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impairment due to uncorrected refractive error: a systematic review.
Chapter 2 Refractive errors & ocular health
37
BMC Public Health. 2023;23. doi: 10.1186/s12889-023-16484-z.
34. Rosner J, Gruber J. Differences in the perceptual skills development of
young myopes and hyperopes. Am J Optom Vis Sci. 1985;62(8):501-4.
35. Rosner J, Rosner J. Some observations of the relationship between the
visual perceptual skills development of young hyperopes and age of first
lens correction. Clin Exp Optom. 1986;69:166-8.
36. Williams WR, Latif AH, Hannington L, Watkins DR. Hyperopia and
educational attainment in a primary school cohort. Arch Dis Child.
2005;90(2):150-3.
37. O'Leary CI, Evans BJW, Edgar DF. The effect of low refractive corrections
on rate of reading. Optometry in Practice. 2014;15(3):87-100.
38. Yammouni R, Evans BJ. An investigation of low power convex lenses
(adds) for eyestrain in the digital age (CLEDA). J Optom. 2020;13(3):198-
209. doi: 10.1016/j.optom.2019.12.006.
39. O'Donoghue L, Rudnicka AR, McClelland JF, Logan NS, Saunders KJ. Visual
acuity measures do not reliably detect childhood refractive error--an
epidemiological study. PLoS ONE. 2012;7(3):e34441. doi:
10.1371/journal.pone.0034441.
40. Evans BJW. Case reports: The need for optometric investigation in
suspected Meares-Irlen syndrome or visual stress. Ophthal Physiol Opt.
2005;25:363-70.
41. Farbrother JE. Spectacle prescribing in childhood: a survey of hospital
optometrists. Br J Ophthalmol. 2008;92(3):392-5.
42. Cotter SA. Management of childhood hyperopia: a pediatric
optometrist's perspective. Optom Vis Sci. 2007;84(2):103-9. doi:
10.1097/OPX.0b013e318031b08a [doi].
43. Lyons SA, Jones LA, Walline JJ, Bartolone AG, Carlson NB, Kattouf V, et al.
A survey of clinical prescribing philosophies for hyperopia. Optom Vis
Sci. 2004;81(4):233-7.
44. Shneor E, Evans BJ, Fine Y, Shapira Y, Gantz L, Gordon-Shaag A. A survey
of the criteria for prescribing in cases of borderline refractive errors. J
Optom. 2016;9(1):22-31. doi: 10.1016/j.optom.2015.09.002.
45. O'Leary CI, Evans BJW. Criteria for prescribing optometric interventions:
literature review and practitioner survey. Ophthal Physiol Opt.
2003;23:429-39.
46. Jennings A. Behavioural optometry: a critical review. Optometry in
Practice. 2000;1:67-78.
47. College of Optometrists. F02: Guidance for the issuing of small
prescriptions and making small changes to existing prescriptions. www
college-optometrists org. 2012.
48. Evans BJW, Drasdo N, Richards IL. Investigation of accommodative and
binocular function in dyslexia. Ophthal Physiol Opt. 1994;14(1):5-19.
Chapter 3
Binocular vision and accommodative
anomalies
Chapter abstract
The main binocular vision and accommodative anomalies are
considered in turn: strabismus, heterophoria, convergence
insufficiency, binocular instability, accommodative insufficiency, and
accommodative infacility. For each of these conditions, any
associations with reading difficulties are reviewed. Each review
includes an assessment as to whether the anomaly may contribute
causally to reading difficulties. The treatment of these conditions is
discussed, also addressing the general question of when optometric
factors need to be treated. The chapter closes with a table
summarising the above conditions. It is concluded that binocular
instability (fusional vergence dysfunction) is correlated with dyslexia,
but is unlikely to be a major cause of the condition.
Introduction
Binocular vision, or binocular co-ordination, describes how the eyes
move together and align on objects of regard, controlled by muscles
outside the eye. Up to about the age of about 50 years, human eyes
can focus on different distances. This ocular accommodation is
controlled by muscles within the eye. Together, binocular vision and
accommodation are sometimes described as ocular motor functions
(confusingly, this has a different meaning to oculomotor, which is
variously used in relation to saccadic eye movements or functioning of
the third cranial nerve).
© The Author(s), under exclusive license to Springer Nature Switzerland AG 2024
A. J. Wilkins and B. J. W. Evans, Vision, Reading Difficulties and Visual Stress,
https://doi.org/10.1007/978-3-031-65568-5_3
39
In this chapter, the main anomalies of binocular vision and
accommodation that could be linked with reading difficulty are
discussed. Typically, this link is investigated in research studies that
compare two groups of children, one group with reading difficulty (e.g.,
dyslexia) and the other with normal reading skills (control). Even when
research of this type identifies a visual factor that is associated with
reading difficulty, this does not necessarily mean that the visual
condition is a cause of the reading difficulty. The headings outlined in
Chapter 1 are used to determine whether there is an association
between a condition and reading difficulty, whether there is a causal
relationship, and how the ocular motor anomaly can be detected and
treated.
