According to the Capitol physician, Johnson suffered bleeding in the brain caused by a congenital arteriovenous malformation.
Sanjay Gupta, CNN's resident brain surgeon / wandering doctor-minstrel, armchair-diagnoses a long recovery, positing effects on the brain's speech center. Get well soon and our best wishes for your family, Senator.
For those of you -- like me -- who think it would be terribly cruel and unfair for South Dakota Republican Governor Mike Rounds to take Johnson's seat away from the Democrats in the eventuality that Johnson can no longer serve, it's happened plenty of times before. Respect for the wishes of the dead does not extend to giving up an opportunity to change a Senate seat to your party (though such a switch has never tipped the Chamber -- the only time that's happened mid-session in Senate history was when Jim Jeffords defected). So don't kid yourselves.