A new maneuver for repetitive nerve stimulation testing in the trapezius muscle

Introduction: The repetitive nerve stimulation (RNS) test in the trapezius muscle is used widely for the evaluation of myasthenia gravis. However, pseudofacilitation is often difficult to avoid in this muscle and may compromise the detection of small decremental responses. We have devised a new maneuver to reduce pseudofacilitation. Methods: Using our maneuver, the shoulder of a supine subject is elevated passively and is held firmly by the examiner. Four conventional maneuvers as well as ours were compared with regard to pseudofacilitation that was maximal at the second wave in 14 control subjects. Results: Pseudofacilitation at the second and fourth waves was the smallest using our maneuver. Up to 15% pseudofacilitation was observed using the other maneuvers. Conclusion: Pseudofacilitation in the trapezius muscle is mainly due to shortening of the muscle belly. It can be reduced greatly by shortening the muscle in advance.