Anaesthetic Considerations for Spine Surgery

1. PREOPERATIVE ASSESSMENT

Patient-Related Factors

• Neurological status: Document pre-existing motor/sensory deficits, myelopathy, radiculopathy. Baseline neurological exam is mandatory - provides comparison for post-op assessment and guides the urgency of surgery.
• Airway assessment: Critically important in cervical spine disease. Evaluate neck ROM, atlanto-axial instability, atlanto-occipital fusion, ankylosing spondylitis, rheumatoid arthritis. Even minor neck movements during laryngoscopy can cause spinal cord injury if the spine is unstable.
• Associated comorbidities: Scoliosis patients may have restrictive lung disease, pulmonary hypertension, cardiac defects (in congenital scoliosis). Assess FVC, FEV1, baseline SpO2.
• Coagulation status: Anticoagulants/antiplatelets should be stopped per guidelines. Spine surgery (especially long fusion with instrumentation) carries high blood loss risk. A type-and-cross is standard.
• Anticipated blood loss: Review imaging (MRI/CT) for vascularity of the lesion, planned levels of fusion, and prior spine surgery (redo surgery bleeds more).

2. AIRWAY MANAGEMENT

Routine Cases

• Standard general anaesthesia with orotracheal intubation (RSI or standard induction depending on aspiration risk).
• Direct laryngoscopy causes C0-C1 and C1-C2 displacement; minimal motion at subaxial levels (C2-C5).

Unstable Cervical Spine - Key Indications for Awake Intubation

• Cervical spine trauma with instability
• Atlanto-axial instability (rheumatoid arthritis, Down syndrome, os odontoideum)
• Severe myelopathy where any further cord compression is unacceptable
• Ankylosing spondylitis (rigid spine - fixed flexion deformity, restricted mouth opening)

Anterior Cervical Surgery (ACDF, Corpectomy)

• Shoulder roll placed under the shoulders to extend the neck.
• Risk of recurrent laryngeal nerve (RLN) injury: surgeons may prefer a left-sided approach as the RLN on the left has a more predictable course.
• Post-op haematoma can cause acute airway compression - always have reintubation equipment at hand.
• Anterior-posterior (AP) combined procedures: highest risk of post-op airway oedema. A leak test before extubation is advisable.

3. PATIENT POSITIONING

Prone Position (Most Common)

• Intubate in supine, then turn with entire team using a log-roll technique.
• Re-check ETT position after turning (flexion of neck can advance the tube; extension can displace it).
• Re-check all lines, pulse oximetry, eye protection.

• Wilson frame: Elevates the abdomen, reduces lumbar lordosis. Risk of VAE.
• Andrews frame ("hinder binder"): Similar principle.
• Jackson spinal table: Allows C-arm access for fluoroscopy; allows abdomen to hang free - best for reducing epidural venous pressure.
• Relton-Hall (four-poster) frame: Used in scoliosis surgery.
• Goal: free the abdomen to prevent IVC compression, which diverts blood to epidural venous plexus and increases surgical blood loss.

Sitting / Semi-sitting Position

• Used for posterior cervical / posterior fossa procedures.
• Major risk: Venous Air Embolism (VAE). Detection by precordial Doppler (most sensitive clinically), ETCO2 decrease, or TEE. Management: flood field with saline, lower head, aspirate via right heart catheter, vasopressors, CPR if needed.
• All patients in sitting position should have a right atrial catheter for air aspiration.
• Risk of haemodynamic instability (venous pooling in lower limbs - use TED stockings and gradual positioning).

Lateral Position

• Used for lateral approaches (XLIF - extreme lateral interbody fusion, disc surgery).
• Axillary roll is mandatory to prevent brachial plexus compression.

4. INDUCTION OF ANAESTHESIA

• Standard induction with propofol + opioid (fentanyl/remifentanil) + neuromuscular blocking agent.
• Succinylcholine is relatively contraindicated in patients with existing denervation/paraplegia (risk of fatal hyperkalaemia).
• Rocuronium is preferred NMB. Important: if intraoperative neuromonitoring (IONM) with MEPs is planned, use a short-acting or low-dose NMB, or use sugammadex for reversal between monitoring epochs - muscle relaxants attenuate MEP amplitude.
• For unstable cervical spine: maintain manual in-line stabilisation (MILS) during laryngoscopy, or perform AFOI.

