Objective: To determine if the primary olfactory cortex (POC) is involved in the olfactory deficits in early-stage Parkinson’s disease (PD) using functional magnetic resonance imaging (fMRI) and voxel-based morphometry (VBM).

Background: Olfactory dysfunction is prevalent in PD, with deficits in odor detection, identification, and discrimination¹. The central olfactory system is highly affected by PD pathology. Lewy body deposition, initiates in the olfactory bulb and anterior olfactory nucleus². To date, the cause of olfactory deficits in PD is still not clear. In this study, we sought to determine if there are functional or morphological changes in the POC in early-stage PD.

Methods: The POC functions related to odor-smelling and sniffing of 21 H&Y stage-1 idiopathic PD patients and 22 age/sex-matched healthy controls (HC) were studied with an olfactory stimulation paradigm on a 3T MRI system. The VBM was conducted based on a high-resolution T₁-weighted imaging of the whole brain. The smell detection threshold and smell identification ability of each participant were evaluated with the OLFACT-C Threshold Test and University of Pennsylvania Smell Identification Test. All participants gave written informed consent.

Results: Compared to the HCs, PD patients had significantly weaker smell identification ability and lower odor detection threshold (two sample t-tests, p < 0.001). Both odor-sniffing and odorless-sniffing induced significant activation in the POC of PD patients, which was comparable to that in HCs. In contrast to the significant odor-related activation in the bilateral POC of HCs, no significant odor-related POC activation was observed in PD at the same statistical threshold. The odor-related BOLD signal change in PD patients was significantly weaker in the right POC than that in the HCs (one-way ANOVA with age and sex as covariates, p < 0.001). There was no significant volumetric or morphological difference in the POC between PDs and HCs.

Conclusions: In early-stage PD, while there was no significant cortical atrophy or sniffing-related activation change in the POC, the odor-related POC activation was significantly reduced. Thus the impairment of smell detection and identification functions in early-stage PD is likely not due to the pathology in the POC, but due to the interference or even breakdown of the transferring of odor signals from the peripheral to the POC.

References: 1. Haehner A, Boesveldt S, Berendse HW, et al. Prevalence of smell loss in Parkinson’s disease–a multicenter study. Parkinsonism Relat Disord. 2009 Aug;15(7):490-4.

2. Braak H, Del Tredici K, Rub U, de Vos RA, Jansen Steur EN, Braak E. Staging of brain pathology related to sporadic Parkinson’s disease. Neurobiol Aging. 2003 Mar-Apr;24(2):197-211.

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MDS Abstracts - https://www.mdsabstracts.org/abstract/the-involvement-of-primary-olfactory-cortex-in-olfactory-deficits-in-early-stage-parkinsons-disease/