Robin Williams' family have revealed the actor had been living with an undetected health condition before he took his own life in 2014.
The star, who would have turned 70 a few days ago, was living with Lewy body dementia but didn't know about it. It was only discovered during the autopsy.
The condition - which can often be misdiagnosed as Alzheimer's or Parkinson's disease - can cause hallucinations as well as problems with understanding, thinking and memory.
Months before his passing, he'd been diagnosed with Parkinson's and it has been previously reported he had feelings of paranoia on the run up to his death.
The star's wife Susan Schneider Williams explained: "My husband had unknowingly been battling a deadly disease.
"Nearly every region of his brain was under attack. He experienced himself disintegrating."
Speaking to the journal Neurology, she went on to add: "Robin was losing his mind and he was aware of it. Can you imagine the pain he felt as he experienced himself disintegrating?"
She continued: "I experienced my brilliant husband being lucid with clear reasoning one minute and then, five minutes later, blank, lost in confusion."
Susan went on to compare the disease to a 'terrorist inside my husband's brain'.
The Mrs Doubtfire star took his own life in September 2014 at the age of 63. On what would have been his 70th birthday, his eldest son Zak shared a tribute to his father on Instagram.
Alongside a black and white picture of the star, he wrote: "Dad, on what would be your 70th birthday, I would want you to know that your incredible spirit lives within us.
"Our family will be celebrating you and your memory today. We miss you and love you always!"
Zak, a mental health advocate, also appeared on podcast The Genius Life to discuss his father's death, including the 'frustration' he saw over the actor's misdiagnosis of Parkinson's disease.
"What he was going through didn't match one to one (with what) many Parkinson's patients experience," he said. "So, I think that was hard for him."
Zak added: "There was a focus issue that frustrated him, there were issues associated with how he felt and also from a neurological perspective he didn't feel great. He was very uncomfortable."Featured Image Credit: PA