Six years after finishing his second term as the president of the United States, Ronald Reagan was diagnosed with Alzheimer's -- a devastating neurological disease that impairs memory, judgment and reasoning. But the former president's son, Ron Reagan, says he saw the early signs of Alzheimer's while his father was still in office.
"It wasn't anything that obvious. It wasn't like, 'Oh my God, he doesn't remember he's president,' Ron Reagan said in an exclusive interview with ABC News. "It was just, I had an inkling that there might be something going on."
Ron Reagan recounts what he calls the early signs in his new book, "My Father at 100: A Memoir."
Alzheimer's disease, which is estimated to affect up to 5.1 million people in the U.S. according to the National Institute on Aging, is an irreversible and progressive brain disease that affects a person's ability to carry out the simplest tasks of daily living. But subtler changes in memory and mood can signal the disease's early stages.
"Most commonly people complain of short term memory issues," said Dr. Gary Small, director of the University of California, Los Angeles Center on Aging. Forgetting plans and having trouble remembering names or words -- the so-called 'tip of the tongue' phenomenon -- are common early symptoms. And although they might not interfere with someone's job in the beginning, they will as they worsen.
"If it really is early Alzheimer's and it progresses over the years, the person's memory and cognitive ability become more impaired," said Dr. David Loewenstein, chief of psychiatry at the University of Miami. This can affect a person's attention to detail and their ability to keep track of situations and react accordingly -- all of which affect a person's ability to do their job. For Ronald Reagan, the job was running the country.
Ron Reagan said his father's confusion during a 1984 debate -- just three years in to what would be an eight year presidency -- prompted his concern.
"There was just something that was off. I couldn't quite put my finger on it," Ron Reagan told "20/20's" Elizabeth Vargas.
Ron Reagan's half-brother, Michael Reagan, has publicly rejected the notion that their father had symptoms of Alzheimer's during his tenure as president.
"Look what he accomplished in the last four years of his presidency: Reykjavik, START agreements, all the things he accomplished. The speech at the Berlin Wall in 1987 on June 12th," Michael Reagan said in an interview on CBS' "The Early Show." "Someone with dementia does not accomplish all of those things."
But depending on the level of support people have in organizing their daily lives, early symptoms of Alzheimer's may go unnoticed, Loewenstein said.
"A lot of people in very high positions -- not just presidents -- are surrounded by people who organize their lives and cover for them," Loewenstein said. "I've seen cases where people are, frankly, demented and actually very impaired in doing their job, but they're covered for so successfully by their staff."
Whether the former president himself noticed any early changes is unclear. Ron wrote in his book: "I've seen no evidence that my father (or anyone else) was aware of his medical condition while he was in office."
"A lot of people do notice changes and get upset," Loewenstein said. "But there are others who don't notice them at all. The changes are really seen by those around them. We don't know what other colleagues saw and we may never know. But even when certain people are aware, they tend to downplay it."
Age is a risk factor for Alzheimer's. And at 69, Ronald Reagan was the oldest man to be elected president. During his campaign, he pledged to resign if he became "senile" – a term that refers to age-related dementia – while in office.
But without a clinical evaluation by a neuropsychologist, Alzheimer's disease in its early stages is difficult to detect, Loewenstein said.
"We're coming up with better biomarkers. And in the future, we may have better medical tests," he said.
But if you do notice cognitive changes in yourself or someone else, talking to a doctor early can make a significant difference.
"Even though there's no cure, there are treatments. The earlier you get started, the better the outcome," said Dr. Small.