(Also known as dyseidetic, surface dyslexia or dyseidesia) If your subject has visual dyslexia, he can sound out words, but has trouble with words that don't sound out regularly e.g. such as 'who' and 'any'. Subjects with this type of dyslexia have to learn words primarily by sight and will need to 'picture' the word for later recall.
Given a piece of text, they will read very slowly and are better able to spell phonetically without registering that the 'appearance' of the words are wrong e.g. throo, skayt, dotter, etc.
To test for Visual Dyslexia, psychologists will ask the subject to read a number of phonetically regular words (e.g. think, wishing, testing, etc.) and irregular words (e.g. who, they, enough, etc.) This will reveal obvious disparities in their reading, by comparing their abilities to read each group or words.
Visual Dyslexia is completely different from 'Visual Stress' or 'Scotopic sensitivity syndrome' SSS, this is where the letters of words apparently seem to jump around on the page.
Visual Strees is sometimes thought to be treatable by special lenses and coloured overlays, but this requires specialist examination and recommendation by a qualified optometrist. There is also some concerns over the real effectiveness of this approach.