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Файл:Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_5253_Библиотеки_им_академика_М_И_Перельмана.pdf
compensated. Although, because of the link between accommodation
and convergence, a long-sighted correction will slightly increase the
exophoria,6 sensory fusion may be sufficiently enhanced to
compensate. These factors can be investigated clinically using the
Mallett Fixation Disparity Test,
17 22 23
described in the next section.
O’Leary and Evans carried out a randomised controlled trial of
patients who were not selected as having any reading difficulty but
whose optometric test results indicated decompensated exophoria.
The researchers investigated whether base in prisms that corrected
the decompensated exophoria improve the reading speed.18 They
found prisms significantly improve reading speed when
the required prism is 2.5 or more. It is interesting to note, the effect
of prisms on the Wilkins Rate of Reading Test is modest compared to
the effect of coloured filters in people with visual stress (see Chapter
7).
Figure 3.5. The Mallett Fixation Disparity Test, enlarged to show the test for
horizontal deviations. The upper green vertical line is seen by one eye and
the lower green vertical line by the other eye (as a result of polarising filters).
Vision, Reading Difficulties and Visual Stress
48
When the two lines are aligned, the eyes are appropriately aligned.
Detection of decompensated heterophoria
In the UK, a test developed by Mallett in the 1960s has
revolutionised the detection, diagnosis, and management of
decompensated heterophoria. This is the Mallett fixation disparity test
(enlarged in Figure 3.5), which is one of the tests on the Mallett unit.
Unlike most binocular vision tests, the Mallett test simulates normal
binocular vision because the central text (O X O) and the text
surrounding the test are seen by both eyes in the normal way.
However, when the patient wears special polarised filters, two features
of the test are seen only by one eye: the green lines in Figure 3.5 known
as Nonius strips. Typically, the green line above the X is seen by the right
eye and the strip below the X by the left eye. A minute misalignment of
the lines (fixation disparity) is suggestive of decompensated
heterophoria. The strength of a type of lens (prism) that is required to
align the lines indicates whether the heterophoria is decompensated
and how it might be corrected.
Pickwell and colleagues in the 1980s showed that the Mallett
fixation disparity test is the best single test for detecting
decompensated heterophoria.6 However, no single test is perfect and
the diagnosis of decompensated heterophoria is usually based on
symptoms and a combination of test results (cover test recovery,
aligning prism on the Mallett Unit, and, in the case of exophoria, the
adequacy of the fusional reserve that opposes the heterophoria).6 This
is discussed further below.
Treatment of decompensated heterophoria
There are three main strategies for the management of
decompensated heterophoria: eye exercises (vision therapy), refractive
Chapter 3 Binocular vision and accommodative anomalies
49
correction, or prismatic correction.6 The first two of these are more
relevant to children with decompensated exophoria,24 and the most
appropriate approach largely depends on the child’s motivation to
complete eye exercises.6 As noted above, prismatic corrections,
prescribed based on the results of the Mallett test,25 can significantly
improve the speed of reading.18 However, the conventional view was
that prisms are not the treatment of choice for young people, since eye
exercises or refractive modification may lead to an improvement in
underlying function.6 The timescale of the improvement is weeks to
months with eye exercises and months to years with refractive
modification. Therefore, many eyecare practitioners took the view that
eye exercises would be a better approach, if the child and parents are
willing.
This conventional view has been thrown into question by a large
and thorough multi-centre research study, known as CITT-ART.
26 27
This
study investigated 310 children aged 9-14 years with a particular type
of decompensated heterophoria: exophoria associated with
convergence insufficiency. The research investigated a very thorough
programme of eye exercises, involving exercises at home and in the
clinic for 16 weeks. Importantly, there was a control group who
received sham exercises, carefully designed to give them the same
time and attention as the children receiving the “genuine” treatment.
Two key clinical tests showed greater improvement in the group
receiving the real treatment than the control, so eye exercises do seem
to change visual function in this condition. Symptoms improved, but
the magnitude of improvement did not differ significantly in the two
groups and nor did the reading performance, which was assessed
thoroughly. In other words, vision therapy was no better than a
placebo at improving symptoms and reading performance.
The CITT-ART study, published in 2019, has important implications.
