It occurs when the arteries link directly with veins, bypassing capillaries. The resulting malformation can rupture and bleed into the cerebral area.
Why an AVM forms remains a mystery. Cerebral AVMS are extremely rare, but once developed tend to vary both in size and location in the brain. Most people with an AVM have no symptoms; consequently, it is often discovered during treatment for an unrelated disorder. Imaging tests to help with diagnosis include EEG, CT scan and MRI.