Robin Williams' widow explains how his rare brain disease left the couple struggling to understand his symptoms, in a new essay published in the medical journal Neurology.

In the moving piece, Susan Schneider Williams says her late husband was initially diagnosed with Parkinson's, until the autopsy revealed the actor was more specifically suffering from Lewy body disease, a little-known and severe form of dementia.

Symptoms went unexplained

"Robin was losing his mind and he was aware of it," she wrote in the essay published this week. "Can you imagine the pain he felt as he experienced himself disintegrating?"

Schneider Williams, a graphic designer and on the American Brain Foundation's board of directors, said she has spent the last year researching the disease since finding out the results three months after the comedian died.

She said in the year prior to her late husband's suicide in 2014, the Patch Adams star was experiencing paranoia, anxiety, depression, insomnia and memory impairment.

'I just want to reboot my brain'

"Robin was having trouble remembering even one line for his scenes," Schneider wrote about Williams' experience on set of Night at the Museum 3, which was shot in Vancouver.

Actor Robin Williams poses for a portrait in 2011. He did not know before killing himself in 2014 that he was suffering from a severe form of dementia called Lewy body disease. (Dan Steinberg/Associated Press)

She compared it with his memory "just three years prior" when he starred for five months in a Broadway production, with "hundreds of lines" and "not one mistake."

"This loss of memory and inability to control his anxiety was devastating to him," she said, later adding: "He kept saying, 'I just want to reboot my brain.'"

Negative test results confusing

Despite "countless blood tests, urine tests, plus rechecks of cortisol levels and lymph nodes" among other tests including a brain scan, all the results came back negative, she said.

She said the late comedian remained clean and sober, turning to psychotherapy, meditation, self-hypnosis and yoga to try to alleviate the symptoms and remove the "terrorist within his brain."

"Robin was growing weary. The Parkinsonian mask was ever present and his voice was weakened. His left hand tremor was continuous now and he had a slow, shuffling gait. He hated that he could not find the words he wanted in conversations."

Williams was found dead after he hanged himself at his Tiburon, Calif., home on Aug. 11, 2014.

Last words

His widow said she believes he was likely experiencing symptoms far worse and painful than he let on, exacerbated by the fact that the couple couldn't get an accurate diagnosis.

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"How I wish he could have known why he was struggling, that it was not a weakness in his heart, spirit or character."

Schneider Williams also shared the final words exchanged with her husband the night before he died, saying she believed at the time he was getting better after the pair had spent "one long date" together.

"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,'" she wrote. "His words still echo through my heart today."