[Advances in the identification of the aetiology of mental retardation].

Despite the advances made in the field of genetics, neuroimaging and metabolic diseases, half the children with mental retardation remain without an aetiological diagnosis. A genetic base is estimated to be present in 40% of cases, environmental teratogens and prematurity in 20%, metabolic diseases in 1-5% and multifactor causes in 3-12%. The family history, the detailed medical records required by dysmorphology and the neurological examination will make it possible to establish or suspect a diagnosis in two thirds of the cases and, in the others, scanning tests will be able to confirm an aetiology. The order of the studies will be guided by the clinical picture: karyotype if a chromosome pathology is suspected, neuroimaging if there is some abnormality in the neurological examination and specific genetic or neurometabolic studies to confirm the clinical presumption. The estimated diagnostic performance of the different techniques is: karyotype, 9%; fragile X, 5%; subtelomeric abnormalities, 4%; neurometabolic diseases, 1%, and new microarray techniques, 19%. As a result of the higher performance and cost-benefit ratio, today the recommended procedure, as the first line of treatment for unexplainable cases of mental retardation, is the study of microarrays. Although the outcomes of these tests are complex and require confirmation and careful interpretation by a specialist in medical genetics, the advances in their technological development and resolution, together with lower costs make this technique a fundamental tool in the identification of the aetiology in these children.