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Scoliosis: Anaesthesia Considerations - Complete Guide

The Cobb Angle X-ray (Scoliosis Severity Measurement)

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PART 1: PATHOPHYSIOLOGY OF SCOLIOSIS

Definition and Anatomy

• Lateral curvature in the coronal plane (the obvious deformity)
• Vertebral rotation (creates the characteristic "rib hump" on forward flexion)
• Sagittal profile changes (loss of normal kyphosis/lordosis)

• < 20° → observation only
• 25-40° → consider bracing

• 40-50° → surgical correction indicated

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Pathophysiological Changes

1. RESPIRATORY CHANGES (Detailed)

A. Mechanical Distortion of the Thoracic Cage

• Ribs on the convex side are pushed posteriorly → create the rib hump → limited excursion
• Ribs on the concave side are crowded anteriorly → reduced intercostal space → limited expansion
• The result is a deformed, rigid thoracic cage that cannot expand normally

B. Restrictive Lung Disease (Primary Mechanism)

C. Ventilation-Perfusion (V/Q) Mismatch

• The compressed concave lung is small and poorly ventilated but may receive disproportionate blood flow → increased shunt (perfusion without ventilation)
• The convex lung is over-distended → increased dead space (ventilation without perfusion)
• Net result: V/Q mismatch → increased alveolar-arterial (A-a) oxygen gradient → hypoxaemia

D. Alveolar Hypoventilation

• The work of breathing increases substantially because of chest wall rigidity
• Respiratory muscles work at mechanically disadvantaged positions
• In neuromuscular scoliosis, the underlying muscle weakness adds to this
• Result: chronic alveolar hypoventilation → CO₂ retention → hypercapnia
• Hypercapnia → respiratory acidosis → compensatory metabolic alkalosis

E. Pulmonary Hypertension (Cascade to Right Heart Failure)

1. Chronic hypoxia → hypoxic pulmonary vasoconstriction (HPV) (Euler-Liljestrand reflex) → widespread vasoconstriction in poorly ventilated lung regions
2. V/Q mismatch perpetuates hypoxia → sustained HPV
3. Sustained HPV → structural changes in pulmonary vasculature → fixed pulmonary hypertension
4. Pulmonary hypertension → increased right ventricular afterload → right ventricular hypertrophy (RVH)
5. RVH → eventual right ventricular failure (Cor Pulmonale)

Chest wall deformity → Restriction → Hypoventilation → Hypoxia + Hypercapnia → HPV → Pulmonary HTN → Cor Pulmonale → Right Heart Failure

F. Reduced Lung Development (Early-Onset Scoliosis)

• Fewer alveoli (reduced alveolar count - "hypoplastic lung")
• Permanently reduced lung volumes even after surgical correction
• This is why early management (casting, growing rods) aims to preserve growth before fusion

2. CARDIOVASCULAR CHANGES

• Right ventricular hypertrophy secondary to pulmonary hypertension
• Right ventricular dysfunction/failure (cor pulmonale) in severe/chronic cases
• The mechanical rotation of the spine also displaces the heart and great vessels (mediastinal shift)
• In neuromuscular scoliosis (Duchenne MD, Friedreich's ataxia): cardiomyopathy is a co-existing finding
• Autonomic dysfunction (in some neuromuscular disorders) → haemodynamic instability under anaesthesia

3. NEUROLOGICAL CHANGES

• Risk of spinal cord injury from the deformity itself or from corrective surgery (stretch/ischemia)
• Syringomyelia may be present (especially in congenital scoliosis - mandates MRI pre-operatively)
• Neuropathic pain in some adults with degenerative scoliosis
• In neuromuscular scoliosis: underlying neurological disease (Cerebral palsy, DMD, SMA, etc.)

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PART 2: PREOPERATIVE EVALUATION

Goals of Preoperative Assessment

Detailed History

• Type and aetiology of scoliosis (idiopathic vs neuromuscular vs congenital vs syndromic)
• Age of onset - earlier onset = more likely severe pulmonary restriction
• Cobb angle - severity of curve
• Functional status: exercise tolerance, dyspnoea on exertion, oxygen requirement, nocturnal symptoms (nocturnal hypoventilation is common)
• Cough effectiveness - particularly in neuromuscular scoliosis (risk of postoperative secretion retention)
• Swallowing and aspiration risk (in cerebral palsy, SMA)
• Cardiac symptoms: palpitations (cardiomyopathy), syncope, ankle oedema (cor pulmonale)
• Previous anaesthetics: complications, difficult airway, malignant hyperthermia history
• Current medications: anticonvulsants (alter anaesthetic drug requirements and IONM signals), steroids, cardiac medications

