Examining the role of intrinsic and reflexive contributions to ankle joint hyper‑resistance treated with botulinum toxin‑A

Journal of NeuroEngineering and Rehabilitation, 2023 · DOI: https://doi.org/10.1186/s12984-023-01141-8 · Published: January 18, 2023

Pharmacology Neurology Neurorehabilitation

Simple Explanation

Spasticity, which increases joint resistance, is often treated with Botulinum neurotoxin-A (BoNT-A) injections. The effects of BoNT-A are not well understood because current clinical measures cannot differentiate between different symptoms affecting joint resistance. This study aims to distinguish between reflexive and intrinsic pathways contributing to ankle joint hyper-resistance in participants receiving BoNT-A injections. The study involved evaluating nine participants with spasticity resulting from spinal cord injury or stroke over three sessions: at the time of BoNT-A injection, and again 6 and 12 weeks later. The evaluations included clinical assessments like the modified Ashworth scale (MAS) and Tardieu Scale, alongside instrumented assessments using the spasticity test (SPAT) and parallel-cascade (PC) system identification. The study found that at a group level, the expected reduction in joint resistance following BoNT-A injections was only observed using the MAS. The instrumented measures, however, did not confirm a clear group-level reduction in reflexive resistance, although the measures showed strong correlations with each other, suggesting validity.

Study Duration

12 weeks

Participants

9 participants with spasticity after spinal cord injury or after stroke

Evidence Level

Not specified

Key Findings

1
At group-level, the expected joint resistance reduction due to BoNT-A injections was only observed in the MAS (overall resistance).
2
This observed reduction could not be attributed to an unambiguous group-level reduction of the reflexive resistance contribution, as no instrumented measure confirmed the hypothesis.
3
Validity of the instrumented measures was supported through a strong association between different assessment methods.

Research Summary

This paper studied the intrinsic and reflexive ankle joint resistance within participants treated with BoNT-A injections to reduce spasticity. On group-level, our hypothesis was only confirmed for the MAS, a measure of overall joint resistance, which showed a significant reduced resistance at week 6. Regarding validity, all instrumented outcome measures showed a strong correlation when quantifying the same resistance contribution.

Practical Implications

Clinical Decision Making

Objective and reliable joint resistance quantification would improve clinical decision making in prescription of BoNT-A.

Heterogeneous Response

Further quantification of the individual contributions to joint resistance changes using instrumented measures across a large sample size are essential to understand the heterogeneous response to BoNT-A injections.

Validity of Instrumented Measures

A moderate to strong correlation between all reflexive measures and a strong correlation between the intrinsic measures supported the validity of the used instrumented measures.

Study Limitations

1
Clinical evaluations were performed by a non-blinded physiotherapist, potentially biasing MAS and Tardieu Scale scores.
2
Spasticity's manifestation in a passive experimental environment may differ from active or functional environments.
3
A low reflexive resistance magnitude at baseline in some participants limited the potential to show a reflex reduction.

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