Ask the Doctors

Dear Doctor: Could one's sense of smell really be used to predict Alzheimer's? I've never had a very good sense of smell, so does that mean I'm doomed?

Dear Reader: It's true that recent research has suggested a link between an impaired sense of smell and an increased likelihood of developing dementia. But before we get into the details, it's important to note the researchers agree that this dysfunction is not a risk factor for, and not a cause of, dementia. Instead, the results of the study suggest that an impaired sense of smell may be an early warning sign that someone is in the process of developing the condition.

In the study, which was published last year in the Journal of the American Geriatrics Society, about 3,000 participants between the ages of 57 and 85 had their sense of smell evaluated. They were asked to sniff a series of "odor pens," which look somewhat like a felt-tip marker. They were then asked to choose the scent they detected from four possible options. The five scents they were asked to identify were orange, rose, peppermint, fish and leather. A solid majority -- 78 percent -- correctly identified four out of the five scents. (Of the five scents, leather proved to be the most challenging.) Three scents were correctly identified by 13.8 percent of participants, 4.9 percent got two scents right, and 2.2 percent correctly named one scent. One percent of study participants were unable to identify any of the five scents.

Five years later, researchers conducted home visits with either the study participants themselves, or a proxy, such as a friend or relative, if the individual was too ill or had passed away. They discovered that those individuals who could not correctly identify at least four of the five scents were twice as likely to have developed dementia. There was also a correlation between the fewer scents identified and an increase in the likelihood of a dementia diagnosis.

What's going on? The researchers suspect that our sense of smell is so closely connected to brain function that when it declines, it can be an indicator of more serious and complex problems. Our sense of smell takes place via the olfactory nerve, which leads from the mechanisms in the nose that identify odors and connects directly to the brain. In addition, the scent sensors in the nose are constantly being replaced, with olfactory stem cells producing new cells when the old ones die off. This has led scientists to wonder whether the decrease in the ability to smell indicates that the brain's ability to rebuild certain vital components may be in decline and could be related to changes connected to dementia.

As for your own poor sense of smell, that appears to be a condition you've lived with for some time. As we mentioned earlier, flunking the smell test doesn't amount to either a risk factor or a cause of dementia. Instead, it means that in your case, the smell test would not be an appropriate diagnostic predictor.

(Send your questions to askthedoctors@mednet.ucla.edu, or write: Ask the Doctors, c/o Media Relations, UCLA Health, 924 Westwood Blvd., Suite 350, Los Angeles, CA, 90095. Owing to the volume of mail, personal replies cannot be provided.)

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