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Chapter Summary
• Context: Intraoperative neurophysiological monitoring (IONM) provides real-time functional assessment of the central and peripheral nervous system during spine surgery, serving as an early-warning diagnostic tool to detect impending iatrogenic neural injury, guide safe corrective maneuvers, and prevent postoperative neurological deficits. Multimodal IONM integrates transcranial motor evoked potentials (TcMEP), somatosensory evoked potentials (SSEP), spontaneous and triggered electromyography (sEMG and tEMG), and dermatomal/reflex responses (D-wave, bulbocavernosus reflex). Each modality monitors distinct neural pathways with specific temporal sensitivities and alarm criteria: SSEPs assess the dorsal column-medial lemniscal sensory pathway (supplied by the posterior spinal arteries), while TcMEPs evaluate the corticospinal motor pathway and anterior horn cells (supplied by the anterior spinal artery). Consequently, selective anterior spinal artery ischemia or direct anterior motor tract contusion can abolish TcMEPs without altering SSEPs. Free-run sEMG detects mechanical nerve root traction, irritation, or compression during instrumentation and decompression, while pedicle screw triggered EMG (tEMG) detects pedicle cortical breaches. Successful IONM requires total intravenous anesthesia (TIVA) without halogenated inhalational agents or neuromuscular blockade (which abolish MEPs and EMG), temperature and blood pressure optimization, and adherence to structured institutional checklists to identify and rapidly reverse intraoperative neurological insults.
• Chapter Objective: To present the neurophysiological principles, modalities, alarm criteria, and anesthetic protocols of multimodal intraoperative neuromonitoring (IONM) in spine surgery. The reader should be able to understand the physiological pathways evaluated by SSEP, TcMEP, EMG, D-wave, and reflex testing; apply multimodal monitoring in deformity, cervical myelopathy, intradural tumor, and minimally invasive lateral surgery; recognize significant neurophysiological alarm threshold criteria; and execute structured intraoperative rescue protocols following acute signal degradation.
• Multimodal IONM Modalities and Neural PathwaysMultimodal IONM combines complementary neurophysiological modalities: 1) Somatosensory Evoked Potentials (SSEP): elicited by peripheral nerve stimulation (median, ulnar, posterior tibial nerves) and recorded over the primary somatosensory cortex and cervical spine, evaluating the ascending dorsal column-medial lemniscal sensory pathways; 2) Transcranial Motor Evoked Potentials (TcMEP): generated by transcranial electrical stimulation over the motor cortex and recorded as compound muscle action potentials (CMAPs) from target limb muscles, evaluating the descending lateral corticospinal tracts and alpha motor neurons; 3) D-Wave: recorded directly from the spinal epidural space via a catheter electrode, evaluating direct pyramidal tract axonal conduction independent of neuromuscular junction transmission; 4) Spontaneous EMG (sEMG): continuous monitoring of root-specific muscles for neurotonic burst and train discharges indicating mechanical root irritation; 5) Triggered EMG (tEMG): delivering electrical current to pedicle screw probes to detect bone breaches; and 6) Bulbocavernosus Reflex (BCR): monitoring the S2–S4 conus medullaris and pudendal reflex arc.
• Validated Alarm Threshold CriteriaNeurophysiological alarm criteria define significant signal loss requiring immediate surgical and systemic action: 1) SSEP alarm: a >=50% decrease in amplitude and/or a >=10% increase in latency compared to stable baseline; 2) TcMEP alarm: a >=50–80% amplitude reduction, significant threshold voltage increase (>100 V), or complete loss of CMAP response in one or more limb muscles; 3) D-Wave alarm: a >=50% reduction in D-wave amplitude during intramedullary tumor resection indicates permanent motor tract injury; 4) sEMG alarm: neurotonic train activity (sustained high-frequency discharge) indicates active mechanical root stretch or compression; 5) Triggered EMG: pedicle screw threshold <6–8 mA in the lumbar spine or <5–6 mA in the thoracic spine indicates medial/inferior pedicle wall breach with direct nerve root contact.
• Anesthetic Requirements: TIVA ProtocolAnesthesia profoundly influences IONM signal acquisition. Halogenated volatile inhalational anesthetics (sevoflurane, desflurane, isoflurane) and nitrous oxide produce dose-dependent suppression of anterior horn alpha motor neurons, markedly blunting or abolishing TcMEP and H-reflex responses. Therefore, Total Intravenous Anesthesia (TIVA)—utilizing continuous infusions of propofol and short-acting opioids (remifentanil, sufentanil, fentanyl), with or without low-dose ketamine/dexmedetomidine—is mandatory. Neuromuscular blocking agents (paralytics) can be used for initial endotracheal intubation but must be completely avoided or reversed during monitoring (train-of-four [TOF] maintained at 4/4 twitches) to permit muscle MEP and EMG recordings.
• Clinical Indications Across Spine SubspecialtiesIONM indications are tailored to surgical risk: 1) Adult and adolescent deformity (scoliosis/kyphosis): continuous TcMEP and SSEP monitoring during correction maneuvers (derotation, translation, distraction) and osteotomies (PCO, PSO, VCR); 2) Cervical and thoracic myelopathy: TcMEP and SSEP monitoring from patient positioning (head positioning and traction) through decompression; 3) Intramedullary spinal cord tumors (ependymomas, astrocytomas): combined TcMEP, SSEP, and D-wave monitoring to establish safe resection margins (stopping resection if D-wave drops >50%); 4) Lateral lumbar interbody fusion (LLIF transpsoas): real-time triggered EMG directional mapping to detect and avoid lumbar plexus branches within the psoas; 5) Lumbosacral instrumented fusion: tEMG screw testing and BCR monitoring to protect roots and bowel/bladder function.
• Structured Intraoperative Rescue ProtocolUpon an IONM alert, a rapid, standardized multidisciplinary checklist must be executed simultaneously: 1) Systemic optimization: verify mean arterial pressure (MAP target >80–85 mmHg to restore cord perfusion), check core temperature (>35.5°C), hematocrit (>28–30%), arterial blood gases, and rule out anesthetic bolus or volatile gas accidental administration; 2) Technical verification: check recording/stimulating electrodes and machine grounding; 3) Surgical intervention: immediately halt active surgical maneuvers, remove compressive retractors, irrigate wound with warm saline, release temporary correction rods or distraction forces, reverse deformity correction, and remove recently placed screws or interbody cages. If signals fail to recover despite optimization, consider an intraoperative wake-up test (Stagnara wake-up test).
• Clinical Application: In spine deformity surgery, TcMEPs detect impending spinal cord ischemia or stretch injury minutes before SSEP changes appear. When a sudden loss of lower extremity MEPs occurs during scoliosis rod derotation or vertebral column resection (VCR), the surgical team immediately raises MAP to >85–90 mmHg, pauses the procedure, and releases rod tension. In over 85–90% of cases, timely release of correction and perfusion augmentation restore MEP signals and prevent permanent paraplegia. In thoracic and lumbar pedicle screw placement, triggered EMG with thresholds <6 mA accurately flags misplaced screws encroaching on the spinal canal or exiting nerve root, enabling immediate repositioning before wound closure. In intramedullary tumor resections, preserving a D-wave amplitude >50% ensures that the patient will maintain long-term ambulatory motor function, even if transient postoperative paresis occurs due to spinal cord stunning.
