Correlates of mental retardation and structural changes of the brain.

Neuropathological findings are analyzed in 299 cases from 3 state institutions for mental retardation (MR) with or without cerebral palsy or seizures. Major lesions were found in 127 patients (42.5%): various forms of post-hypoxic encephalopathy, developmental anomalies, degenerative and metabolic diseases and post-infectious encephalopathy, all showing good clinicopathological correlations. Ninety-seven patients (32.5%) showed minor or non-specific lesions which could not be used to make a specific etiopathogenetic diagnosis and which appeared not concordant with the magnitude of the functional deficiency but nevertheless demonstrated structural imperfection of the brain. This group included subtle developmental anomalies, such as focal heterotopia, polymicrogyria, abnormal convolutional patterns and disproportionate subunits as in Down syndrome (45 cases; 15%) and subependymal germinolysis in which the etiology is varied. The most baffling and elusive group consisted of 75 cases (25%) showing no significant morphological abnormalities. Thirty-six of these had a history of seizures but 39 did not. The causes of MR in this group are heterogenous as in the group with major morphological lesions and may include unrecognized biochemical abnormalities, ultrastructural lesions, psychiatric disorders or cultural factors, but many still remain conjectural. It appears that a morphologically normal brain provides a potential for but does not ensure average functional development, but the reverse is not always true.