Dark adaptation, docosahexaenoic acid and dyslexia

B Jacqueline Stordy, BSc, PhD

Scotia Pharmaceuticals, Guildford, UK

It has been estimated from government sponsored studies in the UK and the USA that 10% of the population suffer to some extent from dyslexia, and that 4% are severely affected.  In the USA there has been a threefold increase in prevalence between 1976 and 1993.  This costly epidemic affects individual lives and educational budgets.  So it is important that the biological basis of the condition is investigated.  Visual and central processing deficits have been found, particularly in the magnocellular visual subsytem^1,2,3

We measured dark adaptation with a Friedmann Visual Field Analyser 2, set for the dark adaptation function, in ten adults with dyslexia and ten controls.  Dyslexics showed poorer dark adaptation than controls especially in the second part of the curve, which corresponds with rod dark adaptation (p < 0.05).  We subsequently tested the possibility that dark adaptation might be influenced by docosa hexaenoic acid (DHA) supplementation.  For one month, five dyslexics and five controls were given 480 mg of DHA daily with no additional vitamin A or vitamin D.  Dark adaptation was then retested.  In four controls DHA had no effect on dark adaptation, although in one (a strict vegetarian) adaptation clearly improved.  By contrast, in the dyslexics with poor scotopic vision, DHA consistently and significantly improved dark adaptation (p < 0.04)⁴.

Within the retina, DHA is heavily concentrated in the lipids of the rod cells, and within the brain neurons at the synapses.  The rod cells are receptors for the magnocellular system, so it is possible that the central processing deficits of the magnocellular system will also be improved following DHA supplementation.  So far, there is only anecdotal and subjective evidence that improvements in reading ability and behaviour are associated with DHA supplementation in dyslexia.  More formal controlled studies are in progress.

1. Grosser GS, Spafford CS.  Light sensitivity in  peripheral retinal fields of dyslexic and proficient readers.  Perceptual Motor Skills 1990:  71:  467-77.

2. Galaburda A, Livingstone M.  Evidence for a magnocellular defect in developmental dyslexia.  Ann NY Acad Sci 1993; 682:  71-81.

3. Eden GF, VanMeter JW, Rumsey JM, Maisog JM, Woods RP & Zeffiro TA.  Abnormal  processing of visual motion in dyslexia revealed by functional brain imaging.  Nature 382:  66-69: 4 July 1996

4. Stordy BJ.  Benefit of docosahexaenoic acid supplements to dark adaptation in dyslexics.  Lancet 1995; 346:385.

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