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reduced divergent fusional reserves in dyslexia,
29 45
and another study
both reduced divergent and convergent reserves in dyslexia.46 These
findings are a sign of the low fusional amplitude in binocular instability
and one research group have linked this with postural instability.46 One
study found no significant difference in the fusional reserves of
dyslexic children compared with controls.14 A study of children with
academic underachievement (but not dyslexia) found low convergent
and divergent fusional reserves, and vergence infacility (impaired
ability to rapidly alter vergence).47 This study did not control for
potentially confounding variables (e.g., intelligence, attention deficit
hyperactivity disorder), but did demonstrate a significant correlation
whereby underachieving children with worse vergence facility reported
more visual symptoms.
Can binocular instability cause reading difficulty?
During normal eye movements, small vergence errors occur48 which
mean that, even in an orthophoric patient, adequate fusional reserves
are required to maintain comfortable fusion. Bucci and colleagues
found evidence of poor binocular co-ordination during saccades in
children with dyslexia, which might contribute to reading difficulties.49
This study did not look at naturalistic measures of static binocular
coordination (e.g., Mallett Fixation Disparity Test), but did evaluate
fusional reserves. Although there was no correlation between fusional
reserves and poor binocular co-ordination during saccades in the
dyslexic or control groups, most dyslexics had abnormal fusional
reserves and also abnormal binocular coordination during and after
saccades. This study did not control for ADHD and if this was present
in some of the dyslexic group then this could have confounded the
results since attentional factors influence vergence control.
50 51
Other studies have found binocular instability increases vergence
Vision, Reading Difficulties and Visual Stress
58
instability during saccades whilst reading.52 A small study, including
eight dyslexics, found poor binocular coordination during saccades
only when reading and argued this is secondary to increased cognitive
demand or reduced engagement of attention when reading.53 Ward
and Kapoula carried out a detailed analysis of vergence eye
movements in dyslexic and control adolescents in a non-reading task
and found abnormal velocity profiles of vergence eye movements,
which was associated with reduced stereoacuity.54 They postulate this
is due to delays in sensory processing, possibly related to a
magnocellular-dorsal deficit (Chapter 5), although magnocellular
function was not measured as such. In this study heterophoria was not
measured, many of the dyslexic group were receiving or had received
orthoptic treatment, and the groups were not matched for potential
confounding variables. Despite these limitations, the finding is
potentially important because post-saccadic vergence drifts were
linked to poor reading.54
A systematic review by Temelturk and Ozer in 2022 noted that
recent investigations of binocular control in dyslexic groups in both
linguistic and non-linguistic tasks had yielded conflicting results.55
However, this study only considered binocular control in eye
movement studies and did not link this to conventional tests of
binocular instability.
Jimenez and colleagues found differences between poor readers
and good readers in the vergence response to flashed presentations of
words, compared with scrambled words or strings of the letter x.56 This
was attributed to differences in visual attention, although this is
speculative because the authors did not measure visual attention or
the confounding variable of heterophoria.
Binocular instability has been linked with slower reaction times in
dyslexic adults.57 There is some evidence that binocular instability may
Chapter 3 Binocular vision and accommodative anomalies
59
contribute to the reading and spelling errors that some children
make.
58-61
Another explanation for the relationship between binocular
instability and reading difficulty deserves consideration. Binocular
instability is sometimes associated with visual stress
62 63
and it is
possible that the perceptual instability that results from visual stress
worsens sensory fusion (Figure 3.4) which in turn causes binocular
instability. According to this idea, sensory instability can cause motor
instability. The clinical implications of this are discussed in Chapter 8.
The letter strokes of words and lines of words cause striped patterns64
and it is possible that binocular instability during reading worsens
pattern glare associated with visual stress (see Chapter 8).
Detection of binocular instability
When searching for visual correlates of reading difficulties it is
useful to measure the fusional reserves (Chapter 10 and Figure 3.8).
Evans and colleagues found that the fusional amplitude (the sum of the
maximum convergence and divergence a person has in reserve) is
significantly lower in dyslexic readers than in controls.21 A reanalysis of
the raw data42 from this study reveals that, as noted above, 15% of the
dyslexic group and 5% of the control group had a fusional amplitude
less than 20. Fusional reserve measurements are influenced by test
conditions and these authors used a Variable Prism Stereoscope
(adjusted at 0.5/s) with a small (2) accommodative target at 30cm.
Other signs of binocular instability are an unstable fixation disparity
and a variable reading in the Maddox Wing Test (>±2). These
diagnostic signs are combined in the scoring system in Table 3.2.
Vision, Reading Difficulties and Visual Stress
60
Figure 3.8. Prism bar used to measure fusional reserves (see Chapter 10).
Joss and Jainta65 found that some clinical tests of binocular co-
ordination only demonstrated a weak to moderate correlation with
dynamic measures of vergence alignment when eye movements were
recorded during reading. However, the 65 participants were all non-
dyslexic with normal visual function and fairly low symptom scores,
and the Mallett test was not included.
