Billy Connolly, who is being treated for the "initial symptoms" of Parkinson's Disease, admitted earlier this year that he had started to forget lines during performances.
Speaking about it, he said: "This is f*****g terrifying. I feel like I'm going out of my mind."
There is no cure for the disease but symptoms can be controlled using a combination of drugs, therapies and occasionally surgery.
Actor Bob Hoskins announced his retirement last year after being diagnosed with the disease.