Displacement of pontine arteries by fungating pontine astrocytomas.

Pontine astrocytomas may fungate, encircling the basilar artery and pontomesencephalic vein and intruding between these structures and the clivus. The tumor thus may prevent anterior displacement of these vascular structures against the clivus or produce paradoxical posterior displacement. However, the pontine arteries are usually swept forward by the tumor nodules to lie anterior to the basilar artery. These arteries are also stretched as they encircle the enlarged pons, losing their normal undulations. An increased posterior extent of the vessels may be seen, presumed due to hypertrophy or displacement or both. These findings seem highly specific for the diagnosis of fungating pontine astrocytoma.