The Influence of Anesthetic and Surgical Factors on Spinal Late Motor Responses: A Systematic Review

Late motor responses, including F-waves and H-reflexes, are commonly recorded during electrophysiological diagnostic evaluation in awake patients. Although these responses could offer added value during intraoperative neurophysiological monitoring, their use has not yet been widely adopted. This systematic review aims to assess current knowledge on how anesthetic and surgical factors influence the recording and monitoring of spinal late responses during surgery. The authors searched PubMed and EMBASE for relevant articles on studies of late response recording. Studies were considered eligible if the influence of one or more of the following parameters on late responses were assessed: (1) ventilation and metabolic parameters, (2) volatile agents, (3) intravenous agents, (4) neuromuscular blocking agents, (5) epidural or intrathecal agents, or (6) surgical factors. The QUADAS-2 tool was used to assess the risk of bias and concerns regarding applicability. Sixty-two studies were included. Respiratory conditions like hypercapnia reduced H-reflex amplitude, whereas hyperoxia and intermittent hypoxia had limited impact. Volatile agents, including isoflurane, sevoflurane, and nitrous oxide decreased late response amplitudes, with dose-dependent effects. Most intravenous anesthetics, particularly propofol, reduced H-reflex amplitude and F-wave persistence in a dose-dependent manner. Conversely, ketamine was found to increase H-reflex amplitude. During spinal and vascular surgeries, H-reflex monitoring proved feasible and provided valuable supplementary data to the conventional motor evoked potential (MEP) and somatosensory evoked potential (SSEP) modalities. This systematic review demonstrates that H-reflexes and F-waves are significantly influenced by anesthetic factors. The reviewed studies indicate that intravenous and volatile anesthetics significantly affect late responses, by reducing H-reflex and F-wave amplitudes, whereas latencies appear unaffected. The methodological quality varied significantly across the included studies and affects the interpretation of their conclusions. These findings support the potential integration of late response monitoring into intraoperative neurophysiological monitoring (IONM) protocols, provided that anesthetic management and surgical context are carefully considered.