When researching binocular vision and accommodation, careful
attention needs to be paid to the way the research is undertaken (the
research methodology). Some studies have been omitted
1-4
because of
methodological difficulties and these are explained in Chapter 12.
There are a variety of approaches to classifying and diagnosing
binocular vision anomalies, so it is important that researchers give
details of the approach they have taken.5
Strabismus
Background
Strabismus (squint; heterotropia) occurs when the eyes are
misaligned (Figure 3.1). Strabismus can be obvious, as in the figure, or
of a very small angle (microtropia) when it will only be detected during
an eye examination. Strabismus affects 2-3% of the population.6
Vision, Reading Difficulties and Visual Stress
40
Figure 3.1. Illustration of normal ocular alignment (orthophoria) and
various types of misalignment of the eyes, known as strabismus, squint, or
heterotropia.
When the angle of deviation (misalignment) remains unchanged in
different position of gaze the strabismus is said to concomitant.
Otherwise the strabismus is incomitant. Strabismus that occurs early
in life does not usually result in double vision because of sensory
adaptations. The simplest form of adaptation can be conceptualised as
the brain “turning off” (suppressing) the strabismic eye to prevent its
(misaligned) image causing double vision. Possibly as a result of this
suppression, strabismus in the first few years of life usually results in
reduced vision (poorer acuity) in the strabismic eye. This reduced vision
in one eye is sometimes called lazy eye, but the preferred term is
amblyopia. The amblyopia typically has a negligible effect on
performance under normal viewing conditions because the amblyopic
eye is suppressed by the brain and the image from the non-amblyopic
eye is preferred.7
Chapter 3 Binocular vision and accommodative anomalies
41
Is strabismus associated with reading difficulty?
Strabismus7 (including microtropia)8 and amblyopia
9 10
do not seem
to be associated with reading difficulties; nor with eyestrain
(asthenopia).11 This applies to both concomitant and incomitant
strabismus.
Can strabismus cause reading difficulty?
One research study found that the maximum reading speed under
binocular conditions (with both eyes) was slower in a group of
participants with amblyopia associated with microtropia than in a
control group.12 However, it is not clear whether a similar effect was
present under normal reading conditions (when people are not trying
to read just for maximum speed) and the result may have been
influenced by treatment that the amblyopic group was receiving.
In general, the authors have found very little evidence suggesting
that strabismus is likely to cause reading difficulty and this is probably
because of sensory adaptations.6 When strabismus develops in
childhood (below the age of 8-13 years), the person usually develops
sensory adaptations that prevent the strabismus from interfering
significantly with visual perception.6
Rarely, unstable strabismus may interfere with visual perception
when reading and these cases usually present with symptoms. The
strabismus or the sensory adaptation may be unstable, for example,
the patient rapidly alternates from one eye to the other. An unstable
response would be revealed by sensory tests (e.g., Mallett Foveal
Suppression Test, Mallett Large OXO Test, Bagolini lenses).6 Some of
these tests are illustrated in Figure 3.2. In these cases, the patient
typically reports text blurring, doubling, or moving, and the symptoms
are alleviated by closing or covering one eye (or reading with the head
at an unusual angle so that the nose occludes one eye). These cases
Vision, Reading Difficulties and Visual Stress
42
are very rare. It was noted above that strabismus affects 2-3% of the
population, and it is only a small proportion of people with strabismus
who have the unstable perception described in this paragraph.
Figure 3.2. The Mallett Large OXO Test (top row, left panel),6 Foveal
Suppression Test (top row, fourth panel);
6 13
and Fixation Disparity Test
6 13
for horizontal (bottom left panel) and vertical (bottom right panel)
deviations.
Detection of strabismus
Most cases of strabismus are detected by a simple test all eye care
practitioners use, called the cover-uncover test.6 Each eye is covered in
turn and the practitioner observes the eye movement. In a small
proportion of cases of strabismus, the angle of deviation may be so
small that it is not apparent on cover testing. These cases are usually
detected by reduced vision in one eye (amblyopia) and poor
performance on tests of stereopsis (stereopsis is a form of depth
perception that results from the brain fusing together the images from
Chapter 3 Binocular vision and accommodative anomalies
43
each eye).
