Olfactory Network Functional Connectivity as a Marker for Parkinson's Disease Severity.

Olfactory impairment was assessed in akinetic-rigid (PD) and tremor-predominant (PD) subtypes of Parkinson's disease (PD), classified based on motor symptoms. Seventeen PD, fifteen PD, and twenty-four cognitively normal (CN) participants completed the University of Pennsylvania Smell Identification Test (UPSIT). Groups were well-matched for age and demographic variables, with cognitive performance statistically controlled. Resting-state fMRI (rs-fMRI) and seed-based functional connectivity (FC) analyses were conducted to characterize olfactory network (ON) connectivity across groups. UPSIT scores were significantly lower in PD compared to PD. Consistently, ON FC values were reduced in PD relative to both PD and CN. FC of the primary olfactory cortex (POC) significantly differed between CN and the PD subtypes. Furthermore, connectivity in the orbitofrontal cortex and insula showed significant differences between PD and PD, as well as between PD and CN. Notably, ON FC between the left hippocampus and the posterior cingulate cortex (PCC) also differed significantly between PD and PD. These findings reveal distinct ON FC patterns across PD and PD subtypes. Variations in UPSIT scores suggest that motor symptom subtype is associated with olfactory performance. Moreover, ON connectivity closely paralleled the UPSIT scores, reinforcing a neural basis for olfactory deficits in PD. Given the accelerated motor and cognitive decline often observed in the PD, these results support the potential of olfactory impairment as a clinical marker for disease severity.