Atrophy of the Nucleus Basalis of Meynert predicts the progression of gait variability in Parkinson’s disease

Abstract Parkinson’s disease (PD) is a systemic brain disorder where the cortical cholinergic network begins to degenerate early in the disease process. Readily accessible, quantitative, and specific behavioral markers of the cortical cholinergic network are lacking. Although degeneration of the dopaminergic network may be responsible for deficits in cardinal motor signs, the control of gait is a complex process and control of higher-order aspects of gait, such as gait variability, may be influenced by cognitive processes attributed to cholinergic networks. We investigated whether swing time variability, a metric of gait variability that is independent from gait speed, was a quantitative behavioral marker of cortical cholinergic network integrity in PD. Twenty-two individuals with PD and twenty-nine age-matched controls performed a validated stepping-in-place (SIP) task to assess swing time variability off all therapy. The PD cohort also underwent structural MRI scans to measure gray matter volume of the Nucleus Basalis of Meynert (NBM), the key node in the cortical cholinergic network. Swing time variability was also measured ON subthalamic nucleus (STN) deep brain stimulation (DBS) in PD individuals. A subset of eleven individuals with PD completed the SIP task again off all therapy after three years of continuous DBS. Clinical motor assessments were performed for each condition. Swing time variability was significantly greater (i.e., worse) in PD compared to controls and greater swing time variability was related to greater atrophy of the NBM. STN DBS significantly improved cardinal motor signs but did not improve swing time variability. Swing time variability worsened in PD, off therapy, after three years of continuous STN DBS, and NBM atrophy predicted the degree of increase. In contrast, cardinal motor signs did not progress. These results demonstrate that swing time variability is a reliable marker of cortical cholinergic health, and support a framework in which higher-order aspects of gait control in PD are reliant on the cortical cholinergic system, in contrast to other motor aspects of PD that rely on the dopaminergic network.