Is Losing Your Sense of Smell an Early Sign of Dementia?
Impaired olfactory function—a declining sense of smell—is associated with mild cognitive impairment (MCI) as well as Alzheimer’s disease (AD). A recent study reveals that olfactory functioning naturally weakens in healthy older adults and is impaired, in terms of odor identification, in people with neurodegenerative conditions such as MCI and AD.
Previous studies have shown that neurodegeneration can affect brain circuits that process olfactory information and can result in a diminished ability to identify odors.
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HOW DOES YOUR SENSE OF SMELL WORK?
According to the National Institutes of Health (NHI), your sense of smell—like your sense of taste—is part of your chemosensory system, or the chemical senses. Your ability to smell comes from specialized sensory cells, called olfactory sensory neurons, which are found in a small patch of tissue high inside the nose. These cells connect directly to the brain.
Each olfactory neuron has one odor receptor. Microscopic molecules released by substances around us stimulate these receptors. Once the neurons detect the molecules, they send messages to your brain, which identifies the smell.
Evidence: Olfactory Function as an Early Sign of Dementia
In a study conducted at Massachusetts General Hospital, published online in November 2016 in Annals of Neurology, principal investigator Mark Albers, MD, PhD, assistant professor of neurology at the Mass General Institute for Neurodegenerative Disease, wrote this summary: “There is increasing evidence that the neurodegeneration behind Alzheimer’s disease starts at least 10 years before the onset of memory symptoms.”
Developing an affordable, accessible, and noninvasive means to identify healthy people who are at risk is an important step to developing treatments that can slow or halt AD progression, he added.
Study Specifics
- Who they tested. Participants numbered 183, divided into four groups: 70 were cognitively normal, 74 tested normal on cognitive tests but were concerned about their cognitive abilities, 29 had mild cognitive impairment, and 10 had been diagnosed with possible Alzheimer’s disease.
- How they tested. Sense of smell was measured using an olfactometer. The first round of testing (OPID-10, Odor Percept IDentification) called for identifying 10 common scents: menthol, clove, leather, strawberry, lilac, pineapple, smoke, soap, grape, or lemon. In a second step, the OPID-20 test added 10 additional odors mixed with the first 10: banana, garlic, cherry, baby powder, grass, fruit punch, peach, chocolate, dirt, and orange.
- Findings. Results of the OPID-20 test showed significant differences among the four groups of participants, which correlated with the thinning of two brain regions: the hippocampus and the entorhinal cortex (both of which are involved in memory), regions which have previously been linked to Alzheimer’s risk.
In the second phase of the test, participants were asked to report whether the scents in the second set of 10 scents also had been present in the first set of 10 scents. The participants’ ability to remember the odors from the first test determined their POEM (Percepts of Odor Episodic Memory) score. Poor POEM performers were found more likely to have the variant of the APOE gene, which is linked with increased risk of AD. These participants also showed thinning of the entorhinal cortex.
The participants’ ability to remember a scent they’d previously smelled (POEM score) also showed meaningful differences between the two cognitively normal groups and the subjects with known Alzheimer’s disease.
What It All Means
“Early diagnosis and intervention are likely to produce the most effective therapeutic strategy for Alzheimer’s disease,” said Dr. Albers, noting that a larger study is planned.
“If these results hold up,” he added, “this sort of inexpensive, noninvasive screening could help us identify the best candidates for novel therapies,” to prevent progression of this disease.
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