Mental retardation in children. Prevalence and etiology

: The prevalence of mental retardation defined as IQ less than 70 has been estimated as ranging between 6.7-11.9 per thousand children in the Nordic countries. Etiological factors can be found in 80% of patients with severe mental retardation (IQ less than 50) and in 50% with mild mental retardation (IQ 50-70). Down's syndrome is the most common specific cause of mental retardation, followed by the Fragile X syndrome, which mainly affects boys. Since the middle of the 1980s Rett's syndrome has been recognized as the most frequent cause of progressive mental retardation in girls. Recent advances in the field of chromosomal disorders, metabolic diseases and clinical syndromes imply a need for new epidemiological research aimed at obtaining an updated picture of the heterogenous causes of mental retardation.