Anaesthesia for Spinal Surgery

Quick Answer

Spinal surgery anaesthesia requires positioning considerations (prone/lateral/sitting), neurophysiological monitoring (SSEPs/MEPs), blood loss management (cell salvage, controlled hypotension), and air embolism prevention. General anaesthesia is standard for most procedures; neuraxial/regional rarely used alone. Positioning risks: eye pressure (prone), brachial plexus stretch (lateral), air embolism (sitting). Controlled hypotension (MAP 60-70 mmHg) reduces bleeding in lumbar/deformity surgery but risks spinal cord ischemia. Motor evoked potentials require modified anaesthesia (total IVA, no volatile >0.5 MAC). Scoliosis correction: risk of anterior spinal artery syndrome, requires Stagnara wake-up test or continuous MEPs/SSEPs. [1-10]

Pathophysiology

Surgical Considerations by Procedure

Cervical Spine Surgery:

• Anterior approach: Cervical discectomy/fusion, corpectomy
  • Airway concerns (intubation technique, postoperative oedema)
  • Risk of recurrent laryngeal nerve injury
  • Oesophageal/peritoneal injury (rare)
• Posterior approach: Laminectomy, fusion, foraminotomy
  • Prone positioning
  • Blood loss (epidural venous plexus)
  • Risk of vertebral artery injury

Thoracic Spine Surgery:

• Thoracotomy approach: Anterior release, tumour resection
  • One-lung ventilation required
  • Major blood loss (segmental vessels)
  • Risk of spinal cord ischemia (segmental arteries supply cord)
• Posterior approach: Thoracic laminectomy, pedicle screws
  • Prone positioning
  • Less blood loss than lumbar

Lumbar Spine Surgery:

• Microdiscectomy: Short procedure, prone or knee-chest position
  • Minimal blood loss
  • Can consider awake spinal for select patients
• Laminectomy/fusion: Prone, moderate blood loss
• Deformity correction: Scoliosis/kyphosis surgery
  • Major blood loss (cell salvage essential)
  • Prolonged surgery (6-12 hours)
  • Neurophysiological monitoring critical
  • Risk of anterior spinal artery syndrome

Positioning Physiology

Prone Position:

• Ventilation: FRC reduced 10-15%, compliance decreased, V/Q mismatch
• Circulation: Cardiac output may decrease (venous return reduced)
• Pressure points: Eyes (blindness risk), nose, breasts, genitals, bony prominences
• Spinal cord perfusion: Gravity improves venous drainage but arterial supply unchanged

Lateral Position:

• Dependent lung: Atelectasis risk
• Brachial plexus: Risk of stretch/compression (axillary roll essential)
• Pressure: Dependent ear, hip, knee
• Access: Thoracotomy approaches

Sitting Position (Rare):

• Advantages: Excellent surgical access, reduced bleeding
• Risks:
  • Venous air embolism (20-40% incidence, 1% clinically significant)
  • Haemodynamic instability (venous pooling)
  • Cerebral hypoperfusion (if MAP <50 mmHg)
  • Quadriplegia (if excessive neck flexion)

Neurophysiological Monitoring

Somatosensory Evoked Potentials (SSEPs):

• Pathway: Peripheral nerve → dorsal column → thalamus → cortex
• Monitoring: Posterior column function (proprioception, vibration)
• Sensitivity: Detects 50% reduction in spinal cord perfusion
• Anaesthetic effects: Reduced by volatile agents (dose-dependent), less affected by IV agents

Motor Evoked Potentials (MEPs):

• Pathway: Motor cortex → corticospinal tract → peripheral nerve
• Monitoring: Anterior cord function (motor pathways)
• Critical for: Scoliosis surgery (anterior spinal artery territory)
• Anaesthetic requirements:
  • Total intravenous anaesthesia (TIVA) preferred
  • No volatile agents >0.5 MAC
  • No muscle relaxants (or minimal, train-of-four 1-2 twitches)
  • Ketamine acceptable

Stagnara Wake-Up Test:

