The use of a circumferential cathode improves amplitude of intraoperative electrical transcranial myogenic motor evoked responses

Measurement of motor evoked responses to transcranial electrical stimulation (tc-MER) is a technique for intraoperative monitoring of motor pathways. Since most anesthetics significantly reduce motoneuronal excitability, optimal stimulation paradigms should be sought. We compared the efficiency of stimulus delivery using two different configurations of the cathode component of the stimulating electrode pair (circumferential: F(z), F₃, F₄, A₁, and A₂ versus a single cathode at F(z)). The anode was positioned at C(z) with both cathode configurations. Fourteen neurologically normal patients undergoing spinal surgery were anesthetized with sufentanil- N₂O-ketamine. Partial neuromuscular blockade (single twitch height 25%) was maintained with vecuronium. Compound action potentials to transcranial stimulation with both cathode configurations were recorded from the tibialis anterior muscle. All recordings were completed before spinal manipulation. The median amplitude response using the F(z) cathode configuration was 256 μV (10th-90th percentiles: 50-641 μV). With the circumferential cathode configuration, tc-MER amplitude increased to 281 (87-1479) μV (P < 0.01). There was no significant difference in onset latency between electrode configurations. The observed tc-MER amplitude augmentation with the use of a circumferential cathode might allow tc-MER monitoring in those patients who do not have sufficiently reproducible responses when a single cathode is used. A possible explanation is that the circumferential cathode alters the direction of electrical currents in the cortex, resulting in more efficient depolarization of cortical motor neurons.