Smell Test May Identify Increased Risk of Parkinson’s Disease

A smell test may identify people at increased risk of incident Parkinson’s disease as many as 10 years before diagnosis, according to research published in the October 3 issue of Neurology. The association between poor olfaction and incident Parkinson’s disease may be stronger among men than among women, and among whites than among blacks, and this possibility requires further investigation, said the authors.

Honglei Chen, MD, PhD

“Earlier studies had shown prediction of Parkinson’s disease about four to five years after the smell test was taken,” said Honglei Chen, MD, PhD, Professor of Epidemiology and Biostatistics at the Michigan State University College of Human Medicine in East Lansing. “Our study shows that this test may be able to inform the risk much earlier than that.”

A Prospective Study of the Elderly

Dr. Chen and colleagues examined data for 1,510 white participants and 952 black participants in the Health, Aging, and Body Composition study. During clinical examinations in 1999 and 2000, the participants underwent the Brief Smell Identification Test (BSIT). The researchers followed the population until the date of Parkinson’s disease diagnosis, death, last contact, or August 31, 2012, whichever came first. Parkinson’s disease was diagnosed retrospectively using several data sources. Dr. Chen and colleagues used multivariable Cox models to estimate hazard ratios for Parkinson’s disease.

Participants’ mean age was 76. About 49% of participants were male, and approximately 39% of the population was black. During a mean follow-up duration of 9.8 years, the researchers identified 42 incident cases of Parkinson’s disease, including 30 white participants and 12 black participants.

Patients in the lowest tertile of BSIT scores (ie, those with the worst olfaction) were older and more likely to be male, black, and current smokers, compared with participants in the highest tertile of BSIT scores. Participants in the lowest BSIT tertile also were less likely to report education beyond high school or optimal health.

Olfaction Predicted Parkinson’s Disease

Overall, poor olfaction was associated with a higher risk of Parkinson’s disease. Compared with the highest BSIT tertile, the hazard ratio for Parkinson’s disease was 1.3 for the middle tertile and 4.8 for the lowest tertile. The association between olfaction and Parkinson’s disease was stronger among whites than among blacks, and stronger among men than among women.

Furthermore, the association between olfaction and Parkinson’s disease was significant for the first five years of follow-up, as well as for the period of follow-up after five years. In lagged analyses that excluded the first years of follow-up, the association remained similarly strong for the first six years of follow-up (hazard ratio, 4.1–5.0), after which point the hazard ratio decreased to 2.9.

“Previous studies have shown that black people are more likely to have a poor sense of smell than whites, and yet may be less likely to develop Parkinson’s disease,” said Dr. Chen. “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. More research is needed to further investigate a possible link.”

Several Factors Could Explain Hyposmia

“The observation that black participants have higher prevalence of hyposmia yet lower incidence of Parkinson’s disease needs further clarification to determine whether this observation is simply methodologic … or if there is a true biologic explanation,” said Gene L. Bowman, ND, MPH, Adjunct Assistant Professor of Neurology at Oregon Health and Science University in Portland, in an accompanying editorial.

One of the study’s limitations is that other factors besides neurodegeneration could explain hyposmia. The cause of the olfactory dysfunction observed in the study could be a subject for further research. “More granularity on the specific aspects of the smell tests that are impaired (eg, detection, identification, intensity) would help time-constrained clinicians focus on the most relevant tests,” Dr. Bowman concluded.

—Erik Greb

Suggested Reading

Bowman GL. Biomarkers for early detection of Parkinson disease: a scent of consistency with olfactory dysfunction. Neurology. 2017;89(14):1432-1434.

Chen H, Shrestha S, Huang X, et al. Olfaction and incident Parkinson disease in US white and black older adults. Neurology. 2017;89(14):1441-1447.