How can Mitch McConnell still hold office with brain cancer?

"Democrats funneled tens of millions of dollars to former fighter pilot Amy McGrath in an attempt to oust the Senate majority leader, but Sen. Mitch McConnell was successful in retaining the seat he first won in 1984."

Since Senators require a rational and sound mind for their judgement of the law, how can Mitch McConnell hold office? Maybe I recall it wrong, but I thought he had brain cancer.