Robin Williams suffered from dementia in his last days, says book

Oscar-winning actor Robin Williams was struggling from diffuse Lewy body dementia in his last days, a new book has revealed.

In a soon-to-be-released biography, titled "Robin", author Dave Itzkoff says the actor did not have Parkinson's disease, as he was diagnosed earlier, New York Post reported.

The actor, who had started having troubles with speech and behavioural dysfunction, did not know he was suffering from the neurogenerative disease.

It was only three months after his death that the autopsy results revealed that he had "diffuse Lewy body dementia".

The inability to express himself, a rarity for one of the most spontaneous performers on-stage and on screen, was eating Williams from the inside during the filming of "Night at the Museum: Secret of the Tomb", the third movie in the successful family franchise in Vancouver in 2014.

The actor committed suicide at the age of 63 on August 11, 2014.

The author spoke to people close to Williams to piece together the actor's troubled last year.

Williams was having trouble remembering his lines while he was filming Night at the Museum: Secret of the Tomb, the third movie in the franchise.

According to make-up artiste Cheri Minns recalled the actor would cry unstoppably in her arms.

"He was sobbing in my arms at the end of every day. It was horrible. Horrible. I said to his people, 'I'm a makeup artist. I don't have the capacity to deal with what's happening to him'," she says in the book.

Minns even asked the actor to return to stand-up comic scene to get back his confidence but he refused.

"He just cried and said, 'I can't, Cheri. I don't know how anymore. I don't know how to be funny'."