 -- The dementia that struck actor Robin Williams shortly before he committed suicide last year can lead to devastating symptoms that include vivid hallucinations and cognitive impairment, the kind of behavior his widow emotionally described in an exclusive interview with ABC News.

His widow, Susan Williams, is speaking out this week for the first time more than a year after his death and raising awareness of how Lewy body dementia had devastating effects on her husband in the weeks before he died.

"Lewy body dementia is what killed Robin," Williams said. "It’s what took his life and that’s what I spent the last year trying to get to the bottom of, what took my husband’s life."

What Is Lewy Body Dementia?

Lewy body dementia results after specific protein bodies cause problems with thinking, mood, movement and behavior, according to the National Institute of Health.

It is fairly common and currently affects about 1 million people in the United States, according to the NIH. Typically, the disease strikes people at age 50 or older.

An autopsy revealed last year that the beloved actor had early-stage Parkinson's disease and Lewy body dementia.

Doctors examining autopsy reports told Susan Williams the disease progression was one of the worst they had ever seen.

What Are the Symptoms of Lewy Body Dementia?

Symptoms of Lewy body dementia (LBD) can be frightening for the patient, according to Dr. Dan Kaufer, director at the University of North Carolina Memory Disorders Program.

"With many different presentations, you can see dramatic effects in thinking, emotions and behavior," Kaufer told ABC News.

People with LBD sometimes appear confused and disoriented and exhibit unusual behavior, said Angela Taylor, the director of programming for the Lewy Body Dementia Association.

Those with this form of dementia can also have extremely graphic hallucinations that can include smell, as well as visual hallucination.

“The dementia usually leads to significant cognitive impairment that interferes with everyday life,” Taylor said last year when Williams’ autopsy was released, adding that people with LBD often struggle with tasks like eating, staying clean and paying bills.

Many with the diagnosis have Lilliputian hallucinations populated by small people or creatures. They usually don’t find these frightening, just very real, she said.

How Is the Disease Diagnosed?

Diagnosing the disease can be frustratingly difficult because in early stages a person's cognition can fluctuate between being aware and having hallucinations. Sometimes the diagnosis can only be confirmed after death, when tests reveal specific proteins deposits called Lewy bodies on the brain's nerve cells.

The autopsy of Williams’ brain showed Lewy bodies, as well as other brain changes that are the hallmarks of Alzheimer’s and Parkinson’s, according to his autopsy report last year.

Williams said her husband also had depression, anxiety and paranoia, and that his dementia symptoms had begun to worsen shortly before his death in August 2014.

"He was keeping it together the best that he could, but the last month he could not," she said. "It’s like the dam broke."

Williams said her husband initially started to report various symptoms including abdominal pain in the fall of 2013, but that it was unclear until May 2014 what his diagnosis was.

"It was this endless parade of symptoms," Williams recounted.

Williams said that as the symptoms worsened and with more tests planned, she thinks her husband was wanting to take control of his life and what happened to him.

"I think he was just saying no and I don’t blame him one bit," Williams said of her husband's suicide.