A sharp drop-off in the ability to smell could be a sign of dementia in otherwise healthy people, researchers at the University of Chicago have learned. Additionally, a worsening sense of smell can be linked to changes in regions of the brain that Alzheimer’s disease affects, suggesting that measuring smell over time may be a useful metric for diagnosis.
The team behind the new study, published Thursday in the journal Alzheimer’s & Dementia, analyzed data from 515 predominantly white patients who were studied as part of the Rush Memory and Aging Project. The patients in this dataset were given repeated smell tests typically over the course of several years; at the end of the study period, they received a clinical diagnosis of normal cognition, mild impairment, or Alzheimer’s. A subset of these patients also underwent MRI scans to measure gray matter volume in the brain, which is important in regulating the function of memories, movement, and emotions—and is known to decrease in people with Alzheimer’s.
The Chicago team wanted to determine whether the rate of olfactory decline affected a patient’s likelihood of receiving an Alzheimer’s or mild cognitive impairment diagnosis, and how it might be linked to gray matter volume.
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“Our idea was that people with a rapidly declining sense of smell over time would be in worse shape—and more likely to have brain problems and even Alzheimer's itself—than people who were slowly declining or maintaining a normal sense of smell,” lead author Rachel Pacyna, a University of Chicago medical student, said in a press release.
Participants were tasked with identifying odor samples of turpentine, chocolate, cinnamon, gasoline, lemon, onion, paint thinner, pineapple, rose, soap, smoke, and banana; and then received scores between 0 and 12 on each test for the number of scents they correctly guessed. The researchers calculated each individual’s rate of olfactory decline, then compared people who maintained normal cognition with those who ultimately were diagnosed with cognitive impairment or Alzheimer’s.
They found that people who experienced faster olfactory decline were nearly twice as likely to develop subsequent Alzheimer’s or cognitive impairment, and also exhibited smaller gray matter brain regions.
A smell test is unlikely to fully predict the onset of Alzheimer’s, the researchers stressed. Sense of smell, they argue, should be considered alongside the multiplex of factors affecting cognitive conditions like Alzheimer’s, which have been found to include diet and genetic predisposition.
Even so, it could become a useful, individualized sign for at-risk patients to monitor at younger ages than before. The researchers said that building off this work may even lead to new treatments and understanding of the complex condition.
“If we could identify people in their 40s, 50s and 60s who are at higher risk early on, we could potentially have enough information to enroll them into clinical trials and develop better medications,” Pacyna said.
