A 61-year-old man fell in...

A 61-year-old man with a history of atrial fibrillation suddenly fell into a coma. He was in the living room when he complained of feeling unwell, saying that he was dizzy and couldn't see. He then lay down on a couch and became unresponsive within a minute. Paramedics arrived several minutes later, and he was found to be in a deep coma. He required intubation in the emergency room due to breathing difficulties. His wife denied any substance abuse or toxic ingestion, and there was no history of trauma. Upon arrival at the emergency room, his vital signs were stable, but he was unresponsive to noxious stimuli and showed anisocoria (one pupil larger than the other), decorticate posturing, and no spontaneous movements. Tests, including lab work and imaging studies (CT and CT angiogram of the head and neck), did not reveal any abnormalities such as hemorrhage, tumor, stroke, or thromboses. Based on these findings, what is the provisional diagnosis?