New research suggests older adults who have difficulty identifying common odors may have a greater risk of developing problems with thinking, learning and memory. The discovery may help scientists determine when individuals are developing mild cognitive impairments.

Mild cognitive impairment—or a decline in thinking, learning and memory abilities—is increasingly recognized as a precursor to Alzheimer’s disease, according to background information in the article.

Impairments in the ability to recognize odors have been associated with more rapid cognitive decline and also with the development transition from mild cognitive impairment to Alzheimer’s disease. However, little is known about factors that predict the development of mild cognitive impairment.

In the report, found in the July issue of Archives of General Psychiatry, Robert S. Wilson, Ph.D., of Rush University Medical Center, Chicago, and colleagues studied 589 older adults (average age 79.9) who did not have cognitive impairment in 1997.

At that time, the participants took a smell identification test, during which time 12 familiar odors were placed under their nose. They were asked to match each odor to one of four possible alternatives, and were scored from one to 12 based on the number of correct responses.

At the beginning of the study and again every year for up to five years, the participants underwent a clinical evaluation that included a medical history, neurological examination and testing of their cognitive function.

During the study, 177 individuals (30.1 percent) developed mild cognitive impairment. Risk of developing mild cognitive impairment increased as odor identification decreased, so that those who scored below average (eight) on the odor identification test were 50 percent more likely to develop the condition than those who scored above average (11).

This association did not change when stroke, smoking habits or other factors that might influence smell or cognitive ability were considered. Impaired odor identification was also associated with lower cognitive scores at the beginning of the study and with a more rapid decline in episodic memory (memory of past experiences), semantic memory (memory of words and symbols) and perceptual speed.

“The neurobiological bases of age-associated olfactory dysfunction are uncertain,” the authors write. Evidence suggests that even before the symptoms of Alzheimer’s disease develop, hallmark tangles develop in certain areas of the brain that may be associated with the processing of smells.

Because difficulty identifying odors is associated with other neurological diseases, including Parkinson’s disease, other mechanisms are likely involved.

“Further clinicopathological and clinicoradiological research on age-related olfactory dysfunction is needed,” they continue.

“Among older persons without manifest cognitive impairment, difficulty in identifying odors predicts subsequent development of mild cognitive impairment,” the authors conclude.

“The findings suggest that olfactory dysfunction can be an early manifestation of Alzheimer’s disease … and that olfactory assessment may be useful for early disease identification.”

Source: American Medical Association (AMA)