Effects of 10-kHz Subthreshold Stimulation on Human Peripheral Nerve Activation.

ABSTRACT

OBJECTIVE: The mechanisms of action of high-frequency stimulation (HFS) are unknown. We investigated the possible mechanism of subthreshold superexcitability of HFS on the excitability of the peripheral nerve. MATERIALS AND METHODS: The ulnar nerve was stimulated at the wrist in six healthy participants with a single (control) stimulus, and the responses were compared with the responses to a continuous train of 5 seconds at frequencies of 500 Hz, 2.5 kHz, 5 kHz, and 10 kHz. Threshold intensity for compound muscle action potential (CMAP) was defined as intensity producing a 100-µV amplitude in ten sequential trials and "subthreshold" as 10% below the CMAP threshold. HFS threshold was defined as stimulation intensity eliciting visible tetanic contraction. RESULTS: Comparing the threshold of single pulse stimulation for eliciting CMAP vs threshold for HFS response and pooling data at different frequencies (500 Hz-10 kHz) revealed a significant difference (p = 0.00015). This difference was most obvious at 10 kHz, with a mean value for threshold reduction of 42.2%. CONCLUSIONS: HFS with a stimulation intensity below the threshold for a single pulse induces axonal superexcitability if applied in a train. It can activate the peripheral nerve and produce a tetanic muscle response. Subthreshold superexcitability may allow new insights into the mechanism of HFS.