If the reports of cranial surgery are correct, the two most likely causes would be blood vessel (e.g. aneurysm/AVM) or a tumor.
An elderly friend and his wife were on a cruise a couple of weeks ago. Five hours into the trip, the gent had what all thought to be a stroke. Later it was determined to be some type of aneurysm. He was helicoptered to a hospital and a stint (sp) was inserted through a vein in his leg and installed into his brain. It was the third procedure of the type ever to be performed. He was in perfect health two days later when I ran into him at a meeting.