His wife Susan told AARP Magazine in an interview published on Monday that the 94-year-old singer, whose first hit "Because of You" was released in 1951, had been losing his ability to make decisions.

In an effort to keep working, Bennett had been hiding his diagnosis, she said. Singing has helped him with the disease.

Bennett remains upbeat but his condition is increasingly deteriorating, his wife said.

"He would ask me, 'What is Alzheimer's?' I would explain, but he wouldn't get it," his wife told AARP Magazine.

Gayatri Devi, a neurologist at Lenox Hill Hospital in Manhattan and author of "The Spectrum of Hope" about Alzheimer's, diagnosed Bennett in 2016.

Devi has strongly encouraged Bennett's family to keep him singing and performing for as long as he can enjoy it.

"It kept him on his toes and also stimulated his brain in a significant way," Devi is quoted as telling AARP Magazine.

The progressive disease, the most common form of age-related dementia, causes a decline in memory, thinking and reasoning skills, according to the Alzheimer's Association.

It leaves its victims dependent on caregivers, although Bennett so far has been spared the disorientation that can sometimes prompt patients to wander from home or experience terror, rage or depression, AARP Magazine said.

"He might never develop these symptoms. But there was little doubt that the disease had progressed. Even his increasingly rare moments of clarity and awareness reveal the depths of his debility," the magazine said.

Australian Associated Press