Robin Williams and his daughter Zelda pose on arrival for the world premiere of the movie Happy Feet Two in Hollywood in this November 2011 file photo.

Reuters/Los Angeles

Robin Williams was suffering from the early stages of Parkinson's disease along with severe depression at the time of his apparent suicide, his widow said on Thursday, drawing public attention to the correlation between the diseases.

Although the gifted comedian had spoken before about his depression, Parkinson's experts have noted how the incurable and debilitating nervous system disorder that causes tremors and slowness of movement also affects people emotionally.

"The neurochemicals that are impacted by Parkinson's disease and the pathways that control motor functions are also integrally involved in the control of mood," said Dr Irene Richard, a neurology professor at the University of Rochester in New York.

More than half of those who suffer from Parkinson's also experience clinical depression, according to the National Parkinson Foundation, which advises all Parkinson's patients to be screened for depression.

The 63-year-old Oscar-winning comedic virtuoso, whose madcap style and dramatic versatility made him one of film and television's top stars, was found hanged at his home in Northern California on Monday.

Williams' widow, Susan Schneider, said the comedian "was not yet ready to share publicly" his struggles with Parkinson's, which affects about 1mn people in the US.

"It is our hope in the wake of Robin's tragic passing that others will find the strength to seek the care and support they need to treat whatever battles they are facing so they may feel less afraid," Schneider said in the statement.

Between 50,000 and 60,000 people are diagnosed with Parkinson's each year in the US. It typically affects people older than 50.

"We actually believe that (depression) is part of the disease itself. It's related to the brain dysfunction in Parkinson's disease," Richard said.

Actor Michael J Fox, boxer Muhammad Ali and singer Linda Ronstadt have all be diagnosed with Parkinson's. Ronstadt said last year that the disease had robbed her of her singing voice.

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

Williams, whose starring roles included Mrs. Doubtfire and Good Will Hunting, had been open about his struggles with alcohol and had gone to a Minnesota rehabilitation centre this summer to "fine-tune" his sobriety, his publicist said in July.

Schneider said Williams was sober when he died and was also suffering from anxiety.

Funeral arrangements are pending, and a full toxicology report will take two to six weeks, local officials said.