Robin Williams' widow speaks: Depression didn't kill my husband

(CNN)  Comedian Robin Williams' widow, Susan Williams, said she and her husband "were living a nightmare" in the months leading up to his death.

"My best friend was sinking," an emotional Williams told ABC's Amy Robach in an interview that aired Tuesday, her first since Robin Williams killed himself in August 2014.

Williams said she's spent the last year trying to get to the bottom of what led him to take his own life. Contrary to what most people think, she said, it wasn't depression, nor was it a re-emergence of his longtime struggles with alcohol and drug addiction. Robin Williams had no alcohol or illegal drugs in his system; he'd been sober for eight years, his wife said.

What drove her husband to suicide, "was what was going on in his brain," Williams said.

"The chemical warfare that no one knew about."

'Chemical warfare'

That "chemical warfare" that doctors conducting Robin Williams' autopsy discovered was Lewy body dementia.

Though not nearly as well known (or talked about) as Alzheimer's disease, which accounts for more than half of dementia diagnoses in the United States, Lewy body dementia, or LBD, is the second most common type of progressive dementia.

Nearly 1.4 million Americans are known to have the disease, but because it's a relatively "young disorder," Angela Taylor, director of programming for the Lewy Body Dementia Association said, that number is likely much higher.

LBD is caused when normal proteins in the brain begin to aggregate, forming clumps called Lewy bodies that, as they spread, "muck up the ability for the brain to transmit signals," said Dr. James Leverenz of the Cleveland Clinic.

Like Alzheimer's disease, symptoms of LBD include cognitive problems like confusion, reduced attention span, and memory loss, Taylor said.

But LBD also affects a patient's movements, as well as their mood, making it a "triple threat," Taylor said.

"It's not just memory, it's not just movement, and it's not just behavior. It's a combination of all three, which makes it difficult to diagnose and difficult to treat," Leverenz said.

'Endless parade of symptoms'

Susan Williams recalls thinking her husband was a hypochondriac, when, starting in November 2013, every month he seemed to complain about a different ailment.

Like a game of "whack-a-mole," Robin Williams was wrought with a severe pain in his gut, sleeplessness and constipation, she said.

After months of heightened anxiety and paranoia about his health, Susan Williams said, Robin Williams felt a small "sense of relief" when he was diagnosed with early onset Parkinson's disease in May 2014.

While Parkinson's disease, which like Alzheimer's has no cure, is hardly good news, Susan Williams said it was nice to have a possible answer for her husband's seemingly "endless parade of symptoms."

Parkinson's, a nervous system disorder that affects movement, could be blamed for the tremor in Robin Williams' left hand, but Susan Williams said it didn't explain everything.

'I miscalculated'

Susan Williams breaks down as she remembers what she witnessed on July 24, 2014, just months after Robin Williams was diagnosed with Parkinson's.

She was in the shower when she noticed her husband lingering by the sink. She opened the door to find him holding a bloodied towel, a severe gash on his head. "Robin, what happened?" she screamed.

She said he motioned toward the door, and said just two words, "I miscalculated."

Though she didn't know it then, Susan Williams said LBD had affected his vision and his ability to recognize and identify objects, like the door.

Susan Williams said despite his diagnosis, her husband of three years was happy.

"Lewy body dementia killed Robin," she told Robach.

Changing capacity to do things

As Lewy bodies form and take over different parts of the brain affecting body movement, mind and mood, patients suffering from LBD experience symptoms of a person with both Alzheimer's and Parkinson's, conditions that alone are devastating.

Because Robin Williams was a very active and very successful person, it's understandable that he would have grown depressed about his "changing capacity to do things he used to do," Leverenz said.

Susan Williams said she believes her husband was losing his mind, and "he was aware of it."

His decision to use a belt to hang himself from his bedroom door was, in Susan Williams' opinion, his way of taking his power back, a painful choice for which she immediately forgave him.

After emergency responders realized they couldn't revive him, Susan Williams got to see him.

"And I got to tell him, 'I forgive you 50 billion percent, with all my heart. You're the bravest man I've ever known.'"