Correlation among olfactory function, motors' symptoms, cognitive impairment, apathy, and fatigue in patients with Parkinson's disease

Although Parkinson's disease (PD) is usually considered as a movement disorder, it is strongly associated with non-motor symptoms (NMS), including smell and taste dysfunctions, cognitive impairment, apathy, fatigue, and autonomic dysregulation. Olfactory deficit is considered the most common NMS in PD preceding the motor symptoms for years. The aim of this study was to investigate olfactory function, cognitive impairment, apathy, and fatigue in patients with PD in comparison with healthy controls, and subsequently to analyse the correlations between these NMS and motor symptoms severity in subjects with PD. One hundred and forty-seven participants were enrolled (96 PD patients, mean age in years 67.5, SD 7.2; 51 healthy controls; mean age 65.1, SD 11.8). Olfactory function was evaluated using the Sniffin' Sticks test (odor detection threshold, discrimination and identification). The Montreal Cognitive Assessment (MoCA) was used to assess cognitive impairment. Apathy was examined by the self-report version of Starkstein Apathy Scale and fatigue was evaluated with the Parkinson's Disease Fatigue Scale. PD patients showed severe impairment in odor detection threshold, discrimination, and identification compared to healthy controls. Moreover, in PD patients, apathy and fatigue scores were significantly increased, while MoCA scores were decreased in comparison with controls. Multivariate linear regression analyses showed that both apathy and Unified PD Rating Scale (UPDRS) were associated with odor identification, discrimination and Threshold-Discrimination-Identification (TDI) score. In conclusion, our results reported changes in apathy and motor disability as significant predictors in alterations of odor identification, discrimination and TDI score. Furthermore, these data suggest that olfactory dysfunction might progress in tight relation with motor impairment UPDRS but also with non-motor symptoms such as apathy.