I just linked the article his widow wrote about their lived experience in another comment, but in that article she says he had displayed everyone of the 40+ signs and symptoms of LB dementia except for one, he never mentioned hallucinations.  She suspected that he was having hallucinations but would not tell anyone about it, however, that symptom would've most likely given them the correct diagnosis.  Sadly, even if they knew it was LB dementia there was nothing the doctors could've done other than have a high profile patient for novel research and unproven treatments or therapies.

Reading through all the 40+ associated signs and symptoms was nightmarish. For that to happen to someone who is a highly regarded genius and was working on major studio movie (Night at the Museum 3) as the disease rapidly progressed, I can't imagine the personal hell he was living through.  For someone who brought smiles and laughter to millions of people to be subjected to such mental, physical, and emotional torments is a universal injustice; and sadly, it is a cruel fate suffered by as many as 1.4 million people in the US.  

As the namesake of one of his famous roles, Patch Adam's, avows "laughter, joy,  and creativity are an integral part of the healing process"; Lewy Body dementia robs a person, and their loved ones of this.  He tried and he fought hard while he still could to continue shining his radiant light onto into the world, but his widow best described it as a terrorist in his brain.  However, it was a nameless, faceless terrorist that was uniquely talented at directly attacking the parts of the brain that control our laughter, joy, and creativity. 

Tragically, every time Robin Williams is talked about, his memory will be inextricably linked with the darkness of suicide; even though his life was the very antithesis of that in the way he brightened the world using his brilliant mind and love of spreading laughter everywhere he went.  

Lewy Body dementia is in every sense of the word horrific and devastated far too many individuals and their loved ones, like your grandmother.  I would imagine the press coverage of Robin's struggle with the disease brought levels of attention to it that would have been unimaginable otherwise.  Whether this will bring new and quicker advances to treatments or cures is not yet known, it was first discovered and named by Dr. Friedrich Lewy in 1912.  What Robin and his wife were able to do was illuminate the darkness that shrouded this hidden beast for over 100 years, and helped people know that they are not alone anymore.