Two years after Robin Williams' passing, his widow, Susan Schneider Williams, continues to serve as an advocate for Lewy body disease, a kind of dementia that led to the famed actor's death.

Susan recently penned a heartbreaking essay titled, "The Terrorist Inside My Husband's Brain," published in the journal Neurology that chronicles the star's final months as he struggled to understand his own failing health.

"Robin is and will always be a larger-than-life spirit who was inside the body of a normal man with a human brain," she writes. "He just happened to be that 1 in 6 who is affected by brain disease."

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It was during those final months of his life that the actor began exhibiting physical symptoms and problems with his memory that were not associated with any one disease. Susan recalls the moment he was not able to remember his lines while filming Night at the Museum 3.

"Robin was losing his mind and he was aware of it," she adds. "Can you imagine the pain he felt as he experienced himself disintegrating? And not from something he would ever know the name of, or understand? Neither he, nor anyone could stop it — no amount of intelligence or love could hold it back."

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Diagnosing his symptoms, however, was difficult, and the scramble to find answers was not simple.

"When we were in the neurologist's office learning exactly what this meant, Robin had a chance to ask some burning questions," Susan writes. "He asked, 'Do I have Alzheimer's? Dementia? Am I schizophrenic?' The answers were the best we could have gotten: No, no, and no. There were no indications of these other diseases. It is apparent to me now that he was most likely keeping the depth of his symptoms to himself."

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It was his final days that proved to be the most devastating.

"As the second weekend in August approached, it seemed his delusional looping was calming down," she explains. "Maybe the switch in medications was working. We did all the things we love on Saturday day and into the evening, it was perfect — like one long date. By the end of Sunday, I was feeling that he was getting better. When we retired for sleep, in our customary way, my husband said to me, 'Goodnight, my love,' and waited for my familiar reply, 'Goodnight, my love.' His words still echo through my heart today."

He was dead the next day.

Susan hopes the tearjerking letter will encourage and recognize doctors who continue the fight to learn more about Lewy body disease.

Our hearts go out to his family and loved ones.