[The neurology of dyslexia]

Developmental dyslexia is a heterogeneous disorder in which the prominent manifestation is a discrepancy between reading achievement and intelligence. There is no serious auditory, visual, psychiatric, social or educational factor that could be responsible of the discrepancy. Males are more often affected than females. It is a pervasive condition but with adequate help and spontaneous compensation, reading ability may improve. Neuroimaging, mainly MRI, allows to demonstrate in two thirds, an absence of the usual symmetry of the planum temporale favoring the left side. Twenty to 25% of the remaining cases show asymmetry of the right side. Etiology is unknown but heredity plays an important role. The pathology of dyslexia has revealed abnormalities of the cerebral cortex focal four-layer microgyria, microdysgenesis and arteriovenous malformations. Galaburda hypothesises that a pre or perinatal adverse event produces a basic cognitive, progressive alteration, that eventually invades the perceptual elements (visual, phonological, semantic-syntaxic difficulties). Heilman, Voeller and Alexander propose a motor-articulatory feedback hypothesis.