Vision, Reading Difficulties and Visual Stress
50
Although the authors only studied one condition and one system of
eye exercises, the condition they studied (decompensated near
exophoria) is the one for which eye exercises are most commonly
prescribed. Some other conditions, such as esophoria, are generally
thought to respond less well to eye exercises. If there is a concern that
a case of decompensated exophoria may worsen to a strabismus, eye
exercises are likely to be worthwhile because the CITT-ART trial found
an improvement in test results that indicate better control of the
exophoria. However, if it is thought that an exophoria is contributing
to symptoms, the CITT-ART results indicate that eye exercises are not
likely to be the best approach. This conclusion applies to the condition
studied in the CITT-ART trial, near decompensated exophoria
associated with convergence insufficiency. The results may not apply
to a simpler condition, when there is a convergence insufficiency that
exists in isolation without a decompensated exophoria. This is
discussed next.
The CITT-ART study excluded children with phonologically based
reading difficulties. Therefore, it is not known whether the results also
apply generally to people with dyslexia. There is some evidence that
cognitive load influences binocular co-ordination.28 It is therefore
possible that the extra cognitive load required by dyslexics when
reading could make them more prone to binocular vision anomalies.
However, this is speculation.
Convergence insufficiency
Background
Convergence describes the turning in of the eyes to maintain
alignment on a target that approaches the nose. Convergence
insufficiency, in its simplest form, is characterised by a remote near
Chapter 3 Binocular vision and accommodative anomalies
51
point of convergence (the nearest point to which the eyes can
converge). The term is also sometimes used to describe a
decompensated exophora for near vision when associated with a
convergence insufficiency (convergence insufficiency exophoria
syndrome or CIES).6 In this book, when the term convergence
insufficiency is used alone it solely refers to a remote near point of
convergence.
Is convergence insufficiency associated with reading difficulty?
A review in 2001 noted controversy as to whether convergence
insufficiency is a correlate of dyslexia.7 Only one study since 2001 has
found a slightly more remote near point of convergence in a group of
children with dyslexia than in a control group.29 Other studies have
found no significant difference in the near point of convergence of a
group of dyslexic children compared with controls.
14 30
Of course,
dyslexia may have many causes, and if convergence insufficiency is in
fact a cause, but only one among many, it is unlikely to be detected in
a group of dyslexic individuals unless the study is very large.
Convergence insufficiency does appear to be associated with attention
deficit hyperactivity disorder (ADHD),31 although this could be an
artefact owing to poor concentration on the task. In any event,
convergence insufficiency is such a prevalent condition that it will be
encountered quite commonly in both good and poor readers.
Can convergence insufficiency cause reading difficulty?
In view of the weak correlation between convergence insufficiency
and reading difficulty it is unlikely that there is a major causal
relationship. However, there is evidence that in some individuals with
dyslexia, convergence insufficiency may be associated with
symptoms,32 which could contribute to the burden a dyslexic child
experiences when reading.
Vision, Reading Difficulties and Visual Stress
52
Figure 3.6. RAF rule used to measure near point of convergence or
accommodative amplitude.
Detection of convergence insufficiency
Convergence insufficiency is easily detected as a remote near point
of convergence.33 This can be measured with a handheld target and
ruler, or with an instrument that has a target on a slide that moves
along a ruler. An example of this type of test is the RAF rule (Figure 3.6).
Treatment of convergence insufficiency
Eye exercises are the usual approach for treating convergence
insufficiency.6 The CITT-ART study investigated a more complex
condition, convergence insufficiency associated with near
decompensated exophoria (CIES)6. In that study, symptoms improved
no more with a thorough system of eye exercises than with placebo
eye exercises, but there was a significant improvement in the near
Chapter 3 Binocular vision and accommodative anomalies
53
point of convergence. It seems likely that the symptoms were
attributable, in large part, to the decompensated exophoria that would
affect people at their normal reading distance. Therefore, in patients
with the simpler problem when the anomaly is a remote near point of
convergence, it seems reasonable to expect that eye exercises will be
effective at improving the near point of convergence.
In cases where the near point of convergence is receding, it may be
helpful to improve the near point of convergence with exercises before
it reaches the stage where symptoms are likely.6 IFS exercises are
designed to be carried out by the patient, usually a child, at home; but
they are more comprehensive and intensive than home exercises
typically used in the UK. There are a series of four target cards which
the patient views and which create a variety of stereoscopic
perceptions. The exercises come with detailed documentation which
gives the parent (or the patient if an adult) a programme of instructions
to take them through a series of tasks, each gradually building their
convergent fusional reserves and other relevant functions. There are
also instructions for the practitioner, who typically sees the patient for
Vision, Reading Difficulties and Visual Stress
54
The IFS programme of exercises (Figure 3.7) was developed by one
of the authors at the Institute of Optometry to treat convergence
insufficiency. An initial open trial found this system of exercises
improved the near point of convergence and symptoms.34 A recent
randomised controlled trial found that IFS exercises improve near
point of convergence and convergent fusional reserves, but not
symptoms.
