A vascular malformation composed of clusters of large, thin walled veins lacking intervening nervous tissue. They are most common in the BRAIN STEM but may also occur in the cerebral hemispheres, diencephalon, cerebellum, and spinal cord. The lesions have a tendency to rupture and cause a variety of clinical deficits (e.g., SEIZURES; hemiparesis) that depend upon the location of the hemorrhage. (From Adams et al., Principles of Neurology, 6th ed, pp851-2).