A sniff test may eventually help identify if a person is at high risk for Parkinson’s disease, according to researchers.
In an analysis of data from the Health, Aging, and Body Composition (Health ABC) study, having a lower Brief Smell Identification Test (BSIT) score was associated with higher disease risk, reported Honglei Chen, MD, PhD, of the National Institute of Environmental Health Sciences in Research Triangle Park, N.C., and colleagues.
The association appeared to be stronger in white than in black participants, and in men than in women, the authors reported wrote in Neurology.
“Poor olfaction predicts Parkinson’s in short and intermediate terms,” they wrote, adding that “the possibility of stronger associations among men and white participants warrants further investigation.”
During an average follow-up of 9.8 years, 42 incident Parkinson’s disease cases were identified in 30 white and 12 black participants. A significant association between sense of smell and disease was observed within the first 5 years of follow-up (HR 4.2), and continued even after adjustment for factors known to increase risk for Parkinson’s disease, such as smoking, coffee consumption, and head trauma (HR 4.1).
When compared with patients with the best sense of smell who had scores in the highest tertile of the BSIT, those in the second tertile had a greater risk of disease (HR 1.3), and those who had the poorest sense of smell and scored in the lowest tertile had the highest risk (HR 4.8).
Although recent studies have demonstrated a link between poor olfaction and incident Parkinson’s disease in Asian and white people, this study is one of the first to look at the association in black people, the authors said.
In general, participants with the lowest BSIT scores tended to be older, male, black, smokers, and less likely to report a post-secondary education or an optimal health status. However, a poor sense of smell appeared to be less predictive of Parkinson’s disease development in black than in white people.
“We found no statistical significance for a link between poor sense of smell and Parkinson’s disease in blacks but that may have been due to the small sample size,” said Chen, who is also with the Michigan State University College of Human Medicine in East Lansing, said in a statement. “More research is needed to further investigate a possible link.”
He also pointed out that earlier studies have demonstrated that Parkinson’s disease could be predicted about 4 to 5 years after the smell test was taken. “Our study shows that this test may be able to inform the risk much earlier than that,” he said.
Chen’s group evaluated BSIT results from 2,544 ABC study participants at the year 3 clinical examination in 1999-2000. All were able-bodied individuals living in the metropolitan areas of Pittsburgh, Pennsylvania, and Memphis, Tenn. Almost half were male, slightly more than 40% were black, and the average age was 75 years. The study included 1,510 white participants and 952 black participants.
After scratching and smelling 12 individual odorants, participants were asked to identify smells such as cinnamon, lemon, gasoline, soap, and onion from four multiple-choice answers. Several data sources were then used to adjudicate Parkinson’s disease cases identified through Aug. 31, 2012.
In an accompanying editorial, Gene Bowman, MD, MPH, of the Nestlé Institute of Health Sciences in Lausanne, Switzerland, noted that the current study has the longest follow-up to date, and includes important data on African Americans.
“Biomarkers that can assist in the differential diagnoses, detect a prodromal neurodegenerative disease early on, and enrich clinical trials with an at-risk population are attractive and require coordinated efforts in Parkinson’s research and development,” Bowman wrote.
Hyposmia can be caused by neurodegeneration of the olfactory bulb and may be the earliest nonmotor symptom of Parkinson’s disease, he pointed out, noting that in sporadic Parkinson’s, the prevalence of olfactory dysfunction is 90%. However, hyposmia can also be caused by chronic rhinosinusitis, he cautioned.
More clarification is needed to explain why the incidence of Parkinson’s disease in the current study was lower in African-American participants even though the prevalence of hyposmia was higher, Bowman stated.
In addition, better insight into specific impairments on the smell tests — such as detection, identification, and intensity — “would help time-constrained clinicians focus on the most relevant tests.”
Before that happens, more research is needed to determine whether olfactory dysfunction might be a useful tool for screening the general population for prodromal or undiagnosed Parkinson’s disease, the authors said. Other key prodromal symptoms, biomarkers or risk factors “may help evaluate its potential to screen for prodromal Parkinson’s in high-risk populations.”
Study limitations included the fact that adjudication of Parkinson’s disease was retrospective and carried out at the end of the follow-up period, thus increasing the risk that some Parkinson’s disease cases may have been missed.
Also, the date of first evidence for a Parkinson’s disease diagnosis had to be estimated by the researchers so this increased the risk that some of the prevalent Parkinson’s disease cases at baseline might have been adjudicated as incident cases.
“For these reasons, interpretation of our results on the temporal relationship requires caution,” Chen’s group stated.
The study was funded by the National Institute on Aging (NIA), the National Institute of Nursing Research, the Intramural Research Program of the NIH, and the National Institute of Environmental Health Sciences.
Chen disclosed relevant relationships with the American Journal of Epidemiology, the International Journal of Molecular Epidemiology and Genetics, and the American Journal of Neurodegenerative Disease.
Bowman disclosed a relevant relationship with the Horizon 2020 European Union–funded PROPAG-AGEING project.
Robert Jasmer, MD Associate Clinical Professor of Medicine, University of California, San Francisco and Dorothy Caputo, MA, BSN, RN, Nurse Planner