Tipping the scales: how clinical assessment shapes the neural correlates of Parkinson's disease mild cognitive impairment

Mild cognitive impairment in Parkinson's disease (PD-MCI) is associated with consistent structural and functional brain changes. Whether different approaches for diagnosing PD-MCI are equivalent in their neural correlates is presently unknown. We aimed to profile the neuroimaging changes associated with the two endorsed methods of diagnosing PD-MCI. We recruited 53 consecutive non-demented PD patients and classified them as PD-MCI according to comprehensive neuropsychological examination as operationalized by the Movement Disorders Task Force. Voxel-based morphometry, cortical thickness, functional connectivity and graph theoretical measures were obtained on a 3-Tesla MRI scanner. 18 patients (32%) were classified as PD-MCI with Level-II criteria, 19 (33%) with the Parkinson's disease Cognitive Rating Scale (PD-CRS) and 32 (60%) with the Montreal Cognitive Assessment (MoCA) scale. Though regions of atrophy differed across classifications, reduced gray matter in the precuneus was found using both Level-II and PD-CRS classifications in PD-MCI patients. Patients diagnosed with the PD-CRS also showed extensive changes in cortical thickness, concurring with the MoCA in regions of the cingulate cortex, and again with Level-II regarding cortical thinning in the precuneus. Functional connectivity analysis found higher coherence within salience network regions of interest, and decreased anticorrelations between salience/central executive and default-mode networks in the PD-CRS classification for PD-MCI patients. Graph theoretical metrics showed a widespread decrease in node degree for the three classifications in PD-MCI, whereas betweenness centrality was increased in select nodes of the default mode network (DMN). Clinical and neuroimaging commonalities between the endorsed methods of cognitive assessment suggest a corresponding set of neural correlates in PD-MCI: loss of structural integrity in DMN structures, mainly the precuneus, and a loss of weighted connections in the salience network that might be counterbalanced by increased centrality in the DMN. Furthermore, the similarity of the results between exhaustive Level-II and screening Level-I tools might have practical implications in the search for neuroimaging biomarkers of cognitive impairment in Parkinson's disease.