Arteriovenous malformation (AVM): An arteriovenous malformation (AVM) is a congenital disorder (one present at birth) of blood vessels in the brain, brainstem, or spinal cord that is characterized by a complex, tangled web of abnormal arteries and veins connected by one or more fistulas (abnormal communications).

The AVM has no capillary bed of its own and the fistulas in the AVM permit high-speed, high-flow shunting of blood from the arterial to the venous side of the circulation. This shunting causes low blood pressure (hypotension) in the arterial vessels feeding the AVM and neighboring areas of the brain that they normally supply with blood.

AVMs typically cause problems before the age of 40. The most common symptoms of AVM include hemorrhaging (bleeding), seizures, headaches, and neurological problems such as paralysis or loss of speech, memory, or vision. The frequency of hemorrhage in various series ranges from 30-82%. AVM rupture accounts for 2% of all strokes.

There are three general forms of treatment for AVM:

• Surgery, which is the best-known and longest-standing treatment for AVM. Surgery for an AVM involves identifying the margins of the malformation, ligating (tying off) or clipping the feeder arterial vessels, obliterating the draining veins, and removing or obliterating the nidus (the nest) of the AVM.
• Endovascular occlusion, which involves closing off the vessels of the AVM by one of various nonsurgical means. Catheters can deliver agents to block the blood vessels that include permanent balloons, thrombosing (clogging) coils, sclerosing (hardening) drugs, and fast-acting embolization glue (embolization is often used before surgery).
• Radiosurgery, which involves focusing multiple radiation beams on the AVM so as to injure and thrombose (clog) the AVM. The effect of radiosurgery takes weeks to months to become fully manifest. A real danger of radiosurgery is damage to neighboring nervous system tissue, normal brain (or spinal cord) tissue around the AVM. Therefore, radiosurgery is usually reserved for AVMs that are relatively small (less than 3 cm in diameter), situated so deep within important brain tissue that surgery is hazardous, or have so many feeder arteries that embolization is not feasible.

Most people (perhaps 80% or more) with AVMs never experience problems due to them. However, AVMs that hemorrhage can lead to serious neurological problems, and sometimes death.