Written informed consent was obtained from the patient for the publication of this case report and any accompanying images. A 71-year-old man who presented with preoperative bilateral lower extremity numbness but preserved motor strength was subjected to L3–5 spinal fenestration for lumbar spinal stenosis, with MEP and somatosensory-evoked potential (SSEP) monitoring. No significant changes were observed at other spinal levels.

The train-of-four (TOF) test as well as MEP and SSEP monitoring was conducted using a neurophysiological monitoring unit (Neuromaster MEE-1232; Nihon Kohden, Tokyo, Japan). For MEP monitoring, transcranial stimulation with a train-of-five pulse at 500 Hz was applied to C3–C4 (International 10–20 system), with the intensity set at the supramaximal level (approximately 500 V). MEP were recorded from the APB on the upper extremities and from the tibialis anterior, gastrocnemius, and abductor hallucis (AH) on the lower extremities. For SSEP monitoring, stimulation was applied over the median nerves bilaterally 3 cm proximal to the wrist and the posterior tibial nerves bilaterally at the ankle with the intensities of 25 and 45 mA, respectively. To record the SSEP, C3′ and C4′ (2 cm posterior to C3 and C4, respectively) were chosen to evaluate the upper limbs and Cpz for the lower limbs, and Fz was established as a reference electrode. A total of 100–200 stimulation repetitions were averaged to record each SSEP. The TOF test was conducted over the mentioned anatomic structures with SSEP-stimulating electrodes. The pattern for the TOF test was composed of four equal stimuli provided at a frequency of 2 Hz, stimulation time of 0.5 ms, and current of 50 mA. The obtained amplitudes of the APB and AH were analyzed. The TOF ratio (T4/T1) was determined by comparing the magnitude of the fourth response (T4) with that of the first (T1).

The patient received total intravenous anesthesia using propofol via target-controlled infusion (TCI). Anesthesia was maintained with propofol (2.0–3.0 µg/mL of TCI), remifentanil (0.2–0.5 µg/kg/min), and intermittent bolus injection of fentanyl. The anesthetic depth was regulated to preserve the bispectral index ranging from 40 to 60. No additional neuromuscular blockade was performed to prevent overlapping with MEP waveform interpretations after the administration of rocuronium (0.6 mg/kg) at anesthetic induction. MEP and SSEP were initially recorded after prone positioning. The TOF test showed a 15–25% TOF ratio in the right APB and bilateral AH 45 min after rocuronium administration, while the left APB responses did not appear in the evaluated timespan (Fig. 1). When the analysis window was extended up to three times, the muscle response from the left APB in the TOF test exhibited a TOF ratio of 10% and a markedly delayed latency and a smaller amplitude than those from the right APB. Subsequently, sugammadex (2 mg/kg) was injected to reverse the residual effects of neuromuscular blockade. Although the TOF ratio at all evaluated muscles returned to 100%, the responses from the left APB still exhibited a substantially smaller amplitude than those from the other limbs. Next, the initial MEP was measured, and the left APB demonstrated a substantially prolonged latency (34.8 ms) and a smaller amplitude (0.263 mV) than those of the right APB (21.7 ms and 1.76 mV, respectively) (Fig. 2). Physiological (blood pressure and body temperature), pharmacological (depth of anesthesia), and technical (wiring and equipment settings) parameters and their impact on the MEP were evaluated and found to be appropriate. Among the muscles analyzed, amplitude suppression and latency prolongation were observed only in the left APB; therefore, the systemic influence of pharmacological and physiological factors was ruled out. Because this phenomenon is considered to be a local anomaly, the possibilities of a technical error or impending peripheral neuropathy due to unfavorable body positioning were taken into account. The anesthesiology and neurosurgery teams checked the absence of extreme traction, flexion, and extension of the neck, shoulder, elbow, and forearm, inspected to ensure their neutral positioning. Subsequently, the SSEP from the bilateral upper and lower extremities exhibited normal values (Fig. 3). For the SSEP monitoring at the upper limb, the stimulation point was on the median nerve about 3 cm proximal to the wrist joint. Considering the normal SSEP findings in the upper limb and the neutral position of the neck, shoulder, elbow, and forearm, we ruled out the possibility of a position-related impending peripheral nerve injury originating from the neck and forearm. Finally, peripheral neuropathy at the wrist joint, presumably carpal tunnel syndrome (CTS), was suspected to be the etiological factor for the abnormal TOF and MEP results. As there had been no remedy for this condition, only MEP from the right APB was used as control MEP to assess the systemic effects of the pharmacological and physiological factors. Thereafter, the surgery was completed uneventfully under MEP monitoring, relying on the control MEP from the right APB. Postoperatively, the bilateral numbness in the lower limbs resolved, and there were no new neurological deficits. Postoperative interview with the patient revealed complaints of numbness in the left middle and fourth fingers. Subsequent examination confirmed the diagnosis of CTS.

Train-of-four bilateral test conducted at the upper and lower extremities. The first train-of-four (TOF) recordings obtained immediately after prone positioning of the patient exhibited a significantly prolonged latency in the left APB. The recording time was adjusted to observe the left APB waveform (second row). The last row indicates that although the TOF already reached 100%, the left APB had a small amplitude and prolonged latency

Initial transcranial electrical stimulation motor-evoked potential. The left APB exhibited lower amplitude (0.263 mV) and prolonged latency (34.8 ms), whereas the right APB showed normal values of these parameters (1.76 mV and 21.7 ms, respectively). The ideal imaginary line indicated that the left APB had a significantly prolonged latency

Somatosensory-evoked potentials. Somatosensory-evoked potentials helped identify the cause of the abnormality by demonstrating normal results in all extremities during the procedure