Visual problems and Dyslexia and other learning difficulties
Optometric correlates of dyslexia
Refractive errors & accommodative dysfunction
Refractive errors
There is some evidence of increased prevalence of hypermetropia (longsightedness) in the reading disabled.
Accommodation (focus)
There is a high prevalence of reduced amplitudes of accommodation (Your optometrist will usually check three functions of focus, amplitude of accommodation, facility of accommodation (ability to change focus) and lag of accommodation (accuracy of focus)
Binocular vision dysfunction and dyslexia
Amplitude of convergence
A normal near point of convergence is considered to be between 6 to 10 cm. Reduced amplitude of convergence is frequently associated with dyslexia (Evans, 1996). Convergene anomalies are usually successfully treated with conventional orthoptics.
Heterophoria
Heterophoria correlates of dyslexia include, associated phoria (also called fixation disparity), reduced vergence ranges and central suppression.
Heterotropia (strabismus)
Although stable unilateral strabismus is unlikely to be a causative factor or a correlate, alternating strabismus can result in unstable perception of print
Binocular instability
This was described and characterised by Gibson (1955) and Giles (1960) as unstable heterophoria with reduced vergence reserves. Both positive and negative vergence reserves are often reduced in dyslexia. It has been shown that binocular vision problems of this type can cause asthenopia (eye strain) and may discourage a child from reading
Ocular dominance and the Dunlop test
There have many theories postulated relating to crossed or unstable ocular dominance and dyslexia. Most authorities now agree that atypical sighting dominance is not a factor in dyslexia. In the past there was a theory that a lack of a dominant or reference eye for near vision was a cause or correlate of dyslexia. An Australian orthoptist developed a test named after her, the Dunlop test to test for near eye dominance. There was considerable debate in the 1980’s over its validity and significance and most authorities believe the test is difficult to administer and probably not reliable.
Eye movements
In the 1980’s and 1990’s Dr Barnard used an eye tracking instrument in the practice to assess the tracking eye movements of all dyslexic children he examined. This was because there was a theory that poor tracking was a cause of reading difficulties. Dr Barnard’s PhD thesis research subsequently used this instrument to look at other aspects of eye movements. By the end of the 1990’s Dr Barnard ceased analysing the tracking eye movements of children with reading difficulties because current sound research had disproved the theories of a causative relationship between tracking and dyslexia.
Yes, poor readers show anomalous eye movements but these are correlates of the poor reading, poor attention or fidgetiness of the child not the cause.
Tinted lenses
In the 1980’s Olive Meares an Australian SEN teacher reported coloured sheets helped some of her pupils. Subsequently Helen Irlen, an educational psychologist from the USA claimed that 50% of dyslexics have a perceptual dysfunction that can be treated with tinted lenses. A double masked placebo control trial by Wilkins et al, in 1994 showed a positive effect on symptoms reported by some children, that could not be solely explained by a placebo effect. Professor Wilkins developed the Intuitive Colorimeter at the Department of Applied Psychology, MRC, Cambridge and this instrument is used by optometrists specialising in visual aspects of dyslexia. Dr Barnard was the first optometrist in the UK to install the Intuitive Colorimeter.
Before Colorimetry is carried out on a child, an initial screening with Intuitive coloured overlays should be done to try and rule out a placebo effect. There is an important role for SEN teachers in this regard.
Optometric management protocol for tinted lens prescribing
Eye examination and visual assessment
- treat any visual or binocular anomalies
- if no other anomalies present then screen with overlays
- overlay screening/trial may have already been instituted by teacher
- following a sustained positive effect with overlay, child should be referred to specialist optometrist for Intuitive Colorimetryand prescribing precision tinted lenses (Cerium Visual Technologies)
Controversial optometric therapies
Both “behavioural” and “developmental” optometry are controversial. There is very little evidence that the diagnostic strategies and treatment interventions of behavioural optometry are valid.
Summary
- There is a high prevalence of visual anomalies amongst dyslexics
- These are correlates not cause but may be contributory factors
- “Significant” anomalies should be treated by the optometrist
- The effect of colour should be considered
- Intuitive Colorimetry should be used when indicated
- The optometrist should work closely with the SEN teacher, Ed Psych, OT, Physiotherapist, GP
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