MENTAL RETARDATION: CAUSES, GENETICS AND INTERVENTION

Key words: Mental retardation, Causes, Classification, Intervention

Mental retardation (MR) consists of: a) a defect of IQ higher than 2 SD (<70), b) a defect in the capacity of adaptation and c) an onset during puberty (18 years). In order to answer to the most mandatory of these three conditions, MR should represent 3% of the population (< 3rd centile). Actually, its prevalence is much higher than 3% in poor countries and sufficiently lower than 3% in rich countries. Because of the marginal rate, many acquired causes, from bad nutrition during pregnancy and afterwards, to infections and neglect, certainly play a marginal role especially for mild mental retardation. In order to understand these cases, the limit of IQ has been moved to 75. MR finds its raison d’être in: A) an unfavourable summation of QTL genes, namely of genes with a quantitative effect that pushes the unlucky possessors in the disadvantage corner of the bell curve that covers ALL the kinds of intelligence (non-syndromic, non-familial MR); B) the presence of a single mutated, destructive gene that not only alters the development of the brain but also the other organs and systems (syndromic MR, more than 1,290 “items”); C) the presence of single mutation, that affects only one function of the brain (familial, non-syndromic MR). In the cases of familial, non-syndromic MR studied up to now, only X-associated genes, which exert their specific function on the cytoskeleton or on the formation and transportation of the synaptic vesicles, have been recognized as the cause. Non-syndromic MR generally falls within the framework of mild mental retardation (IQ 50-75). By nature, the prevalence of mild MR and in particular of marginal MR (IQ 70-75) is very high in the paediatric population and each family paediatrician should take charge of more than one child affected by MR.