ACCESSORY BRAINS (EXTRACEREBRAL HETEROTOPIAS) - UNUSUAL PRENATAL INTRACRANIAL MASS LESIONS

Prenatal ultrasonographic evidence of intracranial mass lesions genera lly results in a diagnosis of primary glial or primitive neuroectoderm al neoplasm. We describe two infants, one who was stillborn at 25 week s' estimated gestational age and one term infant who was born live and died shortly after birth with large intracranial space-occupying lesi ons that exerted significant mass effect. At autopsy, large soft-tissu e spheres of partially organized brain tissue containing neurons, astr ocytes, oligodendroglia, ependyma, and choroid plexus were found adjac ent to intact, fully formed cerebral hemispheres with normal brain ste ms and cerebelli within the cranial cavity. We have termed these extra cerebral heterotopias ''accessory brains.'' The telencephalic vesicles arise as lateral outpouchings at the rostral end of the developing em bryo during the 5th week of embryogenesis. These accessory brains may arise embryologically from an accessory third evagination inferior to the telencephalic vesicles.