Poor sense of smell associated with elevated Parkinson’s disease risk
Olfactory impairment in older adults may point to an elevated risk of developing Parkinson’s disease (PD), particularly in Caucasian men, a small US study found.
Individuals with a poorer sense of smell (demonstrated in this study as lower tertile [scores] of the Brief Smell Identification Test [BSIT]; t1 as the lowest tertile) had higher risks for developing PD than those with a better sense of smell (t3; hazard ratio [HR], 5.1, 95 percent confidence interval [CI], 2.1–11.9 for t1 and HR, 1.4, 95 percent CI, 0.5–3.8 for t2; ptrend=0.00001 for both comparisons vs t3). [Neurology 2017;89:1-7]
The association was more evident in men compared with women (HRt1/[t2 + t3], 5.4, 95 percent CI, 2.3–12.9 and HRt1/[t2 + t3], 2.9, 95 percent CI, 1.1–7.8, respectively), and in Caucasians compared with individuals of African ethnicity (HRt1/[t2 + t3], 4.9, 95 percent CI, 2.3–10.5 and HRt1/[t2 + t3], 2.5, 95 percent CI, 0.8–8.1, respectively).
“We found no statistical significance for a link between poor sense of smell and [PD in individuals of African ethnicity] but that may have been due to the small sample size,” said study author Dr Honglei Chen from the Michigan State University College of Human Medicine, East Lansing, Michigan, US.
“Reasons for this potential racial difference are unclear. One possibility is that, compared to [Caucasian] participants, the aetiology of olfactory dysfunction in [participants of African ethnicity] is more diverse and complex, and that PD-related pathology is a relatively minor contributor,” said the researchers. Another possibility is that the BSIT test, while developed for use in the general population, may work better in Caucasians, they said.
However, as previous studies have shown a lower incidence of PD among individuals of African ethnicity as well as a higher incidence of olfactory loss in individuals of African ethnicity and in women, [Am J Epidemiol 2003;157:1015-1022; Neuroepidemiology 2010;34:143-151; J Gerontol A Biol Sci Med Sci 2014;69:323-329; JAMA 2002;288:2307-2312] a poor sense of smell may be a less accurate predictor of PD in individuals of African ethnicity and in women, said the researchers.
While the association was detected in both the first 5 years of follow-up (HRt1/[t2 + t3], 4.2) and after (HRt1/[t2 + t3], 4.1), lag analyses that excluded the first years of follow-up demonstrated a reduced association with time.
Researchers used the BSIT test to identify sense of smell in 2,462 individuals aged 70–79 years (1,510 and 952 of Caucasian and African ethnicity, respectively) from the Health, Aging, and Body Composition study. There were 42 cases of Parkinson’s disease identified during the 9.8-year follow-up period, 30 and 12 in individuals of Caucasian and African ethnicity, respectively.
According to the researchers, olfactory dysfunction has been proposed as an identifier of prodromal PD. [Mov Disord 2015;30:1600-1611]
“However, its potential usefulness as a screening tool for prodromal or undiagnosed PD in the general population awaits further investigations, in part because of the low PD incidence, the lack of specificity of olfactory dysfunction to PD, and the lack of understanding of temporal relationship of olfactory dysfunction with PD,” they said.
Among the study limitations were potential missed cases of PD, documentation of prevalent cases as incident cases of PD, and as in the case of previous studies, the use of a one-time olfaction test.
“Future studies with repeated measurements over a longer period may help better characterize the temporal relationship of olfactory dysfunction with PD,” said the researchers. The results may also not extend to younger individuals, they cautioned.