La dyslexie est un trouble neurodéveloppemental qui touche 5 à 10 % des enfants d’âge scolaire. Ce trouble spécifique de l’apprentissage de la lecture est d’origine neurologique. Sa physiopathologie demeure inconnue mais les données scientifiques suggèrent des particularités du carrefour temporo-pariéto-occipital. Les données épidémiologiques évoquent l’implication de facteurs génétiques. Le retentissement psychique de la dyslexie se manifeste par un risque accru de troubles des conduites et de troubles anxiodépressifs chez ces enfants. Le diagnostic, la prévention et le soin de ces troubles doivent faire partie intégrante de leur prise en charge. Par ailleurs, la dyslexie survient rarement de façon isolée et les autres troubles des apprentissages (troubles d’acquisition du langage oral [30 %], la dyscalculie [25 %], les troubles du développement moteur [50 %]) et le TDAH (15 à 40 %) sont des comorbidités fréquentes. L’examen des liens de ces pathologies avec la dyslexie éclaire la recherche de sa physiopathologie d’une façon nouvelle et permet d’approfondir l’étude des facteurs étiologiques, notamment génétiques. Pour le clinicien, la connaissance de la fréquence de ces associations doit permettre une meilleure prise en charge.

Dyslexia is a complex neurodevelopemental disorder that affects 5 to 10% of school-age children. This condition consists in a specific learning disability with a neurological origin. These learning difficulties are unexpected in relation to other cognitive abilities and the provision of efficient classroom instruction. A range of neurobiological investigations suggests that disruption of the parieto-temporo-occipital systems underlies a failure of skilled reading to develop. The observation that dyslexia is both a familial and heritable problem was made early on and was confirmed by twin studies. They also suggested that both genetic and environmental factors are involved. Several loci have been implicated in dyslexia, notably on chromosomes 2, 3, 6, 15 and 18 and some candidate genes have been proposed, but no functional mutation has yet been identified.

Dyslexia seldom appears isolated and dyslexic people are very likely to present other kinds of learning disabilities or psychiatric disorders. Specific language impairment, often with a mild outcome, is the most frequently associated with dyslexia. Indeed, late language development is often reported by dyslexic patients and also occurs more frequently among their siblings. Genetic linkage studies suggest some common genetic factor underlying this comorbidity. Dyscalculia is associated with dyslexia in 25% of cases, but most people with dyscalculia do not have any sign of dyslexia. The question of whether dyscalculia associated with dyslexia and dyscalculia itself rely on the same cognitive impairment is still controversial. Impaired motor development is also a common feature that affects nearly 50% of dyslexics and dyslexia is frequent among dyspraxic patients. This association raises the discussion on the role of motor impairment in dyslexia’s physiopathology and the cerebellar theory of dyslexia. Beyond its link with other learning disorders, the study of dyslexia’s comorbidity highlights psychopathological issues. ADHD is the most frequent psychiatric disorder associated with dyslexia. Underpinnings of this link between the two disorders seem to rely on common cognitive and genetic factors. Some authors have proposed a candidate gene ADRA2A to determine the condition including ADHD and dyslexia. In addition, dyslexics are exposed to a higher risk of anxiodepressive and behavioural disorders. Dyslexic children experience three times more behavioural disorders and one third of children with behavioural problems turn out to be affected by dyslexia. The literature study reveals inconsistent findings about depressed mood among dyslexics, but evidence of a persistent increase in the rate of anxiety disorders. The authors put forward the impact of environmental factors to explain these psychiatric comorbidities.

This review emphasizes dyslexia’s comorbidities because they represent an important issue, both from a scientific and clinical point of view. Indeed, for clinicians, children showing multiple learning disabilities have specific reeducation and educational needs and dyslexics have a higher risk of emotional and behavioural disorders. On the other hand, dyslexia’s comorbidity study provides a powerful method for researchers to investigate the still unknown physiopathology of dyslexia.