Arteriovenous malformation (AVM)

Arteriovenous malformations (AVMs) are masses of blood vessels (arteries and veins) without intervening capillaries. Arteries decrease in size as their distance from the heart increases. Ultimately they become so small that they are called capillaries. Capillaries are large enough to allow only one or two red blood cells to flow through them at a time. The decrease in the size of arteries from the heart to the capillaries is accompanied by a large decrease in the pressure within them at these locations. Veins are blood vessels that form from initially from coalescence of capillaries. They hold blood under very low pressure.

In AVMs, because there are no capillaries, high pressure arterial blood empties directly into veins that, as noted above, have thin walls capable of containing only low pressure. If the wall of a vein breaks under pressure venous hemorrhage results.

The other clinically significant consequence of arterial blood flowing directly into veins without intervening capillaries is that the tissues through which the blood flows cannot adequately extract oxygen and glucose necessary for their functioning and survival. This lack can result in abnormal activity of neurons causing seizure or stroke.

Nidus:  the portion of the malformation with the densest concentration of arteries.  The portion most prone to bleeding.

Early draining vein:

Pathophysiology of arteriovenous malformations

An AVM is a complicated surgical problem. The surgeon must figure out the best strategy for removing the malformation without causing a hemorrhage (which can be difficult due to abnormally weak blood vessels). Some small AVMs (less than 3 cm in diameter can be treated by Gamma Knife or other radiosurgery but the malformation disappears very slowly after Gamma Knife and is at risk for rupture until completely gone (takes more than a year). 
Embolization (filling the AVM with pellets or glue injected into it through a catheter) can occlude many of the arteries that flow into the malformation which may decrease its size or eliminate it entirely.
Many neurosurgeons will have their neurointerventional colleagues embolize an AVM before doing a craniotomy to remove the malformation.

Arteriovenous malformation resection - procedure

Spinal AVM

(compare: Cavernous malformation)

(compare: Venous angioma)

(compare: Aneurysm)

Vascular malformations

Stroke and Cerebrovascular Disorders Specialty Center

Large left parietal arteriovenous malformation ruptured several days before this patient's death. Gregpry Pinsky MD USC Neuropathology

figure: arteriovenous malformation (AVM) angiogram

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