• Robb G. Best

More Senses, Lower Dementia Risk

Dementia is a devastating condition, and as our population ages, a looming fear for many. For decades, science has sought to better understand dementia and the factors surrounding it.

A recently concluded study by UC San Francisco followed 1,794 subjects, all in initially their seventies, and tracked them for ten years to try to better understand the relationship between their sensory functioning--their ability to see, hear, smell, taste, and touch--and whether or not they developed dementia. At the start, none of the 1,794 had the condition, but 328 people developed it over the course of the study. How did their sensory functioning fit into this picture?

Between the third and fifth year of the study, researchers tested each subject's hearing (sans hearing aids), sight (glasses were allowed), ability to sense vibrations via the big toe, and smell. In the smell test, participants had to identify the scent of things like paint thinner, lemons, onions, roses, and turpentine. Each subject's total results were considered good, middling, or poor.

Of those who scored poorly in their sensory tests, 27% developed dementia over the course of the decade. This compares with 12% in the good range and 19% in the middle range. In other words, those who demonstrated a fuzzier grasp on their senses were more than twice as likely to get dementia as those who maintained excellent sensory functioning. Why? We don't know yet.

It's possible that the senses could be harmed by the same neurodegeneration that brings about conditions like dementia, said first author Dr. Willa Brenowitz of the UCSF Department of Psychiatry and Behavioral Sciences, and the Weill Institute for Neurosciences. “Alternatively," she added, "sensory impairments, particularly hearing and vision, may accelerate cognitive decline, either directly impacting cognition or indirectly by increasing social isolation, poor mobility and adverse mental health.”

However, it seems that having a keen sense of smell has a stronger association against dementia than sight or hearing. “The olfactory bulb, which is critical for smell, is affected fairly early on in the course of the disease,” Brenowitz said. “It’s thought that smell may be a preclinical indicator of dementia, while hearing and vision may have more of a role in promoting dementia.”

In general, those who didn't develop dementia were found to have higher cognition and no sensory impairments at the outset, while those in the middling range tended to have either a few mild sensory impairments or one fairly major one, and those at higher risk for dementia had multiple moderate-to-severe impairments.

There could also be confounding factors at play, however. Those with fewer sensory impairments also tended to be in better health than those with moderate-to-severe impairments, which suggests that lifestyle habits may also be connected.

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