Robin Williams' widow claims he was suffering from Lewy Body Dementia (LBD) before his death.
Susan Schneider believes the late actor committed suicide in August 2014 as a result of the debilitating brain disease, which is the second most common neurodegenerative dementia after Alzheimer's and causes heightened levels of anxiety, delusions and impaired movement.
She said: "It was not depression that killed Robin. Depression was one of let's call it 50 symptoms and it was a small one."
While the Oscar winner's illness progressed rapidly in the months before he killed himself, Susan, who married the comedian in 2011, believes there were some "miscalculations" in his diagnosis before an autopsy confirmed he had the disease.
She told PEOPLE magazine: "I know now the doctors, the whole team was doing exactly the right things. It's just that this disease was faster than us and bigger than us. We would have gotten there eventually."
The graphic designer added: "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.'"
Susan believes the Good Will Hunting star, 63, killed himself in order to regain a sense of control over his life.
She told Good Morning America: "He was aware of it. He was keeping it together as best as he could, but the last month he could not. It was like the dam broke.
"I think he was just saying no and I don't blame him one bit."