Dyslexia: An update on recent developments on aetiology, pathogenesis and therapy
KOTSOPOULOS S.

Important contributions have in recent years been made towards a better understanding of the aetiology, pathogenesis and therapy of dyslexia by psycholinguistics, pathological anatomy, neuropsychology, genetics and language therapy interventions. Dyslexia is a severe delay in the acquisition of the written language and is observed at the child's early years in school. Dyslexia is related to other language disorders such as phonological disorder and specific language impairment (developmental dysphasia). The essential deficit in dyslexia is severe impairment in phonological awareness. Phonological awareness is the ability of the child to distinguish that the spoken language consists of sentences, the sentences of words, the words of syllables and the syllables of basic sound units pronounced in sequence, the phonemes, and that the phonemes correspond to distinct written symbols, the graphemes. The dysfunction observed in dyslexia also includes alphabetic mapping, phonological decoding, verbal short term memory, name encoding and retrieval. The dyslexic child is characteristically unable to read pseudowords.

Pathological anatomy studies of brains of dyslexic individuals by Galaburda and associates have shown cytoarchitectonic anomalies that are scattered in selected areas predominantly in the left hemisphere. These are "microgyria" located in the perisylvian cortex and "microdysgenesis" clustered in the foot of the inferior frontal gyrus in the vicinity of the Broca's area. Cytoarchitectonic anomalies have also been reported in the left medial geniculate nucleus and the lateral geniculate nuclei.

Studies using visualizing brain techniques, such as fMRI and "magnetic source imaging" (MSI) have shown that the dyslexic individual while performing on language processing tasks (phonological awareness), presents with cortical dysfunction which is observed on the language module. The dysfunction consists of activation of the right temporoparietal region instead of the left homologous region as observed in normal controls. In addition to phonological processing, dysfunction has also been reported in auditory and visual perception and in the cerebellum.
Dyslexia is familial and heritable. Dyslexic parents are likely to have dyslexic children. Linkage studies and molecular genetics have pointed to genetic loci (on chromosomes 6 and 15 by several studies) which probably contain genes associated with dyslexia. A recent study has pointed to gene DYX1C1 at chromosome 15 as a candidate for dyslexia.

Intensive remedial language therapy that focuses on phonological awareness has shown very good results. Studies of treated subjects with MSI and fMRI have shown that following intensive treatment the brain activation pattern becomes normalized. The activation is transferred from the right to the left temporoparietal region. This obviously occurs as a result of the brain plastic ability.

Dyslexia is often comorbid with attention deficit-hyperactivity disorder. This may pose difficult management problems and requires special attention. The unattended educational needs of the dyslexic child may place the child at risk for emotional and behaviour problems. Part of the treatment is informing the child on the nature of his academic difficulties and supporting him to cope with his difficulties, and providing counseling to the parents for a more adequate management of the child's problems.

Key words: Dyslexia, phonological awareness, language module, genetics, remedial language therapy.