Brain Arteriovenous Malformation (AVM) | Symptoms and Causes

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Brain arteriovenous malformations (AVMs) are tangles of arteries and veins in the brain that are abnormally connected, usually from birth, leading to a variety of neurological symptoms. In rare cases, they are associated with conditions that run in families.

In normal blood circulation, oxygen-rich blood from the heart travels at high pressure and speed through arteries that branch into smaller and smaller vessels, ending in capillaries. In the capillaries, blood flow slows down and loses pressure, allowing the body to take the oxygen it needs. The depleted blood then moves into the veins and returns to the heart and lungs to pick up more oxygen.

In brain AVMs, the pattern is abnormal. Blood travels directly from the arteries to the veins through a tangle of abnormally formed, small, very irregular vessels. This direct connection, bypassing the capillaries, prevents the blood from slowing down, losing pressure and releasing its load of oxygen. This can cause several problems:

  • Brain tissue near the AVM can become dysfunctional and sometimes even undergo stroke due to a lack of oxygen.
  • Blood flow and fluid balance throughout the brain become abnormal.
  • The area where arteries and veins connect, known as the nidus, contains blood under high pressure—normally found only in arteries. The pressure can cause the arteries leading to the AVM to become enlarged and even develop aneurysms, while veins leading away may either enlarge and balloon out or become narrowed (stenosis). These abnormal vessels—leading to the AVM, within the AVM and leading away from the AVM—all can become weak and susceptible to rupture.
  • The tangle of AVM vessels itself can put pressure on nearby structures or block the normal flow of cerebrospinal fluid through and around the brain.
  • If the AVM is large, or if it blocks the flow of cerebrospinal fluid, hydrocephalus can develop.
  • Rarely, in newborns with a very large brain AVM, the flow may be so high that the baby experiences heart failure, as the heart struggles to pump enough blood both to supply the body and to flow through the AVM.
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