Patterns of cortical thickness and surface area in early Parkinson's disease

Idiopathic Parkinson's disease (PD) is a neurodegenerative disorder diagnosed on the basis of motor symptoms, but that also includes cognitive and visuo-spatial deficits. Though PD is known to initially affect subcortical regions, the cortex also exhibits neuronal loss in the course of the disease as post mortem studies have shown. So far, PD-related pattern of cortical damage remains unclear, because of disease-caused heterogeneity, and also in part because of methodological issues such as the limitations of Voxel Based Morphometry. Here corticometry was used, a technique that decouples local surface from thickness, to obtain a better picture of PD corticomorphometric patterns. We acquired MRI volumes for 33 healthy controls (HC) and 49 PD patients, extracted local cortical thickness and surface area and modeled both of them as a function of group and age for each participant. Cortical thickness averaged on the whole cortex did not differ between the two groups while mean surface area was significantly larger in the PD group. The bilateral parietal lobule, the right superior frontal gyrus, the left cingulate cortex and the left insular cortex exhibited larger local surface area in the PD group. The right precuneus exhibited cortical thinning associated with age in the PD group and not in the HC group. Furthermore, cortical thinning was observed in the PD group compared with the control group in the left medial supplementary motor area (SMA) and in the right dorsal pre-SMA. Finally, we found the left temporal pole thickness to correlate with disease duration, as well as the bilateral occipital cortex and Broca's area. These results suggest that PD etiology is associated with specific cortical alterations, which could account for cognitive deficits that arise as the disease evolves. Finally, our results observed in the occipital cortex as a function of disease duration may indicate the increase in PD-related visuo-spatial deficits, which can sometimes result in hallucinations later on in the disease. In the future, MRI-generated corticometry, combined with additional behavioral markers, may prove to be a useful diagnosis tool to characterize the evolution of motor and cognitive deficits in PD.