diplopia.
In practice, the intervention that the
optometrist is most likely to prescribe
is a refractive correction. The effect of
this on the angle and on the sensory
adaptation can be predicted simply by
placing the proposed prescription in
a trial frame (Figure 2) and asking the
patient if they experience diplopia. If
not, then the correction is likely to be safe
to prescribe, but the patient should be
warned to return if any diplopia occurs.
Management
Esotropia
As noted earlier in this article, when
esotropia is found then hypermetropia
must be suspected and a cycloplegic
refraction is indicated. It is not
uncommon to see primary school
children with moderate degrees of
hypermetropia who start to suffer
decompensation
or
intermittent
strabismus
owing
to
increased
accommodative demand with school
work (Figure 2). If the esotropia has not
become well-established then correction
with spectacles is usually successful.
If the esotropia is not corrected with
the full plus prescription, then it still
might be possible to straighten the visual
axes at least at near by using multifocal
spectacles. The effect of a near add
can be investigated, but obviously not
whilst the cycloplegic is still having an
effect. As noted above, the proposed
prescription can be placed in a trial
frame to ensure that it does not cause
diplopia. Often, patients whose esotropia
is at near only can be effectively treated
with multifocal spectacles, the aim being
to gradually reduce the add over time,
as long as a reduction does not cause
the esotropia to return or an esophoria
to decompensate. Any amblyopia will
need to be treated as outlined below.
Exotropia
The most common form of exotropia
to occur under the age of 19 years
is intermittent exotropia. As noted
earlier in this article, an unexplained
new deviation will require referral to a
neuro-ophthalmologist to investigate
the aetiology. The cases that are
amenable to optometric management
are those where there is a long-standing
exophoria that may be decompensating
into an exotropia, perhaps owing to an
increase in near visual tasks at school.
Exotropia can also be associated with
neurological abnormalities (eg, cerebral
palsy). Exotropia can be divided into three
main categories: divergence excess (worse
at distance), convergence weakness (worse
at near), and basic exotropia (a significant
deviation at both distance and near).
Divergence
excess
characteristically
presents as a deviation that occurs
intermittently when the patient is looking
in the far distance. It is usually associated
with suppression so that the patient may
be unaware that one eye has diverged
markedly. It is more likely to occur with
far distance fixation (eg, looking out of a
window) than with the usual optometric
6m test distance. Eye exercises or
negative lenses are sometimes helpful;
more severe cases may need surgery.
Near vision exotropia (convergence
weakness) may respond to optometric
intervention and the prognosis for this
is more favourable if the angle is less
than 20∆ and intermittent. The usual
optometric interventions can be tried:
eye exercises, refractive modification
(negative lenses), or prisms. Larger
angles, especially if permanent, may need
referral for surgery. Similar approaches
can be tried in basic exotropia.
Hypertropia & cyclotropia
Hypertropia and cyclotropia are usually
the result of incomitant deviations and new
cases will require referral to the HES for
an investigation of the underlying cause.
Occasionally, cases of long-standing (eg,
congenital) superior oblique palsy will
be seen where the patient is just able to
compensate for the deviation at distance
but may have an intermittent or constant
hypertropia of the affected eye for reading.
In some cases, a vertical prism may allow
comfortable binocular single vision under
a wider range of viewing conditions.
Before prescribing, the prism should be
tried in a trial frame to see if it improves
symptoms and, most importantly, to
check that it does not cause diplopia.
Strabismic amblyopia
Amblyopia affects about 3% of the
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1. The amplitude of movement should always
be estimated (in Δ) and recorded during cover
testing.
2. It is easy to train yourself to be quite accurate
at this, and to regularly ‘calibrate’ your estimations.
On a typical Snellen chart, the distance from
a letter on one end of the 6/12 line to a letter
on the other end is about 12cm (measure this
on your chart to check). If the distance is 12cm,
this means that when the patient changes their
fixation between these two letters the eyes make
a saccade of 2Δ (1Δ is equivalent to 1 cm at 1 m).
