Olfactory impairment in Parkinson's disease is a consequence of central nervous system decline

Early diagnosis and timely treatment of Parkinson's disease are essential factors to provide these patients with a longer period of a better quality of life. Olfactory loss is among the first non-motor symptoms of the disease; however, in light of the many causes of smell loss, it is a very unspecific biomarker and should only be used as part of a diagnostic test battery. In this study, we investigated the olfactory response in 71 subjects, consisting of Parkinson's disease patients, hyposmic and anosmic patients of other causes, and normosmic individuals searching for sensitive, distinct biomarkers for which we used scalp event-related 64-channel electroencephalography and psychophysical tests. The analysis of the global field power indicated significant measurable differences between patients with Parkinson's disease and otherwise olfactory dysfunctional and normosmic individuals. The localization of brain sources, in particular, provides evidence for differences in mainly late EEG-components suggesting a decline of central brain networks as a causal factor for olfactory loss in Parkinson's disease. The findings indicate a different pattern of olfactory processing in patients with Parkinson's disease compared to olfactory dysfunctions of other origin, which provide further insights into the mechanisms behind olfactory dysfunction in Parkinson's.