Temporal Summation of Subthreshold Stimuli in Human Motor Axons: Implications for Intraoperative Neuromonitoring
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Objectives
To examine how stimulus amplitude and width influence subthreshold superexcitability of peripheral axons and to provide evidence-based recommendations for minimizing inadvertent compound muscle action potential (CMAP) generation during intraoperative corticobulbar monitoring.
Methods
Fifteen healthy participants received median nerve stimulation under nine conditions combining three amplitudes (80%, 85%, and 90% of threshold) and three pulse widths (0.1 ms, 0.5 ms, 1.0 ms). Trains of 1–7 subthreshold pulses (2 ms interpulse interval) were delivered 10 times per condition. CMAPs were recorded from the abductor pollicis brevis (APB), and the probability of a response exceeding 100 µV (baseline-to-negative-peak) was calculated. Persistent sodium current was estimated using the latent addition test (LAh).
Results
Higher pulse amplitudes and wider pulse widths significantly increased CMAP probability, with a significant interaction (F(4,56) = 4.853, p = .002, partial η² = .257). All pairwise comparisons were significant (p ≤ .023). When controlling for rheobase, LAh was positively correlated with response probability (r partial (12) = .539, p = .047).
Conclusions & Significance
Subthreshold trains activate motor axons in a predictable manner depending on amplitude, width, and train length. These findings challenge current IONM assumptions and highlight the need for threshold-referenced, standardized protocols.
