Postural instability in Parkinson Disease: To step or not to step
Introduction
Postural instability (PI) is a cardinal manifestation of Parkinson disease (PD) that is associated with falls and increased morbidity [6], [7], [18], [46]. PI usually becomes problematic approximately a decade after the onset of PD symptoms [48]. PI, along with speech and gait deficits, does not generally respond to dopaminergic medications [4], [8], [32], [47]. PI and falls have been associated with the akinetic–rigid as opposed to the tremor-dominant subtype of PD [30], [43]. A recent quantitative research study found that axial rigidity, especially involving the neck, is associated with balance and gait problems in subjects with PD [15]. In a meta-analysis of six studies, Pickering et al. [38] found that, except for tremor, all items from United Parkinson's Disease Rating Scale (UDPRS; [13], [26], [27]) were related to falls with the strongest association between gait and speech abnormalities.
A number of studies have sought to establish an effective means to clinically evaluate PI. The most common method is the backward Pull Test, which is a standard part of the UDPRS (item 30). The Pull Test [3], [5], [35] and related PI tests [28], such as the Push and Release Test [44] and the Steady Stance Test [46], have been analyzed for ability to predict PI. Some studies have suggested that the Pull Test is not a particularly effective method for predicting PI and falls [7], [29], [44] whereas others have shown relatively good overall predictive accuracy [23], [46], especially when using an unexpected [6], [24] rather than an expected pull [13], [40]. Although the mechanics and purpose of the Pull Test have been refined and standardized in the MDS-UPDS (Movement Disorder Society-UPDS [17]), it is not clear if one of the factors in testing relates to how much force is needed to displace a subject and, if the subject takes a step to maintain balance, does this constitute evidence for mild PI [26]. Finally, because of safety issues, the Pull Test is only done in the backward direction so that the examiner is able to catch a subject should they fall [17], [26], thus it may not capture PI in other pull directions.
Our research objective was to acquire quantitative data using a modified Pull Test apparatus. We expected that the results would lead to a better understanding of the mechanisms of PI in individuals with PD. We also hoped to develop a more reliable clinimetric that also included an assessment of PI in the forward and side directions. Based on previous literature [23], [26], [38] and personal observations, we expected that PI in individuals with PD would 1) be related to a reduced amount of force needed to pull them off-balance compared to Controls and 2) be worse for backward compared to other pull directions.
Section snippets
Methods
Two research apparatuses were constructed from 3/4 inch plywood reinforced by 2 × 4 beams: one to obtain quantitative pull-force measures (Fig. 1a, b) and the other to measure the center of mass (COM; Fig. 2a, b; [16]). We anticipated, based on the physics of pulling an object off balance that some variability in the force needed to pull a subject off balance would relate to height, weight, and body mass index (BMI).
Results
As expected, the PD group scored significantly worse than the CON group on the ABC Scale [55 ± 14% versus 95 ± 4%; t(7.8) = 8.2; p = .00004; r2 = .90], explaining approximately 90% of the data variance. There were no overlapping scores between groups. It should be noted that this result was based on the AM results only since Controls did not fill out a PM ABC Scale. Also, the t-test was based on 7.8 rather than 19° of freedom because variances between groups were unequal [Levene's test: p = .0001].
Discussion
Although the study lacked statistical power because of the small number of subjects, the major findings are both statistically and behaviorally very robust and instructive. We had expected that PD subjects would require less force to be pulled off balance because of PI. Yet, the data showed no statistical differences in Pull Force between the CON and PD groups in any pull direction (Table 2). However, when inspecting the mean AM and PM Pull Forces displayed in Table 2, the least force of
Conclusion
Based on our results, the utility of the backward Pull Test does not relate to the force needed to pull a PD subject off-balance but rather to their behavioral response to being pulled off-balance. Stepping in the direction of pull to maintain balance without falling is a normal response and does not necessarily indicate that the subject has PI. Because PD subjects did not show a learning response to being pulled off balance, there is no need to pull subjects unexpectedly [6], [24], which could
Conflict of interest
The authors have no conflict of interest to report.
Acknowledgments
This research was supported in part by a Medical Student Summer Research Scholarship from the American Academy of Neurology, St. Paul, MN (USA) to K. Kimmell with E. Ross serving as primary advisor.
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