“This is a personal story, sadly tragic and heartbreaking,” writes Susan Schneider Williams, Robin Williams's widow, in the latest issue of the medical journal Neurology. Her essay—titled "The Terrorist Inside My Husband's Brain"—chronicles the late actor's experience with Lewy body disease, a relatively common type of dementia he didn't know he had. (About 1.5 million people in the U.S. have it.)

Often missed or misdiagnosed, Lewy body disease can resemble or overlap with Parkinson's or Alzheimer's. It may cause muscle stiffness and movement difficulties, as well as hallucinations and confusion, and problems with attention and complex thoughts. But what's less known is just how profoundly the illness can alter a person's life, a point that Susan makes clear throughout her candid piece.

In the fall of 2013, Robin had been suffering from a range of seemingly unrelated symptoms: "Constipation, urinary difficulty, heartburn, sleeplessness and insomnia, and a poor sense of smell—and lots of stress." By winter, things had gotten worse; "paranoia, delusions and looping, insomnia, memory, and high cortisol levels—just to name a few—were settling in hard."

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That spring crippling anxiety began to interfere with Robin's ability to work. While filming Night at the Museum 3, the actor struggled to remember "even one line for his scenes," Susan writes, which left him devastated: “Robin was losing his mind and he was aware of it.”

On May 28, after countless medical tests, Robin was diagnosed with Parkinson's disease. Susan took the diagnosis as a sign of hope for the future. But somehow she knew "Robin was not buying it."

His health continued to deteriorate in the coming months. “At times, he would find himself stuck frozen in a stance, unable to move, and frustrated when he came out of it. He was beginning to have trouble with visual and spatial abilities in the way of judging distance and depth," Susan writes. "I experienced my brilliant husband being lucid with clear reasoning 1 minute and then, 5 minutes later, blank, lost in confusion."

In August of 2014, Robin committed suicide. It wasn’t until several months later that an autopsy revealed the Lewy bodies—abnormal deposits of a protein called alpha-synuclein—ravaging his brain. Experts told Susan that Robin's case was one of the most severe they had ever seen.

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“This is where you come into the story,” writes Susan near the end of her essay, addressing the medical professionals reading Neurology. "Hopefully from this sharing of our experience you will be inspired to turn Robin’s suffering into something meaningful through your work and wisdom."

"I know you have accomplished much already in the areas of research and discovery toward cures in brain disease. And I am sure at times the progress has felt painfully slow. Do not give up," she writes. "Trust that a cascade of cures and discovery is imminent in all areas of brain disease and you will be a part of making that happen."