When Ronald Reagan ran for president in 1980, pundits and political opponents criticized him for making contradictory statements and described him as absent-minded and inattentive. The portrayals raised questions about the health, and particularly the mental faculties, of a man who at age 69 would become the oldest to be elected president. During the campaign, Mr. Reagan agreed to let me talk to all his doctors, examine his records and then interview him about his health.
On a plane from San Francisco to Denver, Mr. Reagan told me that his mother, Nellie, had been senile "for a few years before she died" of a stroke at age 80. Mr. Reagan said he fully expected his White House doctors to check his mental status, and he pledged to resign if he became senile while in office.
Then he asked me about senility. He, like many people, including doctors at the time, believed that senility usually developed as a complication of strokes. Strokes can cause senility, or dementia. And Alzheimer's disease was not the household term it is today.
(snip)
In 1980, despite his mother's history of senility, it was only a hypothetical possibility that Mr. Reagan would develop Alzheimer's. The disease's hereditary pattern was, and is, not precisely known. As it turned out, the disease is believed to have afflicted Mr. Reagan's brother, Neil, too. Whether their mother's dementia was from strokes or Alzheimer's, or both, is not known.
more…
http://nytimes.com/2004/06/15/health/15docs.html