A brain blood-vessel tangle—doctors call it an arteriovenous malformation, or AVM—is a knot of arteries and veins that formed the wrong way before you were even born. Most people never feel it, but if the tangle starts to act up it can shout at you in five clear ways. Sudden “thunderclap” headacheThe pain hits like a switch flipped inside your head—zero to worst in seconds. It can feel like you’ve been hit with a bat, and it usually keeps you from finishing whatever you were doing. Seizure out of the blueYou might jerk, stiffen, or blank out for a minute.... Learn more