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