This is the latest piece of information to emerge as many try to figure out why Williams took his own life.
Dr. Ben Walter, with University Hospitals, said there's a strong relationship between Parkinson's and depression.
Williams' family released this statement following his suicide:
"Robin spent so much of his life helping others. Whether he was entertaining millions on stage, film or television, our troops on the frontline, or comforting a sick child- Robin wanted us to laugh and to feel lees afraid.
Since his passing, 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.
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.
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."
Williams had been active in the last year of his life, acting in four films that have yet to hit theaters. He checked into a rehab facility last month and authorities confirmed he was seeking treatment for depression.
"Other significant neurodegenerative diseases that have a lot worse prognosis, like, something like ALS or Lou Gehrig's disease, has a much less likelihood of causing depression. Depression in ALS is about 18 percent, where it's 50 percent for those with Parkinson's disease," said Walter.
It's not clear whether the early-stage Parkinson's disease affected Williams' ability to work.
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