(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.

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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 Cleveland Clinic neurologist Dr. James Leverenz.

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.

Photos: Remembering Robin Williams Photos: Remembering Robin Williams Robin Williams died August 11, 2014, at age 63. Williams' peers regarded him as a brilliant actor and comedian. His friend David Letterman remembered him as "nothing we had ever seen before." Click through to see moments from the beloved actor's remarkable life. Hide Caption 1 of 28 Photos: Remembering Robin Williams Williams first shot to stardom with Pam Dawber in the sitcom "Mork & Mindy" in September 1978. Hide Caption 2 of 28 Photos: Remembering Robin Williams Williams attends the Robin Williams Opening Party on April 11, 1979, at Studio 54 in New York City. Hide Caption 3 of 28 Photos: Remembering Robin Williams This 1982 file photo originally released by Warner Bros. Pictures shows Williams as T.S. Garp from the film "The World According to Garp." Hide Caption 4 of 28 Photos: Remembering Robin Williams Williams and his first wife, Valerie Velardi, join a huge number of photographers packed into singer Paul Simon's apartment to celebrate Simon's wedding to actress Carrie Fisher in New York City on August 16, 1983. Hide Caption 5 of 28 Photos: Remembering Robin Williams Williams, center, takes time out from rehearsal at NBC's "Saturday Night Live" with cast members Eddie Murphy, left, and Joe Piscopo on February 10, 1984. Williams would appear as guest host on the show. Hide Caption 6 of 28 Photos: Remembering Robin Williams Robin Williams was honored during 2014's Emmy telecast with a tribute led by friend Billy Crystal, who hosted the "Comic Relief" benefits with Williams and Whoopi Goldberg (seen here in 1986). Hide Caption 7 of 28 Photos: Remembering Robin Williams Williams enjoys music through a headset in a scene from the film "Good Morning, Vietnam" in 1987. Hide Caption 8 of 28 Photos: Remembering Robin Williams Williams portrayed a teacher in the movie "Dead Poets Society" in 1989, one of his first mostly dramatic roles. Hide Caption 9 of 28 Photos: Remembering Robin Williams Williams went to all lengths to stay with his children in the 1993 movie "Mrs. Doubtfire." Hide Caption 10 of 28 Photos: Remembering Robin Williams From left, Bonnie Hunt, Bradley Pierce, Kirsten Dunst and Williams hold one another in a scene from the 1995 film "Jumanji." Hide Caption 11 of 28 Photos: Remembering Robin Williams Comedians and co-hosts, from left, Billy Crystal, Whoopi Goldberg and Williams sing and dance the 1940s-era opening number to "Comic Relief VII" on November 11, 1995, in Los Angeles. Hide Caption 12 of 28 Photos: Remembering Robin Williams Jay Leno laughs as Williams jokes around during a taping of "The Tonight Show with Jay Leno" on November 13, 1995, at the MGM Grand Hotel in Las Vegas. Hide Caption 13 of 28 Photos: Remembering Robin Williams Williams and Nathan Lane starred in the film "The Birdcage" in 1996. Hide Caption 14 of 28 Photos: Remembering Robin Williams Williams visits a sick child in a scene from the film "Patch Adams" in 1998. Hide Caption 15 of 28 Photos: Remembering Robin Williams Williams and his wife, Marsha, pose for photographers with their daughter, Zelda, as they arrive at the premiere of the film "Patch Adams" in December 1998 in New York City. Hide Caption 16 of 28 Photos: Remembering Robin Williams Williams wears a clown nose as he places his hands in concrete during a ceremony outside Mann's Chinese Theatre in Hollywood in 1998. In the critically acclaimed "Patch Adams," he played a doctor who used humor to help heal his patients Hide Caption 17 of 28 Photos: Remembering Robin Williams Actor-writers Matt Damon, left, and Ben Affleck, right, pose with Williams, holding the Oscars they won for "Good Will Hunting" at the 70th annual Academy Awards in 1998. Damon and Affleck won for best original screenplay, and Williams won for best supporting actor. Hide Caption 18 of 28 Photos: Remembering Robin Williams Williams donates blood at the Irwin Memorial Blood Center in San Francisco on September 11, 2001. Hide Caption 19 of 28 Photos: Remembering Robin Williams U.S. Postal Service team rider Lance Armstrong rides with Williams during training on a rest day of the 89th Tour de France cycling race in Vaison La Romaine on July 22, 2002. Hide Caption 20 of 28 Photos: Remembering Robin Williams Williams plays Rainbow Randolph in the 2002 black comedy "Death to Smoochy." Hide Caption 21 of 28 Photos: Remembering Robin Williams Williams and Oscar host Billy Crystal perform at the 76th Academy Awards show in 2004. Hide Caption 22 of 28 Photos: Remembering Robin Williams Williams poses for pictures with U.S. soldiers at the main U.S. base at Bagram, Afghanistan, on December 16, 2004. Hide Caption 23 of 28 Photos: Remembering Robin Williams Williams in 2007's "License to Wed." Hide Caption 24 of 28 Photos: Remembering Robin Williams Williams and Susan Schneider arrive at the premiere of "World's Greatest Dad" in Los Angeles on August 13, 2009. Hide Caption 25 of 28 Photos: Remembering Robin Williams Williams at the pre-premiere party for "Happy Feet Two" at the Grand Connaught Rooms in London on November 20, 2011. Hide Caption 26 of 28 Photos: Remembering Robin Williams Williams poses for photographs in Sydney, Australia, on December 5, 2011. He and Australian director George Miller were in Australia to promote "Happy Feet Two." Hide Caption 27 of 28 Photos: Remembering Robin Williams Upon his death, Williams' wife, Susan Schneider, said, "This morning, I lost my husband and my best friend, while the world lost one of its most beloved artists and beautiful human beings. I am utterly heartbroken." Hide Caption 28 of 28

