Cortical Complexity Alterations in Motor Subtypes of Parkinson's Disease: A Surface-Based Morphometry Analysis of Fractal Dimension

Background: Based on motor symptoms, Parkinson's disease (PD) can be classified into tremor dominant (TD) and postural instability gait difficulty (PIGD) subtypes. This study aimed to investigate differences in cortical complexity and gray matter volume (GMV) between TD and PIGD. Methods: We enrolled 36 TD patients, 27 PIGD patients, and 66 healthy controls (HC) from the PPMI (Parkinson's Progression Markers Initiative) database. Voxel-based morphometry (VBM) and surface-based morphometry (SBM) were utilized to assess differences in GMV, cortical thickness, and cortical complexity. Additionally, correlations between clinical data and structural changes were examined. Results: In comparison with HCs, PIGD patients exhibited a significant fractal dimension (FD) decrease in many cortical regions, such as the bilateral insula, right superior temporal, and left rostral middle frontal. Moreover, PIGD patients showed significant FD reduction in various regions, including the left supramarginal gyrus, left lateral orbitofrontal gyrus, right superior temporal gyrus, left lateral occipital gyrus, and bilateral insula, compared to the TD group. A significant negative correlation between age and FD was observed in the left insula for the PIGD patients and in the bilateral insula for the TD patients. However, no significant differences were found in GMV, cortical thickness, or other complexity indices. Conclusion: Altered FD, particularly in bilateral insula, indicates that postural instability and gait disturbances in PD may result from a failure to integrate information from various structures, whereas parkinsonian tremor is not associated with this integration.