Objective: To investigate the interaction between the olfactory and trigeminal systems in Parkinson’s disease (PD) as opposed to other forms of olfactory dysfunction and controls in order to identify a PD-specific pattern of alteration.

Background: Olfactory dysfunction (OD) is a highly frequent non-motor symptom appearing as an early biomarker of PD. However, it is not specific to PD, as 20% of the general population also exhibits it. Therefore, it is important to differentiate between PD-related OD and non-parkinsonian OD (NPOD) to use olfactory measures as early biomarkers to help early disease diagnosis. To do so, the trigeminal system, which is closely related to the olfactory system, and responsible for the perception of warmth, burning, stinging, and freshness sensations, holds particular promise. These systems convey complementary information that is merged via mutually enhancing and/or suppressive interactions, and these interactions appear to be differently affected in PD.

Method: We assessed the influence of olfactory-trigeminal co-stimulation on the localization of a pure trigeminal stimulus (which consists of identifying the stimulated nostril in 40 trials) in 18 PD patients (67.5 ± 8.13 years; 4 women), 20 NPOD patients (68.5 ± 7.1 years; 12 women), and age- and sex-matched controls. Using an olfactometer, we administered stimuli under four conditions: (1) CO₂ (pure trigeminal stimulus), (2) phenylethanol (PEA; pure olfactory stimulus), (3) PEA and CO2 in the same nostril (ipsilateral), and (4) PEA and CO₂ in opposite nostrils (contralateral). The intensities of the stimuli were 45% for PEA and 40% for CO₂. The brain responses were recorded using the 64-channel electroencephalography (EEG) system.

Results: Preliminary results show that ipsilateral olfactory co-stimulation enhances the localization of a pure trigeminal stimulus across all groups (p < 0.05). Ongoing analyses of event-related potentials will further investigate these interactions at the central level.

Conclusion: We show that ipsilateral olfactory co-localization enhances trigeminal perception across all groups, confirming an interaction between both systems. While behavioral results alone do not reveal a PD-specific pattern, electrophysiological measures will allow to better understand this interaction at the central level and how it may specifically be affected in PD.

References: Kobal, G., S. Van Toller, and T. Hummel. 1989. “Is There Directional Smelling?” Experientia 45 (2): 130–32. https://doi.org/10.1007/BF01954845.

Tremblay, Cécilia, Rosa Emrich, Annachiara Cavazzana, Lisa Klingelhoefer, Moritz D. Brandt, Thomas Hummel, Antje Haehner, and Johannes Frasnelli. 2019. “Specific Intranasal and Central Trigeminal Electrophysiological Responses in Parkinson’s Disease.” Journal of Neurology 266 (12): 2942–51. https://doi.org/10.1007/s00415-019-09517-4.

Tremblay, Cécilia, and Johannes Frasnelli. 2018. “Olfactory and Trigeminal Systems Interact in the Periphery.” Chemical Senses 43 (8): 611–16. https://doi.org/10.1093/chemse/bjy049.

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MDS Abstracts - https://www.mdsabstracts.org/abstract/olfactory-trigeminal-interactions-in-parkinsons-disease-compared-to-other-olfactory-disorders/