Comedian Robin Williams had health problems that would have killed him within three years if he had not ended his own life, his widow has said.

In her first interview since the actor died last August, Susan Williams said her husband was "disintegrating before my eyes" in the weeks before his death.

"We were living a nightmare," she told ABC's Good Morning America.

He had been diagnosed with Parkinson's disease and had signs of a condition known as dementia with Lewy bodies.

The condition is caused by deposits of an abnormal protein called Lewy bodies inside brain cells, which disrupt the brain's normal functions.

It can interfere with memory, judgement, movement, concentration and visual perception, according to the NHS.

"If Robin was lucky, he would've had maybe three years left," Mrs Williams said. "And they would've been hard years. And it's a good chance he would've been locked up."

Williams was "completely clean and sober" when he died, his widow said, but was struggling with depression and anxiety.

His physical symptoms included stiffness, slumping, a shuffling gait and "losing his ability in his voice", she said.

"It's one minute, totally lucid… And then five minutes later, he would say something that wasn't... it didn't match."

The actor was dealing with his problems as well as he could, she said, describing him as "the bravest man I've ever known".

"But the last month he could not. It was like the dam broke."

'I don't blame him'

Asked whether her husband's suicide was his way of taking back control, she replied: "In my opinion, oh, yeah.

"I mean, there are many reasons. Believe me. I've thought about this. Of what was going on in his mind, what made him ultimately commit... you know, to do that act.

"And I think he was just saying, 'No.' And I don't blame him one bit. I don't blame him one bit."

Robin Williams was one of America's most popular comedians and actors, thanks to roles in films like Mrs Doubtfire, Good Morning, Vietnam, Dead Poets Society and Good Will Hunting. He was 63.

Robin Williams in 1979. The photo was taken by Michael Dressler and used for the March 12, 1979 cover of Time magazine. It was installed in the National Portrait Gallery to commemorate Williams after his death. Photo collected from Wikipedia .