Olfactory decline and brain volume
Dr. 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,” Dr. 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, Dr. 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. Dr. Pinto disclosed receiving consulting fees from Sanofi/Regeneron, Optinose, and Genentech not related to this work.