Blood returning from the upper body is supposed to arrive on the right side of the heart, where the lungs get the first pass at filtering out anything small and solid before it reaches the brain. In a rare congenital arrangement called Raghib syndrome, that safety check is bypassed.
A vein that should have disappeared before birth persists, and instead of draining into the right atrium, it empties directly into the left. Anything traveling in that blood, including a clot, goes straight out to the arteries and, potentially, to the brain.
