Investigating Inducible Muscle Overactivity in Acquired Brain Injury and the Impact of Botulinum Toxin A.

OBJECTIVE: To investigate the pattern of change in muscle overactivity during repetitive grasp/release using dynamic computerized dynamometry (DCD; objective 1) and the effect of botulinum toxin A (BTX-A; objective 2). DESIGN: Secondary analysis of an observational cohort study. SETTING: Hospital outpatient spasticity management service. PARTICIPANTS: A convenience sample (N=65), comprising adults with upper motor neuron syndrome affecting the arm after acquired brain injury (ABI; n=38) and participants without ABI (n=27). INTERVENTIONS: After clinical assessment, a subgroup of participants with ABI (n=28) underwent BTX-A injections as part of their spasticity management. MAIN OUTCOME MEASURES: Post hoc DCD data processing extracted the values of minimum force generation between 10 sequential contractions. The pattern of change was analyzed. RESULTS: The ABI injected group exerted greater force at baseline than both other groups (ABI injected=1.04 kg, ABI noninjected=0.74 kg, participants without ABI=0.53 kg; P=.011). After the first contraction, minimum force values increased for all groups and were greatest in the ABI injected group. With subsequent cycles, the group without ABI showed a linear pattern of decreasing force generation, whereas both ABI groups showed a quadratic increasing pattern, which was of greater magnitude in the ABI injected group. After injection, values for the ABI injected group showed a 51% reduction in inducible muscle overactivity (P=.003) to magnitudes similar to those of the ABI noninjected group. CONCLUSIONS: This study showed that hand relaxation deteriorated during repetitive movements in people with spasticity, a feature hypothesized to adversely influence everyday hand function. After BTX-A injection, the magnitude but not the pattern of this inducible muscle overactivity improved.