Spinal cord monitoring

Surgical approach (where spinal cord monitoring is used)

• Common operations requiring monitoring
  • Spinal deformity correction (scoliosis): instrumentation, rod placement, correction manoeuvres, possible osteotomies
  • Intramedullary/extramedullary spinal cord tumour surgery: laminectomy, durotomy, myelotomy, tumour debulking
  • Thoracoabdominal aortic aneurysm (TAAA) repair / descending thoracic aorta: cross-clamping, intercostal artery interruption/reimplantation, CSF drainage may be used
  • Cervical spine decompression/fusion: traction, decompression, instrumentation
• Key cord-risk moments (communicate with monitoring team)
  • Positioning (prone/neck flexion/traction), deformity correction manoeuvres, pedicle screw placement, temporary vessel occlusion/cross-clamp, hypotension/anaemia

Anaesthetic management (to facilitate spinal cord monitoring)

• Type of anaesthesia
  • Usually GA with TIVA to optimise SSEP/MEP signal quality
  • Regional techniques generally avoided if they interfere with monitoring (e.g., neuraxial local anaesthetic blocks SSEPs/MEPs); consider wound infiltration/erector spinae plane blocks as alternatives depending on surgery and local practice
• Airway
  • ETT preferred (prone surgery, long duration, need for controlled ventilation, stable CO2/O2, access limitations)
• Duration
  • Often prolonged: scoliosis 4–8+ h; tumour 3–6 h; TAAA 6–12 h (variable)
• How painful?
  • Moderate–severe (spine instrumentation and osteotomies particularly painful); plan multimodal analgesia while preserving monitoring
• Core anaesthetic technique (typical)
  • Induction: propofol + short-acting opioid; consider lidocaine/ketamine adjuncts; avoid long-acting sedatives
  • Maintenance: propofol infusion + remifentanil (or fentanyl/alfentanil); aim stable depth to avoid signal drift
  • Neuromuscular blockade: allow for intubation/positioning; then minimise/avoid for MEPs (or maintain a stable low level if required for surgical conditions and agreed with neurophysiology)
  • Ventilation: normocapnia; avoid hypoxia; stable PaCO2 and temperature (both affect latency/amplitude)
  • Haemodynamics: maintain spinal cord perfusion (MAP targets agreed; commonly ≥75–85 mmHg for spine, higher if signals deteriorate or high-risk vascular cases)
  • Blood management: anticipate major blood loss (cell salvage, TXA where appropriate, large-bore access, warming, point-of-care coagulation)

Aims and rationale

• Detect impending spinal cord injury early enough to allow intervention and prevent permanent neurological deficit
• Different modalities assess different pathways: dorsal columns (SSEP) vs corticospinal tracts/anterior cord (MEP)
• Monitoring is adjunctive: does not replace meticulous surgical technique, perfusion management, and postoperative neurological assessment

Modalities: what they monitor and how

• SSEPs (somatosensory evoked potentials)
  • Stimulate peripheral nerve (e.g., posterior tibial/median) and record cortical/subcortical responses
  • Assesses dorsal column–medial lemniscus pathways; less sensitive to anterior spinal artery territory ischaemia
  • Alarm criteria often: amplitude ↓ >50% and/or latency ↑ >10% from baseline (institution-dependent)
• MEPs (motor evoked potentials)
  • Transcranial electrical stimulation (TES) of motor cortex; record compound muscle action potentials (myogenic MEPs) in limb muscles
  • Assesses corticospinal tract/anterior cord function; more sensitive to ischaemia and mechanical injury affecting motor pathways
  • Highly sensitive to anaesthetic agents and neuromuscular blockade; requires stable technique and minimal NMB
  • Alarm criteria: loss of MEPs or major amplitude reduction; interpretation is context-specific and muscle-specific
• EMG (electromyography)
  • Free-run EMG: detects spontaneous neurotonic discharges suggesting nerve root irritation/traction
  • Triggered EMG: pedicle screw stimulation to detect breach/close proximity to nerve root (thresholds vary with system and anatomy)
• D-wave (epidural spinal cord evoked potential)
  • Recorded from epidural electrode on spinal cord after transcranial stimulation; reflects direct corticospinal tract volley
  • Less affected by anaesthetic agents and neuromuscular blockade than myogenic MEPs; used mainly in intramedullary tumour surgery
  • Preservation of D-wave with loss of myogenic MEPs may predict transient deficit; D-wave loss suggests higher risk of permanent motor deficit

Anaesthetic effects on monitoring (high-yield)

