Comparison of forward and backward postural perturbations in mild-to-moderate Parkinson's disease
Introduction
Postural instability in Parkinson’s disease (PD) is clinically evaluated with a sudden backward pull on the shoulders (the “pull test”). The response is rated on an ordinal scale (0–4), heavily weighting the number of steps required to recover balance [1], often with imperfect standardization [2] and a poor ability to discriminate fallers from non-fallers [3]. Laboratory balance studies in PD are more standardized and quantitative, but methods are heterogeneous: some have tested reactive backward steps, others forward [[4], [5], [6], [7], [8]]. The best direction to detect Parkinsonism remains unclear. In this study we: (1) evaluated backward versus forward reactive stepping to compare performance between PD and controls and (2) evaluated the utility of using principal components analysis to capture the dynamics of the reactive response and differences between groups.
Section snippets
Materials and methods
We studied 62 individuals (40 mild-to-moderately severe PD and 22 controls, Table S1). All gave written informed consent. PD subjects were tested after overnight withdrawal from immediate release PD medications (24 h for sustained release formulations). No PD participant had been treated surgically. The protocol was approved by the University of Minnesota Institutional Review Board.
Reactive steps were induced by rapid translations of the support surface using a servocontrolled treadmill (Cmill,
Results
Forward-stepping perturbations required significantly fewer steps than backward (p < 0.001). The main effect of group was nonsignificant (p = 0.84). The group by direction interaction for number of steps was significant (p < 0.01) reflecting that the magnitude of the difference between PD and controls was greater in the forward than the backward direction (Fig S1; mean ± SD: Forward: PD: 1.9 ± 0.8; control: 1.7 ± 0.5) (Backward: PD: 2.4 ± 0.9; control: 2.5 ± 1.0). Equivalently, the differences
Discussion
For both PC1 projections and number of steps, the magnitude of the group difference was greater in the forward than the backward direction, as reflected in a significant group x direction interaction. The difference between PD and controls in the forward direction was statistically significant for PC1 projections, but not for number of steps, which we attribute to the coarse integer step-count.
PC1 projections were inversely related to the number of steps (Fig. 2B); thus, a higher projection
CRediT authorship contribution statement
Chiahao Lu: Methodology, Software, Data curation, Visualization, Writing - original draft, Writing - review & editing. Sommer L. Amundsen-Huffmaster: Methodology, Investigation, Writing - review & editing. Kenneth H. Louie: Investigation, Writing - review & editing. Robert Lowe: Data curation, Validation, Writing - review & editing. Reme Abulu: Data curation, Validation, Writing - review & editing. Robert A. McGovern: Data curation, Validation, Writing - review & editing. Jerrold L. Vitek:
Declaration of Competing Interest
None.
Acknowledgement
This work was supported by the National Institutes of Health [grant numbers: P50NS098573, R01NS088679, R01NS081118, R01NS094206] and the State of Minnesota’s MnDRIVE Brain Conditions Initiative.
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