5. MAINTENANCE OF ANAESTHESIA

Choice: TIVA vs Inhalational

• N2O: Avoided in sitting position (risk of expanding air emboli), pneumocephalus after posterior fossa surgery, or if bowel surgery is co-performed.
• Dexmedetomidine (alpha-2 agonist): useful adjunct - provides sedation, analgesia, reduces volatile/opioid requirements, does not suppress MEPs significantly.
• Avoid hyperventilation (normocarbia maintained). Hypocapnia causes vasoconstriction and can compromise spinal cord perfusion in vulnerable patients.

Blood Pressure Management

• Maintain MAP ≥70-80 mmHg (or 20 mmHg above baseline) to preserve spinal cord perfusion pressure, especially in myelopathic patients.
• Deliberate hypotension (MAP 55-65 mmHg) was historically used to reduce blood loss in scoliosis surgery but is now discouraged due to risk of spinal cord ischaemia and POVL.
• Vasopressors (phenylephrine, noradrenaline) should be readily available.

6. INTRAOPERATIVE NEUROMONITORING (IONM)

Modalities

Response to Neuromonitoring Alert

1. Inform the surgeon immediately.
2. Increase MAP (vasopressors) to improve spinal cord perfusion.
3. Correct anaemia (transfuse if Hb <8-9 g/dL in at-risk patients).
4. Check anaesthetic depth - ensure it is not too deep (agents suppressing signals).
5. Check positioning - relieve any potential cord compression.
6. Surgeon to reverse the corrective manoeuvre (e.g., reduce the degree of scoliosis correction).
7. Consider wake-up test (Stagnara test) as a backup if IONM is equivocal - lighten anaesthesia, ask patient to move feet/hands.

Pedicle Screw Testing

• Pedicle screw misplacement (breach of pedicle wall) causes radicular pain and can injure nerve roots.
• Stimulate the screw: if the threshold for EMG response is <6 mA (or <8-10 mA in some protocols), pedicle breach is suspected.
• Must ensure NMB is reversed at this point.

7. FLUID AND BLOOD MANAGEMENT

Strategies to Reduce Blood Loss:

1. Patient positioning: Free abdomen (reduces epidural venous pressure).
2. Controlled hypotension: Limited to MAP 65-70 mmHg; avoid in myelopathic patients.
3. Tranexamic acid (TXA): Loading dose 10-15 mg/kg IV, followed by infusion 1-2 mg/kg/h. Evidence supports reduction in blood loss and transfusion requirements in spine surgery.
4. Intraoperative cell salvage (autologous blood transfusion): Standard for major spine procedures; pooled blood is washed and returned.
5. Preoperative autologous donation (PAD): Less used now.
6. Acute normovolaemic haemodilution (ANH): Blood harvested at start of surgery, replaced with crystalloid/colloid; returned after major haemorrhage.
7. Recombinant EPO: For Jehovah's Witnesses or pre-op Hb optimisation.
8. Surgical techniques: Bipolar electrocautery, bone wax, topical haemostats (surgicel, thrombin).

Transfusion Triggers:

• Standard threshold: Hb <7-8 g/dL (or <9-10 g/dL in spinal cord compromise, elderly, cardiorespiratory disease).
• With IONM - keep Hb ≥9 g/dL to optimise neural tissue oxygenation.
• Maintain normothermia to preserve coagulation function.

Fluid Type:

• Balanced crystalloids (Plasmalyte, Hartmann's) preferred over normal saline (avoids hyperchloraemic acidosis).
• Colloids can be used to reduce total volume load and possibly reduce POVL risk.
• Avoid hypotonic fluids.

8. SPECIFIC SURGICAL SCENARIOS

Cervical Spine Surgery

• Anterior approach (ACDF): Supine with shoulder roll. Monitor for RLN injury, oesophageal injury, vertebral artery injury. Post-op haematoma = airway emergency.
• Posterior approach: Prone with neck flexed or in pin fixation. Check chin-to-chest clearance.

