In one of her first interviews since the actor's passing, Susan Williams has shared with PEOPLE the real reason behind his death. According to the 51-year-old, Robin suffered from a debilitating brain disease called Diffuse Lewy Body Dementia or Dementia with Lewy bodies (DLB).
"Depression was one of let's call it 50 symptoms and it was a small one," says Susan.
According to Alzheimer's Society, Diffuse Lewy Body Dementia is often misdiagnosed and is a type of dementia that shares symptoms with both Alzheimer's disease and Parkinson's disease.
It causes problems with memory and judgement, recurrent visual hallucinations and sleep disturbances.
The disease took a toll on the Mrs Doubtfire actor in the last year of this life, but symptoms worsened in the months leading up to his death.
"I know now the doctors, the whole team was doing exactly the right things," Susan tells PEOPLE. "It's just that this disease was faster than us and bigger than us. We would have gotten there eventually."
Susan first spoke out about the Robin's passing this week, in a interview with ABC news. In the one-on-one, which will be aired on Good Morning America on Tuesday, the graphic designer talks about her relationship with the actor. "It's the best love I ever dreamed of," she says, smiling fondly.
Recounting the awful moment that she discovered her husband had died, she says: "I just screamed 'Robin! What happened? What did you do?'"
The voiceover for the preview also reveals that the actor’s third wife will share never before seen home videos of the couple’s wedding and snapshots of the life that they shared together.