Effect of botulinum toxin injections on stretch reflex responses of spastic elbow flexors in hemispheric stroke survivors: Case study

Spasticity, characterized by hyperreflexia, is a common motor impairment following hemispheric stroke. Botulinum toxin (BT) injections are widely used to reduce spasticity. BT acts by disrupting action potential propagation between a motor neuron and its innervated muscle fibers. Here we quantified the magnitude and time course of a BT injection on the stretch reflex responses, by analyzing the root mean squared amplitude of tap evoked surface EMG (sEMG) potentials from stroke affected biceps brachii (BB), before the BT injection and up to 18 weeks post-injection. We used a 16 × 8 sEMG electrode grid and mapped the tap-evoked potentials. We found, (a) the maximum reduction of the RMS EMG amplitude of the reflex response to occur six weeks post-BT injection and (b) a gradual recovery of the RMS EMG amplitude by 18 weeks post-injection. Unexpectedly, even at 18 weeks post injection, the tap-evoked reflex activity did not fully recover to its baseline (pre-injection recordings). These findings have a potential impact on the timing of BT administration for repeated injections.