Does failing sense of smell predict Alzheimer's?
On his website (www.doctoroz.com), Mehmet Oz, MD, host of TV's "The Dr. Oz Show," suggests that the loss of smell in older people may be an early
warning sign of Alzheimer's disease.i A recently published VA study, however, indicates that this may not
be the case.
"Smell tests have been touted as a possible way of predicting Alzheimer's dementia because of a reported association with decreased sense of smell," says
Gordon Sun, MD, a general otolaryngologist and Robert Wood Johnson/VA scholar at the Ann Arbor VA Medical Center and the University of Michigan. "Our team
set out to determine whether these beliefs are based on existing high-quality evidence."
Sun and his team looked at nearly 1,200 studies in the medical literature on the subject of smell testing and Alzheimer's disease in a review sponsored by
VA and the Robert Wood Johnson Foundation. Their study, titled "Olfactory identification testing as a predictor of the development of Alzheimer's dementia: a systematic review," appeared online May 2, 2012, on
the website of the journal Laryngoscope.ii
The team found 30 studies that compared the sense of smell in patients with Alzheimer's and in those without the disease, or with mild cognitive
impairment, a precursor condition. They also reviewed two longitudinal studies that looked at whether or not older patients who had lost their sense of
smell eventually developed Alzheimer's.
One of the two longitudinal studies found that people from 41 to 85 years old who had lost their sense of smell had about a 50 percent chance of developing
Alzheimer's during their lifetime. The other found that people who developed Alzheimer's tended to do worse on a smell test than those of the same age who
did not, but that study had too few patients at follow-up for more conclusive results.
The comparison studies found that people with Alzheimer's did worse on smell tests than people who did not have the illness or those whose cognition was
mildly impaired. But many studies did not take into account other factors besides the disease itself that might have caused that result, including age; a
decreased level of hydration; the thinning of the mucous lining in the nasal passages; and possible exposure to toxic agents.
Because of this, Sun and his team concluded that while there is an association between loss of smell and Alzheimer's, there is not enough evidence
to conclude that loss of smell can, by itself, predict if the disease is present. There are simply too many other reasons—including the aging process—that
might explain why people might lose their sense of smell for the lack of that ability to be definitively associated with the disease.
According to Sun, "A nonspecific association between poor smell function and Alzheimer's dementia is not the same as actually being able to use a smell
test to predict Alzheimer's. Unfortunately, this misinterpretation of the research has led to the promotion of these tests by the medical community and
public figures like Dr. Oz. This study helps set the record straight about where the evidence currently stands."
Sun believes clinical trials can resolve the issue. At present, however, no one has designed such a trial.
Alzheimer's disease is a type of dementia that causes problems with memory, thinking and behavior. The symptoms of Alzheimer's usually develop slowly and
get worse over time, eventually becoming severe enough to interfere with daily tasks. More than five million Americans today have an Alzheimer's diagnosis.
VA provides a full range of care for Veterans with Alzheimer's disease, including home-based primary care; homemakers and home health aides; respite care;
adult day care; and impatient hospital, nursing home or hospice care. VA's 20 Geriatric Research Education and Clinical Centers, or GRECCs, conduct basic
laboratory research on the origins of aging and the diseases commonly associated with aging, including Alzheimer's.
GH Sun, CA Raji, MP MacEachern, JF Burke, "Olfactory identification testing as a predictor of the development of Alzheimer's dementia: A systematic
review." Pub online: 2 May 2012, DOI: 10.1002/lary.23365.