Longitudinal MRI study of hippocampal subfields Morphometry in early Parkinson's disease

BACKGROUND: Parkinson's Disease (PD) is a neurodegenerative disorder characterised by motor and non-motor symptoms. Cognitive impairment is a prominent non-motor feature. OBJECTIVE: This study investigated hippocampal subfield shape deformation and atrophy in early-stage PD using spherical harmonic-based shape analysis. METHODS: Our cohort of early PD patients (n = 146; disease duration = 14.9 ± 9.5 months; 44% Female) and matched healthy controls (HC) (n = 59; 56% Female) underwent magnetic resonance imaging (MRI) and cognitive assessments at baseline and 2-year follow-up. Hippocampal subfields were segmented in FreeSurfer; shape analysed with SPHARM-PDM. RESULTS: Baseline shape analysis identified morphological changes in Cornu Ammonis 1 (CA1), CA3, subiculum, and fimbria in early PD patients compared to HCs, despite no baseline volumetric differences. Longitudinally, volume decline/atrophy was observed in the right whole hippocampus, right CA1, and left fimbria. In the PD-Mild Cognitive Impairment (PD-MCI) subgroup, localised shape deformations in CA1, CA3, Granule Cell Layer and Molecular Layer of the Dentate Gyrus (GC-ML-DG), and fimbria were observed at baseline when compared to PD-Normal Cognition (PD-NC), with longitudinal left fimbria decline observed and associated with cognitive deterioration in this cohort, especially with visuospatial and executive function domains. CONCLUSION: These findings suggest that, in this cohort, shape analysis may capture baseline subfield differences that are not apparent using volumetric measures in early-stage PD. This study suggests hippocampal subfield shape analysis may help characterize early structural differences related to cognition in PD. The left fimbria warrants further study as a candidate imaging marker of cognitive vulnerability in PD.