Treatment of binocular instability
The management of binocular instability is first to remove any
obstacle to sensory fusion.6 For example, any significant refractive
errors should be corrected. If binocular instability persists, if a direction
for the deviation in binocular instability is apparent (e.g., exophoria),
the management of the condition is the same as the management of
decompensated heterophoria described above.
If there is no significant heterophoria then the next stage usually is
to improve the motor fusion (Figure 3.4) with fusional reserve
exercises. In binocular instability, the underlying problem is low
fusional reserves and it has been shown in well-controlled studies that
fusional reserves improve with a programme of eye exercises.
26 27 35
It
seems likely that any symptoms directly attributable to the low fusional
Chapter 3 Binocular vision and accommodative anomalies
61
reserves would improve when the fusional reserves improve. However,
the effect of eye exercises on symptoms in binocular instability has not
been investigated in large randomised controlled trials, so the
effectiveness of this treatment awaits validation.
Accommodative anomalies
Background
Ocular accommodation is the ability to change the focus of the eye
between its relaxed state, when looking in the distance, and its
accommodated state, focussed for near vision. There are three main
clinical methods of assessing accommodation:
1. Accommodative amplitude is the maximum accommodation that
can be exerted, measured as the nearest point to which the
person can accommodate.33 It is often measured with the RAF rule
(Figure 3.6), the limitations of which recently have been
highlighted.
66 67
2. Accommodative lag is the extent to which the person’s actual
accommodation lags that required for the target. Usually, it is
measured by objective methods, requiring no patient response
other than viewing a target.6
3. Accommodative facility is the rate at which the accommodation
can be changed.6
As noted in Chapter 2, young people with uncorrected
hypermetropia can attempt to compensate for this with
accommodation, which would cause an abnormal response on tests of
accommodation. Therefore, accommodative anomalies can only be
Vision, Reading Difficulties and Visual Stress
62
diagnosed with certainty once latent hypermetropia has been ruled
out, typically requiring drops that relax the accommodative muscles
(cycloplegic refraction).6
Are accommodative anomalies associated with reading
difficulty?
Evans and colleagues found that a group of children with dyslexia
had a slightly lower median amplitude of accommodation (16D) than a
group of control children without dyslexia (20D).21 More recent
research also finds marginally lower amplitude of accommodation in
dyslexia than in controls.
14 30
Two studies of children with non-dyslexic
reading difficulties find lower amplitudes of accommodation and
accommodative facility than controls, but neither study controlled for
potentially confounding variables such as intelligence and attention
deficit disorder.47
Pattern glare is discussed in Chapter 8 as several lines of evidence
point to its role in visual stress, the condition that appears to be helped
by coloured filters. Research that measured accommodation
objectively indicates that symptoms of pattern glare are not strongly
associated with accommodation.68
Can accommodative anomalies cause reading difficulty?
One of the studies that found slightly reduced amplitude of
accommodation in dyslexia21 found no difference between the groups
in accommodative lag whilst the marginally reduced median
accommodation amplitude was not found to significantly impair
performance at a visual search task. In the other studies cited above
where reduced amplitude of accommodation was found to be
associated with dyslexia, the difference in mean binocular amplitude
in the two groups was only 1-2D. In general, such a small difference is
Chapter 3 Binocular vision and accommodative anomalies
63
unlikely to be clinically significant.
14 30
However, this finding might
indicate that children with reading problems are slightly more likely to
suffer from accommodative insufficiency compared to good readers,
and there is some evidence that accommodative insufficiency in
dyslexia can cause symptoms when reading.32 Therefore, it is sensible
for children with reading difficulties to have a careful assessment of
their accommodative function. Two studies, one of accommodative
facility69 and one of combined accommodative and vergence facility,21
have failed to find a relationship between infacility and dyslexia.
In summary, children with dyslexia are a little more likely to have
accommodative insufficiency. This is unlikely to be a major cause of
dyslexia, but could cause symptoms when trying to read, in which case
treatment is likely to be of some help.
Detection of accommodative anomalies
The clinical protocol for testing accommodation is described in
more detail in Chapter 10. The most common test is the push up test,
which measures the amplitude of accommodation (Figure 3.6).
33 66 67
A
method of measuring accommodative lag is the monocular estimation
method (MEM), when the clinician uses light reflected from the back of
the eye to measure the lag. This has the advantage of not requiring a
subjective response. Accommodative facility is especially relevant if a
child has problems copying from the board (i.e., changing focus from
distance to near and vice versa) although this symptom also can result
from non-optometric problems, such as poor short-term memory.
