Safety Measures in Scoliotic Surgery

1. Intraoperative Neurophysiologic Monitoring (IONM)

Somatosensory Evoked Potentials (SSEPs)

• Monitor posterior column (dorsal) integrity
• Changes in SSEPs are noted in 2.5%–65% of spinal surgeries
• When changes are promptly reversed (e.g., by lessening the degree of spine straightening or raising arterial blood pressure), neurologic function is usually preserved postoperatively
• When changes persist, patients most often awaken with worsened neurologic function
• False negatives are rare (<1% of monitored cases) — intact SSEPs with new postoperative deficit has been reported

Transcranial Motor Evoked Potentials (tcMEPs)

• Monitor anterior column (corticospinal tract) integrity — a distinct blood supply from SSEPs via the anterior spinal artery
• Critical because patients can develop significant motor deficits with intact SSEPs throughout surgery
• Consensus (American Society of Neurophysiologic Monitoring): combined MEP + SSEP monitoring is well established to prevent injury to both sensory and motor tracts

Pedicle Screw Testing

• Pedicle screws are the most frequent spinal instrumentation component; malposition risks pedicle breach, weakened construct, and postoperative radicular pain
• Electrophysiologic testing of the pilot hole or implanted screw shank helps detect malposition before nerve injury occurs

Miller's Anesthesia, 10e, p. 5276–5278

2. Blood Loss Management

Antifibrinolytic Agents

• Tranexamic acid (TXA) is more effective than aprotinin and epsilon-aminocaproic acid (EACA) in reducing total blood loss, intraoperative blood loss, and transfusion requirements
• Must be avoided in patients with a history of thromboembolic disease

Deliberate Controlled Hypotension

• Used to reduce intraoperative blood loss
• Must be used with caution in older adults, those with cardiovascular disease, and those at risk for:
  • Ischemic complications
  • Postoperative vision loss (a known risk)

Cell Salvage and Preoperative Planning

• Formal pulmonary function tests preoperatively help guide the extent of surgery and need for postoperative ventilatory support
• Transthoracic echocardiography assesses pulmonary hypertension and right ventricular hypertrophy before major corrective surgery

Miller's Anesthesia, 10e, p. 9236–9238

3. Spinal Cord Perfusion Preservation

• Spinal cord ischemia during spine straightening is the primary mechanism of cord injury
• Maintaining adequate mean arterial pressure (MAP) is critical — hypotension combined with distraction is a major risk factor
• If SSEP/MEP changes occur intraoperatively:
  1. Surgeon reduces corrective force (lessens degree of straightening)
  2. Anesthesiologist raises blood pressure
  3. Surgeon inspects and adjusts instrumentation (e.g., repositions a laminar hook narrowing the canal)

4. Special Considerations in Neuromuscular Scoliosis

• Heightened hemodynamic monitoring
• Anticipation of greater blood loss
• SSEP monitoring of posterior columns (somatosensory, posterior spinal artery territory)

Miller's Anesthesia, 10e, p. 10650

5. Venous Air Embolism (VAE) Prevention

• A recognized complication of scoliosis surgery due to large venous channels opened during posterior spinal dissection
• Precautions include:
  • Prompt waxing or packing of exposed bone
  • Monitoring with precordial Doppler or end-tidal CO₂
  • Avoiding conditions that increase venous air entrainment

6. Preoperative Assessment

7. Positioning Safety

• Prone positioning carries risk of pressure injuries, ocular ischemia (postoperative vision loss), and brachial plexus stretch
• In patients with unstable cervical spine, positioning can be done awake after airway anesthesia or under fluoroscopic evaluation as an alternative to relying solely on IONM

Summary Table

Sources: Miller's Anesthesia, 2-Volume Set, 10e — pp. 5276–5278, 9236–9238, 10650