3. If you place two markings on the wall near the
letter chart that are 24cm apart, when the patient
changes their fixation between these two marks
then the eyes are moving by 4Δ.
4. After you have done the cover test and
estimated the amplitude (in Δ) of the strabismic
or heterophoric movement, remove the cover
and have the patient look between these two
marks, or between the two letters on the 6/12
line whilst you watch their eyes. Compare the
amplitude of this eye movement with the
amplitude of movement that you saw during
cover testing, to check the accuracy of your
estimate.
5. A similar method can be used for larger
amplitudes.
6. At near, this task is even easier. Use as your
fixation target the numbers on a centimetre ruler
which you hold at 1/3 m. If the patient looks from
the 1 to the 2 then the patient’s eyes are moving
by 1cm which, at 1/3 m, equates to 3Δ
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Figure 3
Stages to the treatment of strabismic amblyopia. Research suggests that many cases improve with
spectacles alone, or with part-time (2hrs a day) occlusion, and do not need full-time occlusion.
population
and the detection of
amblyopia is an important role for
primary eye care practitioners. There
has been a great deal of research
in recent years on this subject and
there is only room here to briefly
summarise some of the key findings.
Where there is a strabismic component
to the amblyopia then treatment before
the age of about 7-8 years is important
for two reasons. Firstly, treatment before
this age is likely to be more successful
than treatment after this age. Secondly,
treatment after this age is associated with a
small risk of causing intractable diplopia,
and so should only be undertaken
cautiously by practitioners who can
carefully monitor the sensory status.
These two reasons together explain why
most practitioners do not recommend
treatment of strabismic amblyopia
after about the age of eight years.
There is a long history of patients with
strabismic amblyopia being treated
in community optometric practices
and this is just as appropriate today as
it has been in the past, as long as the
practitioner has the appropriate expertise
and is used to working with children.
Of course, if the heterotropia requires
neuro-ophthalmological
investigation
then referral is required, and guidance
on when this is appropriate was given
earlier in this article. Even when referral
is required, then there is no reason why
the community optometrist cannot start
treatment of the amblyopia so that this
can be underway whilst the patient
is awaiting a hospital appointment.
There are four stages to the treatment
of strabismic amblyopia (Figure 3),
not all of which are necessary in a
given person. For example, some cases
will be cured of amblyopia simply by
refractive correction, and will never
progress beyond stage 1.6 Similarly,
many cases do not require full-time
occlusion. The timings in Figure 3 are
debated: some experts would suggest
longer intervals and others shorter. It is
wise to see patients frequently during
the treatment of amblyopia, to begin
with every 4-6 weeks. Penalisation
is a viable option to occlusion, for
example using
a cycloplegic7 or a
spectacle or contact lens that blurs
the good eye at one or all distances.
Whoever treats amblyopia, whether
an
optometrist,
orthoptist,
or
ophthalmologist, will need to give
clear instructions to the patient and
should be aware that patient and parent
compliance is often the key to success.
This does not just mean using a patch
when advised, but also making sure that
the patch fully occludes the eye and that
spectacles fit properly. Contact lenses
are a viable option in many cases.8 The
idea of patching is to cause an eye that
has fallen into disuse to become used, at
least when the patch is worn. It therefore
helps to encourage the child to carry out
some detailed visual task when the patch
is worn. This is made much easier if the
child likes the task, such as a favourite
DVD or computer game. If the parents
carefully control the viewing distance then
the child can be encouraged to monitor
their high score on computer games as
a way of helping them to appreciate
the improvement in the amblyopia.
It must be remembered that, if the
amblyopia is quite marked, a patient
may be rendered partially sighted during
the time when the good eye is occluded.
This may impact on education, safety,
and quality of life and all these factors
will need to be carefully discussed with
the parent. At the very least, it is helpful
to ask for the child to sit near the front
in class when the occluder is worn.
Not every case of amblyopia responds to
treatment and some of those who do not
respond may have subtle degrees of optic
nerve hypoplasia. However, since the
diagnosis of amblyopia is one of exclusion
then the index of suspicion of pathology
being present must be raised in those cases
who do not respond to treatment. If there
is any deterioration in acuity, or any other
risk factors, then referral is indicated.
