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Williams had been diagnosed with early stages of Parkinson's disease before he died by suicide August 2014. But an autopsy revealed he also suffered from Lewy body dementia (LBD), a brain disorder that affects more than a million Americans.
According to The Alzheimer's Association, LBD is a type of progressive disease that causes a decline in thinking, reasoning and independence, linked to the buildup of microscopic deposits - so-called Lewy bodies - in the brain, which damage the cells over time. It's the third most common type of progressive dementia after Alzheimer's disease and vascular dementia.
LBD shares similarities with Parkinson's and Alzheimers, and scientists think all three disorders may have a common cause.
The symptoms of LBD include changes in thinking or reasoning, day-to-day confusion, hunched posture and rigid movements, hallucinations, and other problems.
"Lewy body dementia is what killed Robin," Williams told ABC News. "It's what took his life and that's what I spent the last year trying to get to the bottom of, what took my husband's life."
The week of Williams' death, his doctors were planning to check him into a facility where he would undergo neurocognitive testing.
The complete interview airs tonight on "World News Tonight with David Muir" and "Nightline," and again Friday on "The View."