Do you smell the roses? Lilacs in spring rain? The alarming odors of things burning or rotting? The answer may be more important than you think. Scientists are discovering that an impaired sense of smell is one of the earliest signs of Alzheimer's and Parkinson's and other neurodegenerative diseases.

The Quality Standards Subcommittee of the American Academy of Neurology has endorsed smell testing as an aid to the diagnoses of these diseases, writes Richard L. Doty, though such testing is still not routinely performed in neurology clinics. In an article in The Scientist, Doty, director of the Smell and Taste Center at the University of Pennsylvania's Perelman School of Medicine, describes recent research that shows difficulty smelling - a condition called hyposmia - is often an important early warning signal. He cites a pioneering study by Amy Bornstein Graves and colleagues at the University of South Florida who administered smell tests to 1,604 senior citizens who had no symptoms of dementia. Overall, people who had no sense of smell and one genetic risk factor for dementia were five times more likely to develop cognitive decline in the next two years than people whose sense of smell was not impaired. Further, Doty notes, the smell test was more predictive than cognitive test scores.

Doty, who has developed smell and taste tests, writes that olfactory test results can help doctors with diagnosis and treatment. Alzheimer's and Parkinson's diseases (AD and PD) are often misdiagnosed in patients suffering from other afflictions, including severe depression or supranuclear palsy, which are not accompanied by loss of smell and are not helped by drugs used to treat AD and PD. In some patients with mild AD, he adds, smell tests can indicate responsiveness to a drug that does improve cognitive function in some patients.

Is olfactory dysfunction the result of damage that comes with neurodegenerative diseases, or does loss of smell precede the damage? Can damage to the olfactory system induce disease in those disposed to neurodegenerative disorders? Doty says further research is needed to answer those questions, and further an understanding of the relationship between smell and health. Watch Doty's slide presentation on the sense of smell. He begins it with a picture of a Lady and the Unicorn tapestry showing the lady weaving a garland of carnations to illustrate the sense of smell. Five of the fifteenth century tapestries depict the five senses and a sixth is believe to represent love or understanding. 

Doty's article is one of several in The Scientist issue devoted to examining our sense of smell. Another by Ron Yu discusses pheromones. These elusive molecules, and the scents associated with them, are known to influence mating and other behavior in insects and some mammals. When it comes to human behavior, there's disagreement. If pheromones do exist in humans, the molecular machinery that would make them work is not clear. There is also evidence that smells can leave afterimages in the brain, even after the stimulus is no longer present, that influence memory. Marcel Proust, remembering the madeleines of his childhood, wrote that tastes and smells of the past "remain poised a long time, like souls, ..."

"Smell is a potent wizard that transports you across thousands of miles and all the years you have lived." Helen Keller

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