Does severity of motor impairment affect reactive adaptation and fall-risk in chronic stroke survivors?

Wrong question. What are the protocols that will prevent falls in chronic stroke survivors?

Does severity of motor impairment affect reactive adaptation and fall-risk in chronic stroke survivors?

• Tanvi BhattEmail author,
• Shamali Dusane and
• Prakruti Patel

Journal of NeuroEngineering and Rehabilitation201916:43

https://doi.org/10.1186/s12984-019-0510-3

©  The Author(s). 2019

• Received: 27 August 2018
• Accepted: 4 March 2019
• Published: 22 March 2019

Abstract

Background

A single-session of slip-perturbation training has shown to induce long-term fall risk reduction in older adults. Considering the spectrum of motor impairments and deficits in reactive balance after a cortical stroke, we aimed to determine if chronic stroke survivors could acquire and retain reactive adaptations to large slip-like perturbations and if these adaptations were dependent on severity of motor impairment.

Methods

Twenty-six chronic stroke participants were categorized into high and low-functioning groups based on their Chedoke-McMaster-Assessment scores. All participants received a pre-training, slip-like stance perturbation at level-III (highest intensity/acceleration) followed by 11 perturbations at a lower intensity (level-II). If in early phase, participants experienced > 3/5 falls, they were trained at a still lower intensity (level-I). Post-training, immediate scaling and short-term retention at 3 weeks post-training was examined. Perturbation outcome and post-slip center-of-mass (COM) stability was analyzed.

Results

On the pre-training trial, 60% of high and 100% of low-functioning participants fell. High-functioning group tolerated and adapted at training-intensity level-II but low-functioning group were trained at level-I (all had > 3 falls on level-II). At respective training intensities, both groups significantly lowered fall incidence from 1st through 11th trials, with improved post-slip stability and anterior shift in COM position, resulting from increased compensatory step length. Both groups demonstrated immediate scaling and short-term retention of the acquired stability control.

Conclusion

Chronic stroke survivors are able to acquire and retain adaptive reactive balance skills to reduce fall risk. Although similar adaptation was demonstrated by both groups, the low-functioning group might require greater dosage with gradual increment in training intensity.

Keywords

• Perturbation training
• Adaptation
• Motor impairment
• Stroke

Introduction

Approximately more than 800,000 individuals annually suffer from stroke and its associated detrimental long term disability in the USA [1]. The primary deficits associated with stroke, such as sensorimotor impairment, postural dysfunction, and cognitive impairment, result in secondary complications such as falls [2, 3, 4]. The high risk of falls during the acute phase persists even into the chronic phase when people with chronic stroke (PwCS) regain their ambulatory ability; especially predisposing them to falls from unexpected environmental perturbations such as slips or trips [5].

Reactive balance control plays a crucial role in recovering from large unexpected perturbations, thereby lowering fall-risk [6, 7, 8, 9, 10]. A rapid and sufficiently large stepping response helps to regain postural stability by restoring the displaced base of support (BOS) and providing the necessary lever arm to generate adequate rotational counter-torque at step touchdown to decelerate the body’s forward or backward moving center of mass (COM) [11]. Studies examining reactive responses to stance perturbations in PwCS have reported delayed onset latencies of lower extremity muscles, with smaller amplitude and altered sequence of activation [12, 13]. Moreover, PwCS often show delayed compensatory step initiation with a short compensatory step, or they exhibit an aborted step or multiple stepping responses; all of which compromise postural stability and increase fall-risk [14, 15, 16, 17, 18, 19, 20].

Considering the above-mentioned evidence, training reactive responses in PwCS could be critical for fall-risk reduction in this population. Leveraging the principle of task specificity, perturbation training elicits reactive motor adaptations and induces learning of effective recovery strategies by improving COM state stability control and compensatory stepping responses throughout trials [21, 22]. Even a single session of perturbation training has shown to induce longer-term reduction in laboratory-induced and real-life falls in healthy older adults [23]. However, limited evidence exists on the effect of perturbation training on acquiring adaptations for fall-risk reduction in PwCS.

The role of neuroplasticity, specifically reorganization of the sensorimotor cortex, in optimizing motor recovery and function for both skilled volitional and locomotor tasks in PwCS is well known [24, 25, 26]. It is also established that the cerebellum and cerebral cortex play a crucial role in acquiring locomotor-balance adaptations [27, 28]. While cerebellar stroke can impair acquisition of motor adaptation, this ability has been shown to be intact in cortical stroke [27, 29, 30]. Nonetheless, little is known whether adaptations within the reactive balance control system are possible post-stroke. A preliminary training study employing therapist-induced, small magnitude external pull-push perturbations showed reduction in daily falls for sub-acute stroke patients [31]. Such low intensity perturbations, while appropriate for individuals in the early phases of recovery, might not be challenging enough to mimic real-life perturbations faced by community-dwelling PwCS.

Previously, it has been established that there is a need for different dosage considerations when training patients with varying degrees of impairment in order to improve locomotor balance control [32]. However, there is lack of evidence on recommendations for an optimal perturbation training intensity for PwCS with different severity of motor impairment to suitably match their motor capabilities and ultimately induce reactive adaptation. It is also unknown if PwCS with varying levels of motor impairments could safely tolerate the perturbation intensity dosages provided to healthy young and older adults for training.

This study aimed to examine if PwCS could acquire reactive adaptation to large slip-like stance perturbations, and if adaptive gains differed based on the perturbation intensity and severity of motor impairment. We also examined if the adaptive gains could be scaled when exposed to a higher perturbation intensity and then retained over several weeks.

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