UPDATE: Things aren't as rosy as the initial reports. From Aaron Blake at the Washington Post:
@FixAaron: Rep. Mark Kirk (R-Ill.) update: They had to remove 4x8-inch section of his skull. Full mental recovery odds "pretty good," per doctor.
@FixAaron: But Kirk sounds as though he will have limited use of the left side of his body, particularly his arm.
HuffPost has it. It was an "ischemic stroke," and there was swelling around his brain. It sounds like this kind of ischemic stroke, since they appear to have found it in his neck:
An embolic stroke is also caused by a clot within an artery, but in this case the clot (or emboli) forms somewhere other than in the brain itself. Often from the heart, these emboli will travel in the bloodstream until they become lodged and cannot travel any farther. This naturally restricts the flow of blood to the brain and results in near-immediate physical and neurological deficits.From Mike O'Brien of MSNBC:
@mpoindc: Kirk was "beginning to deteriorate neurologically" on Sunday evening, which precipitated surgery to relieve brain swelling, says doc.