Microstructural network alterations of olfactory dysfunction in newly diagnosed Parkinson's disease

Olfactory dysfunction is a robust and early sign for Parkinson's disease (PD). Previous studies have revealed its association with dementia and related neural changes in PD. Yet, how olfactory dysfunction affects white matter (WM) microstructure in newly diagnosed and untreated PD remains unclear. Here we comprehensively examined WM features using unbiased whole-brain analyses. 88 newly diagnosed PD patients without dementia (70 with hyposmia and 18 without hyposmia) and 33 healthy controls underwent clinical assessment and diffusion tensor imaging (DTI) scanning. Tract-based special statistics (TBSS), graph-theoretic methods and network-based statistics (NBS) were used to compare regional and network-related WM features between groups. TBSS analysis did not show any differences in fractional anisotropy and mean diffusivity between groups. Compared with controls, PD patients without hyposmia showed a significant decrease in global efficiency, whilst PD patients with hyposmia exhibited significantly reduced global and local efficiency and additionally a disrupted connection between the right medial orbitofrontal cortex and left rectus and had poorer frontal-related cognitive functioning. These results demonstrate that hyposmia-related WM changes in early PD only occur at the network level. The confined disconnectivity between the bilateral olfactory circuitry may serve as a biomarker for olfactory dysfunction in early PD.