Spinal Cord Protection

Paraplegia following descending aortic surgery:

• Occurs in 2-6% of patients
  
• Due to occlusion of feeder vessels to the anterior and posterior spinal arteries
  • Anterior spinal artery and therefore motor function is at ↑ risk
  • Source vessels from the left subclavian and vertebral arteries
  • Anterior spinal arteries have additional supply:
    • Superiorly from the intercostal and lumbar segmental arteries, and the artery of Adamkiewicz
      
    • Inferiorly from the mesenteric, internal iliac, and sacral arteries
• Occlusion may occur from:
  • Aortic cross-clamping
  • Destruction/occlusion of intercostal arteries
  • Embolisation
    • Air
    • Plaque
• Risk is related to:
  • Extent of aneurysm
    ⩾20% of aneurysm stented.
  • Less common with:
    • Infrarenal aneurysms
      ~0.25%.
    • Endovascular repair
• Continues despite major improvements in surgical technique
• May occur:
  • Immediately
    Generally poor prognosis.
  • Delayed
    Better prognosis. Manged with:
    • CSF pressure drained to 10mmHg
    • MAP ↑ to 100mmHg

Mechanisms for Spinal Cord Protection

Consider:

• Defend MAP and spinal perfusion pressure
  Maintenance of spinal cord perfusion is crucial and is the single most important factor. Requires:
  • Maintain an adequate mean systemic filling pressure
  • ↓ CVP
  • Avoiding CSF pressure ↑
• Lumbar drain
  • Recommended in patients with high risk of ischaemia
    Carries a spinal cord injury risk of ~1-3%. Discuss with the surgeon for:
    • Thoracic stents
    • Non-fenestrated stents
    • Open AAA
    • Previous aortic hardware
  • Reverses poor perfusion caused by spinal cord oedema
  • Also facilitates regional hypothermia of cord, if desired
    Aim to cool at the watershed area of T8-L1.
  • Technical aspects:
    • Use an epidural kit to place a spinal drain
    • Optimise spinal perfusion pressure by draining CSF
      Drain to 10mmHg. Limit drainage to:
      • 20mL in first hour of surgery
      • 40mL in any four hour period
    • If nerve injury indicated on SSEPs or MEPs, drain 10ml of CSF and augment MAP
    • Leave in for 1-3 days post operatively
• Hypothermia
  May be:
  • Active
    With CPB circuit.
  • Passive
    Hypothermia of anaesthesia.
• Partial CPB
  • Bypass of left heart
  • Access cannula placed into the left atrium, with a return cannula into the femoral artery
    Oxygenator can be used, but is not required.
  • Anterograde flow is provided to vessels distal to cross clamp
• Nerve monitoring
  
  • Include:
    • SSEPs
      Indicate dorsal column function by stimulating the posterior tibial nerve and recording cortical response.
    • MEPs
      Indicate spinothalamic tract function by stimulating motor cortex.
      • Performed by:
        • Recording anterior tibial muscle response
        • Monitor hands as controls
  • Indicator of ischaemia
  • Allows surgical technique to be adapted
  • Significant false positives
    
    • High specificity but poor sensitivity
    • Thresholds for intervention are not defined
• Surgical techniques
  Of controversial benefit, but include:
  • Intercostal artery reimplantation
  • Vessel preservation
  • Side-branch stenting

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References

1. Scott DA, Denton MJ. Spinal cord protection in aortic endovascular surgery. Br J Anaesth. 2016 Sep 1;117(suppl_2):ii26–31.