Physical Examination

• Airway assessment - cervical spine mobility may be limited; some syndromic scoliosis (Marfan's, Down syndrome) may be at risk for atlantoaxial instability
• Chest wall inspection - rib hump, asymmetry, respiratory rate, use of accessory muscles
• Auscultation - areas of reduced air entry, added sounds
• Cardiovascular - JVP, peripheral oedema, murmurs (RV heave in pulmonary HTN)
• Neurological baseline - document ALL pre-existing neurological deficits (medicolegally critical, as any deficit found postoperatively must be distinguished from a pre-existing one)
• Nutritional status - in neuromuscular patients; poor nutrition = increased surgical risk and poor wound healing

Investigations

Respiratory Investigations

Cardiac Investigations

Haematological and Biochemical Investigations

• Full blood count - baseline haemoglobin (blood loss can be massive)
• Coagulation profile (PT, aPTT, INR, fibrinogen) - needed for blood conservation planning
• Group & Screen / Cross-match - for anticipated major blood loss (typically 2-4 units for posterior spinal fusion)
• Electrolytes, renal function - baseline; relevant if antifibrinolytics (TXA) to be used
• Liver function tests - if on hepatic metabolised medications
• HbA1c - if diabetic (relevant for wound healing)

Imaging

• Standing PA and lateral whole-spine X-ray - Cobb angle measurement
• MRI spine - rule out syringomyelia, tethered cord, spinal cord compression (especially in congenital/neuromuscular cases)

Preoperative Optimisation

• Chest physiotherapy and incentive spirometry (especially in neuromuscular scoliosis)
• Nocturnal NIV/BiPAP if nocturnal hypoventilation is present - ideally started weeks before surgery
• Chest infections must be cleared - surgery should be delayed
• Nutritional supplementation if malnourished (PEG feeding in severe neuromuscular cases)
• Cardiac optimisation - treat cor pulmonale, optimise heart failure
• Blood conservation strategies: Preoperative autologous donation (PAD), erythropoietin, iron therapy (oral or IV) to maximise haemoglobin
• Preoperative TIVA assessment - if neuromuscular disease is present (risk of malignant hyperthermia-like reactions with volatiles)
• Discussion with neurophysiology team - baseline neurological recording for IONM
• Detailed consent - including risk of neurological injury (0.5-7.5%), visual loss, massive transfusion, re-intubation, ICU admission

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PART 3: INTRAOPERATIVE MANAGEMENT

Positioning

• All pressure points carefully padded (knees, ankles, elbows, forehead)
• Eyes must be protected - direct pressure on the eye is strictly prohibited (risk of CRAO and perioperative visual loss)
• Head in neutral position - no extreme flexion, extension, or rotation (risk of vertebral artery injury and vision loss)
• Abdomen must hang free - reduces epidural venous pressure and reduces blood loss
• Arms positioned carefully - shoulder at < 90° abduction to prevent brachial plexus stretch

Anaesthetic Technique

Induction

• Standard IV induction with propofol + opioid (fentanyl/remifentanil)
• Avoid succinylcholine in neuromuscular scoliosis (risk of hyperkalaemic cardiac arrest, malignant hyperthermia trigger)
• Use rocuronium (reversible with sugammadex) as the neuromuscular blocker for intubation
• Standard ETT (not LMA) mandatory - surgery is long and in prone position
• Fibreoptic intubation if difficult airway anticipated (syndromic patients, limited neck movement)
• For anterior thoracic approaches: double-lumen ETT or bronchial blocker for lung isolation

Maintenance

• Propofol + Remifentanil infusion (TCI) - minimal effect on SSEP/MEP signals
• Volatile agents (especially halothane/isoflurane/desflurane) significantly suppress SSEP and MEP signals in a dose-dependent manner - particularly dangerous with MEPs
• If TIVA is not available: low-dose volatile (< 0.5 MAC) may be acceptable but is suboptimal
• Nitrous oxide should be avoided - suppresses MEPs and increases the risk of venous air embolism
• Ketamine (subanesthetic doses) and dexmedetomidine are useful adjuncts - preserve IONM signals and provide analgesia/sedation

