Accessory cerebral ventricle of the occipital lobe. Morphogenesis and clinical and pathological appearance.

Small separated accessory ventricles in the occipital lobe were observed in 21.3% of 404 patients, as seen by computerized tomogram. There was no significant preponderance in regard to sex or laterality. The accessory ventricles were clinically not significant. As seen at autopsy, accessory ventricles were found in the subcalcarine white matter, posterior to the occipital horn of the lateral ventricle, in 29.5% of 200 "normal" brains. Again, there were no significant sex and laterality differences. Accessory ventricles were never found in brains of fetuses or newborn babies. The youngest child in whom an accessory ventricle was found was 1 month old. No accessory ventricles were larger than 1 cm in diameter; they were slit-like, triangular or oval in shape. Histologically, they showed subtotal loss of the ependymal layer, subependymal gliosis, and/or fibrosis, and, in some cases, hyalinofibrotic capillary degeneration. Electron microscopy of the remaining ependymal cells in the accessory ventricle showed marked atrophy. Accessory ventricles are formed at the tip of the occipital horn postnatally through the expansion of the deep calcarine fissure, increase in brain volume in the region, and subsequent fusion of the mediolateral ventricular walls.