Objective: To evaluate the olfactory functions in the preoperative and postoperative period in Parkinson’s Disease (PD) patients who underwent deep brain stimulation (DBS) of bilateral subthalamic nucleus (STN)

Background: Although the mechanism of the olfactory disorders in the PD is not fully elucidated, abnormal proteins in the olfactory epithelium, cell loss and presence of the Lewy bodies in the olfactory bulb (OB) are the potential factors. The investigation of the obtained data from the limited number of studies revealed that STN DBS did not cause any acute change in the postoperative period, but the data related to the long-term follow-up was contradictive.

Methods: Fourteen PD patients were enrolled. Patients underwent Sniffin’ Sticks Test one week before bilateral STN DBS and the 3rd month of the operation (on medication/off stimulation). The odor threshold (OT), odor discrimination (OD) and odor identification (OI) scores were determined. Odor thresholds for n-butanol were assessed with triple-forced choice procedure. Odor discrimination was performed by means of triplets of odorants. Odor identification was assessed by means of 16 common odors. 

Results: Nine of the participating patients were male and 5 were female. The mean age was 61.4±10.4 years. The mean duration of the disease was 9.4±4.3 years. The preoperative OT and total olfactory score was significantly improved (p=0.026; p=0.028), OD and OI scores remained unchanged (p=0.505, p=0.140). 

Conclusions: The previous studies focused on the correlation of olfactory disorders and STN DBS showed that there was not a significant effect on OT, but OD and OI scores were significantly high in stimulation “on”. OI sores remained unchanged in an recent study. As distinct from these studies, we observed a significant improvement in OT scores. This finding might be evaluated from 3 aspects: 1-It was supposed that the OT score was mostly correlated with the OB and anterior olfactory nucleus (AON). AON is connected with the pyriform nucleus, amygdala and implicitly with thalamus. STN DBS does not only affect pallidal activity but also the functions of the thalamus 2-Post DBS neuroplasticity was  shown in the hippocampus and OB in animals 3-The modulation of the centrifugal control on the OB with the stimulation after STN DBS might make an improvement in the OT values. In conclusion, in this preliminary study it was shown that STN DBS has positive effects on the olfactory functions. 

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MDS Abstracts - https://www.mdsabstracts.org/abstract/deep-brain-stimulation-and-olfactory-function-in-parkinsons-disease/