(2) Visual correlates of dyslexia have been identified and include, magnocellular deficit and binocular instability
. (3,4) These factors can be associated with letter or word confusions, (5-7) but are unlikely to be major causes of dyslexia.
After moving from primary to secondary school, Billie began having problems with her eyesight and was diagnosed with binocular instability
The six-year-old, of Whitley Bay, had struggled to keep up with his classmates at Appletree Gardens First School in Monkseaton until he was diagnosed with binocular instability
Research has identified visual correlates of dyslexia, most notably a magnocellular deficit (a sensory factor) and binocular instability
(a motor condition); (4) which are associated with each other.
The two most common visual correlations of dyslexia--visual stress and binocular instability
--can both cause perceptual instability.
The evidence Professor Evans presented indicated that binocular instability
is prevalent in 15% of dyslexics, compared to 5% in those with higher literacy.
As described in the second article of this series (see OT, April 19, 2013), the most appropriate method is perhaps the Mallett unit, whereby the minimum amount of prism required to neutralise any misalignment of the nonius markers (fixation disparity) and/ or reduce binocular instability
(18) Movement or intermittent flashing of the nonius markers indicates binocular instability
. Displacement of the nonius markers on the Mallett unit indicates a fixation disparity, and the prism required to align the markers is an indication of the associated phoria.
Patients with convergence or accommodation deficiencies, or binocular instability
, typically attend complaining of similar symptoms.
In terms of convergence and fusional abilities, a patient with ABI might present with decompensated heterophoria, convergence insufficiency, (27) reduced fusional ranges, reduced stereopsis, fixation disparity and binocular instability
. Optometric intervention includes provision of an appropriate spectacle correction including the use of ground prism or Fresnel prisms and training of convergence and divergence where appropriate.
(18) Fixation disparity assessment particularly included a report of whether the patient observed any movement of the nonius markers; the presence of movement but no misalignment indicated binocular instability
. Ocular dominance was determined by the sighting/pointing method and colour vision was assessed using the Isihara plates.
The main visual problems that are correlated with dyslexia are Meares-Irlen syndrome/visual stress (MISVIS), binocular instability
, and accommodative insufficiency.