Olfactory dysfunction and parasympathetic dysautonomia in Parkinson's disease

Objective Olfactory impairment occurs early in Parkinson's disease (PD), as may dysautonomia. We investigated the relationship between olfaction and dysautonomia as well as other non-motor manifestations of PD. Methods Olfaction [University of Pennsylvania Smell Identification Test (UPSIT)], autonomic function in the pupillary (constriction and redilation velocity) and cardiac systems (resting low- and high-frequency heart rate variability (LF and HF HRV), positional changes in systolic blood pressure), neuropsychiatric function [Mini-mental Status Exam (MMSE)], Hamilton Depression Scale, activities of daily living [(ADLs), Schwab and England ADLs scale], quality of life [Short Form-36 health survey, PD Questionnaire 39 (PDQ-39)], and other non-motor symptoms [Non-motor Symptoms Scale (NMSS)] were simultaneously assessed in 33 participants (15 PD, 18 controls). Group comparisons, Spearman's coefficients and non-parametric rank-based regression were employed to characterize relationships between olfaction and non-motor features. Results Smell scores were lower in the PD group and correlated positively with pupil constriction velocity and HF HRV. Smell scores were correlated negatively with PDQ-39 and gastrointestinal items of the NMSS and positively with MMSE and Schwab and England ADLs. These correlated measures were not significant terms in regression models of smell scores in which age and PD diagnosis were significant and accounted for over half of the variability (R-squared 0.52-0.58). Interpretation This study suggests olfactory involvement occurs with parasympathetic dysautonomia in the pupillary and cardiovascular systems, involving both age-related and PD-related processes. Other non-motor features are concurrently involved, supporting the notion that aging and PD have widespread effects involving discrete portions of the autonomic and olfactory systems.