Robin Williams’ widow has revealed that a little-known disease he was suffering from when he died was like a "terrorist within his brain".

The Oscar-winning actor took his own life in 2014 without knowing what was wrong with him.

Writing in the Neurology journal, Susan Schneider says her husband had been suffering from Lewy body disease, which has characteristics common to both Alzheimer's and Parkinson's. 

For all the people who judged Robin Williams, an essay by his wife revealing the full agony of his brain disease https://t.co/7njWc71Ptu

— WendyBrandes (@WendyBrandes) October 1, 2016

"How I wish he could have known why he was struggling, that it was not a weakness in his heart, spirit, or character," Ms Schneider writes. 

Describing the disease's progression, she says her husband's "larger than life spirit" was crushed by "skyrocketing" fear and anxiety.

There was also a "shuffling gait", "terrible insomnia" and a "loss of basic reasoning" and memory which began to affect his work. 

While filming Night At The Museum 3, Ms Schneider says: "Robin was having trouble remembering even one line for his scenes, while just three years prior he had played in a full five-month season of the Broadway production Bengal Tiger At The Baghdad Zoo, often doing two shows a day with hundreds of lines - and not one mistake."

She says: "This loss of memory and inability to control his anxiety was devastating to him."

In her essay, called The Terrorist Inside My Husband's Brain, Ms Schneider also recalls the last words they said to each other.

"When we retired for sleep, in our customary way, my husband said to me 'Goodnight, my love,' and waited for my familiar reply: 'Goodnight, my love'.