35
The authors of this trial cautioned
that this finding may
have occurred because the symptoms were reassessed too soon after the
eye exercises stopped. Nonetheless, this
finding is similar to the CITT-ART
findings described above and adds to the evidence that eye exercises
(vision therapy) should not be advocated as a widespread approach to
treating symptoms and are not likely to improve reading performance.
a first follow-up about three weeks after the exercises are dispensed.
The IFS exercises, like other orthoptic exercises, should only be
prescribed by eye care professionals who can select appropriate
patients and monitor progress.
Figure 3.7. Institute Free-Space Stereogram (IFS) exercises.
Sheard’s criterion (an assessment of the adequacy of the fusional
reserves that are used to overcome the heterophoria) has been shown
to be useful in the diagnosis of decompensated exophoria;
36 37
but
does not seem to be a useful method of prescribing prisms.38
Dusek and colleagues evaluated children who had reading
difficulties that were not attributed to a learning difficulty but due
instead to a convergence insufficiency.39 Some children opted for eye
exercises and others for spectacles with prisms and both these
interventions were found to be helpful for reading compared with a
Chapter 3 Binocular vision and accommodative anomalies
55
group who opted for no treatment. However, the authors point out
that this was not a randomised controlled trial39 and the lack of any
intervention for the control group means that placebo effects could
account for the results.
Binocular instability (fusional vergence
dysfunction)
Background
Binocular instability is a condition characterised by low fusional
reserves and an unstable heterophoria.
40 41
In North America, the
condition is called fusional vergence dysfunction. Binocular instability
is not the same as decompensated heterophoria, although sometimes
the conditions overlap and are both present in the same individual
(Table 3.1). An unstable heterophoria can be detected with tests of
heterophoria However, it is most likely to be clinically significant if it
is present under more natural conditions, using the Mallett Fixation
Disparity Test (Figure 3.5). With the Mallett test, it is important to ask
patients whether there is any movement of one or both Nonius lines;
such a finding is suggestive of binocular instability and often associated
with symptoms.17
Table 3.1. Differential diagnosis of binocular instability & decompensated
heterophoria (reproduced with permission from Pickwell’s Binocular Vision
Anomalies, 6th edition, Bruce Evans, Elsevier, 2021).6
Sign
Binocular instability
Decompensated
heterophoria
Heterophoria
May be present, or may be
orthophoric (no strabismus or
Heterophoria must be
present
Vision, Reading Difficulties and Visual Stress
56
heterophoria)
Stability of
heterophoria6
Unstable: movement of arrow in
Maddox wing test usually 2 or
more
Stable or unstable:
movement of arrow in
Maddox wing test
usually less than 2
Cover test6
Recovery may or may not be normal
Recovery usually slow
and hesitant
Fusional
reserves
Usually both convergent and
divergent reserves are low, so
fusional amplitude <20. Result
may worsen markedly as patient
tires
Fusional reserve
opposing the
heterophoria is usually
low
Fixation
disparity/
aligning prism
One or both Nonius lines move.
There may be an aligning prism, or
the movement may be similar in
both directions
Nonius lines are
misaligned, but are not
necessarily moving
Foveal
suppression6
Often present, likely to be transient,
may be alternating
May be present, likely to
be constant during
binocular viewing,
usually unilateral
Is binocular instability associated with reading difficulty?
One straightforward sign of binocular instability is when the
fusional amplitude is less than 20 (see below). Using this method,
binocular instability has been found to affect about 15% of people with
dyslexia compared to 5% of controls.42 This does not mean that 15% of
people with dyslexia require treatment of binocular instability, since
more than one diagnostic sign is usually required to reach a firm
diagnosis (see later). This research, showed that binocular instability is
associated with the magnocellular deficit in dyslexia (see Chapter 5).43
A small study, of 13 dyslexic and 13 control participants found no
significant difference between the groups in vergence accuracy or
speed during a dynamic task.44 Subsequent studies have found
Chapter 3 Binocular vision and accommodative anomalies
57