• Should be avoided during IONM as they suppress MEPs
• Single-dose for intubation only, then allow full reversal
• Monitor NMB with a train-of-four before IONM readings

Monitoring

Standard Monitoring

• ECG, SpO₂, EtCO₂, temperature (core - nasopharyngeal or oesophageal)
• Invasive arterial line (A-line) - mandatory for continuous BP monitoring, frequent ABG sampling, blood conservation
• Central venous access - for fluid management, CVP monitoring, vasopressor infusion
• Urinary catheter - hourly urine output
• Warm air blanket / fluid warmer - normothermia essential (hypothermia suppresses IONM, exacerbates coagulopathy)

Intraoperative Neurophysiological Monitoring (IONM)

1. Immediately communicate with surgeon and neurophysiologist
2. Implement predetermined checklist:
   • Optimise mean arterial pressure (MAP ≥ 65-70 mmHg; consider higher target ≥ 80 mmHg)
   • Correct anaemia - transfuse to Hb > 8-10 g/dL
   • Correct temperature, pH, PaCO₂ abnormalities
   • Lighten the anaesthetic plane (check TIVA doses)
   • Ensure no NMB in effect
3. Wake-up test (Stagnara test) if IONM changes persist despite optimisation:
   • Lighten anaesthesia
   • Ask patient to move hands (control) then feet (test)
   • Inability to move feet = cord injury → immediate surgical reversal (remove instrumentation, decompress)
   • Risks: inadvertent extubation, dislodgement of instrumentation, air embolism on deep inspiration, patient awareness

Blood Conservation and Haemostasis

Strategies:

1. Antifibrinolytics:
   • Tranexamic acid (TXA) - drug of choice; more effective than EACA and aprotinin in reducing total blood loss, intraoperative blood loss, and transfusion requirements. Administer as a loading dose + infusion.
   • EACA (epsilon-aminocaproic acid) - alternative when TXA is contraindicated
   • Avoid in patients with thromboembolic history, coronary stents, or significant renal impairment
2. Cell salvage (intraoperative autologous blood recovery) - blood from the surgical field is processed and re-infused
3. Deliberate controlled hypotension:
   • Reduces blood loss by lowering perfusion pressure at the surgical site
   • Target MAP 55-65 mmHg (used judiciously)
   • Caution: must not be used during IONM monitoring periods (hypotension compromises spinal cord perfusion); avoid in elderly, cardiovascular disease, prior stroke; avoid in patients at risk for perioperative visual loss (ION)
   • Mean arterial pressure must be maintained ≥ 65-70 mmHg to protect the spinal cord
4. Preoperative autologous donation (PAD) - patient donates their own blood weeks before surgery for re-infusion intraoperatively
5. Positioning - free-hanging abdomen reduces IVC compression and epidural venous engorgement → less blood loss
6. Normovolaemia - must be maintained throughout; hypovolaemia in prone position can rapidly cause cardiovascular collapse; patients with neuromuscular scoliosis are 3 times more likely to have intraoperative cardiac arrests than idiopathic scoliosis patients

Transfusion Trigger:

• Hb < 7-8 g/dL in otherwise healthy patients
• Hb < 10 g/dL in patients with pulmonary hypertension or cardiac disease (they tolerate anaemia poorly)
• Guide transfusion with TEG/ROTEM for coagulation status

Fluid Management

• Balanced crystalloids (Hartmann's/PlasmaLyte) as maintenance
• Colloids - albumin/Gelofusine alongside crystalloids when blood loss is substantial (reduces risk of perioperative visual loss)
• Avoid excessive crystalloid - worsens tissue oedema, can impair IONM signals
• Consider vasopressors (noradrenaline/vasopressin) to maintain MAP without fluid overloading

Temperature Management

• Active warming throughout (forced air warming blanket, warmed IV fluids)
• Hypothermia depresses IONM signals, causes coagulopathy, increases infection risk, prolongs NMB
• Target normothermia (core temperature 36-37°C)

Venous Air Embolism (VAE)

• Risk is significant during wide paravertebral dissection and vertebral distraction
• Detection: fall in EtCO₂, hypotension, mill-wheel murmur on precordial doppler
• Management: pack wound, flood field with saline, stop N₂O if in use, left lateral decubitus + Trendelenburg if possible (difficult in prone - have plan ready)

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PART 4: POSTOPERATIVE MANAGEMENT

