TUESDAY, July 3, 2007 (HealthDay News) -- Losing your sense of smell might be an early sign of cognitive decline, a new study finds.
"It has been reported before that people who already have mild cognitive impairment have difficulty with odors," noted lead researcher Robert S. Wilson, a professor of neuropsychology at the Rush Alzheimer's Disease Center in Chicago.
However, "no one has started [a study] with people with no cognitive impairment at all," he said. But the new research, published in the July issue of the Archives of General Psychiatry, did just that.
In the study, Wilson's group enlisted 589 older people, averaging close to 80 years of age, to take a test in which 12 common odors were placed under their noses. The participants were scored from one to 12 on their ability to match a smell to one of four alternatives.
They then were evaluated once a year for five years in tests of neurological and cognitive function.
Over that time, 177 of the participants developed the kind of mild cognitive impairment that can be a warning sign of future Alzheimer's disease. The risk of developing such mental impairment was associated with a decrease in odor identification ability. Those who scored below the average score on the odor identification test were 50 percent more likely to develop mild cognitive impairment than those who scored above average, the researchers say.
The study results fit in with one leading theory surrounding the development of Alzheimer's disease, Wilson said. That notion centers on the idea that Alzheimer's begins with trouble in certain specialized areas of the brain, then spreads more widely until it involves the major thinking areas.
"There could be several years in which you wouldn't expect to see problems with thinking but problems with functions such as smell," he said.
The theory received lukewarm support from Dr. Sam Gandy, chairman of the Alzheimer's Association's National Scientific Advisory Council. Gandy is also director of the Farber Institute for Neurosciences at Thomas Jefferson University in Philadelphia.
"The idea is attractive, since the smell area of the brain is involved in the course of the disease," Gandy said.
And yet, "reports that problems with a person's sense of smell may be an early indicator of memory and thinking problems have appeared in the scientific literature at several occasions over the past 20 years," he pointed out. Still, "no observation has been particularly strong nor have they been replicated by other scientists," Gandy said.
More detailed studies are needed to assess the possibility of a connection between smelling ability and changes in mental function, he said.
"These tests should be tested on one group of people and then subjected to double-blind assessment in a second or third population before scientists begin to make claims that a particular smell test is associated with or predictive of imminent or incident cognitive dysfunction," he said.
But Richard L. Doty, director of the Smell and Taste Center of the University of Pennsylvania Medical Center in Philadelphia -- who developed the smell test used in the Wilson study -- said the test has already been used on a limited basis in clinical practice.
"It was given to relatives of persons with both Parkinson's disease and Alzheimer's disease who were normal," Doty said. "It showed that there was a loss of the ability to distinguish odors in those who eventually converted to the disease. Some neurologists have been using it in differential diagnosis."
The test used in the Rush study can be administered (or taken by the person being tested) in about five minutes, Wilson said. "But I'm not recommending that everyone run out and have this test administered," he said.
Diagnosis using smell testing or other methods would do a patient no real good, Wilson reasoned, because there's still no effective treatment to halt the progression of Alzheimer's disease. However, "the field is expecting that that will change in the next few years," he said.
Medications that target the accumulation of amyloid deposits in the brain, which many believe to be involved with the development of Alzheimers disease, are showing promise, Wilson said. "If these agents are effective in combating the accumulation of amyloid in the brain, they could alter the course of the disease," he said.
Detection of the condition in its earliest stage would then be desirable, because "the treatments should be applied as early as possible," Wilson said.
Even then, a smell test alone would not be the best early detection method, he stressed. "There are several other strategies looking at other ways to detect it early," Wilson said. "There would likely be a variety of methods, a smell test in conjunction with others."
There's more on Alzheimer's disease at the Alzheimer's Association.