• Volatile agents (sevo/des/isoflurane)
  • Dose-dependent depression of SSEP amplitude and prolongation of latency; profound suppression of MEPs even at low MAC
  • If used, keep very low MAC (e.g., ≤0.3–0.5) and stable; but TIVA is preferred for reliable MEPs
• IV anaesthetics
  • Propofol: reduces amplitudes (esp. MEP) but predictable and workable with stable infusion; avoid boluses during critical monitoring periods
  • Opioids (remifentanil/fentanyl): minimal effect on SSEPs/MEPs; useful to reduce hypnotic requirement
  • Ketamine: may increase SSEP/MEP amplitudes; useful rescue adjunct if signals poor (consider psychomimetic emergence; manage with propofol/benzodiazepine if needed)
  • Dexmedetomidine: generally modest effects; can reduce requirements for propofol/volatile; watch bradycardia/hypotension
• Neuromuscular blockade
  • Myogenic MEPs and EMG require intact neuromuscular transmission; avoid continuous paralysis if MEPs/EMG needed
  • If some relaxation required, use short-acting agent and maintain a consistent low level with monitoring (e.g., TOF target agreed) to avoid confounding changes
• Physiology and technical factors
  • Hypotension, anaemia, hypoxia, hypocapnia/hypercapnia extremes, hypothermia all worsen signals and/or indicate cord risk
  • Electrode displacement, diathermy interference, poor impedance, limb ischaemia (e.g., positioning/pressure) can mimic neurological change

Practical conduct: set-up, communication, and baseline acquisition

• Pre-op planning
  • Confirm modalities required (SSEP/MEP/EMG/D-wave) and anaesthetic constraints (avoid NMB/volatile; stable TIVA)
  • Review neuro status, myelopathy, neuropathy, diabetes, prior deficits (baseline may be absent/poor)
  • Discuss haemodynamic targets and response plan to signal change with surgeon and neurophysiology
• Intra-op workflow
  • Obtain stable baselines after induction, positioning, and before major surgical steps; document anaesthetic concentrations/infusion rates and MAP at baseline
  • Avoid boluses of propofol/volatile changes around monitoring checks; if changes necessary, warn neurophysiology
  • Maintain normothermia, stable ventilation, and adequate perfusion; treat blood loss early
• Safety with MEP stimulation
  • Bite block to prevent tongue/lip injury; secure airway and eyes; anticipate patient movement during stimulation (protect surgical field)
  • Relative contraindications: uncontrolled epilepsy, raised ICP, unstable intracranial lesions; consider pacemakers/ICDs (liaise; risk usually low with modern systems but must be assessed)

Response to intraoperative signal deterioration (structured approach)

• Immediate actions (first 1–2 minutes)
  • Stop surgical manipulation/correction; inform surgeon and neurophysiology; check timing vs recent events (traction, screw placement, clamp, hypotension, bolus drugs)
  • Check artefact: electrodes/leads, impedance, diathermy, stimulation settings; confirm whether change is unilateral/bilateral and SSEP vs MEP
• Optimise spinal cord perfusion and oxygen delivery
  • Increase MAP (vasopressors, reduce anaesthetic depth if safe); treat hypotension promptly
  • Correct anaemia/major blood loss; ensure adequate oxygenation; consider ABG to confirm PaO2/PaCO2 and acid-base
  • Ensure normothermia; avoid extremes of PaCO2; correct hypocalcaemia/hyperkalaemia if massive transfusion
• Anaesthetic-specific troubleshooting
  • Reduce/stop volatile agent; avoid propofol boluses; consider lowering propofol infusion slightly while maintaining hypnosis (use processed EEG if available)
  • Ensure no residual neuromuscular blockade for MEP/EMG (check TOF; allow recovery; avoid top-ups)
  • Consider ketamine bolus/infusion as rescue to improve MEP/SSEP amplitudes (local protocol dependent)
• Surgical/position-related interventions
  • Reverse recent correction/traction; remove/adjust offending instrumentation; check pedicle screws; decompress if needed
  • Check positioning: neck alignment, avoid excessive flexion/rotation, relieve limb pressure/ischemia; ensure no abdominal compression in prone (venous congestion)
• Escalation
  • If persistent loss: consider wake-up test (Stagnara) if feasible and agreed; plan postoperative imaging and neurocritical care
  • In vascular cases: consider CSF drainage optimisation, distal perfusion, reimplantation of intercostals, reduce clamp time (surgeon-led); maintain higher MAP

Wake-up test (Stagnara): key points

• Indication: unresolved concerning monitoring change or when monitoring unavailable/unreliable; used mainly in scoliosis surgery
• Technique: lighten anaesthesia to allow purposeful movement; ask patient to move hands/feet; maintain airway/ventilation and haemodynamic stability
• Limitations/risks: awareness/distress, movement risking instrumentation, time delay, not feasible in all patients (language barrier, developmental delay), may be confounded by residual NMB

Postoperative considerations

• Immediate neuro exam and documentation; low threshold for imaging if deficit suspected
• Maintain cord perfusion post-op (avoid hypotension), optimise Hb/O2, manage pain without excessive sedation
• High-risk cases may need HDU/ICU (major blood loss, prolonged surgery, neurology concerns, TAAA repair)