I'll speculate that he popped an aneurism and they clipped it during surgery.
If the reports of cranial surgery are correct, the two most likely causes would be blood vessel (e.g. aneurysm/AVM) or a tumor.
That's what I was just thinking.
I agree, think it was an aneurysm. In that event, Sen. Johnson has really superb docs working on him. I know a woman who had brain aneurysm there a few years ago.
Let us not forget the new hospital building at GW Hospital is the Ronald Reagan bldg, with some $7M contribution from a former Congressman from Wyoming. This is the same hospital where they took Reagan after he was shot, and where they bring VP Cheney whenever he has a heart problem.