Endplate spike morphology: A clinical and simulation study

Objective: To describe the various morphologic appearances of endplate spikes, define the theoretical volume conduction basis of these waveforms' morphologies, and simulate 'atypical' endplate spike waveforms documented by other investigators. Design: Endplate spikes were recorded from the biceps brachii in healthy individuals using a monopolar needle electrode. The morphologies of these waveforms were compared with those obtained from a computer simulation. Previously documented endplate waveforms were simulated using two fundamental types of biphasic initially negative and positive waveform morphologies. Setting: University clinic outpatient electrodiagnostic medicine facility. Subjects: Five subjects without history or physical evidence of neuromuscular disease. Main Outcome Measures: Endplate potential morphologies were assessed with respect to overall waveform shape and number of phases. Computer-generated waveforms for individual endplate spike waveforms were qualitatively compared with those recorded from the subjects. Results: Three fundamental waveforms were documented to arise from the endplate regions of all subjects and were successfully simulated: (1) biphasic initially negative potential from the endplate itself and up to 0.2mm from the endplate, (2) triphasic initially positive potential from within 0.2mm of the endplate up to 0.5mm from the musculotendinous junction, and (3) biphasic initially positive potential from the last 0.4mm of the fiber or from impulse blocking. Two biphasic endplate spike waveforms could be summated to generate all other endplate waveforms described in previously documented literature. Conclusion: The combination of clinical and simulation studies suggests that endplate spike potentials can have quite varied morphologies. Triphasic initially positive and biphasic initially positive endplate spikes may be mistaken for fibrillation potentials and positive sharp waves, respectively. The triphasic waveforms most likely arise from an action potential propagating past the recording electrode adjacent to the endplate, while the biphasic initially positive potential is simulated to arise from the needle electrode blocking action potential propagation.