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C CLINICAL RESEARCH

ACCOMMODATION
Accommodation is the ability to concentrate and bring near-vision into focus. It ensures the sharpness of images at dif-
ferent viewing distances. Accommodative amplitudes are usually at their maximum during childhood up to the age of 10,
and accommodative power naturally decreases with age. Therefore, it is rare to observe accommodative insufficiency
44
in a child. An accommodative disorder can also occur in a child with an uncorrected elevated hypertropia with a history
of viral infection, brain or ocular trauma, or brain stem pathology, or as a side-effect of medications. The hypothesis of
4
accommodative weakness in dyslexic or learning-disabled children has led several professionals to promote the use of
bifocals to compensate for it. Although some studies have shown a slightly lower range of accommodation for dyslexic
children compared to normal children, the amplitude in dyslexics was within the expected norms for their age. There-
8,45
fore, there is no significant difference between the accommodative capacity of patients with reading disabilities and that
of normal readers. Furthermore, there is no scientific evidence that an increase in magnification increases reading
8,13
efficiency. One study even showed that the correction of accommodative insufficiency has no significant impact on eye
movement and fluidity during reading. Therefore, any therapy designed to decrease the accommodative effort of a child
46
with a learning disability, suggesting that the child’s disorder is the cause, is scientifically unfounded. 2,4,6,20
BINOCULAR VISION
Perfect oculomotor balance is rare in both the paediatric and general populations. Most individuals have low eso-
phoria or asymptomatic exophoria that are considered to be within normal limits. 4,17,19 Several studies have investi-
gated binocular function and accommodation in children with learning disabilities and dyslexia. No causal relation-
ship has been found. 17,19,47

Convergence insufficiency presents as a difficulty in merging an object at a close distance correctly and effectively.
When a convergence effort is difficult to overcome, it can cause various symptoms of visual discomfort such as vi-
sual fatigue, headache, blurred reading, near-vision diplopia, and difficulty concentrating for prolonged periods of
close work. In addition, certain factors such as lack of sleep, illness and general fatigue can aggravate the problem.
4
The prevalence of convergence insufficiency is around 3% to 5% of the population. However, because of the differ-
4
ence in diagnostic criteria, some studies report different results. Both an accommodative difficulty and convergence
insufficiency can interfere with reading comfort. 3,8,45 These visual disorders must be treated if they are recognized
as being problematic according to the criteria used for the general population. As a result, the treatment of con-
vergence insufficiency can help with reading comfort and near-vision work by making extended reading easier. 20,48
Also, if the reading difficulties of an individual are secondary to an anomaly of accommodation or convergence,
those difficulties will disappear once the visual impairment is treated. However, since these visual disorders are not
4
the cause of dyslexia, training will not impact reading decoding and comprehension skills.
4
Like most children, those with learning disabilities enjoy playing video games. Playing video games requires good
eye-hand coordination, prolonged concentration, effective accommodation, active convergence and good visual
perception. Therefore, if these deficits were the major cause of reading impairment, these children would avoid this
task, which also requires the intensive use of their visual abilities.
4,6
COLOURED LENSES AND FILTERS
The use of coloured lenses or filters to improve reading comfort and performance in individuals with learning
disabilities is highly controversial. Some believe that the use of lenses or yellow filters can improve control over
visual attention and eye movements in some children through brain stimulation. The use of blue lenses or filters,
on the other hand, supposedly improves concentration and therefore reading. 49,50 Others suggest that control of
accommodation and convergence are influenced by sensitivity to certain wavelengths of light, which create visual
stress when reading. The use of certain coloured filters corresponding to these different wavelengths would allow
for a reduction of this stress and enable more efficient reading. 49,51-53 However, several studies have determined that
the appropriate selection of the beneficial filter colour for each individual is inconsistent and non-repeatable. 6,54-59
Other studies have shown that the use of lenses or coloured filters has no beneficial effect on visual function or read-
ing performance. 55,60 Since no consensus has been reached to date, the use of coloured filters to treat children with
learning disabilities is not clinically justified at this time. 4,6,14,54,56-59,61-63

PRISMS
The use of prisms in patients with learning disabilities has been reported. Some argue that top-base prisms could
be used to treat exophoria or convergence insufficiency while bottom-base prisms could be used to treat esophoria
or excess convergence. Bottom-base prisms are sometimes even used to facilitate the adaptation of individuals for
20
whom a slight hypermetropy correction is prescribed. 20

66 CANADIAN JOURNAL of OPTOMETRY | REVUE CANADIENNE D’OPTOMÉTRIE VOL. 80 NO. 3

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