Vision, Reading Difficulties and Visual Stress
44
Treatment of strabismus
Since strabismus is not likely to contribute to reading difficulty, its
treatment will not be considered here. Detailed information on the
diagnosis and treatment of strabismus can be found in the textbook
Pickwell’s Binocular Vision Anomalies, Sixth Edition (2021).6
Heterophoria and decompensated heterophoria
Background
Only about 2-3% of people have strabismus. For most people, the
eyes are aligned for distance vision and converge appropriately on
near objects. However, when one eye is covered the eye behind the
cover will often misalign, even if the eyes are usually aligned when
both are open
heterophoria are illustrated in Figure 3.3. There is no consistent
evidence of a predisposition towards any type of heterophoria in
children with reading problems, including dyslexia.
an increased prevalence of small (less than one prism dioptre) vertical
heterophoria (hyperphoria) in dyslexia,15 but this study tested
participants at a distance of four metres. Reading material is usually
placed closer than half a metre from the eye and it is not clear whether
the hyperphoria was present at this distance.
Another way of describing heterophoria is a dissociated deviation,
because the eyes deviate when they are dissociated by covering one
eye, or by forcing the eyes to view dissimilar objects. Heterophoria can
be associated with symptoms (see later), but the magnitude of
heterophoria is a poor predictor of symptoms.6 In a study of normal
. This is called heterophoria and the main types of
7 14
One study found
readers in 2019, Jainta and Joss showed people tend to read more
quickly with both eyes than with one eye and this binocular advantage
tends to be reduced when there is a large heterophoria.16
Figure 3.3. Illustration of main types of heterophoria, revealed by the
position of an eye behind a cover (shown as if transparent). When there is
no heterophoria (orthophoria), the eye behind the cover maintains
alignment. In horizontal heterophoria, the eye behind the cover turns in
(esophoria) or out (exophoria). In vertical heterophoria, the eye behind the
cover turns up (hyperphoria), or down (hypophoria; not shown).
For most people, three factors prevent heterophoria from breaking
down into strabismus (Figure 3.4). During everyday vision, we exert
motor fusion to align the eyes so that both eyes obtain a similar view
and sensory fusion can take place (Figure 3.4). Sensory fusion is
possible if each eye obtains similar views, but as the dissimilarity
increases the processes of fusion eventually break down, leading to
double vision (diplopia). The dissimilarity depends in part on the
Chapter 3 Binocular vision and accommodative anomalies
45
alignment of the eyes. The power to converge the eyes (turn them
inwards) and diverge the eyes (turn them outwards) as necessary to
avoid diplopia is referred to as fusion reserves or motor fusion.
Figure 3.4. A simple model of binocular vision. Phoria, heterophoria.
Sensory fusion is the process whereby the two monocular images
are fused into a single percept, and this provides information on the
relative depth of objects. The role of sensory and motor fusion in
helping a person to compensate for heterophoria is summarised
schematically in Figure 3.4. If the dissociated deviation is excessive
and/or if the sensory and motor fusion are inadequate, then
heterophoria may break down to a strabismus. In other cases,
although the person does not develop strabismus, because the eyes
remained aligned, the person must strain excessively to maintain that
alignment. In these cases, there is a decompensated heterophoria.
The most common type of decompensated heterophoria
(decompensated near exophoria) affects near vision: the two eyes are
misaligned because they are insufficiently converged. This can be
Compensated phoria
or
Decompensated
phoria
or
Strabismus
Sensory fusion
(fusion lock)
Size of
deviation
Motor fusion
(fusional
reserves)
Vision, Reading Difficulties and Visual Stress
46
corrected with spectacles with base in prisms, with concave (minus)
lenses, or by eye exercises.6 These methods of correction are discussed
in more detail later.
Decompensated heterophoria causes symptoms,17 including blur,
double vision, visual perceptual distortions (e.g., text appearing to
move) and eyestrain and headaches. It is therefore not surprising that
it can cause decreased visual performance, in the form of a reduced
rate of reading.18
Is decompensated heterophoria associated with reading
difficulty?
In the UK decompensated heterophoria is typically detected using a
test called the Mallett unit, which is described below. This detects a
minute tendency for the eyes to misalign, detected as a fixation
disparity and measured by an aligning prism (see below).6 Although one
early study found fixation disparity to be associated with poor
reading,19 subsequent studies found that that the presence of fixation
disparity
20 21
and magnitude of aligning prism21 are not associated with
dyslexia. Although decompensated heterophoria is not a strong
correlate of reading problems, it is quite common, may occur in any
child and could make reading more difficult.
Can decompensated heterophoria cause reading difficulty?
If a person has a refractive error (Chapter 2) or visual stress (Chapter
7) that is causing blur or other visual perceptual distortions, this could
interfere with sensory fusion, Figure 3.4. In some cases, correction of
the refractive error, or possibly of visual stress, may be all that is
required to render the heterophoria compensated. This may explain
why, exceptionally, patients may be encountered where correction of
a low degree of long-sightedness can render an exophoria
Chapter 3 Binocular vision and accommodative anomalies
47