Seniors
who can identify smells like roses, turpentine, paint-thinner and lemons, and
have retained their senses of hearing, vision and touch, may have half the risk
of developing dementia as their peers with marked sensory decline.
In a
study by UC San Francisco, researchers tracked close to 1,800 participants in
their seventies for a period of up to 10 years to see if their sensory
functioning correlated with the development of dementia. At the time of
enrollment, all participants were dementia-free, but 328 participants (18
percent) developed the condition over the course of the study.
Among
those whose sensory levels ranked in the middle range, 141 of the 328 (19
percent) developed dementia. This compares with 83 in the good range (12
percent) and 104 (27 percent) in the poor range, according to the study, which
publishes in Alzheimer's and Dementia:
The Journal of the Alzheimer's Association on July 20, 2020.
Previous
research has centered on the link between dementia and individual senses, but
the UCSF researchers' focus was on the additive effects of multiple impairments
in sensory function, which emerging evidence shows are a stronger indicator of
declining cognition.
"Sensory
impairments could be due to underlying neurodegeneration or the same disease
processes as those affecting cognition, such as stroke," said first author
Willa Brenowitz, PhD, of the UCSF Department of Psychiatry and Behavioral
Sciences, and the Weill Institute for Neurosciences. "Alternatively,
sensory impairments, particularly hearing and vision, may accelerate cognitive
decline, either directly impacting cognition or indirectly by increasing social
isolation, poor mobility and adverse mental health."
While
multiple impairments were key to the researchers work, the authors acknowledged
that a keen sense of smell, or olfaction, has a stronger association against
dementia than touch, hearing or vision. Participants whose smell declined by 10
percent had a 19 percent higher chance of dementia, versus a 1-to-3-percent
increased risk for corresponding declines in vision, hearing and touch.
"The
olfactory bulb, which is critical for smell, is affected fairly early on in the
course of the disease," said Brenowitz. "It's thought that smell may
be a preclinical indicator of dementia, while hearing and vision may have more
of a role in promoting dementia."
The
1,794 participants were recruited from a random sample of Medicare-eligible
adults in the Health, Aging and Body Composition study. Cognitive testing was
done at the beginning of the study and repeated every other year. Dementia was
defined by testing that showed a significant drop from baseline scores,
documented use of a dementia medication or hospitalization for dementia as a
primary or secondary diagnosis.
Multisensory
testing was done in the third-to-fifth year and included hearing (hearing aids
were not allowed), contrast-sensitivity tests for vision (glasses were
permitted), touch testing in which vibrations were measured in the big toe, and
smell, involving identifying distinctive odors like paint-thinner, roses,
lemons, onions and turpentine.
The
researchers found that participants who remained dementia-free generally had
higher cognition at enrollment and tended to have no sensory impairments. Those
in the middle range tended to have multiple mild impairments or a single
moderate-to-severe impairment. Participants at higher risk had multiple moderate-to-severe
impairments.
"We
found that with deteriorating multisensory functioning, the risk of cognitive
decline increased in a dose-response manner," said senior author Kristine
Yaffe, MD, of the UCSF departments of Psychiatry and Behavioral Sciences,
Epidemiology and Biostatistics, and Neurology, as well as the San Francisco VA
Health Care System. "Even mild or moderate sensory impairments across
multiple domains were associated with an increased risk of dementia, indicating
that people with poor multisensory function are a high-risk population that
could be targeted prior to dementia onset for intervention."
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 risks for dementia. The former
group was more likely to have completed high school (85 percent versus 72.1
percent), had less diabetes (16.9 percent versus 27.9 percent) and were marginally
less likely to have cardiovascular disease, high-blood pressure and stroke.
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