Abstract
Postural instability marks a prevalent symptom of Parkinson’s disease (PD). It often manifests in increased body sway, which is commonly assessed by tracking the Center of Pressure (CoP). Yet, in terms of postural control, the body’s Center of Mass (CoM), and not CoP is what is regulated in a gravitational field. The aim of this study was to explore the effect of early- to mid-stage PD on these measures of postural control in response to unpredictable visual perturbations. We investigated three cohorts: (i) 18 patients with early to mid-stage PD [Hoehn & Yahr stage (1–3); 1.94 ± 0.70]; (ii) a group of 15 age-matched controls (ECT); and (iii) a group of 12 young healthy adults (YCT). Participants stood on a force plate to track their CoP, while the movement of their entire body was recorded with a video-based motion tracking system to monitor their CoM. A moving room paradigm was applied through a head-mounted virtual reality headset. The stimulus consisted of a virtual tunnel that stretched in the anterior-posterior direction which either remained static or moved back and forth in an unpredictable fashion.We found differences in mean sway amplitude (MSA) and mean velocities of CoP and CoM between the groups under both conditions, with higher MSA of CoP and CoM for PD and higher mean velocities of both variables for PD and ECT when compared with YCT. Visual perturbation increased mean CoP velocity in all groups but did not have effects on mean CoM velocity or MSA. While being significantly lower for the young adults, the net effect of visual perturbation on mean CoP velocity was similar between patients with PD and age-matched controls. There was no effect of the visual perturbation on mean CoM velocity for any of the groups.Our simultaneous assessment of CoP and CoM revealed that postural control is reflected differently in CoM and CoP. As the motion of CoM remained mostly unaffected, all groups successfully counteracted the perturbation and maintained their balance. Higher CoP velocity for PD and ECT revealed increased corrective motion needed to achieve this, which however was similar in both groups. Thus, our results suggest increased effort, expressed in CoP velocity, to be an effect of age rather than disease in earlier stages of PD.
Highlights
Due to the ongoing demographic transition and subsequent over-aging of modern societies in many industrialized countries, neurodegenerative diseases are gaining in prevalence, with Parkinson’s disease (PD) being the second most common after Alzheimer’s disease (Lange and Erbguth, 2017; Tysnes and Storstein, 2017)
As the motion of the tunnel led to an increase in mean velocity in each respective group and of mean sway amplitude (MSA) of Center of Pressure (CoP) in ECT and YCT, we investigated the net effect of visual perturbation (UC) on MSA and mean velocity of both parameters
We investigated potential disease- and age-specific changes in postural control during quiet stance in a static environment as well as in response to random visual perturbations by simultaneously assessing mean sway amplitude and mean velocity of CoP and Center of Mass (CoM)
Summary
Due to the ongoing demographic transition and subsequent over-aging of modern societies in many industrialized countries, neurodegenerative diseases are gaining in prevalence, with Parkinson’s disease (PD) being the second most common after Alzheimer’s disease (Lange and Erbguth, 2017; Tysnes and Storstein, 2017). In addition to the cardinal symptoms of rigidity, hypokinesia and resting tremor, as the disease progresses, most patients develop postural instability (Jankovic, 2008). A common indicator to predict the individual risk of future falls is whether the patient had experienced more than two falls in the preceding year (Pickering et al, 2007). This implies, that patients already had a history of falls and hampers the usefulness as a predictor to take preventive measures. Identifying reliable biomechanical indicators of postural instability is crucial to monitor the development of this symptom and allow early intervention
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