Increased supernormality in patients with multiplet discharges: Evidence for a common pathophysiological mechanism behind multiplets and fasciculations

•In patients with motor neuron disease, distal stimulation can evoke multiplet discharges (MDs).•In patients showing electrically evoked MDs, motor axonal excitability changes suggest K+ conductance impairment.•Distally evoked MDs may arise from the same pathophysiological mechanism as fasciculations of distal origin in motor neuron disease.

ObjectiveTo determine whether there is a relation between electrically evoked multiplet discharges (MDs) and motor axonal excitability properties. We hypothesized that electrically evoked MDs share their underlying pathophysiological mechanism with fasciculations.MethodsHigh-density surface EMG and motor nerve excitability recordings of the thenar muscles were performed in 22 patients with motor neuron disease (MND) in their differential diagnosis and who were referred for EMG examination.ResultsSupernormality (hyperexcitable phase following the refractory period) was significantly increased in patients with MDs (n = 10) compared to patients without MDs (n = 12) (25.5% vs 17.0%; p = 0.02). Depolarizing threshold electrotonus differed significantly between both groups as well (TEdpeak, 76.6% vs 66.6%, p < 0.01; TEd90-100 ms, 51.7% vs 44.3%, p < 0.01)ConclusionsOur findings imply that the same pathophysiological excitability changes are involved in generating MDs and fasciculations. Yet, MDs may be quantified more easily, and may be more specific for abnormal distal excitability than fasciculations, because fasciculations may originate along the motor axon as well as in the neuron cell body.SignificanceMDs are potentially useful as objective measure of increased distal axonal excitability at individual motor unit level and might complement clinical studies in MND.