Anisometropic amblyopia
Overview
There are some important differences
between
anisometropic
amblyopia
and strabismic amblyopia, particularly
concerning management. The main
difference is that strabismic amblyopia
should be treated before the age of
about eight years, whilst anisometropic
amblyopia can be treated at any age.5
Quite often, anisometropia coexists
with heterotropia and, from the
perspective
of
treatment,
these
mixed cases need to be considered as
39
strabismic amblyopia. This means that
it is important to detect strabismus,
particularly microtropia, in cases of
suspected anisometropic amblyopia.
Investigation
In anisometropic amblyopia, it is
necessary to correct the full degree of
anisometropia. Therefore, in young
patients a cycloplegic refraction is
required. It was noted earlier in this article
that cases of microtropia will usually be
associated with eccentric fixation. They
are also likely to fail stereopsis tests,
particularly those with random dot
targets. Some, but not all, microtropic
cases will show a heterotropic movement
on cover testing. The four prism dioptre
base out test is sometimes recommended
as the diagnostic test for microtropia,
but this test can give confusing results.5,9
Management
As already noted, there are some
differences between the management of
orthotropic anisometropic amblyopia
and strabismic amblyopia. The first
major difference is that, with orthotropic
anisometropic amblyopia, stage 3 of the
treatment for strabismic amblyopia that
was outlined in Figure 3 is inappropriate,
since full-time occlusion could cause
the binocular vision to break down. A
second difference is that, since nonstrabismic anisometropic amblyopia
is essentially a refractive problem, it
is very likely that it will respond to
simple refractive correction. A seminal
paper in 1988 showed that for all types
of anisometropia, contact lenses are
better than spectacles at controlling
aniseikonia.10 These authors noted that
in children with anisometropia, contact
lenses provide a more potent stimulus to
the binocular system. Silicone hydrogel
contact lenses have made continuous
wear possible for these cases and this
can be an option, if the usual criteria
for safe contact lens wear are met.8
The third major difference between
anisometropic and strabismic amblyopia
is that the majority of studies have
indicated
that
in
non-strabismic
anisometropic amblyopia the amblyopia
can respond to treatment at any age.5
Quite commonly in optometric practice
one comes across adults who have one
moderately hypermetropic eye that has
never received correction. In young
adulthood these patients can become
alarmed from increasing blur as they
become less able to accommodate for the
hypermetropia when they cover their
good eye. These cases can be offered a
correction, ideally a contact lens, for
their more hypermetropic eye. A contact
lens trial can be used to confirm that
diplopia does not occur, and indeed
such an outcome would be very unlikely.
Incomitancy
Overview
An incomitant deviation is one in which
the angle of deviation varies depending
on the direction of gaze and on which eye
is fixating. A new or changing incomitancy
can be a sign of pathology and requires
referral to the HES. Long-standing cases
require monitoring by the community
optometrist in case they decompensate.
Investigation
The first indication that an incomitant
deviation may be developing usually
comes from the symptoms. Adult patients
will probably report diplopia and for
child patients, a parent will usually
notice a ‘turning eye’ and questioning
may reveal that this is more noticeable
when they look in a certain direction.
The key test for diagnosing an incomitant
deviation is the ocular motility test. This
is a very simple test to do with a comitant
patient, but it is much harder to use the
test to reveal information about the type
and severity of an incomitancy. Really,
there are three motility tests and when
evaluating a patient with an incomitancy
it is useful to do the test three times,
each time looking for a different feature
of the test. A fairly bright pen light is the
best fixation target and in the first test
the corneal reflection of the pen light is
used to evaluate the eye movements and
detect any under- or over-actions. Care
should be taken that the nose does not
occlude the pen light, when its corneal
reflection will disappear. In the second
version of the motility test, an alternating
cover test is used in peripheral gaze to
determine the position of gaze where
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Figure 4
Successive computerised Hess plots of a resolving right lateral rectus palsy. Reproduced with permission from Evans5
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there is the maximum deviation, and
what type of deviation is present in
this position. In the third version of
the test the patient is asked to report
any diplopia. This can be instructive in
some cases, but very confusing in others.