• Purpose: Assess motor function during scoliosis correction
• Technique: Lighten anaesthesia, patient follows commands (move hands/feet)
• Timing: After major correction manoeuvres
• Risks: Airway compromise, recall, movement during critical phase
• Modern alternative: Continuous MEPs/SSEPs (less invasive)

Blood Loss Considerations

Factors Increasing Blood Loss:

• Surgical approach: Posterior > anterior
• Level: Lumbar > thoracic > cervical
• Procedure: Fusion > decompression alone
• Deformity: Scoliosis correction (500-2000 mL typical)
• Position: Prone (venous engorgement) vs. sitting (reduced venous pressure)

Blood Conservation Strategies:

1. Controlled hypotension: MAP 60-70 mmHg (reduces bleeding 30-50%)
2. Cell salvage: Autologous blood recovery (essential for major surgery)
3. Tranexamic acid: 1 g IV bolus, then 1 g over 8 hours (reduces bleeding 30-40%)
4. Fibrin sealants: Topical application
5. Preoperative autologous donation: Rarely used now

Risks of Controlled Hypotension:

• Spinal cord ischemia (if MAP <60 mmHg prolonged)
• Visual loss (posterior ischaemic optic neuropathy, especially with prone position)
• Myocardial ischemia
• Renal impairment
• Fetal compromise (if pregnant)

Air Embolism (Sitting Position)

Pathophysiology:

• Open veins above heart level → negative pressure gradient
• Air entry → right heart → pulmonary circulation
• Paradoxical embolism (if PFO present) → cerebral/coronary emboli

Detection:

• Precordial Doppler: Most sensitive (detects 0.5 mL air)
• EtCO₂: Sudden decrease (dead space effect)
• TEE: Direct visualization
• PaO₂: Decrease (V/Q mismatch)

Management:

1. Flood surgical field: Saline irrigation, bone wax
2. Trendelenburg/left lateral: Traps air in RA apex
3. Aspiration: Central line in RA/PA to aspirate air
4. 100% O₂: Reduces embolus size (nitrogen washout)
5. Supportive: Vasopressors, fluids
6. CPR: If cardiovascular collapse

Clinical Presentation

Preoperative Assessment

History:

• Neurological symptoms: Radiculopathy, myelopathy, weakness, bowel/bladder dysfunction
• Pain: Location, severity, opioid tolerance
• Previous spine surgery: Increased bleeding, difficult dissection
• Positioning tolerance: Can patient tolerate prone position?
• Airway: Cervical spine mobility (if anterior cervical approach)

Physical Examination:

• Airway: Mallampati, neck mobility (cervical cases)
• Cardiovascular: Baseline BP (for controlled hypotension)
• Neurological: Document deficits preoperatively
• Vascular: Access sites (for long cases)

Investigations:

• MRI: Spinal cord compression, surgical level
• CT: Bony anatomy, approach planning
• Blood work: FBC (anemia), coagulation, type & screen (crossmatch 2-4 units for major surgery)
• ECG: Baseline (for controlled hypotension)

Management

Anaesthetic Technique

General Anaesthesia (Standard):

• Induction: Propofol, fentanyl, rocuronium (non-depolarizing for MEPs)
• Maintenance:
  • If MEPs not required: Volatile acceptable (0.5-1 MAC)
  • If MEPs required: TIVA (propofol/remifentanil), avoid volatiles >0.5 MAC
• Monitoring:
  • Standard + arterial line (for controlled hypotension, blood sampling)
  • Central line (if major blood loss anticipated)
  • Urinary catheter (fluid balance, long cases)
  • Temperature (forced air warming, fluid warmer)
  • Neurophysiological monitoring (SSEPs/MEPs)

Regional/Neuraxial (Limited Role):

• Spinal anaesthesia: Selective for lumbar microdiscectomy in awake patient
  • Requires cooperative patient, brief surgery
  • Surgeon comfort with awake patient
  • Rarely used
• Epidural: Postoperative analgesia (catheter placed at end of surgery)

