Decline of olfactory function in Parkinson's disease: A ten-year longitudinal study

Abstract

Background Olfactory dysfunction is a prodromal sign of Parkinson's disease (PD) present in up to 90% of patients. However, it is unclear whether or not olfactory function worsens over the course of the disease. Objective In this study we examined whether the rate of decline of olfactory function in PD patients exceeds the expected age-related decline. Methods Olfactory function was tested in 90 PD patients at baseline (age at baseline 58.3 years, 68.9% males) and an average 10 years later using the 40-item University of Pennsylvania Smell Identification Test (UPSIT). To screen for concomitant cognitive deficits as a potential confounder, the Mini-Mental State Examination (MMSE) was used. Results At baseline, the mean UPSIT score was 22 points. Over the average 10-year follow-up period olfactory function decreased in 81.1% of PD patients, even in the youngest patients in whom no age-related decline was expected. The mean decrease was six UPSIT points ( p < 0.001), which exceeds the expected age-related decline derived from a previous study. When excluding patients with an MMSE score below 24, reflecting cognitive deficits that might interfere with olfactory test performance, UPSIT score still decreased by almost 7 points over the follow-up period. Conclusions Olfactory function in PD declines more rapidly with increasing disease duration than can be explained by aging or cognitive decline alone. As such, olfactory function appears to be a clinical marker of disease progression in PD that can be measured non-invasively and deserves consideration as part of multimodal phenotyping to monitor disease progression.