Actor and comedian Robin Williams was battling early stages of Parkinson's disease, which he had not shared publicly before he was found dead from an apparent suicide Monday at his home in Tiburon, his wife said Thursday in a statement.

The revelation came as fans around the Bay Area and across the globe struggle to make sense of the death of the 63-year-old performer, who had fought drug and alcohol addiction and depression.

"Robin's sobriety was intact and he was brave as he struggled with his own battles of depression, anxiety as well as early stages of Parkinson's disease, which he was not yet ready to share publicly," his wife, Susan Schneider, said.

An estimated 1 million people in the United States have Parkinson's disease, a neurodegenerative brain disorder that progresses slowly and affects movement and mood, according to the National Parkinson's Foundation.

Tremors, slowed speech and sometimes dementia are among the symptoms of the incurable disease.

Several celebrities have waged public battles with Parkinson's, including singer Linda Ronstadt, who revealed in 2013 that she had the disease, and Michael J. Fox, who was diagnosed with Parkinson's in 1991 before publicly disclosing his condition in 1998.

"Stunned to learn Robin had PD. Pretty sure his support for our (foundation) predated his diagnosis. A true friend; I wish him peace," Fox tweeted Thursday.

Depression is sometimes a symptom of Parkinson's. Nearly 16 percent of adults reported experiencing major depression in the past year, but for Parkinson's patients that percentage is roughly twice as high, said Patricia Arean, a professor in psychiatry at UCSF and licensed clinical psychologist.

Effects on mood

In some cases, the disease influences the brain's ability to produce serotonin - a chemical thought to contribute to happiness.

"If he was already suffering from depression, the Parkinson's would probably make it worse or harder to treat," she said. "In his case, he already had several risk factors (for suicide). He was an older white man, he had a history of substance abuse, and a new illness that's known to affect mood - and a history of depression."

Older white men are more likely to take their own lives than any other demographic, and suicide rates among that group have shot up in the past decade, Arean said. Experts can't pinpoint one factor in the spike, but Arean said it might be linked to the stigma around admitting a struggle with a mood disorder.

Williams was last seen alive at 10:30 p.m. Sunday when Schneider went to bed, said Marin County sheriff's officials. On Monday, his personal assistant became concerned after he failed to answer knocks at his door, then discovered his body hanging from a belt around 11:45 a.m., said Lt. Keith Boyd, the assistant chief deputy coroner.

'Love and respect'

Schneider did not directly address the suspected suicide.

"Robin spent so much of his life helping others. Whether he was entertaining millions on stage, film or television, or troops on the front lines, or comforting a sick child - Robin wanted us to laugh and to feel less afraid," she said.

Williams' three children - Zachary, 31, Zelda, 25, and Cody Williams, 22 - had earlier released statements expressing sadness and confusion over their father's death. Cody Williams said there were "no words strong enough to describe the love and respect I have for my father." He expressed hope that he would see him in the afterlife.