CHICAGO, July 29 -- The rate of new cases of mild cognitive impairment in patients over 70 is higher than previously expected, results from the Mayo Clinic Study of Aging showed.

Initially healthy participants developed mild cognitive impairment at a rate of 5.3% a year (95% CI 4.3% to 6.5%), two to three times higher than the rate of new cases of dementia in the same population, Ronald Petersen, M.D., Ph.D., of the Mayo Clinic in Rochester, Minn., reported at the International Conference on Alzheimer's Disease here.

"If we extrapolate Alzheimer's incidence rates to mild cognitive impairment, we would expect perhaps 1% to 2% per year," said Dr. Petersen, who is also the vice chair of the Alzheimer's Association's medical and scientific advisory council, "but our findings were substantially higher than that."

The rate increased from 3.5% (95% CI 2.4% to 5%) in participants ages 70 to 79 to 7.2% (95% CI 5.5% to 9.3%) in those ages 80 to 89.

In addition, men were nearly twice as likely as women to develop mild cognitive impairment (HR 1.92, 95% CI 1.22 to 3.02).

Although mild cognitive impairment may represent a transitional phase to various forms of dementia, it is unknown how often it occurs in a population-based setting, the researchers said.

To find out, they turned to the Mayo Clinic Study on Aging, which randomly selected participants ages 70 to 89 from Olmsted County, Minn., in 2004; 1,786 participants were available for analysis.

Each participant underwent a baseline examination that included an assessment of cognitive function and a neurological exam. In addition, the researchers interviewed a close acquaintance of each participant. Follow-up was conducted in 15-month intervals.

The rates of mild cognitive impairment in men and women were 6.2% (95% CI 4.7% to 8.1%) and 4.4% (95% CI 3.1% to 6%), respectively.

Compared with women, men were more likely to have amnestic mild cognitive impairment (HR 2.00, 95% CI 1.12 to 3.57) as well as the non-amnestic subtype (HR 1.82, 95% CI 0.87 to 3.81), although the latter comparison did not reach statistical significance.

"These results underscore the urgency of developing new and better strategies to create disease-modifying therapies for Alzheimer's," Dr. Petersen said. "In addition, for public health purposes, we need to know how many people are cognitively impaired and potentially on the road to Alzheimer's."

Ralph Nixon, M.D., Ph.D., of New York University, and a member of the medical and scientific advisory council of the Alzheimer's Association, said, "This both magnifies the urgency of the problem of Alzheimer's disease and extends it to an even larger population but also gives some hope of early intervention that would target this population at an earlier stage of the disease."

According to Dr. Petersen, the findings may not be generalizable to populations outside of the predominantly white population of northern European descent of Olmsted County.

He noted, however, that the results of worldwide studies of mild cognitive impairment were not "drastically" different from those of the current study.

The findings are particularly important considering the shifting population distribution associated with aging baby boomers, Dr. Petersen said.

The study was supported by grants from the National Institute on Aging and by the Robert H. and Clarice Smith and Abigail Van Buren Alzheimer's Disease Research Program.

Dr. Peterson declared being a consultant for GE Healthcare and chair of the safety monitoring committee for Elan Pharmaceuticals.

His co-authors declared potential conflicts of interest with Elan Pharmaceuticals, sanofi-aventis, and Myriad.