No standard test

Actor Michael J. Fox, who has long had the disease and is known for his efforts to fund research into it, tweeted that he was shocked to hear that Williams had early symptoms.

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

Boxer Muhammad Ali, the late radio personality Casey Kasem and the late Pope John Paul II are among other well-known figures diagnosed with the disease.

Parkinson's affects about 6 million people globally. The cause isn't known, but genes are thought to play a role.

There is no standard test for Parkinson's; doctors rely on symptoms, medical history and neurological exams to make the diagnosis.

Dr. Tanya Simuni, director of the Parkinson's Disease and Movement Disorders Center at Northwestern University's medical school in Chicago, said patients often react to the diagnosis with surprise and despair.

Depression is often present even in early stages and can sometimes precede tremors that help doctors make the diagnosis, Simuni said.

It's important to emphasize that not everyone who is depressed has Parkinson's or is likely to develop it, she said, especially given "this tragic case" involving Williams.

She said that many can live for years without severely debilitating symptoms but also that decades after diagnosis, as many as 80 percent of patients develop dementia. Antidepressants are among drugs commonly prescribed for the disease, along with medication to help control jerky movements.

Williams had publicly acknowledged periodic struggles with substance abuse, including alcohol. Recently, depression prompted him to enter rehab.

Schneider said that those who loved Williams are taking solace in the world's massive outpouring of affection and admiration for him.

"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,"she said.

Celebrities, fans mourn the Robin Williams' death	Chinese film fans mourn for star