📑
Chapter Summary
• Context: Intraoperative neurophysiological monitoring (IONM) functions as an early warning system for neural pathway distress during spinal procedures. Its greatest clinical utility occurs when signal changes are recognized while the insult is still reversible. The chapter emphasizes multimodal monitoring—combining motor evoked potentials (MEPs), somatosensory evoked potentials (SSEPs), and electromyography (EMG)—because each modality evaluates distinct neural tracts. Signal loss triggers an immediate diagnostic challenge: differentiating technical failure, anesthetic suppression, or systemic hemodynamic changes from true mechanical compression, stretch, or spinal cord ischemia. Signal interpretation depends on the alert pattern, the affected modality, and, crucially, temporal correlation with the latest surgical step. Utilizing D-wave recording in intramedullary or deformity cases adds critical prognostic value regarding corticospinal tract integrity.
• Chapter Objective: To present the fundamentals of major IONM modalities, guide intraoperative signal interpretation, and establish a rapid response checklist for alerts. The chapter addresses anesthetic interference, spinal cord perfusion optimization (MAP targets), surgical maneuver reversal, D-wave prognostic value, and artificial intelligence in waveform interpretation.
• Modalities and Neural PathwaysSSEPs monitor dorsal sensory columns; transcranial MEPs evaluate anterior and lateral corticospinal motor pathways; and triggered/spontaneous EMG monitors nerve root irritation and integrity. Multimodal monitoring eliminates blind spots in high-risk surgery. D-wave monitoring directly records spinal cord axonal volleys, distinguishing transient synaptic muscle MEP loss from irreversible structural corticospinal injury.
• Global vs. Focal AlertsThe alert pattern is diagnostic: bilateral, simultaneous loss across multiple channels points to systemic causes (hypotension, hypothermia, deep anesthesia, technical disconnect). Conversely, focal or asymmetric signal loss limited to a specific limb or myotome strongly suggests localized mechanical insult (screw misplacement, excessive distraction, compression, or focal vascular compromise). Correlation with the surgical timeline is critical.
• Stepwise Rescue ProtocolThe rescue protocol requires: 1) Pause the surgical maneuver immediately; 2) Check technical integrity of electrodes and stimulators; 3) Alert anesthesia to optimize MAP (> 85-90 mmHg), ensure 100% FiO2, and eliminate volatile agents/muscle relaxants; 4) Warm saline irrigation of the surgical field; 5) Reverse the last surgical maneuver (release rod distraction, remove offending screw, release correction); 6) Administer IV corticosteroids if indicated; 7) Perform a wake-up test (Stagnara test) if signals do not recover and uncertainty persists. Neurological function takes absolute precedence over radiographic curve correction.
• Limitations and Future HorizonsFalse positives and false negatives can occur. Preoperative baseline deficits require tailored stimulation parameters. Machine learning and automated real-time waveform analysis are emerging to differentiate true neural events from electrophysiological artifacts.
• Clinical Application: When an alert occurs, the team must rapidly convert electrophysiological data into a causal hypothesis. Immediate communication between surgeon, anesthesiologist, and neurophysiologist is essential. Global loss triggers hemodynamic and anesthetic correction; focal loss prompts immediate surgical field inspection and maneuver reversal. Anticipating high-risk steps (osteotomies, derotation, traction) by informing the neurophysiologist establishes clean baselines. In intramedullary tumor resections, preserving D-wave amplitude above 50% guarantees long-term motor preservation.
