[Diagnostic investigations for an unexplained developmental disability]

Developmental disability/mental retardation is a major public health problem and a common cause of consultation in pediatrics, neuropediatrics, and genetics. Etiologies of mental retardation are highly heterogeneous. Diagnostic strategies have been explored in a small number of consensus publications, essentially from English-speaking countries. In these publications, the utility of the conventional karyotype, fragile X screening, metabolic workup, and brain imaging were discussed. Recently, investigations in mental disabilities have been dramatically modified by molecular cytogenetics and the emergence of new metabolic pathologies. Based on the published experiments, the Reference centers for rare disease network "mental deficiencies with rare causes" elaborated an updated protocol for the investigation of nonsyndromal mental disability that takes into account recent innovations in genetics and genomics. Whenever local facilities make it possible, we recommend array CGH investigation as the first step or, when CGH is not available, a combination of classic karyotype with systematic screening of telomeric and interstitial rearrangements by MLPA, fragile X screening in both sexes, and a reorientation of metabolic screening toward certain diseases that have recently been described: congenital disorders of glycosylation (CDG), thyroid hormone carrier deficiency, and creatine metabolism deficiency. We recommend MRI imaging only if head size is abnormal, if neurological examination is abnormal, or regression occurs if walking is not achieved by 2 years, or if development is severely delayed.