What is an arteriovenous malformation (AVM)?
Normally blood flows from the heart through large arteries to all areas of the body. The arteries branch and get smaller until they become a capillary, which is just a single cell thick. The capillary bed is where the blood exchanges oxygen and nutrients with the body tissues and picks up waste. The blood travels from the capillary bed back to the heart through veins. In an AVM, arteries connect directly to veins without a capillary bed in between. This creates a problem called a high-pressure shunt or fistula. Veins are not able to handle the pressure of the blood coming directly from the arteries. The veins stretch and enlarge as they try to accept the extra blood. The weakened blood vessels can rupture and bleed and are also more likely to develop aneurysms. The surrounding normal tissues may be damaged as the AVM “steals” blood from those areas. There are four types of AVMs:
Arteriovenous malformation – abnormal tangle of blood vessels where arteries shunt directly into veins with no intervening capillary bed; high pressure.
Cavernoma – abnormal cluster of enlarged capillaries with no significant feeding arteries or veins; low pressure.
Venous malformation – abnormal cluster of enlarged veins resembling the spokes of a wheel with no feeding arteries; low pressure, rarely bleed and usually not treated.
Capillary telangiectasia – abnormal capillaries with enlarged areas (similar to cavernoma); very low pressure.
AVMs can form anywhere in the body and cause symptoms based on their location. Brain AVMs can occur on the surface (also called cortical), deep (in the thalamus, basal ganglia, or brainstem), and within the dura (the tough protective covering of the brain). Dural AVMs are more accurately called arteriovenous fistulas (AVF). The veins of the brain drain into venous sinuses, blood filled areas located in the dura mater, before leaving the skull and traveling to the heart. In an AVF there is a direct connection between one or more arteries and veins or sinuses. Dural AV fistulas and carotid-cavernous fistulas (CCF) are the most common AVFs.
Spinal AVMs can occur on the surface (extramedullary) or within the spinal cord (intramedullary) and are classified into 4 groups:
Type 1 – (most common) is a dural arteriovenous fistula. They usually have a single arterial feeder and are thought to cause symptoms by venous hypertension.
Type 2 – (also called glomus) is intramedullary and consists of a tightly compacted nidus over a short segment of the spinal cord.
Type 3 – (also called juvenile) is an extensive AVM with abnormal vessels both intramedullary and extramedullary.
Type 4 – are intradural extramedullary arteriovenous fistulas on the surface of the cord
What are the symptoms?
The symptoms of AVMs vary depending on their type and location. While migraine-like headaches and seizures are general symptoms, most AVMs do not show symptoms (asymptomatic) until a bleed occurs. Common signs of brain AVMs are:
Sudden onset of a severe headache, vomiting, stiff neck (described as “worst headache of my life”)
Bruit: an abnormal swishing or ringing sound in the ear caused by blood pulsing through the AVM
Common signs of spinal AVMs are:
-Sudden, severe back pain
-Weakness in the legs or arms
An AVM is a tangled bundle of blood vessels where arteries connect directly to veins with no capillary bed between. This creates a system of multiple feeding arteries, the tangle or nidus, and enlarged draining veins. Intracerebellar hematomas (ICHs) and tumors can also cause bleeding into brain tissue.
During an endovascular procedure, a microcatheter is inserted into the arteries feeding the AVM. Materials, such as glue or coils, are inserted into the abnormal arteries to block the flow of blood into the AVM nidus.
National Institute of Neurological Disorders and Stroke
Aneurysm & AVM Support
Toronto Brain Vascular Malformation Study Group