Beyond spasticity reduction: botulinum toxin-a injection is associated with changes in abnormal flexor synergies of the upper limb in chronic stroke

OBJECTIVE: This study aimed to investigate the association between botulinum toxin type A treatment and changes in spasticity and abnormal flexor synergies in patients with chronic stroke. SUBJECTS: Twenty-eight patients with chronic stroke (mean age 56.5 years; mean time since onset 6.3 years) who received botulinum toxin type A injections into upper-arm flexor muscles (biceps brachii, brachialis, or brachioradialis) were enrolled. METHODS: This was a retrospective, single-centre cohort study. Assessments were performed before and approximately 2 months after injection. Primary outcomes were changes in spasticity measured by the Modified Ashworth Scale and the abnormal flexor synergy index based on kinematic analysis. Secondary outcomes included changes in voluntary shoulder flexion angle and subgroup analyses according to baseline upper extremity motor severity and responder/non-responder status based on changes in the Modified Ashworth Scale scores of the elbow flexors. RESULTS: Generalized estimating equation analyses showed significant changes in Modified Ashworth Scale elbow flexor scores and the abnormal flexor synergy index after injection. In severity-stratified analyses, significant Time × Severity interactions were observed for Modified Ashworth Scale scores of the elbow and wrist flexors, whereas no significant interaction was found for the abnormal flexor synergy index. In responder/non-responder analyses, a significant Time × Responder-status interaction was observed for the abnormal flexor synergy index. Non-responders showed no significant changes in Modified Ashworth Scale scores or the abnormal flexor synergy index and showed reduced maximum voluntary shoulder flexion angle. CONCLUSION: This study suggests that botulinum toxin type A treatment was associated with changes in abnormal flexor synergies, in addition to reducing muscle tone. These findings highlight the value of incorporating kinematic assessments such as the abnormal flexor synergy index into the clinical evaluation of botulinum toxin treatment.