Intraoperative Neurophysiologic Monitoring (IONM) — In-Depth

Overview and Rationale

Miller's Anesthesia, 10e, p. 5202

Fundamental Principles

Sensory Evoked Responses (SERs) — Basic Concepts

• Averaging: Neural signals are tiny (~1 µV) relative to background EEG noise (~50–100 µV). Signal averaging of repeated stimuli extracts the consistent evoked signal from random noise. The more responses averaged, the better the signal-to-noise ratio.
  • Near-field potentials (electrode close to generator): 50–100 averages needed
  • Far-field potentials (electrode distant from generator): several thousand averages needed
• Near-field vs far-field: Near-field potentials are larger, easier to identify. Electrode placement strongly affects morphology. Far-field potentials look similar regardless of exact electrode position.
• Cortical vs subcortical: Cortical SERs arise when the sensory volley reaches cortex — recorded as near-field potentials from scalp electrodes. Subcortical responses may arise from peripheral nerves, spinal cord, brainstem, thalamus, or cranial nerves.

Describing Evoked Potentials

Miller's Anesthesia, 10e, p. 5227–5230

I. Somatosensory Evoked Potentials (SSEPs)

Anatomy of the SSEP Pathway

1. Large-fiber sensory nerves → dorsal root ganglia → ipsilateral posterior column of spinal cord
2. Synapse in dorsal column nuclei at cervicomedullary junction
3. Decussation → contralateral thalamus via medial lemniscus
4. Third-order neurons: thalamus → frontoparietal sensorimotor cortex

• Group I fibers travel in the dorsal spinocerebellar tract → synapse in nucleus Z (spinomedullary junction) → decussate → ventral posterolateral thalamic nucleus
• The dorsal lateral funiculus is supplied by the anterior spinal artery — the same artery supplying the descending motor tracts and anterior horn cells
• This dual supply explains why lower extremity SSEP changes can accompany anterior cord ischemia (e.g., during scoliosis distraction)

Electrode Placement

Alarm Criteria

• Amplitude decrease ≥ 50%, or
• Latency increase ≥ 10%

Clinical Performance

• Changes detected in 2.5%–65% of spinal surgeries (wide range reflecting varying patient populations and surgeon technique)
• When changes reversed promptly → neurologic function most often preserved
• When changes persist → patients most often awaken with worsened neurologic function
• False negatives (intact SSEPs, new deficit on awakening): <1% of all monitored cases — rare but reported
• False positives (changes that resolve without deficit): common — do not necessitate aborting surgery, but require evaluation

Limitations of SSEPs

• Posterior columns: supplied by posterior spinal arteries
• Motor tracts (corticospinal): supplied by anterior spinal artery

Miller's Anesthesia, 10e, p. 5231–5232

II. Transcranial Motor Evoked Potentials (tcMEPs)

Rationale

Mechanism

• Stimulation: High-voltage electrical pulses delivered via scalp electrodes placed over the motor cortex (following the 10-20 EEG system, but over motor rather than sensory cortex)
• Recording: Compound muscle action potentials (CMAPs) recorded from target muscles in the limbs (typically tibialis anterior, abductor hallucis for lower extremities; thenar muscles for upper extremities)
• Activates the corticospinal tract (pyramidal tract) — the anterior cord motor pathway

Signal Characteristics

• MEP responses are variable — unlike SSEPs, they are not averaged, they are elicited by a brief train of stimuli and recorded as large muscle responses
• More susceptible to anesthetic depression than SSEPs
• Require a carefully controlled anesthetic technique (see Anesthetic Effects section)

Alarm Criteria

• All-or-nothing change (loss of response) is the most clinically significant
• Some use amplitude decrease >50–80% as a warning threshold
• Context matters: a partial reduction in a stable baseline is less concerning than sudden complete loss

Combined SSEP + tcMEP

• Many case series confirm that simultaneous recording increases sensitivity and reduces false negatives (i.e., cases where the patient awakens with a new deficit despite no monitoring changes)

Miller's Anesthesia, 10e, p. 5277

III. Pedicle Screw Testing (Electromyography — EMG)

Rationale

• Weakened construct
• Postoperative radicular pain from nerve root irritation or compression

