EXCLUSIVE: Diane Keaton's Secret Dementia Battle Revealed After Her Shocking Death at 79 — 'It Was Heartbreaking to See'

Shocking new details have emerged about the sudden death of beloved actress Diane Keaton – RadarOnline.com can reveal she was secretly battling hereditary dementia in her final months and that the final downswing was so rapid and severe that even her closest friends didn't realize what was happening.

Although the family announced that the final cause of death on October 11 at age 79 was from pneumonia, sources said loved ones chose to keep the news of the Annie Hall Oscar winner's debilitating brain disease from the public.

"She'd been struggling quietly for some time," one longtime friend said. "The people around her knew, but she didn't want the world to."

Insiders said she spent her last months at the ultra-exclusive Palm Springs retreat Smoke Tree Ranch to keep her decline secret from public view, and that it got so bad she didn't even recognize former lover and mentor Woody Allen, 89, when he and wife Soon-Yi Previn visited her this summer.

Signs of Keaton's struggle could be glimpsed in 2022 on the set of Maybe I Do, where crew members say she had her lines fed to her through an earpiece – which is how Bruce Willis, 70, managed to eke out his last few roles before his forced retirement from Hollywood, also in 2022, after a diagnosis of aphasia and frontotemporal dementia.

"It was heartbreaking to see," said a production source about the filming of Maybe I Do. "[Costar] William H. Macy was incredibly kind and supportive, but if there was a break for a meal or anything, it was like starting from scratch."

Songwriter Carole Bayer Sager said she was shocked by her longtime friend's appearance when they last saw each other after the California wildfires earlier this year.

"She had lost so much weight," says Sager. "She'd been in Palm Springs for months."

That Keaton allegedly succumbed to rapid cognitive decline is doubly sad – given that her mother, Dorothy, suffered from Alzheimer's for 15 years before dying in 2008.