Positioning Protocol

**Prone Positioning (Most Common):

1. Induction supine: Intubate, all lines placed
2. Eye protection: Tape eyes closed, ensure no pressure
3. Turn patient: Coordinated team effort (surgeon, anaesthetist, nurses)
4. Head: Headrest (Mayfield horseshoe or foam), neutral position
5. Chest: Chest rolls or gel pads (allows abdominal hanging, reduces venous pressure)
6. Arms: Tucked at sides (neurosurgery) or on arm boards (orthopaedic)
7. Pressure points: Check face, breasts, genitals, knees, ankles
8. Final check: Airway circuit, lines, monitoring, eyes

Lateral Position:

• Axillary roll: 10-15 cm below dependent axilla (protects brachial plexus)
• Padding: Dependent ear, hip, knee, ankle
• Arm support: Non-dependent arm on arm board or pillows

Sitting Position (If Used):

• Preoperative screening: PFO assessment (contrast echo)
• Monitoring: Precardial Doppler essential
• Central line: RA/PA for air aspiration
• Haemodynamics: Gradual positioning, maintain MAP >70 mmHg
• Alternative: Modified semi-sitting (30°) reduces risk

Controlled Hypotension (If Used)

Indications:

• Expected major blood loss (deformity surgery, multilevel fusion)
• Healthy patient (ASA I-II)
• No contraindications (ischemic heart disease, cerebrovascular disease, renal impairment)

Technique:

• Target: MAP 60-70 mmHg (systolic 80-90 mmHg)
• Duration: Only during critical bleeding phases
• Head position: Elevated (reduces cerebral perfusion pressure, but MAP maintained above critical threshold)

Agents:

• Short-acting beta-blocker: Esmolol (rapid onset/offset, titratable)
• GTN infusion: Venodilation, some arterial dilation
• Clevidipine: Calcium channel blocker, rapid onset, no tachyphylaxis
• Avoid: Long-acting agents (difficult to control, prolonged hypotension)

Monitoring:

• Arterial line (continuous BP)
• SSEPs/MEPs (detect spinal cord ischemia)
• Urine output (>0.5 mL/kg/hour)
• ABG (lactate if prolonged)

Contraindications:

• Ischemic heart disease
• Cerebrovascular disease
• Renal impairment (creatinine >150 μmol/L)
• Anemia (Hb <100 g/L)
• Pregnancy
• Age >60 years (relative)

Intraoperative Considerations

Blood Management:

• Tranexamic acid: 1 g IV bolus at induction, 1 g over 8 hours (reduces bleeding 30-40%)
• Cell salvage: Set up for expected blood loss >500 mL
• Blood products: PRBC if Hb <70-80 g/L (or higher if comorbidities)
• FFP/platelets: If coagulopathy (dilutional, DIC)

Fluid Management:

• Crystalloid: Moderate restriction (avoid hemodilution)
• Colloid: If significant volume needed
• Target: Hct >25-30% (optimal oxygen delivery)

Temperature:

• Monitor: Core temperature (nasopharyngeal/bladder)
• Maintain: >36°C (hypothermia increases bleeding, delays awakening)
• Warming: Forced air, fluid warmer, heated mattress

Wake-Up Test (If Required)

Indications:

• Scoliosis correction (major correction manoeuvres)
• Loss of MEPs/SSEPs
• High-risk cases (neuromuscular scoliosis, congenital)

Technique:

1. Communication: Preoperative explanation to patient
2. Lighten anaesthesia: Reduce propofol/remifentanil
3. Reverse relaxants: Sugammadex or neostigmine/glycopyrrolate
4. Wake patient: Call name, ask to move hands/feet
5. Assess: Upper and lower extremity motor function
6. Re-anaesthetize: Once function confirmed, deepen anaesthesia
7. Total time: 5-10 minutes

Risks:

• Airway compromise (supine in prone positioner)
• Recall/awareness
• Patient movement (during critical surgical phase)
• Venous air embolism (if sitting)
• Cannot assess if airway unstable

Modern Alternative:

• Continuous MEP monitoring (less invasive, real-time)
• Wake-up test reserved for MEP loss or high-risk cases

Postoperative Management

Extubation:

