Cognitive fluctuations, or episodes when train of thought temporarily is lost, are more likely to occur in older persons who are developing Alzheimer's disease than in their healthy peers, according to scientists at Washington University School of Medicine in St. Louis.

Cognitive fluctuations include excessive daytime sleepiness, staring into space and disorganized or illogical thinking.

"If you have these lapses, they don't by themselves mean that you have Alzheimer's," says senior author James Galvin, M.D., a Washington University neurologist at Barnes-Jewish Hospital. "Such lapses do occur in healthy older adults. But our results suggest that they are something your doctor needs to consider if he or she is evaluating you for problems with thinking and memory."

The study appears in the Jan. 19 issue of Neurology.

Earlier research had associated cognitive fluctuations with another form of dementia called dementia with Lewy bodies, but little information existed on the potential for links between Alzheimer's and such lapses.

Data for the new study came from Alzheimer's disease evaluations of 511 older adults with memory problems. Average age of the participants was 78. Researchers gave participants standard tests of thinking and memory skills. They also interviewed participants and a family member, checking for prolonged daytime sleepiness, drowsiness or lethargy in spite of sufficient sleep the night before, periods of disorganized or illogical thinking, or instances of staring into space for long periods of time.

A total of 12 percent of the participants had at least three of these symptoms, meeting the criteria for cognitive fluctuations. Those with mental lapses were 4.6 times more likely to be diagnosed with Alzheimer's. Of 216 diagnosed with very mild or mild dementia, 25 had mental lapses; of the 295 with no dementia, only two had mental lapses. In addition, participants with mental lapses did worse on tests of memory and thinking than people without mental lapses.

Of the 295 people with no dementia, only two had mental lapses.

"We have some ideas about why the biology of dementia with Lewy bodies causes these mental lapses, but nothing comparable for Alzheimer's," Galvin says. "It's possible that some of the patients who were diagnosed with Alzheimer's disease in this study will go on to develop dementia with Lewy bodies, but at the time of the study, they weren't showing any of the Lewy body dementia's core features."

Lewy body dementia, which causes clumps of proteins known as Lewy bodies to form in neurons, is thought to be the second most common form of dementia after Alzheimer's. Clinically, it can overlap with Parkinson's disease and Alzheimer's disease. Pronounced cognitive fluctuations are a hallmark of Lewy body dementia, as are visual hallucinations and rapid eye movement behavior sleep disorder.

Galvin suggests that further study is needed to determine the best way to include mental lapses in diagnostic procedures for Alzheimer's.

Funding from the National Institute on Aging supported this research.