Recordings of the tcMEPs and SEPs
abductor pollicis brevis muscle, pollicis longus muscle, tibialis anterior muscle, quadriceps femoris muscle, biceps brachii muscle
A 32-channel Cadwell Cascade neurophysiological monitoring system (Cadwell Laboratories, Kennewick, WA) was used for stimulation and recording. The scalp electrodes were positioned in accordance with the international 10–20 montage system.SEPs were obtained continuously by stimulating the upper and lower extremities during the operation. Patch electrodes were used to stimulate the median nerve, ulnar nerve, and posterior tibial nerve at the wrist and ankle, respectively. The anode was at the distal end, and the cathode was at the proximal end. Recording electrodes were placed on Cz (both lower limbs), C3', C4' (both upper limbs). Stimulus parameters were set as follows: intensity 15-25 mA, stimulus interval 100-300 ms, and stimulus frequency 2.1–4.7 Hz. The following recording parameters were used: the filter bandwidth was set to 30-600 Hz, the 50 Hz notch was turned off by default, the average number of signals was set to 100–400, and the analysis time was set to 50 ms.TcMEPs were performed with spiral electrodes to stimulate the C3, C4 or C1, C2 region (C1 is 10% to the left of Cz, C2 is 10% to the right of Cz) with eight pulses (biphasic square pulses). According to different surgical requirements, compound muscle action potentials were recorded by straight needle electrodes on deltoid muscle, biceps brachii muscle, extensor carpi longus muscle, abductor pollicis brevis muscle and abductor digitorum minatus muscle of upper limb, quadriceps femoris muscle, tibialis anterior muscle, extensor pollicis longus muscle, abductor pollicis muscle and calf muscle of lower limb. The stimulus parameters were as follows: stimulus intensity ranged from 100 to 300 V, interval time was set to 2 ms, and stimulus interval was set to 0.1 ms. Recording parameters were set as follows: the filter bandwidth was set to 30-2500 Hz, the 50 Hz notch was turned off by default, the average number of signals was one, and the signal analysis time was 100 ms. The current's power and intensity were set at the outset and maintained throughout the procedure.During the operation, SEPs were routinely recorded every 10 min, and tcMEPs were recorded every 10 min (not at the same time). Multiple somatosensory and motor waveforms were recorded if the surgeon needed to record or monitor the abnormal state. The recordings were made in the following order: first, motor stimulation (left and right), then somatosensory stimulation. The order was left upper limb, right upper limb, left lower limb, and right lower limb.When the monitoring signal(s) of SEPs or (and) MEPs reached the warning value, the surgeons were notified and the operation was temporarily stopped. If the waveform did not recover, the Dex infusion was terminated and a wake-up test was carried out. The true positive case referred to an electrophysiological warning signal and postoperative neural injury. The false-positive case referred to an electrophysiological warning signal without postoperative neural injury. The false-negative case referred to a normal electrophysiological signal but postoperative neural injury. The true negative case referred to a normal electrophysiological signal with no postoperative neural injury.