Author: Tanner Marshall, MS
Editor: Rishi Desai, MD, MPH, Tanner Marshall, MS
Arterio refers to the arteries, which are the the blood vessels that take blood away from the heart, and venous refers to the veins, which carry blood toward the heart. Mal- means “badly”, so an arteriovenous malformation, or AVM, is some sort of “bad” or abnormal formation between the arteries and veins.
Typically, oxygen-rich blood is carried to the body’s tissues via arteries. Large arteries split into smaller and smaller arteries, and feed into capillary beds, where cells exchange oxygen for carbon dioxide. The capillaries then feed into larger and larger veins, which carry deoxygenated blood away from the tissues. Normally, arterial blood is under a high-pressure system, and as it goes through smaller and smaller vessels, ultimately into the capillary bed, pressure in the vessels becomes significantly lower, and blood drains into the lower pressure systemic veins. So this capillary bed dampens the arterial flow a bit.
With AVM, this whole capillary bed isn’t there, and a group of arteries directly link up with a group of veins instead. The vessels in the AVM can start to tangle up and are called a nidus, which is Latin for “nest”. When a single artery and a single vein link up abnormally like this, it’s called an arteriovenous fistula.
In AVM the arteries and the veins are both under high systolic blood pressures because there are no capillaries to dampen the pressure, which means that the AVM can expand in size over time and can put pressure on the surrounding tissue. This pressure on surrounding capillaries can pinch them shut and prevent that functional tissue from getting blood flow. Also, the high pressure causes the arteries supplying blood to dilate, while the veins thicken and undergo fibrosis. Vessel walls are also prone to forming aneurysms, which are these balloon-like protrusions; and because the vessel walls are weakened and stretched out, they are also at risk for ripping and tearing.
Although AVMs can form anywhere in the body, they most often form in certain parts of the body like the brain, the spinal cord, and also the lungs. If the AVM ruptures in the brain, blood escapes into the surrounding brain tissue. Sometimes the blood-loss can be small, called a microbleed, which doesn’t typically cause much damage, but sometimes when blood loss is severe enough, people might experience what is called an intracerebral or subarachnoid hemorrhage. Depending on the location, this can cause several neurological complications like headaches or seizures,
as well as other neurological symptoms that can relate to the part of the brain affected—like for example, if it affects the hippocampus, it might affect the person’s memory, or if it’s the basal ganglia, it might affect their ability to move. In the spinal cord, AVMs can, by the same mechanisms, damaged nerve fibers, which can cause sensory disturbances, muscle weakness, or even paralysis in the parts of the body controlled by the affected nerves.
In the lungs, an AVM might result in shortness of breath or coughing up blood.
It’s also important to remember that having blood flow out of the heart to an artery, then through the AVM to a vein, and directly back to the heart is a lot of wasted work, since that oxygen’s not going to the tissue’s capillaries like it should and it’s just rerouting back to the heart, which ultimately causes the heart to work harder, and over time, an AVM can contribute to heart failure because the heart’s doing extra work with each beat.
Diagnosis might involve several imaging techniques, one of which is angiography, which is actually the gold standard, but others are CT scans and MRI. Aside from imaging, a distinctive clinical sign of AVM is a bruit, which is French for “noise”. When blood flows rapidly flow of blood through the AVM, it causes a whooshing noise. Sometimes this noise can get so loud, especially if the AVM is in the brain, that the person can hear it themselves, which can cause serious psychological stress, not to mention making it hard to sleep or hear other things.
The cause of AVMs isn’t well understood, although it’s thought to be congenital and caused by some mistake in blood vessel growth during embryonic or fetal development, some of which seems to be genetic. One well known relationship is with Osler-Weber-Rendu disease which is an autosomal dominant disorder that causes problems with blood vessels throughout the body.
Most AVMs are treated even if the person’s not symptomatic, because of the risk of complications, like bleeding. Medications can be used to treat certain symptoms from an AVM like headaches. But also they might also have certain procedures, like radiosurgery, where beams of radiation are targeted at the AVM, causing the vessels to scar and close off. Or they might have endovascular embolization, where a catheter is used to obstruct the vessels. Because the location of the AVM varies so widely, usually the procedure varies on a case-by-case basis.
Sources edit
https://en.wikipedia.org/wiki/Arteriovenous_malformation
http://www.ninds.nih.gov/disorders/avms/detail_avms.htm