Intraoperative direct cortical stimulation motor evoked potentials: Stimulus parameter recommendations based on rheobase and chronaxie

•Optimal direct cortical stimulation MEP parameters are the ISI with lowest rheobase2×chronaxie, and D at its chronaxie.•Based on 20 patients, 4 ms ISI and 0.2 ms D are most consistently optimal or near-optimal.•Two-point rheobase and chronaxie estimation is accurate enough for quick individual optimization.

ObjectiveTo determine optimal interstimulus interval (ISI) and pulse duration (D) for direct cortical stimulation (DCS) motor evoked potentials (MEPs) based on rheobase and chronaxie derived with two techniques.MethodsIn 20 patients under propofol/remifentanil anesthesia, 5-pulse DCS thenar MEP rheobase and chronaxie with 2, 3, 4 and 5 ms ISI were measured by linear regression of five charge thresholds at 0.05, 0.1, 0.2, 0.5 and 1 ms D, and estimated from two charge thresholds at 0.1 and 1 ms D using simple arithmetic. Optimal parameters were defined by minimum threshold energy: the ISI with lowest rheobase2×chronaxie, and D at its chronaxie. Near-optimal was defined as threshold energy <25% above minimum.ResultsThe optimal ISI was 3 or 4 (n = 7 each), 2 (n = 4), or 5 ms (n = 2), but only 4 ms was always either optimal or near-optimal. The optimal D was ∼0.2 (n = 12), ∼0.1 (n = 7) or ∼0.3 ms (n = 1). Two-point estimates closely approximated five-point measurements.ConclusionsOptimal ISI/D varies, with 4 ms/0.2 ms being most consistently optimal or near-optimal. Two-point estimation is sufficiently accurate.SignificanceThe results endorse 4 ms ISI and 0.2 ms D for general use. Two-point estimation could enable quick individual optimization.