A congenital malformation of veins which drain normal brain characterized by a caput medusae or an umbrellalike convergence of multiple venules on a single, or occasionally multiple, enlarged parenchymal or medullary vein, like the trunk of a tree or the shank of an umbrella. This dilated terminal vein penetrates the cortex to drain either (a) superficially to cortical veins or sinuses, (b) deeply to subependymal veins of the lateral ventricle and then into the galenic system, (c) to the fourth ventricle and then to the pontomesencephalic vein, or (d) to the precentral cerebellar vein and into the galenic system. [from
HPO]