Congenital vascular anomalies of the brain characterized by tangles of thin walled arteries which communicate directly with veins without intervening capillaries. The malformations vary greatly in size and location, and produce symptoms through rupture (see also INTRACRANIAL HEMORRHAGES), mass effect, and vascular steal effect. Clinical presentation is often delayed until the fourth or fifth decade when affected individuals may develop headaches, seizures, and focal neurologic deficits. Large hemorrhages may result in coma or death. (From Adams et al., Principles of Neurology, 6th ed, p848)