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.

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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."

Photos: I met Robin Williams Photos: I met Robin Williams When Williams came to Oklahoma City during his Weapons of Self-Destruction tour in 2009, Dan McMaster got to see the star. "I told him that I have waited 30 years for this moment," he said. Afterward, McMaster saw Williams had signed his pass, "Dan, an honor for me too, Robin." Hide Caption 1 of 14 Photos: I met Robin Williams While serving in Afghanistan in 2002, Dan Shelor attended one of the comedian's performances for the USO. After the show, Shelor had the privilege of sitting next to Williams on a flight to Manas, Kyrgyzstan. During the flight, Shelor told Williams he was flying to Manas to find a woman he had met before, and was hoping to win her heart over. Hide Caption 2 of 14 Photos: I met Robin Williams Jonathan Robbins first met Williams as an extra on the set of "Good Will Hunting," but saw him again years later in New York after the 2011 performance of "Bengal Tiger at the Baghdad Zoo," a play Williams acted in. After the show, Williams signed a photo of the pair of them together on the set. Hide Caption 3 of 14 Photos: I met Robin Williams Aurora Contreras of the U.S. Navy says meeting Robin Williams aboard the USS Harry S. Truman was one of her fondest memories from her 2004 deployment in the Persian Gulf. Contreras is shown kneeling to the left of the soldier whom Williams was standing behind. "To see him there was a major highlight of our time onboard," she said. Hide Caption 4 of 14 Photos: I met Robin Williams Eric Forst met Williams and former U.S. astronaut Buzz Aldrin at a charity function in Canada in 2005. He loved watching Williams and Aldrin interact. "(Williams) was this humble person who found himself speechless when another American icon had asked him for his autograph," he said. Hide Caption 5 of 14 Photos: I met Robin Williams After finishing a set at Los Angeles' Comedy Store in 2006, Kirk Bovill realized Williams was watching from the back entrance. Their producer invited Williams to come on stage, and he did a set with the whole group. Hide Caption 6 of 14 Photos: I met Robin Williams In 1996, Mark Webb was working with Robin Williams on the movie "Father's Day." Webb's son, Tyler, was playing with his Genie puppet on set when Williams walked over and did an impromptu puppet show for the boy and the 300 people who had gathered to watch. Hide Caption 7 of 14 Photos: I met Robin Williams Todd Tuttle took what he called "a special selfie" with Williams in 1999. Tuttle ran into Williams at the Los Angeles restaurant Spago. Afterward, he said he witnessed Williams giving money to a homeless man nearby. Hide Caption 8 of 14 Photos: I met Robin Williams Nancy Marron-Asti met Williams while working on the set of the 1989 film "Dead Poets Society." On set, the pair shared jokes, laughs and family stories with one another. After filming ended, she was inspired to go into teaching. Hide Caption 9 of 14 Photos: I met Robin Williams At age 9, Stephanie Wilson , left, met Robin Williams while at a camp for sick children. In 1999, she got to perform with the comedian at a fundraising gala. "He was hilarious. I wish he had known how much we appreciated him," she said. Hide Caption 10 of 14 Photos: I met Robin Williams Serni Solidarios ran into Robin Williams in 1986 on the streets of New York and asked him if he would be interested in performing at a university concert. Months later Williams honored the invitation. "I've staged other great legends, but Robin's zaniness and sheer improvisational speed was unmatched," he said. Hide Caption 11 of 14 Photos: I met Robin Williams During the filming of 1991's "Hook," Devra Hamilton's son Daryl played one of the "Lost Boys." She and her family met Williams while on set. "He struck me as a man who was kind and cared about children," she said. Hide Caption 12 of 14 Photos: I met Robin Williams While stationed in Pakistan in 2002, "lifelong fan" Del Dayrit was thrilled to meet the comedic actor. Although they only spoke briefly, Dayrit said he could tell "how genuine and sincere [Williams] is as a human being." Hide Caption 13 of 14 Photos: I met Robin Williams After being diagnosed with a rare form of cancer, David Buist , pictured here in 2004, connected with Williams through his uncle, getting occasional calls from the actor while in treatment. The two met in person after Williams invited Buist to a stand-up performance. Hide Caption 14 of 14

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.'"