Extubation Criteria

• Preoperative FVC > 50% predicted
• Short surgery with minimal blood loss
• Warm, haemodynamically stable
• Reversible NMB confirmed (TOF ratio > 0.9)
• Good cough reflex
• No significant intraoperative complications

• Preoperative FVC < 35% predicted
• Neuromuscular scoliosis with baseline respiratory muscle weakness
• Intraoperative blood loss > 33% of total blood volume
• Massive transfusion
• Prolonged surgery (> 6 hours)
• Multiple vertebral levels fused (> 6 levels)
• Significant intraoperative haemodynamic instability
• Low body weight / severe malnutrition
• Baseline hypercapnia (PaCO₂ > 45 mmHg preoperatively)

Respiratory Management

• Supplemental oxygen in all patients postoperatively
• Incentive spirometry - early, to prevent atelectasis
• Chest physiotherapy - particularly important in neuromuscular cases
• Non-Invasive Ventilation (NIV/BiPAP) - preferred over re-intubation in patients who fail to maintain ventilation; particularly useful in DMD, SMA
• Cough assist devices (mechanical insufflation-exsufflation) - in patients with neuromuscular weakness and ineffective cough
• Anticipate significant postoperative atelectasis - it will not improve immediately after scoliosis correction; in fact, may worsen transiently
• Early and successful weaning from ventilator should be prioritised to reduce risk of ventilator-associated pneumonia (VAP) and tracheal stenosis

Pain Management

Haemodynamic Monitoring

• Continuous ECG, SpO₂, non-invasive BP postoperatively
• Invasive arterial line maintained until haemodynamically stable
• In unstable patients: invasive BP, CVP, hourly urine output monitoring
• Watch for postoperative haemorrhage - surgical wound drainage, falling Hb, haemodynamic instability
• Neurological checks - regular lower limb neurological assessment (can patient move toes/feet?)

Neurological Assessment

• Immediate postoperative neurological exam - assess lower limb power, sensation; compare with preoperative baseline
• Any new neurological deficit = surgical emergency → urgent imaging and possible return to theatre
• Wake-up test (if not done intraoperatively) can be performed in recovery

Fluid and Renal Management

• Maintain adequate urine output (> 0.5 mL/kg/hr)
• Avoid hypovolaemia (risk of spinal cord ischaemia persists postoperatively)
• Monitor for acute kidney injury - especially if antifibrinolytics used, prolonged hypotension, contrast media

ICU/HDU Admission Criteria

• Neuromuscular scoliosis (particularly DMD, SMA)
• FVC < 35% preoperatively
• Prolonged surgery or massive transfusion
• Intraoperative complications (haemodynamic instability, IONM changes)
• Baseline hypercapnia

Specific Complications to Watch For

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SUMMARY FRAMEWORK

SCOLIOSIS ANAESTHESIA: THREE PILLARS

1. RESPIRATORY                    2. BLOOD                    3. SPINAL CORD
   Assessment (PFT, ABG)             Conservation (TXA,           Protection (IONM,
   ↓                                  cell salvage)                SSEP + MEP)
   Optimise pre-op                    ↓                            ↓
   (chest physio, NIV)             Normovolaemia                TIVA (propofol +
   ↓                                  Controlled                   remifentanil)
   Post-op ventilation plan           hypotension                  ↓
   (extubate vs ICU)                  (avoid if cord risk)      Wake-up test
                                                                 if needed

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Key Points to Remember

• Scoliosis surgery carries 0.5-7.5% risk of neurological injury - IONM is non-negotiable
• TIVA (propofol + remifentanil) is the gold standard for maintenance when IONM is used - volatile agents suppress MEPs
• Succinylcholine is contraindicated in neuromuscular scoliosis - risk of hyperkalaemic arrest and malignant hyperthermia
• TXA is the most effective antifibrinolytic - reduces blood loss and transfusion significantly
• Postoperative respiratory failure is the most common serious postoperative complication, especially in neuromuscular scoliosis
• Patients with neuromuscular scoliosis are 3x more likely to suffer intraoperative cardiac arrest compared to idiopathic scoliosis patients
• The "rib hump" is caused by vertebral rotation - not just lateral curvature
• Early-onset scoliosis is more devastating because it restricts alveolar development during childhood - a permanent effect
• Cor pulmonale is the end-stage of untreated severe scoliosis respiratory disease - echocardiography is mandatory before surgery