The brain responsiveness to chemosensory stimuli in Parkinson's disease

The relationship between Parkinson's disease (PD) and olfactory dysfunction has been investigated via psychophysical and electrophysiological assessments. Despite the increasing number of electrophysiological studies focusing on olfactory function, there are still some limitations to observe the chemosensory event-related potentials (CSERP), which are electrophysiological responses of the brain to olfactory and trigeminal stimulations, because of the low sensitivity (low signal-to-noise ratio). Recent studies attempted to establish new techniques to increase the sensitivity for evaluating the CSERP and brain responsiveness. We aimed to inspect CSERP via entropy analysis in assessing chemosensory related brain responses that has been used for the first time. Twelve newly diagnosed and non-medicated PD patients and 12 healthy subjects participated in the study. Psychophysical and electrophysiological evaluation of olfaction were assessed via Sniffin' Sticks Test (SST) and entropy analysis on CSERP in three time windows. The scores of odor threshold, odor identification and total scores of SST were lower (hyposmic) in PD patients compared to healthy subjects. Electrophysiological assessments revealed a significant change in entropy among time windows for olfactory stimulation with phenyl ethyl alcohol and trigeminal stimulation with carbon dioxide (both p < 0.05) in healthy subjects but not in PD patients. Entropy findings indicate that the brain operates in ordered state among healthy subjects in response to olfactory/trigeminal stimuli, whereas the PD patients displayed a chaotic pattern. This pattern in the PD patients suggests the lack of proper smell function. It should be studied if this pattern can be used as a biomarker for PD.