Some patients suppress and so do not
report diplopia and others provide
confusing descriptions of diplopia,
sometimes because their convergence
is breaking down for the pen light,
which is a poor stimulus to fusion.
In uncertain cases, or when there is a preexisting incomitancy that the optometrist
wishes to monitor, it can be very useful to
obtain a plot of the incomitant deviation
that can be repeated after a few weeks
or a few months. Hess or Lees screens
are designed for this and any Windows
PC can be used with the Thomson
Software Solutions Hess Screen (Figure
4). Other diagnostic approaches are
described in more detail in other texts.5
The three most common incomitancies
seen in optometric practice are Duane’s
syndrome, superior oblique palsy, and
lateral rectus palsy. Duane’s syndrome
is a congenital mechanical incomitancy:
there is a restriction of horizontal
movement in one or both eyes. The
affected eye(s) may fail to abduct, adduct,
or both: the eye looks as if it is tethered.
Classically (but not always), there is a
retraction of the eye and associated lid
closure on adduction. Patients usually
adapt to the condition well, experience
no diplopia, and typically adopt a head
position that allows normal binocular
vision. The condition can be associated
with other congenital abnormalities, so
it is sensible to refer the condition if
an optometrist first detects it in a child.
The lateral rectus and the superior
oblique muscles are each innervated by
their own nerves: the sixth and fourth
nerves respectively. These nerves have
long pathways and are prone to damage,
for example from high blood pressure,
diabetes, stroke, trauma, and other
vascular and neurological anomalies.
If a patient presents with diplopia of
recent onset then a problem with one
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of these nerves should be suspected.
If the lateral rectus is affected then the
major component of the diplopia will
be horizontal and the diplopia (and
the eso-deviation) will be worse with
distance fixation and when the patient
looks to the side of the affected muscle.
If the superior oblique is affected then the
diplopia will be predominantly vertical
and cyclotorsional and is often worse
on downgaze. There are exceptions to
this pattern, because secondary sequelae
to the palsy may occur over time.
Management
The most important points to stress
about incomitant deviations are that they
can be a sign of pathology and new or
changing incomitant deviations require
referral. The speed of onset of symptoms
is a good guide to the required speed of
referral. If a patient woke up today with
constant diplopia that they have never
had before and an incomitant deviation
is apparent on testing then an emergency
referral is required. If they have been
experiencing intermittent diplopia for
some years which is gradually becoming
more frequent and an incomitant
deviation is found then a more routine
referral is likely to be appropriate.
A sudden onset third nerve palsy requires
emergency referral, but thankfully is
only seen rarely in optometric practice.
Indeed, the community optometrist is
much more likely to see patients with
long-standing incomitancies. In these
cases, if the patient is asymptomatic then
it is best to try not to change the status quo.
Some cases of fourth nerve palsy manage
to maintain binocular single vision in
the primary position, but are likely to
have a hyperphoria in the affected eye.
If this is causing symptoms, then it can
be helped by a small vertical prism,
which can be prescribed with the Mallett
unit (see Part 3). Since these cases may
have particular difficulty when looking
down and in, multifocal spectacles or
translating designs of multifocal contact
lenses may be contraindicated. This is
because forcing the patient to look in the
field of action of the affected muscle could
cause the incomitancy to decompensate
(worsen).
Indeed,
any
incomitant
deviation can decompensate after a
number of years of stability, and so it is
best not to do anything to interfere with
their ocular motor status. For example,
monovision is contraindicated for longstanding cases of incomitancy, or indeed
for unilateral comitant strabismus.11
Some cases of long-standing lateral rectus
palsy can be helped by a base out prism
for the affected eye in distance vision
spectacles. These cases may benefit from
vocational advice: for example, it would be
best for an affected child to be positioned
in the classroom so that they look away
from the side of the affected muscle
when they look at the board or teacher.
Similarly, most children with a congenital
superior oblique palsy would find it
helpful when using a computer at home
to place the monitor higher than usual.
