People who will develop dementia may begin to lose awareness of their memory issues two to three years before the actual onset of the disease, according to new research published online by the journal Neurology.

“Lack of awareness of memory loss is common in dementia, but we haven’t known much about how common it is, when it develops or why some people seem more affected than others,” said study author Robert S. Wilson, PhD, of Rush University Medical Center in Chicago. “Most studies of memory unawareness in dementia have focused on people [in whom the disease had] already been diagnosed. In contrast, this new study began following older adults before they showed signs of dementia.”

Researchers analyzed data from 2,092 participants in three ongoing studies that each have followed older adults for more than 10 years. The participants, who were an average of 76 years old when the research began, underwent annual tests of memory and thinking abilities. They also were asked how often they had trouble remembering things and how they would rate their memory compared with 10 years earlier.

For the 239 people in whom dementia was diagnosed during the study, memory awareness was stable and then began to drop sharply an average of 2.6 years before the onset of dementia. This drop followed several years of memory decline.

“This study underscores the importance of family members looking for help from doctors, and doctors getting information from friends or family, when making decisions about whether a person has dementia, since people may be unable to give reliable reports about the history of their own memory and thinking abilities,” Wilson said.