Neurobiology of developmental dyslexia: Part 1: A review of evidence from autopsy and structural neuro-imaging studies

S O Wajuihian

doi:10.4102/aveh.v70i4.117

Abstract

Developmental dyslexia (DD) is a language-based neurological disorder which impairs readingability but does not result from low intelligence,lack of motivation, sensory impairment, or inadequate instruction. Although the neurological basisof dyslexia has long been assumed, the exact natureof the altered brain structure associated with DD remains unknown and has been a subject of autopsyand neuro-imaging research. Autopsy studies provide consistent evidence of symmetry of the planum temporale (PT), thalamus and cortical malformations, whereas results from structural imaging studies such as computed tomography (CT) and magnetic resonance imaging (MRI) are inconsistent. To address the possible etiology of DD, this paper reviews evidence from autopsy and structural imaging studies on developmental dyslexia and discusses possible methodological sources of some inconsistent results. The role of the optometrist in the multidisciplinary management of dyslexia is highlighted. (S Afr Optom 2011 70(4) 191-202)

Highlights

In 1968, the World Federation of Neurologists defined dyslexia as “a specific learning disability that is neurological in origin
The results showed a mean leftward planum temporale (PT) asymmetry in both the dyslexic and the control group, with no significant difference for the degree of PT asymmetry
Autopsy studies on dyslexia have been conducted by the same group of researchers (Galaburda et al) and the results are fairly consistent with all subjects examined having symmetrical PT and several with cortical malformation, mainly ectopias and dysplasia[21, 23,24]

Summary

Introduction

In 1968, the World Federation of Neurologists defined dyslexia as “a specific learning disability that is neurological in origin. It is characterized by difficulties with accurate and/or fluent word recognition and by poor spelling and decoding abilities. These difficulties typically result from a deficit in the phonological component of language that is often unexpected in relation to other cognitive abilities and the provision of effective classroom instruction. Karl Wernicke, in 1874 (cited by Nakada)[3], reported that a lesion in the left hemisphere resulted in a unique language disorder characterized by comprehension difficulties (Wernicke’s aphasia). According to Habib[2], it was first reported by Dejerine in 1891 that damage to the same region of the brain (angular gyrus) resulted

Findings

Discussion

Conclusion