An intracranial arteriovenous malformation (AVM) is a tangle of arteries and veins that short-circuits normal blood flow. Because the walls of these vessels are thinner than usual, they can leak or burst, producing symptoms that range from quiet warnings to sudden emergencies. The most dramatic first sign is a sudden, severe headache—often described as a “thunderclap” that hits in seconds. It may be followed by nausea, vomiting, stiff neck, or loss of consciousness if the AVM bleeds. Seizures are another common clue. A person may jerk, stare blankly, or black out for a few minutes, even with no history of... Learn more