Thoracic Spine Surgery

• Posterior approach most common; prone position.
• May require one-lung ventilation (OLV) for anterior thoracic (transthoracic) approach - use double-lumen tube (DLT) or bronchial blocker.

Lumbar Spine Surgery

• Most common spine surgery. Prone or lateral.
• Relatively lower risk than cervical/thoracic.
• Regional anaesthesia (spinal or epidural) is an option for lumbar discectomy/laminectomy - has advantages: reduced blood loss, better post-op analgesia, reduced PONV, maintains ability to assess neurology.

Scoliosis Surgery

• High blood loss (may exceed 1-2 blood volumes in severe cases).
• Full IONM (SSEP + MEP) mandatory.
• Wake-up test (Stagnara) as backup.
• Staged procedures in severe cases.
• Prone on 4-poster (Relton-Hall) or Jackson frame.

Spinal Cord Tumours / Intramedullary Surgery

• D-wave monitoring used in addition to SSEPs and MEPs.
• Strict avoidance of hypotension, anaemia, hypoxia.
• May require dexamethasone for cord swelling.

9. EMERGENCE AND EXTUBATION

Criteria Before Extubation:

• Fully awake, obeying commands.
• Adequate respiratory effort (TV ≥6 mL/kg, RR 12-20).
• Haemodynamically stable.
• No significant facial/airway oedema.
• Normothermic and normoglycaemic.

Special Concerns:

• Post-prone oedema: After prolonged prone procedures, significant facial and oropharyngeal oedema can develop. Perform cuff-leak test before extubation. If leak is absent, delay extubation and head-up position to reduce oedema; consider extubation over an airway exchange catheter (AEC) or Aintree catheter.
• Anterior cervical surgery: Post-op haematoma is an emergency. If suspected, open wound immediately at bedside to decompress.
• Macroglossia: Can develop insidiously after prolonged prone surgery with neck flexion. Causes acute post-extubation airway obstruction. Keep patient intubated until resolved.
• All patients should have an immediate neurological assessment in the operating theatre after extubation to document baseline.

10. POSTOPERATIVE CARE AND COMPLICATIONS

Postoperative Visual Loss (POVL)

• Incidence: ~0.1% for spine surgery (highest among non-ophthalmic surgeries - ASA POVL Registry).
• Posterior ischemic optic neuropathy (PION): More common cause than direct orbital pressure. Associated with prolonged surgery, large blood loss, hypotension, anaemia, large fluid administration, Wilson frame, male sex, obesity.
• Central retinal artery occlusion (CRAO): Direct orbital pressure - preventable by checking eyes every 15 minutes.
• Prevention: Maintain MAP, avoid anaemia (keep Hb >9), head-neutral position (not dependent), avoid Wilson frame where possible, use colloids, limit surgical duration.

Pain Management

• Multimodal analgesia is standard:
  • Paracetamol (regular, IV or PO)
  • NSAIDs/COX-2 inhibitors (if no contraindications)
  • Gabapentin/pregabalin (particularly for neuropathic component)
  • Intrathecal morphine (where applicable, e.g., lumbar cases)
  • Epidural analgesia (for thoracic approaches)
  • Wound infiltration with local anaesthetic (ropivacaine/bupivacaine)
  • IV opioids (PCA)
  • Ketamine infusion (sub-anaesthetic dose 0.1-0.2 mg/kg/h) - reduces opioid consumption, prevents central sensitisation.

Other Postoperative Concerns

• Neurological deficit: Immediate post-op neuro exam; any new deficit requires urgent imaging (CT/MRI) and possibly return to theatre.
• PONV: Common with opioids and prolonged procedures - prophylactic ondansetron, dexamethasone.
• VTE prophylaxis: Early mobilisation, TED stockings, LMWH once surgical haemostasis confirmed (usually 24-48h post-op).
• ICU admission: For major reconstruction, long fusions, high-cervical surgery, intraoperative instability, significant comorbidities.

SUMMARY TABLE: Quick Revision