Conclusion
This article has described the features and
investigation of heterotropia, primarily in
children and in new-borns. Identification
of the presence and type of heterotropia
is vital in such patients, to ensure normal
development of binocular single vision
and appropriate management. Adult
cases of heterotropia are more likely to
be due to pathological causes, and this is
dealt with in the next part of this series.
About the Author
Bruce Evans is Director of Research
at the Institute of Optometry and
Visiting Professor to City University
and London South Bank University.
He spends most of his working week
seeing patients in an independent
optometric practice in Brentwood,
Essex. Professor Evans has authored
many books on the topics of binocular
vision and dyslexia/reading difficulties.
References
See www.optometry.co.uk and search
‘references’
Course code: C-14568 O/D
1.Concerning neonatal misalignments, which of the following observations
should cause a practitioner to be MOST concerned?
a. If they are brief
b. If they become less frequent in the second month of life
c. If the size of the deviation remains constant
d. If they are intermittent
7. If you discover a 30∆ constant exotropia in a two-year-old child, what is
the MOST likely management for the community optometrist?
a. Cycloplegic refraction and full plus prescription
b. Cycloplegic refraction and bifocals
c. Convergence exercises
d. Prompt referral to the HES
2. Which of the following is the MOST accurate description of typical
infantile esotropia syndrome?
a. Constant large angle esotropia with an age of onset before one year
b. Intermittent esotropia with an age of onset in the first three months of life
c. Exotropia in the first year of life which develops into an esotropia after surgery
d. Apparent esotropia in the first year which results from epicanthus
8. Which of the following types of strabismus may be difficult to detect with
the cover test?
a. Alternating esotropia
b. Microtropia
c. Exotropia
d. Convergence weakness exotropia
3. What is the MOST appropriate course of action for an optometrist who
sees a six-month-old child with a constant large angle esotropia, assuming
that cycloplegic refraction reveals +1.50DS each eye?
a. Monitor to see if the condition resolves spontaneously
b. Refer promptly for ophthalmological opinion
c. Prescribe bifocals since most of child’s world is at near
d. Prescribe occlusion and see again in three months to see if the eye has
straightened
9. Which of the following is NOT a typical feature of microtropia?
a. Deviation less than 10∆
b. Esotropia
c. Diplopia
d. Deeply ingrained sensory adaptation
4. Which of the following is NOT a common consequence of infantile
esotropia syndrome?
a. Dissociated vertical deviation (DVD)
b. Latent nystagmus
c. Inferior oblique overaction
d. Convergence insufficiency
5. Which of the following is NOT a feature of latent nystagmus?
a. It is only present or greatly increased on monocular occlusion
b. It beats towards the uncovered eye
c. It always beats in the same direction (always to the left or always to the right)
d. It is always a horizontal nystagmus
6. Which of the following statements about esotropia that develops at an
age of 1.5 years is FALSE?
a. It is a form of infantile esotropia syndrome
b. Cycloplegic refraction will be required
c. If significant hypermetropia is found, then spectacles are required
d. If spectacles fully straighten the eyes, then management is within the scope
of the community optometrist
10. Which of the following is NOT a typical characteristic of divergence
excess exotropia?
a. A deviation that is greater at distance than near
b. An intermittent deviation
c. A larger deviation in the far distance (looking out of a window more than
six metres)
d. Diplopia
11. Which of the following statements about a superior oblique palsy is
FALSE?
a. Accompanying presbyopia should be corrected with varifocals
b. It often causes a hyperphoria
c. It often causes a cyclophoria
d. Resultant symptoms can be assisted by base down prism in the affected eye
12. Which of the following statements about amblyopia is FALSE?
a. Orthotropic anisometropic amblyopia can be treated at any age
b. Orthotropic anisometropic amblyopia should be treated with full-time
occlusion
c. Both anisometropic amblyopia and strabismic amblyopia in some cases
can be treated just with refractive correction
d. Occlusion for strabismic amblyopia is not usually recommended over the
age of about eight years
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Module questions