Functional Connectivity in the Sensory Olfactory Subnetwork in REM Sleep Behavior Disorder and Parkinson's Disease: A Pilot Study.

BACKGROUND: Parkinson's disease (PD) is a neurodegenerative disorder whose diagnostic motor symptoms appear only after significant progression of neurodegeneration. Identification of preclinical markers is essential. Idiopathic rapid eye movement sleep behavior disorder (iRBD) has a high risk of conversion to PD. Olfactory impairment (hyposmia) is present in both PD and iRBD; hyposmia in iRBD may be an additional clue indicating the development of PD. The processes underlying hyposmia in iRBD are unknown. Using resting-state functional connectivity (rsFC), a "sensory olfactory subnetwork" (SOS) has been identified that is thought to represent the processing of basic sensory olfactory information. We investigated whether changes in the SOS are seen in both PD and iRBD and whether changes are associated with hyposmia in both conditions.

METHODS: The University of Pennsylvania Smell Identification Test (UPSIT) and a seed-based approach to analyze SOS region rsFC in early PD, iRBD and healthy controls (HC) were employed. Our SOS regions included (right hemisphere) anterior piriform cortex, dorsal insula (INSd), ventral insula (INSv), posterior insula (INSp) and ventral posterior thalamus (THLvp).

RESULTS: Compared to HC, idiopathic iRBD and PD participants performed significantly worse on UPSIT and exhibited lower FC between INSd and INSv and higher FC between INSd and THLvp and INSv and THLvp. UPSIT scores were negatively correlated with FC between INSv and THLvp and INSp and THLvp.

CONCLUSION: Idiopathic iRBD may be associated with similar functional and perceptual olfactory alterations and potential compensatory changes as early PD, which may show promise as additional preclinical biomarkers of PD.