Eshwar is right, it is a narrowing of the vessel. It isn't a sudden thing but happens over time. In people that just have bad health habits (don't exercise or eat right) two things happen; one is their arteries lose the elasticity (atherosclerosis) and the other is the become narrower when clogged with fatty deposits, plaques and calculus (stenosis). All this makes blood flow poorer and eventually, if they become narrow enough, the symptomology becomes something that you cannot ignore.
In radiated patients, it can be a combination of scar tissue forming in the artery from the radiation, narrowing it, and if they also came into cancer treatments with a poor diet etc. the plaques in addition to. Radiation itself causes atherosclerosis, so you get a double whammy.
Strokes happen when part of the deposits break off and travel downstream until they reach a blood vessel too small for them to pass through, and that's where the blood flow stops. Everything past that dies in very short order, and you have about 3 hours to introduce clot busting drugs to even hope for life to go back to something normal. They of course can be fatal. You can also have a stroke from a thrown blood clot, which often happens in people that have a bad heart valve, or damaged arterial wall. Depending on where the clot or plaque stops, things can get really dire quickly or not. Obviously in your brain, upstream from a carotid issue is a bad thing. Downstream from the heart to the femoral artery and leg is another thing.
My dad had a bad valve which threw a clot and he came home one day from walking the dog limping and rubbing his numb leg. He had no idea what had happened, until I called 911 and told them my dad had a stroke. He thought I was nuts and that it was a pinched nerve. After that was resolved, 9 months later he threw another clot which went to his brain and he lost his ability to talk, reason, and so much more. Days later he had a massive MI and that was that. Men in my family do not live past their early 70's going back for generations, and all die from the same thing. This is one of those nature and nurture things. Knowing all this I have been pretty careful about my health during my adult life. But even when you are doing everything possible, and armed with lots of knowledge, you can't change your genetic make up. But I never calculated that I would end up with an 80+% carotid stenosis a decade early� from radiation, not genetics.
