On Thursday, the late actor's widow, Susan Schneider, revealed Williams had been struggling, before his death at age 63, with the early stages of the neurodegenerative disorder.
Michael J. Fox has said that he felt depressed and turned to alcohol when he was first diagnosed with the disease in 1991. "My first reaction to it was to start drinking heavily," he admitted last year. "I used to drink to party, but now I was drinking alone and … every day."
"Once I did that it was then about a year of like a knife fight in a closet, where I just didn't have my tools to deal with it," he said. Fox went to Alcoholics Anonymous and therapy to help him cope.
Fox Tweeted his reaction Thursday to the news of Williams's own struggle with the diagnosis, and noted that the Academy Award winner supported his foundation for Parkinson's research.
"Stunned to learn Robin had PD. Pretty sure his support for our Fdn predated his diagnosis. A true friend; I wish him peace."
Stunned to learn Robin had PD. Pretty sure his support for our Fdn predated his diagnosis. A true friend; I wish him peace.— Michael J. Fox (@realmikefox) August 14, 2014
The disease causes many patients to "suffer from major depression," according to Dr. Michael S. Okun, a professor at the University of Florida Center for Movement Disorders and Neurorestorations.
"In Parkinson's, you get a lot of degeneration in brain circuitry, dopamine, seratonin and epinephrin," says Okun, who also authored Parkinson's Treatment: 10 Secrets to a Happier Life. "[When you] start to take away some of those chemicals that help keep the balance from depression, you could expect … that the depression would be worse in scenarios where it's contributing to a pre-existing problem."
For patients like Williams, who openly struggled with severe depression throughout his life, "aggressive treatment" for depression caused by Parkinson's is of utmost importance.
"It's the largest unmet hurdle in the treatment of Parkinson's," explains Okun. "You feel hopeless and you don't feel like there's an answer, [but] if we're aggressive in these cases, we can often make patients feel better."
Treatment for depression caused by Parkinson's includes an "interdisciplinary approach" involving psychiatrists, neurologists and counseling psychologists, along with the antidepressants.
"It's important to keep in mind that there is a way to lead a more hopeful and happy life," says Okun. "People are living long lives with Parkinson's."
For more on Robin Williams's tragic death and his legacy as a comic genius, pick up the latest issue of PEOPLE, on newsstands Friday
If you or someone you know is considering suicide, please contact the National Suicide Prevention Lifeline at 1-800-273-TALK (8255).
The Williams family is asking well-wishers to send contributions to charities close to the actor's heart in lieu of flowers. Suggested organizations include St. Jude Children's Research Hospital, Challenged Athletes, USO, the Muhammad Ali Parkinson Center, the Christopher and Dana Reeve Foundation and Glide Memorial Church in San Francisco.