The much-loved actor took his own life in August, 2014 at the age of 63, and it subsequently emerged he had been suffering from depression, anxiety, and the early stages of Parkinson's disease.

However, his wife Susan has now revealed an autopsy on the Mrs. Doubtfire star also detected signs of Diffuse Lewy Body Dementia, also known as Dementia with Lewy bodies (DLB), which can cause hallucinations, motor skills issues, and a fluctuating mental state.

She blames the illness for driving Williams to suicide and reveals it had left him with "maybe three years" to live.

Susan tells Good Morning America, "Lewy Body Dementia killed Robin. It's what took his life... It's chemical warfare in the brain, and we can't find it till someone dies, definitively. There's no cure."

She expands on the issue to People magazine, saying, "I've spent this last year trying to find out what killed Robin. To understand what we were fighting, what we were in the trenches fighting, and one of the doctors said, 'Robin was very aware that he was losing his mind and there was nothing he could do about it.'

"This was a very unique case and I pray to God that it will shed some light on Lewy bodies for the millions of people and their loved ones who are suffering with it. Because we didn't know. He didn't know.

"It was not depression that killed Robin. Depression was one of let's call it 50 symptoms, and it was a small one."

DLB is often misdiagnosed because the symptoms are so difficult to pinpoint. For Robin, they presented as increased anxiety, problems moving and delusional episodes, which worsened over the last 12 months of his life and became increasingly bad shortly before he hanged himself.

Even though he was in regular contact with doctors, it was only his autopsy that allowed them to discover what was wrong.

"I know now the doctors, the whole team was doing exactly the right things," Susan adds to People. "It's just that this disease was faster than us and bigger than us. We would have gotten there eventually."

Susan also used her appearance on Good Morning America on Tuesday (03Nov15) to reveal she does not blame her husband for taking his own life, saying, "I got to tell him, 'I forgive you 50 billion per cent, with all my heart. You're the bravest man I've ever known'... You know, we were living a nightmare."