Rapid Smell Loss a Biomarker of Alzheimer's Disease Risk?

Jennie Smith

August 03, 2022

Rapid deterioration in sense of smell is a strong predictor of both Alzheimer's-related cognitive impairment and loss of volume in specific brain regions linked to both Alzheimer's disease and smell, according to new research findings.

Olfactory dysfunction is common in late life and well documented among people with Alzheimer's disease. However, it was unknown whether faster olfactory decline predicts either onset of Alzheimer's disease or structural brain changes associated with Alzheimer's disease.

Dr Jayant Pinto

In a study published online in Alzheimer's and Dementia, Jayant M. Pinto, MD, and his colleagues at the University of Chicago Medical Center reported that among older adults with normal cognition at baseline, people who experienced rapid loss of sense of smell were more likely to be subsequently diagnosed with mild cognitive impairment (MCI) or dementia, compared with those who did not.

Participants were recruited from Rush University's Memory and Aging Project, a longitudinal cohort of older adults who undergo yearly cognitive and sensory exams, including a scratch test of 12 common smells to identify. The Rush study "was ahead of the curve in looking at smell," Pinto said in an interview. "It gave us a very valuable resource with which to attack these questions."

Pinto has long investigated links between smell and accelerated aging; in 2014 his group published the finding that olfactory dysfunction could predict death within 5 years in older adults, and in 2018 they reported that olfactory dysfunction could predict dementia.

Smell and Cognition Over Time

For the current study, Pinto said, "we were able to look at the question not just using a single point in time, but a more granular trajectory of smell loss. Measuring change year by year showed that the faster people's sense of smell declined, the more likely they were to be diagnosed with MCI or Alzheimer's disease."

Pinto and his colleagues evaluated results from 515 adults (mean age 76.6, 78% female, 94% White) with no cognitive impairment and at least 3 years of normal results on smell tests at baseline. The subjects were followed for a mean 8 years. One hundred subjects (19%) were diagnosed with MCI or dementia by the end of the study period. A subset of the cohort (n = 121) underwent structural magnetic resonance imaging (MRI) between their final smell tests and the study's end. Of these, most still had normal cognition; 17 individuals had MCI.

Patients' individual trajectories of smell loss were mapped as slopes. After adjusting for expected differences in age and sex, the investigators found steeper decline associated with greater risk of incident MCI or dementia (odds ratio, 1.89; 95% confidence interval, 1.26-2.90; P < .01). The risk was comparable to that of carrying an apo E ε4 allele, the key risk variant for late-onset Alzheimer's disease, but was independent of apo E status. The association was strongest among subjects younger than 76 years.

Olfactory Decline and Brain Volume

Pinto and his colleagues, including lead author Rachel R. Pacyna, a 4th-year medical student at the University of Chicago, also sought to identify brain volume changes corresponding with olfactory decline and Alzheimer's disease. The researchers hypothesized that certain brain regions not seen affected in Alzheimer's disease would remain unchanged regardless of olfactory status, but that regions associated with smell and Alzheimer's disease would see smaller volumes linked with olfactory decline.

Faster olfactory decline did predict lower gray matter volume in olfactory regions, even after controlling for apo E status and other known risk factors. Conversely, cognitively unimpaired patients undergoing MRI saw more gray matter volume in primary olfactory and temporal brain regions, compared with those with cognitive symptoms.

Taken together, the findings suggest that "change in sense of smell is better than looking at sense of smell at one time point," Pinto commented. "There are other reasons people have impaired sense of smell: car accidents, COVID, other viruses and infections. But if you identify on a time course those who are starting to lose it faster, these are the people on whom we need to focus."

Not Yet Diagnostic

More work needs to be done to establish thresholds for smell loss that could be useful in clinical or investigative settings as a marker of dementia risk, Pinto acknowledged. "Everyone gets their hearing tested; everyone gets their vision tested. It's not as easy to get your sense of smell tested. But this study is telling people that if we were to start measuring it routinely, we could actually use it."

Smell testing "could become a component of a diagnostic battery that includes things like genotyping and cerebrospinal fluid markers, but adds a little more information. It could be useful in clinical prevention trials to identify people at the highest risk, as smell loss presents quite a few years before MCI or Alzheimer's disease."

The investigators acknowledged that their findings need to be replicated in more diverse cohorts that better represent the Alzheimer's population in the United States. Another limitation of their study, they said, was that the method used to calculate the rate of olfactory decline "was based on slope of measured time points assuming linearity, which may oversimplify the complexity of olfactory changes in normal aging and during the preclinical Alzheimer's disease period." The study was funded by the National Institutes of Health. Pinto disclosed receiving consulting fees from Sanofi/Regeneron, Optinose, and Genentech not related to this work.

This article originally appeared on MDedge.com, part of the Medscape Professional Network.

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