Accessory brains (extracerebral heterotopias): unusual prenatal intracranial mass lesions.

Prenatal ultrasonographic evidence of intracranial mass lesions generally results in a diagnosis of primary glial or primitive neuroectodermal neoplasm. We describe two infants, one who was stillborn at 25 weeks' estimated gestational age and one term infant who was born live and died shortly after birth with large intracranial space-occupying lesions that exerted significant mass effect. At autopsy, large soft-tissue spheres of partially organized brain tissue containing neurons, astrocytes, oligodendroglia, ependyma, and choroid plexus were found adjacent to intact, fully formed cerebral hemispheres with normal brain stems and cerebelli within the cranial cavity. We have termed these extracerebral heterotopias "accessory brains." The telencephalic vesicles arise as lateral outpouchings at the rostral end of the developing embryo during the 5th week of embryogenesis. These accessory brains may arise embryologically from an accessory third evagination inferior to the telencephalic vesicles.