MRI volumes of amygdala and hippocampus in non-mentally retarded autistic adolescents and adults

Objective: To determine whether volumes of hippocampus and amygdala are abn ormal in people with autism. Background: Neuropathologic studies of the lim bic system in autism have found decreased neuronal size, increased neuronal packing density, and decreased complexity of dendritic arbors in hippocamp us, amygdala, and other limbic structures. These findings are suggestive of a developmental curtailment in the maturation of the neurons and neuropil. Methods: Measurement of hippocampus, amygdala, and total brain volumes fro m 1.5-mm coronal, spoiled gradient-recalled echo MRI scans in 14 non-mental ly retarded autistic male adolescents and young adults and 14 individually matched, healthy community volunteers. Results: Amygdala volume was signifi cantly smaller in the autistic subjects, both with (p = 0.006) and without (p = 0.01) correcting for total brain volume. Total brain volume and absolu te hippocampal volume did not differ significantly between groups, but hipp ocampal volume, when corrected for total brain volume, was significantly re duced (p = 0.04) in the autistic subjects. Conclusions: There is a reductio n in the volume of amygdala and hippocampus in people with autism, particul arly in relation to total brain volume. The histopathology of autism sugges ts that these volume reductions are related to a reduction in dendritic tre e and neuropil development, and likely reflect the underdevelopment of the neural connections of limbic structures with other parts of the brain, part icularly cerebral cortex.