One study argued that accommodative insufficiency is the primary
source of symptoms in children diagnosed with convergence
insufficiency,70 although this has been disputed.71
Vision, Reading Difficulties and Visual Stress
64
Treatment of accommodative anomalies
Accommodative anomalies can be treated with spectacles or eye
exercises.72 The prescription for spectacles can be determined with
usual clinical methods.6 A variety of approaches are available for
accommodative exercises, and it seems most sensible to concentrate
on the area(s) of accommodative function where the child has most
difficulty.6
When is optometric treatment necessary?
The views expressed in this chapter might result in about 5-15% of
children with reading problems receiving optometric intervention,
such as spectacles or eye exercises. Some optometric philosophies
argue that a much higher proportion of children with reading problems
should be treated, reflecting a wide variety of views concerning which
cases require treatment. One argument is ‘If there is even a faint
possibility that optometric treatment will help, then why not have a go?’
But there are some difficulties with this approach, that will now be
discussed.
Optometrists have a duty to treat patients who need treatment, but
also to only treat those cases that are likely to benefit from
intervention. Parents of children with reading problems are often
desperate to do anything they can to help their children, but financial
resources and time will inevitably be limited. Time is especially relevant
because these children will often require extra teaching and
homework. Specialist teaching is well known to help with reading
problems, and this can be especially effective if it is systematic and in
small groups or on a one-to-one basis.73
Chapter 3 Binocular vision and accommodative anomalies
65
and who will carry out an in-depth assessment of visual function (see
Chapter 10). This in-depth assessment, sometimes called a ‘special
investigation’, will require about a dozen different tests of binocular
and accommodative function. The probability of having a problem
detected increases with the number of tests undertaken. So, if
practitioners prescribe spectacles or eye exercises to every patient who
fails one test from a large battery of tests, they will treat a high
proportion of cases, probably including many who do not need
treatment. In other words, a normal patient is just a patient who has
not been tested enough!
The solution to this problem is to combine test results and this
approach has been used in the algorithm in Table 3.2 for the diagnosis
of decompensated heterophoria and binocular instability. No
approach or algorithm is perfect and experienced clinicians will make
a careful decision about whether to treat based on the complete
clinical picture, and on the results of repeated measurements over
time.
People with reading or other academic problems should be referred
Vision, Reading Difficulties and Visual Stress
66
to eye care practitioners who have specialised in vision and learning
†
This algorithm is for horizontal heterophoria. For vertical cases, if aligning
prism of 0.5 or more, after checking trial frame alignment, measure the vertical
dissociated phoria. If this is more than the aligning prism and there are
symptoms, diagnose decompensated heterophoria.
DISTANCE / NEAR (delete)
score
1. Does the patient have one or more of the symptoms of decompensated heterophoria
(headache, aching eyes, diplopia, blurred vision, distortions, reduced stereopsis, monocular comfort, sore eyes, general irritation)?
If so, score +3 (+2 or +1 if borderline)
Are the symptoms at D or N
All the following questions apply to D or N, as ticked (if both ticked, complete 2 worksheets)
2. Is the patient orthophoric on cover testing?
Yes or No If no, score +1
3. Is the cover test recovery rapid and smooth?
Yes or No If no, score +2 (+1 if borderline)
4. Is the Mallett horizontal aligning prism: <1 for patients under 40, or <2 for pxs over 40?
Yes or No If no, score +2
5. Is the Mallett aligning prism stable (Nonius strips stationary with any required prism)?
Yes or No If no, score +1
6. Using the polarised letters binocular status test, is any foveal suppression < one line?
Yes or No If no, score +2
Add up score so far and enter in right hand column
if score: <4 diagnose normal, >5 treat, 4-5 continue down table adding to score so far
7. Sheard’s criterion:
(a) measure the dissociated phoria (e.g., Maddox wing, prism cover test); record size & stability
(b) measure the fusional reserve opposing the heterophoria (i.e., convergent, or base out, in
exophoria). Record as blur/break/recovery in .
Is the blur point, or if no blur point the break point, [in (b)] at least twice the phoria [in (a)]?
Yes or No If no, score +2
8. Percival’s criterion: measure the other fusional reserve and compare the two break points.
Is the smaller break point more than half the larger break point?
Yes or No If no, score +1
9. When you measured the dissociated heterophoria, was the result stable, or unstable (varying
over a range of 2 or more). (e.g., during Maddox wing test, if the Hz phoria was 4 XOP and
the arrow was moving from 2 to 6, then result unstable)
Stable or Unstable If unstable, score +1
10. Using the fusional reserve measurements, add the divergent break point to the convergent
break point. Is the total (=fusional amplitude) at least 20?
Yes or No If no, score +1
Add up total score (from both sections of table) and enter in right hand column. If total score: <6 then
diagnose compensated heterophoria, if >5 diagnose decompensated heterophoria or binocular
instability).
Table 3.2. Algorithm for diagnosing decompensated heterophoria† and
Chapter 3 Binocular vision and accommodative anomalies
67
binocular instability. Reproduced under STM Permissions Guidelines from
Evans (2021) Pickwell’s Binocular Vision Anomalies, 6th edition, Elsevier.