Neuropathological findings in autism.

Autism is currently viewed as a largely genetically determined neurodevelopmental disorder, although its underlying biological causes remain to be established. In this review, we examine the available neuropathological literature on autism and discuss the findings that have emerged. Classic neuropathological observations are rather consistent with respect to the limbic system (nine of 14 studied cases showed increased cell packing density and smaller neuronal size), the cerebellum (21 of 29 studied cases showed a decreased number of Purkinje cells, and in all of five cases that were examined for age-related morphological alterations, these changes were found in cerebellar nuclei and inferior olive) and the cerebral cortex (>50% of the studied cases showed features of cortical dysgenesis). However, all reported studies had to contend with the problem of small sample sizes, the use of quantification techniques not free of bias and assumptions, and high percentages of autistic subjects with comorbid mental retardation (at least 70%) or epilepsy (at least 40%). Furthermore, data from the limbic system and on age-related changes lack replication by independent groups. It is anticipated that future neuropathological studies hold great promise, especially as new techniques such as design-based stereology and gene expression are increasingly implemented and combined, larger samples are analysed, and younger subjects free of comorbidities are investigated.