Arterio-venous Malformations
Arteriovenous malformations (AVM) are congenital vascular malformations that can occur in brain tissue. AVMs are often asymptomatic and discovered incidentally during brain imaging. Less frequently, a seizure or brain hemorrhage may lead to the discovery of an AVM.
AVMs consist of a nidus (literally, "nest") where feeding arterial vessels short-circuit directly into the draining veins without an intervening capillary bed. Since the vessel walls of arteries and veins differ significantly, high blood pressure transmitted into a vein can lead to its dilatation and, in some cases, rupture causing cerebral hemorrhage.
In many cases, when an AVM bleeds, it results in sudden onset of severe headache, often accompanied by functional deficits (e.g., paralysis, loss of sensation, speech, balance, or vision); in cases of severe hemorrhage, affected patients may fall into a coma. The likelihood of bleeding from an AVM depends on many factors, not all of which are understood. Overall, the bleeding risk is estimated to be between 1 - 4% per year.
