Definition of Arteriovenous malformation
A brain (intracranial) arteriovenous malformation (AVM) is an abnormal tangle of arteries and veins in your brain. These are congenital lesions, meaning that they can develop before you’re born. Arteriovenous malformations can also occur in your spinal cord (spinal arteriovenous malformation).
Oxygen-rich blood normally enters your brain through arteries, which branch into smaller arterioles and even smaller blood vessels (capillaries). Your brain uses oxygen removed from the blood in your capillaries. Oxygen-depleted blood then passes into small venules and then into larger veins that drain the blood from your brain to your heart and lungs.
In a brain arteriovenous malformation, your blood passes directly from your arteries to your veins, bypassing capillaries. The arteries and veins in an AVM can rupture, causing bleeding into the brain.