Early interpretation of evoked potential and H-reflex in thoracoabdominal aortic replacement

Objective. To analyze intraoperative neurophysiological monitoring modalities (somatosensory evoked potentials, motor evoked potentials or H-reflex) as the earliest signs of spinal cord ischemia in open thoracoabdominal aortic replacement, and to evaluate contribution of each modality to the first alert signal. Material and methods. A single-center prospective observational study included 51 patients who underwent open thoracoabdominal aortic replacement with intraoperative neurophysiological monitoring. Somatosensory evoked potentials were recorded in 50 (98%) patients, motor evoked potentials — in 46 (90%), H-reflex — in 21 (41%) cases. With involvement of neurophysiologist at each stage, we analyzed evoked potentials, including intraoperative signal decrease, complete disappearance of response, signal recovery by the end of surgery, and time to signal decrease and recovery after onset of spinal cord ischemia. The first modality that generated an alert signal (potential amplitude decrease by more than 50%) was analyzed separately. Results. Postoperative spinal cord ischemia was detected in 4 (8%) patients: 3 (6%) cases of monoparesis and 1 (2%) case of paraplegia. Intraoperative decreases in somatosensory evoked potentials were observed in 39 (78%) patients, decreases in motor evoked potentials — in 39 (85%), decreases in H-reflex — in 15 (71%) cases. Median period between onset of spinal cord ischemia and significant decrease was 29 [17—50] min for somatosensory evoked potentials, 10 [1—29] min for motor evoked potentials and 8 [1—20] min for H-reflex. Somatosensory evoked potentials contributed to the first alert signal in 17/39 patients with decrease (44%), motor evoked potentials in 35/39 (90%), H-reflex in 13/15 (87%). When motor evoked potentials and/or H-reflex were combined, they contributed to the first alert signal in 39/40 patients (97%). Conclusion. Motor evoked potentials and H-reflex demonstrated earlier response to intraoperative spinal cord ischemia than somatosensory evoked potentials and more often contributed to the first alert signal. This indicates greater sensitivity of motor and segmental responses to early reduction in spinal cord perfusion. These findings support motor evoked potentials and H-reflex as components of multimodal intraoperative neurophysiological monitoring.