A case of adult microcephaly.

A microcephalic woman who walked, spoke a few words, and was capable of some self-care, died at age 20. The brain weighed 260 g--the equivalent of that of an infant of eight months' gestation. The remarkable neuropathological features in our case included very small cerebral hemispheres, normal cerebral cortex, myelinated white matter, and large neuronal heterotopias situated along the ventricular walls and deep in the white matter adjacent to the intact nuclei of the basal ganglia, amygdala, and thalamus. The brainstem and cerebellum were relatively less reduced in size and showed normal structure and no heterotopias. In rats, microcephaly can be induced experimentally by giving carcinogens during sensitive periods of embryogenesis. Here, the microcephaly appears to result from necrosis of the neuroblasts lining the ventricles, and from a concomitant reduction in DNA synthesis. The pathogenesis of the microcephaly in our patient and others remains undefined. Our study shows that a great discrepancy may exist between brain size and level of function.