It was announced in the news today that Senator Johnson did not have a stroke. He had a congenital cerebral arteriovenous malformation (AVM.)
"AVMs are often silent, asymptomatic, and therefore undiagnosed until a hemorrhage occurs, or some other symptom occurs that results in the person getting a CT or MRI scan of the brain that reveals the lesion."
Arteriovenous malformations are congenital vascular abnormalities, and are so called because they consistent of tangles of abnormally connected vessels, with fistulas rather than capillaries forming arterial-to-venous connections, resulting in overly rapid blood flow that does not allow for proper oxygenation of tissues.
The abnormal vessels are also subject to leakage and rupture, and can lead to intracranial bleeding that can vary in severity according to where the defect occurs.
Here's a few other links to information on AVMs.
Radiology Picture of the Day: Cerebral Arteriovenous Malformation
Discover Magazine's Vital Signs: What's That Noise In Her?
A diagnostician facing his own medical troubles tracks down a killer defect.
- I really like this one because it addresses the problems that a doctor has in his diagnosis (which I already gave away the punchline for) so the main point deals with how he goes about dealing with the strange noise he hears and his own insecurities.