November 3, 2015 11:18 AM EST

In a new interview with People, Robin Williams’s widow Susan opened up about the actor’s suicide in August 2014. “It was not depression that killed Robin,” she said, but frustration over the debilitating symptoms of Lewy Body Dementia (LBD).

It’s a progressive brain disorder in which microscopic protein deposits, called Lewy bodies, develop in the brain. LBD has symptoms that often include changes in thinking, problem solving, memory and movement. It’s the second most common type of dementia, after Alzheimer’s disease, and accounts for about 20% of all cases of dementia.

She told People, “One of the doctors said, ‘Robin was very aware that he was losing his mind and there was nothing he could do about it.'”

Williams added that she wanted to talk about their experience in order to raise awareness about LBD, which affects more than 1 million Americans, mostly over the age of 50.

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