A summary of the forensic evaluation conducted on Williams' brain after he committed suicide notes that he had Lewy body dementia, a degenerative condition in which nerves cells in the brain are blocked by protein clumps (bodies) that interfere with function, USA Today reported.
About 1.3 million people have Lewy body dementia, according to the Lewy Body Dementia Association.
People with Lewy body dementia develop memory and language problems and also experience vivid hallucinations, Gayatri Devi, a neurologist and memory disorder specialist at Lenox Hill Hospital in New York, told USA Today.
However, there is no research showing that hallucinations caused by Lewy body dementia can lead to suicide, Devi said.
Williams died in his Tiburon, Calif. home in August. On Friday, the Marin County coroner said the official cause of death was suicide by hanging. Williams had no alcohol or drugs in his system and only normal levels of prescribed medications.
Williams suffered from chronic depression and was diagnosed with Parkinson's disease in November 2013.
Patients with Lewy body dementia are sometimes misdiagnosed with Parkinson's, which can lead to doctors to prescribe medications that make Lewy body dementia symptoms worse, according to Devi.
"Patients with Lewy body disease can hallucinate on their own but give them a Parkinson's drug and it can make hallucinations worse," she told USA Today. "And drugs used to treat Lewy body hallucinations can cause symptoms of Parkinson's."
While reports suggest that Williams' family believes Lewy body disease was a critical factor in his suicide, Devi said that would be unusual.
"Suicide does not usually occur in Lewy body dementia patients," Devi told USA Today. "I see a fair amount of it because Lewy body disease is the second-most-common kind of dementia after Alzheimer's."