Anomalies of motor unit amplitude and territory after botulinum toxin injection

Objective. Botulinum toxin (BT) induced cholinergic denervation of hyperactive motor units (MUs) is a clinically accepted and extensively practiced way of managing focal spasticity after stroke. The denervation potentially initiates a temporary reorganization of the MU activation patterns and structures by inducing the emergence of a large number of newly innervated muscle fibers. In this study, we quantify the effect of the BT on MU action potential (MUAP) amplitudes and on the MU territory areas (MUTAs) as seen on the surface of the skin over the biceps brachii (BB) muscle.Approach. We have used a 128-channel high-density surface electromyography (HDsEMG) grid on the spastic and contralateral BB muscle and recorded the myoelectric activity along with the contraction force during isometric contraction of the elbow muscles. We have decomposed the recorded EMG signal into individual MU potentials and estimated the MUAP amplitudes and territory areas before and two weeks after a BT injection.Main result. There were significantly larger median (47 ± 9%) MUAP amplitudes as well as reduction of MUTA (20 ± 2%) two weeks after the injection compared to the respective pre-injection recording.Significance. The observed covariation of the amplitude and the territory area indicates that the large amplitude MUs that appeared after the BT injection have a relatively smaller territory area. These results provide a rare insight into the BT-induced changes of MU characteristics and have the potential to improve spasticity treatment. We discuss the potential contributing factors to these changes subsequent to the injection in the context of the investigated subject cohort.