RESUMO
OBJECTIVE: To investigate etiological factors in severe mental retardation (SMR). METHODS: An etiological study is presented of 512 SMR patients in five specialized institutions in Havana. RESULTS: Prenatal, perinatal and postnatal causes were apparent in 58.0, 24.8 and 11.1% of the patients, respectively; infantile psychosis was determined in only 0.4%. The remaining 5.6% were classified as having SMR of undeterminable origin, i.e. patients with apparently normal pre-, peri- and postnatal histories who had neither dysmorphism nor affected first-degree relatives, and had a normal karyotype and metabolic screen. Among prenatal causes, genetic factors were the most frequent (82.8%), while environmental factors were apparent in only 5.3% of these cases. Of the cases with prenatal genetic etiology, chromosomal aberrations were present in 86.5% (Down syndrome 96.2% and 3.7% other chromosomal aberrations), monogenic disorders in 11.3% [neurocutaneous diseases (32.1%) and fragile X syndrome (25%) were the most frequent], and multifactorial disorders in 2.0%. Thirty-five patients (11.7%) presented multiple congenital anomalies of 'prenatal unknown' causes. The latter group may include unidentifiable chromosomal aberrations, uniparental disomy, de novo mutations and multifactorial or teratogenic factors. CONCLUSION: Accurate determination of the etiology of SMR is important not only for genetic counseling purposes, but also in identifying prenatal events which make infants more vulnerable to perinatal risk factors.