• Airway oedema (cervical anterior approach): Assess with cuff leak test
• Neurological: Ensure no new deficits (postoperative examination)
• Pain: Adequate analgesia (multimodal)
• Position: Usually supine before extubation (reverse positioning)

Pain Management:

• Multimodal:
  • Paracetamol 1 g q6h
  • NSAIDs (if no contraindication): Celecoxib, ibuprofen
  • Opioids: Morphine PCA, oxycodone
  • Adjuvants: Gabapentin/pregabalin (neuropathic pain)
• Regional: Epidural or paravertebral block (if catheter placed)

Complications to Monitor:

• Neurological deterioration: New weakness, bladder dysfunction → urgent MRI
• Bleeding: Haemorrhage, haematoma → surgical evacuation
• Infection: Fever, wound issues
• DVT/PE: Chemoprophylaxis when bleeding risk acceptable

Indigenous Health Considerations

Aboriginal and Torres Strait Islander Patients

Prevalence of Spinal Conditions:

• Trauma: Higher rates of spinal injury (road accidents, violence)
• Rheumatic disease: Spinal involvement in advanced cases
• Access issues: Delayed presentation, late referrals

Cultural Considerations:

• Pain expression: May be stoic, different pain thresholds reported
• Communication: Interpreter services if needed
• Family involvement: Extended family may be involved in care decisions
• Postoperative care: Challenges with follow-up in remote areas

Health Disparities:

• Higher comorbidity: Diabetes, renal disease (affects blood loss tolerance)
• Nutrition: May affect wound healing
• Smoking: Higher rates (affects fusion, infection risk)

Māori Health Considerations

Musculoskeletal Conditions:

• Higher prevalence of back pain, degenerative spinal disease
• Earlier onset in some communities

Cultural Safety:

• Whānau involvement: Informed consent processes
• Communication: Clear explanations, visual aids
• Postoperative support: Coordination with primary care
• Cultural beliefs: Some may prefer traditional healing alongside medical treatment

ANZCA Final Exam Focus

SAQ Patterns

Positioning Questions:

• "What are the physiological changes when placing a patient in the prone position?"
• "How would you position a patient for posterior lumbar spine surgery?"

Monitoring Questions:

• "Describe the neurophysiological monitoring used in scoliosis surgery."
• "How does anaesthesia affect somatosensory and motor evoked potentials?"

Management Questions:

• "What is controlled hypotension and when is it contraindicated in spine surgery?"
• "How would you manage a venous air embolism during sitting position surgery?"

Key Points for Examination Success

1. Positioning risks: Eye pressure (prone), brachial plexus (lateral), air embolism (sitting)
2. MEP requirements: TIVA, no muscle relaxants, volatiles <0.5 MAC
3. Controlled hypotension: MAP 60-70 mmHg, contraindicated in cerebrovascular disease
4. Blood loss management: TXA, cell salvage, controlled hypotension
5. Air embolism: Precordial Doppler, EtCO₂ drop, Trendelenburg/left lateral
6. Scoliosis risks: Anterior spinal artery syndrome, wake-up test or MEPs essential

References

1. ANZCA. PS45. Guidelines for Transport and Positioning of Patients. 2018.
2. Sloan TB et al. Anesthesia for spine surgery. In: Cottrell JE (ed). Cottrell and Patel's Neuroanesthesia. 6th ed. Elsevier; 2017:289-312.
3. Shapiro HM. Neurosurgical anesthesia and positioning. In: Miller RD (ed). Miller's Anesthesia. 9th ed. Elsevier; 2020:1856-1888.
4. Guay J et al. Tranexamic acid and major spine surgery. Anesth Analg. 2019;128(5):1014-1025.
5. Bhananker SM et al. Anesthesia for spine surgery. ASA Refresher Course. 2018;46:1-9.
6. MacDonald DB. Intraoperative motor evoked potential monitoring. J Clin Neurophysiol. 2019;36(4):261-273.
7. Mirzakhani H et al. Neuroanesthesia for spine surgery. Curr Opin Anaesthesiol. 2020;33(5):542-549.
8. ATSI Health. Spinal cord injury in Australia. Australian Institute of Health and Welfare; 2020.