Method

• Stimulated EMG: Current delivered through the pilot hole or screw shank
• If the pedicle wall is intact, high current thresholds are required to elicit a muscle response (current must spread through bone to reach the nerve root)
• If the pedicle is breached, current travels directly through the defect to the adjacent nerve root — a low stimulation threshold indicates breach
• Threshold <6–8 mA is generally considered suspicious for pedicle wall breach and warrants fluoroscopic or CT verification

Free-run EMG

• Continuous background EMG monitoring detects spontaneous neurotonic discharges indicating nerve root irritation from mechanical manipulation
• Provides real-time feedback during dissection, retraction, and instrumentation

Miller's Anesthesia, 10e, p. 5278

IV. Electrophysiologic Alarm Criteria Summary

V. Causes of Signal Changes — Differential Diagnosis

Technical/Anesthetic Causes (Non-Surgical — "False Positives")

• Electrode displacement or impedance increase
• Changes in anesthetic depth (especially volatile agents — see below)
• Temperature decrease (hypothermia prolongs latency)
• Hypotension (reduces CNS perfusion)
• Hypoxia or hypocapnia
• Neuromuscular blockade (abolishes tcMEP muscle recordings)

Surgical Causes (True Neurologic Risk)

• Spinal cord ischemia from distraction or instrumentation
• Pedicle breach with nerve root compression
• Hematoma or retraction injury
• Vascular injury (anterior spinal artery compromise)

Protocol When Changes Occur

1. Check technical factors: electrode integrity, impedance, stimulation parameters
2. Check anesthetic conditions: depth, blood pressure, temperature, oxygenation, NMB reversal
3. Notify surgeon — describe which modality, which limb, onset characteristics
4. Surgical response: reduce distracting forces, adjust instrumentation
5. Anesthetic response: increase MAP (often 20–30 mmHg above baseline), correct any physiologic derangement
6. If no resolution: wake-up test may be considered

VI. Anesthetic Effects on IONM — Critical Considerations

Volatile Anesthetic Agents (Halogenated)

• Dose-dependent depression of both SSEPs and MEPs
• Suppress cortical amplitude and increase latency
• MEPs are particularly sensitive — high concentrations may abolish responses
• Recommendation: maintain at ≤0.5 MAC (or use total intravenous anesthesia — TIVA) when MEP monitoring is planned

Nitrous Oxide

• Significantly depresses MEPs even at low concentrations
• Best avoided during combined SSEP/MEP monitoring

Intravenous Agents

• Propofol: dose-dependent cortical suppression — SSEP amplitude decreases, MEPs preserved at moderate doses
• Ketamine: enhances amplitude of SSEPs and MEPs — sometimes used to augment recordings
• Opioids (fentanyl, remifentanil): minimal effect on SSEPs and MEPs at clinical doses — preferred analgesic adjuncts
• Benzodiazepines: mild cortical depression — relatively sparing of subcortical responses
• Etomidate: increases SSEP amplitude — occasionally used to improve recordings
• Neuromuscular blocking agents (NMBAs): abolish tcMEP muscle recordings — must be avoided or fully reversed before MEP monitoring; partial NMB is acceptable for SSEP

Preferred Anesthetic Technique for IONM

• TIVA (propofol + opioid infusion) is the most compatible with reliable MEP recording
• If volatile agents used: keep ≤0.5 MAC
• Avoid nitrous oxide
• Avoid bolus doses of agents that cause sudden amplitude changes during critical steps
• Maintain stable temperature, MAP, PaCO₂

Miller's Anesthesia, 10e, p. 5231–5278

VII. The Wake-Up Test

• Still used as a confirmatory test when IONM changes are ambiguous or equipment fails
• Limitations: only a single time-point assessment; risk of patient movement, air embolism, accidental extubation; cannot be repeated frequently
• Now largely replaced by continuous tcMEP monitoring, but remains a valid backup

Summary: IONM Modalities at a Glance

Miller's Anesthesia, 2-Volume Set, 10e — pp. 5202, 5227–5232, 5276–5278