The study found that people in their 70s who were unable to identify odors from such items as roses, lemons, onions and turpentine were significantly more likely to develop dementia over the next decade than their peers who could depict the smells.
Declines in other sensory functions — vision, touch and hearing — were also found to be associated with an increased risk of dementia, but not nearly as strongly.
“The olfactory bulb, which is critical for smell, is affected fairly early on in the course of the disease,” says Willa Brenowitch, the study’s lead author and a neuroscientist at UCSF, in a released statement. “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.”
Brenowitch and her colleagues believe that assessing people’s sensory impairments “could be a useful tool to help identify or prevent dementia.”
An estimated 5 million adults aged 65 and older are living with dementia in the United States — a number that is expected to grow to 14 million by 2060, according to the Centers for Disease Control and Prevention (CDC).
For the study, Brenowitz and her colleagues followed 1,794 adults aged 70 to 79 for up to 10 years. None of the participants had dementia when they entered the study, but 328 (18 percent) developed the condition by the study’s end.
In addition to undergoing annual cognitive testing, the participants had four sensory functions — hearing, vision, touch and smell — assessed between years three and five of the study. (Eyeglasses were permitted during the tests, but not hearing aids). Based on the results of those assessments, each participant received an overall sensory function “score” from 0 to 12 (0-3 points for each sensory domain). The “best” score was 0 (no impairments).
The study found that the participants who remained dementia-free not only tended to score higher on the cognitive tests at the start of the study, their sensory functions tended to remain unimpaired as well. Specifically, people with a poor (high) multi-sensory function score were two times more likely to develop dementia over the course of the study than those with a good (low) score.
The risk rose as the impairment scores fell. Each point less on the 12-point assessment scale was associated with a 14 percent higher risk of dementia.
But loss of smell had the strongest effect on the risk. For each 10 percent decline in smell, the risk of dementia rose by 19 percent. That compared to a 1 percent to 3 percent higher risk for similar declines in vision, hearing and touch.
These findings held even after adjusting for underlying medical conditions.
The study did find, however, that people with good multisensory function were more likely to be healthier than those with poor function. They also tended to have better mobility and were less likely to be depressed. These findings suggest that healthy lifestyle habits, such as a nutritious diet, not smoking and exercising regularly — as well as the socioeconomic factors that promote healthier habits — may play a role in reducing the risk of dementia.
Limitations and implications
The study’s participants were all healthy at the start of the study and within a specific age range. The findings may not be generalizable, therefore, to younger (middle-aged) or older populations. Another limitation of the study is that the sensory functions were measured only once. The participants’ scores might have been different if they had been assessed on an alternative day.
Still, this isn’t the first study to link sensory decline with an increased risk of dementia (although it appears to be one of the first to try to establish an assessment tool for measuring that risk). The reasons for the association are not yet clear, however.
“Sensory impairments could be due to underlying neurodegeneration or the same disease process as those affecting cognition, such as cerebrovascular disease,” Brenowitz and her co-authors write. “Alternatively, sensory impairments, particularly those due to hearing or vision, may accelerate cognitive decline, either directly impacting cognitive load or indirectly through the effects of sensory impairment on other health-related outcomes.”
Of course, no one should jump to the conclusion that they’re developing dementia simply because they can no longer tell the difference between the odors of an onion and a rose. Many factors — ones having nothing to do with dementia — can trigger a loss of smell, including sinus infections, growths in the nasal cavities, dental problems and certain medications. This year, COVID-19 has been added to that list as well.
If a loss of smell persists, however, you should have it checked out by your doctor.