Robin Williams was in the early stages of Parkinson's disease, his widow, Susan Schneider, said Thursday. She also said the comedian was sober at the time he took his life on Monday.
"Robin's sobriety was intact and he was brave as he struggled with his own battles of depression [and] anxiety as well as early stages of Parkinson's disease, which he was not yet ready to share publicly," Schneider said in a statement. "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."
Acknowledging the tributes from celebrities, political figures and fans worldwide, she said: "Robin spent so much of his life helping others. Whether he was entertaining millions on stage, film or television, our troops on the frontlines, or comforting a sick child -- Robin wanted us to laugh and to feel less afraid.
"Since his passing," she continued, "all of us who loved Robin have found some solace in the tremendous outpouring of affection and admiration for him from the millions of people whose lives he touched. His greatest legacy, besides his three children, is the joy and happiness he offered to others, particularly to those fighting personal battles."
Schneider did not offer details on when the actor had been diagnosed or his symptoms.
Parkinson's disease is an incurable nervous system disorder that involves a loss of brain cells controlling movement. Tremors, sometimes starting out in just one hand, are among the early symptoms.
It can also cause rigid, halting walking, slowed speech and sometimes dementia. Symptoms worsen over time and can often be treated with drugs.
Actor Michael J. Fox, who has long had the disease, is known for his efforts to fund research into it. Fox wrote on Twitter Thursday: "Stunned to learn Robin had PD. Pretty sure his support for our Fdn [foundation] predated his diagnosis. A true friend; I wish him peace."
Pop star Linda Ronstadt revealed in 2013 that she had Parkinson's and said the disease had robbed her of her ability to sing. 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 1 million people nationwide, 6 million globally. The cause isn't known, but genetics 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 in which the two diseases occurred.
She noted that many can live for years without severely debilitating symptoms, but also that 20 years after diagnosis, as many as 80 percent develop dementia. Antidepressants are among drugs commonly prescribed for Parkinson's, along with medication to help control jerky movement.
Williams had publicly acknowledged periodic struggles with substance abuse, including alcohol. Recently, depression prompted him to enter rehab.
Meanwhile, Williams' publicist, Mara Buxbaum, disputed reports that the "Good Will Hunting" Oscar-winner had financial issues that may have contributed to his acknowledged depression.
"Robin had no financial problems," she told TheWrap.com. "We should be blessed to have Robin's financial status."