Declining intellectual function among people over age 75 has been previously underestimated. For most people, it is an inevitable part of aging, according to a new study. Previous studies underestimated the rate of intellectual decline or dementia among older people, largely by excluding older adults who did not return for follow-up testing, the study said.
The study led by Dr. Carol Brayne, researcher at the University of Cambridge, England, used the standard Mini-Mental State Examination (MMSE) and evaluated cognitive functioning in 2,106 subjects age 75 and older. Follow-up examinations were scheduled two, six and nine years later. For subjects who completed all follow-up examinations, the average score decreased only slightly. But the number of participating subjects decreased dramatically over the years -- by the nine-year follow-up examination, only 368 participants remained in the study. The lower their initial MMSE score, the more likely subjects were to drop out before final examinations.
Previous studies appear to have underestimated the prevalence of cognitive decline in people age 75 and older by ignoring missing data from subjects who failed to return for follow-up examinations. When all study dropouts were accounted for, the rate of intellectual decline appeared greater.
"The main message of the study is that previous estimates of dementia are probably underestimated," said Dr. Marco Pahor, professor of medicine at Wake Forest University in Winston-Salem, N.C. "The actual prevalence is much higher than previously thought. The main reason is that those people who have a problem tend to drop out of the study. This is particularly relevant in older persons and persons with dementia."
Dementia will continue to be a major public health concern as the life expectancy of the population continues to increase, according to the study. In 1997, 2.32 million Americans were affected by Alzheimer's disease, which accounts for almost 50 percent of all cases of dementia. The annual number of new cases of Alzheimer's disease is expected to increase from 360,000 in 1997 to 1.14 million in 2047, a threefold increase.
The results of the study will have a powerful impact on future studies on aging, according to Pahor. "This is something we really need to watch carefully," he said. "It serves as a warning for future studies to really be careful for the dropout rate. Even a small dropout rate can make a big difference in the results of a study."
The study is published in the November issue of the Journal of the American Geriatrics Society. The researchers concluded: "Although we may wish that aging is not necessarily accompanied by cognitive decline, these data suggest that for the vast majority of current populations, it is."
