In sensory evaluations, the sense of smell is usually taken as an indicator of sensory abilities in general. These tests, in turn, could be used as markers for different pathologies, including dementia: one study found that if an older person retains their senses of hearing, sight and touch, then they may have half the risk of developing dementia than their peers with marked sensory impairment (Brenowitz et al., 2020).
Because it is important? Sensory impairments could accelerate the process of cognitive decline, directly or indirectly (for example, by increasing social isolation, poor mobility, and adverse mental health).
The researchers note that smell could be a preclinical indicator of dementia, because it is affected quite early in the course of the disease, while “hearing and vision may play a more important role in promoting dementia.”
Methodology: The research team followed 1,794 participants aged 60 years for up to 10 years. Their goal was to see if their sensory functioning correlated with the development of dementia. At enrollment, all participants were dementia-free, but 328 of them (18%) developed the condition over the course of the study.
The study focused on the additive effects of multiple impairments in sensory function, which emerging evidence shows are a strong predictor of declining cognition.
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Participants were recruited from a random sample of Medicare-eligible adults in the Health, Aging and Body Composition study. Cognitive testing was performed at the beginning of the study and repeated every two years. Dementia was defined by tests showing a significant drop in baseline scores, documented use of a dementia medication, or hospitalization for dementia as a primary or secondary diagnosis.
Multisensory testing was performed in the third to fifth year and included hearing (headphones were not allowed), contrast sensitivity testing for vision (glasses were allowed), touch tests in which vibrations in the big toe were measured of foot and smell, which involves identifying distinctive odors such as paint thinner, roses, lemons, onions and turpentine.
Results: the following data were collected from all participants:
- Among those who had sensory levels within the medium range, 19% developed dementia (that is: 141 of the 328 who made up the subgroup);
- Among those who were in the good range of sensory levels, 12% developed dementia (that is: 83 people)
- Of the people who were in the poor range, 27% developed dementia (that is: 104 subjects).
Findings: The research team found that participants who remained dementia-free generally had higher cognition at enrollment and tended not to have sensory disabilities. The sense of smell or acute smell has a stronger association with dementia than touch, hearing or sight.
More precisely: Participants whose sense of smell was reduced by 10% were 19% more likely to have dementia, compared with a 1% to 3% increase in risk related to declines in vision, hearing and touch. .
The 780 participants with good multisensory function were more likely to be healthier than the 499 participants with poor multisensory function, suggesting that some lifestyle habits may play a role in reducing dementia risks. The first group was more likely to have completed high school (85% vs. 72.1%), had less diabetes (16.9% vs. 27.9% of the second group), and was marginally less likely to have cardiovascular disease , high blood pressure and strokes.
Finally, the authors highlight that even mild to moderate multisensory deficiencies had a higher risk of dementia. This would mean that such a group of people are a high-risk population who could be subjects for early intervention.
Brenowitz, W.D., Kaup, A.R., & Yaffe, K. (2020). Incident dementia and faster rates of cognitive decline are associated with worse multisensory function summary scores. Alzheimer’s & Dementia: The Journal of the Alzheimer’s Association. https://doi.org/