Senator Mark Kirk did not have plaque in his artery -- instead he suffered a dissection of the carotid, the main artery carrying blood to the brain tore. Doctors say it can happen simply by coughing.

Dr. James Castle, Stroke Neurologist, NorthShore University HealthSystem: "About half of the people who get a carotid dissection remember a trauma like a car accident or a contact sport collision where there was a major whiplash. About half of the people that get these there's no recalled trauma and the thought is a lot of them suffered a trauma without realizing it. They may have had a very bad cough, cold, flu where they were sneezing or coughing in such a way and whipping their neck around that way they actually cause a tear in the artery."

The tear leads to decreased blood flow.

Dr. Castle: "And it causes the artery itself to become very narrowed, and you can get clotting in this area that you see."

The clot travels up the carotid into the brain where it causes a stroke -- represented by the darker area in this scan. Like a bruise, it swells.

Dr. Castle: "The swelling on other parts of the brain can be fatal if there's swelling on what's called the brain stem, which is deeper down into the brain. And also when the swelling is pushed into other areas of the brain, the other areas of the brain are very sensitive to that pressure and it can damage them as well.

Senator Mark Kirk's doctors relieved the swelling but not before the stroke impacted the right side of the brain. Unlike congresswoman Gabrielle Giffords, who suffered a left brain injury, speech and mental acuity are not the biggest concerns here -- vision and movement are.

Dr. Castle: "I think a right brain injury would be a lot less likely to cause a problem with being a legislator. Of course, there are some cognitive issues that come up when you have a right side injury, and I think only time will tell how important these are. Fortunately it was not on the side of the brain that involves language. There are some visual special abilities that will be lost by having that type of a stroke, and he may not be able to see as well on that side or he may not be able to move his left side very well."

And how well do most patients recover from a stroke of this magnitude?

Dr. Castle: "A small minority of patients with a stroke make a complete recovery, but I would think a stroke large enough to cause that type of swelling there will be some long term deficit he will have to deal with."

For now the big concern is that Senator Kirk may develop pneumonia post surgery. Also, since he'll be in bed constantly, there is a risk for blood clots. Doctors will likely give him blood thinners to reduce the risk as they wait for improvement.