Williams was diagnosed with Lewy body dementia after his death.

In the months before he died in 2014, Williams was misdiagnosed with Parkinson's disease based on his battery of physical and neurodegenerative symptoms.

"Not until the coroner's report, three months after his death, would I learn that it was diffuse LBD [Lewy body disease or Lewy body dementia] that took him," Schneider Williams shared in a 2016 article published by the journal Neurology. "All four of the doctors I met with afterwards and who had reviewed his records indicated his was one of the worst pathologies they had seen."

Though the actor's official cause of death was suicide, his widow views the "intense, confusing, and relatively swift persecution" of his symptoms as his true reason behind his tragic passing.

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He experienced every symptom of the disease.

Schneider Williams says that in October of 2013, around the couple's second wedding anniversary, Williams' symptoms began developing rapidly. "He had been struggling with symptoms that seemed unrelated: constipation, urinary difficulty, heartburn, sleeplessness and insomnia, and a poor sense of smell—and lots of stress. He also had a slight tremor in his left hand that would come and go," Schneider Williams recalled.

From there, new symptoms arose: gut discomfort, fear, depression, and anxiety, Parkinsonian mask, language difficulty, sensitivity to medications, cognitive difficulties, and a shuffling gait. The couple found themselves "chasing symptoms for nearly a year" as they would appear and disappear seemingly at random, Schneider Williams explained during a talk at the Life Itself conference.

"None of the doctors knew that there was this ghost disease underlying all of this," Schneider Williams told CNN in an interview. "When that was revealed, that was like essentially finding out the name of my husband's killer."

His widow says this one symptom was "incredibly scary."

One symptom that Schneider Williams found particularly troubling was her husband's delusional looping: obsessive or recurring fixation on beliefs that flatly defy reality. "Your brain is concocting a story of what you think reality is," she explained in her conference talk. "And the people around you are unable to rationalize with you and bring you back into what is actually real. So it's incredibly scary for everyone around someone who's deluded as well as the deluded person.”

For Schneider Williams, the beloved actor's main source of support, this symptom felt paralyzing. "As a caregiver, you feel incredibly powerless when you realize, 'Oh my gosh, nothing I say or do anymore can bring him back to what's real.' And that's a very scary place," she said.

Exacerbated by his severe insomnia, Williams' symptoms would worsen after dark, said his widow. "Our house was like Night at the Museum at night," Schneider Williams said. Pulling him back from nighttime delusions would take hours, sometimes days, she added. "Imagined fear on fire—that is what it is."

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She now says "diagnosis is everything."

When she first opened up about her late husband's disease, Schneider Williams expressed skepticism that an accurate diagnosis would have made a difference. "I am not convinced that the knowledge would have done much more than prolong Robin's agony while he would surely become one of the most famous test subjects of new medicines and ongoing medical trials," she wrote at the time. "Even if we experienced some level of comfort in knowing the name, and fleeting hope from temporary comfort with medications, the terrorist was still going to kill him. There is no cure and Robin's steep and rapid decline was assured," she added.

Now six years later, Schneider Williams says she’s had a profound change of heart. "When I wrote that editorial, The Terrorist Inside My Husband's Brain, I was convinced that a diagnosis wouldn't matter anyway, because there is no cure," Schneider Williams said at Life Itself. "But my thinking since then has completely changed. Diagnosis is everything—not just for the patients and caregivers, but for the doctors, clinicians, and researchers. If we had an accurate diagnosis, we could have sought specialized care."

"Whoever has hope has many days of feeling the darkness," Schneider Williams said. "But the thing about hope is that no matter what, you dust yourself off, you pick yourself up and you go forward. And you don't do that alone."