FACTBOX-US senator's condition is rare and dangerous

(Reuters) - U.S. Democratic Sen. Tim Johnson was in critical condition on Thursday after undergoing brain surgery for a rare condition, congenital arteriovenous malformation.
-- A congenital arteriovenous malformation occurs when the arteries connect directly to the veins in a part of the brain, without a cushion of tiny capillaries to slow the flow of blood from the arteries.
-- When they burst and bleed, they often kill patients before they can get to the hospital.
-- Quick surgery can stop the bleeding and prevent brain damage.-- Like an aneurysm, these malformations cause few or no symptoms unless they burst. They can occasionally cause headaches or seizures.
-- Corrective treatment closes the connection, sending blood to stronger vessels in the brain. Full recovery is possible.