What is external brain herniation?

External Brain Herniation

External brain herniation is a life-threatening condition where brain tissue protrudes through a surgical defect or craniectomy site, requiring prompt neurosurgical intervention to prevent irreversible neurological damage and death. 1

Types and Pathophysiology

External brain herniation differs from internal herniation syndromes (like uncal or tonsillar herniation) as it involves the outward displacement of brain tissue through a skull defect rather than compression of brain structures within the skull.

• Incidence: Occurs in approximately 25% of patients following decompressive craniectomy 2
• Mechanism: Results from the combined effects of:
  • Gravity
  • Atmospheric pressure
  • Changes in intracranial pressure dynamics
  • Loss of normal skull containment

Clinical Presentation

• Visible bulging at the craniectomy site
• Progressive neurological deterioration
• Worsening headache
• Changes in level of consciousness
• New focal neurological deficits
• Seizures

Monitoring and Assessment

• Serial neurological examinations to detect early signs of increasing ICP:

  • Decreased level of consciousness (early sign)
  • Deterioration in motor function
  • Visual disturbances
  • Changes in vital signs
  • Pupillary abnormalities (late sign) 3

• Imaging studies to identify:

  • Mass effect
  • Frontal horn compression
  • Shift of septum pellucidum or pineal gland 3

Management Approach

Immediate Interventions

1. Airway and Oxygenation:

   • Maintain patent airway
   • Elevate head of bed 20-30° with neutral neck position to facilitate venous drainage 3
   • Avoid Trendelenburg position unless blood pressure is unstable

2. Reduce Intracranial Pressure:

   • Osmotic Therapy:

     • Mannitol: 0.5-1 g/kg IV over 15-20 minutes (duration of action: 2-4 hours) 1
     • Hypertonic saline (3%): 2 ml/kg as alternative 1

   • Temporary Hyperventilation:

     • Target PaCO₂ of 30-35 mmHg for short-term management only
     • Caution: Prolonged hyperventilation can cause cerebral ischemia 1

3. Hemodynamic Management:

   • Maintain MAP > 80 mmHg or SBP > 100 mmHg
   • Ensure cerebral perfusion pressure (CPP) ≥ 60 mmHg 1
   • Avoid vasodilators like nitroprusside that can increase ICP 3

Definitive Management

Urgent Neurosurgical Intervention:

• Decompressive surgery to relieve pressure
• Repair of the defect
• Cranioplasty may be required for paradoxical brain herniation 4

Special Considerations

1. Paradoxical Brain Herniation:

   • Rare but life-threatening complication where brain herniates in direction opposite to craniectomy site
   • Can occur after CSF drainage procedures (lumbar puncture, ventriculostomy, or drainage of subdural collections) 4
   • Requires emergency cranioplasty if conservative measures fail

2. External Brain Tamponade:

   • Occurs when subgaleal fluid accumulates under pressure and compresses the brain across craniectomy defect 5
   • Requires drainage of the collection

3. Sinking Skin Flap Syndrome:

   • When atmospheric pressure overwhelms intracranial pressure causing the brain to appear sunken
   • May require cranioplasty for definitive treatment 5

Prevention Strategies

• Proper surgical technique during initial craniectomy
• Appropriate timing of cranioplasty
• Careful management of hydration status
• Avoiding unnecessary CSF drainage in patients with craniectomy defects
• Vigilant monitoring for early signs of complications

Prognosis

The outcome depends on:

• Promptness of intervention
• Underlying cause
• Extent of neurological damage before treatment
• Presence of associated complications

Early recognition and aggressive management are essential to prevent irreversible neurological damage and death from external brain herniation.