Anesthesia for Infratentorial Craniotomy

Infratentorial Basics - Brainstem Backyard

• Location: Posterior cranial fossa; contains cerebellum, brainstem (midbrain, pons, medulla), and origins of CN V-XII.
• Vital Structures: Houses critical cardiorespiratory centers in medulla & pons. CSF pathways (4th ventricle, aqueduct) prone to obstruction.
• Common Lesions:
  • Tumors: Acoustic neuroma (CN VIII), medulloblastoma (children), ependymoma, brainstem glioma.
  • Vascular: Aneurysms (e.g., PICA), AVMs.
  • Congenital: Chiari malformations.
• Pre-op Red Flags:
  • Obstructive hydrocephalus (common, leading to ↑ICP).
  • Signs of brainstem dysfunction (e.g., dysphagia, ataxia, gaze palsies).
  • Pre-existing cranial neuropathies.

⭐ The "Cushing reflex" (hypertension, bradycardia, irregular respirations) indicates significantly ↑ICP and brainstem compression, a critical warning sign in infratentorial pathology.

Anesthetic Game Plan - Deep Dive Drugs

• Induction Agents:
  • Propofol: 1.5-2.5 mg/kg (↓ICP, ↓CMRO₂).
  • Etomidate: 0.2-0.3 mg/kg (stable, ↓ICP).
  • Opioids: Fentanyl 1-3 mcg/kg / Remifentanil (blunt response).
  • Relaxant: Rocuronium 0.6-1.2 mg/kg (no ↑ICP).
• Maintenance Strategy:
  • Balanced: Volatile (<1 MAC) + Opioid (Remifentanil 0.05-0.2 mcg/kg/min).
  • TIVA: Propofol + Remifentanil (good for neuromonitoring).
  • Adjuncts: Dexmedetomidine (↓MAC), Lidocaine (↓airway reactivity).

⭐ Propofol TIVA is favored for infratentorial surgery: reliably ↓CMRO₂, ↓CBF, ↓ICP; offers good brain relaxation.

Positioning & Perils - High Stakes Seats

• Common Positions & Associated Risks:

  • Sitting: Best surgical view, ↓ ICP, ↓ bleeding.
    • ⚠️ Cons: High VAE risk, hypotension, nerve injuries (sciatic, peroneal), macroglossia, cervical cord ischemia (extreme neck flexion).
  • Prone/Lateral (Park-bench): ↓ VAE risk vs sitting; lateral offers better hemodynamics.
    • Cons: Airway access (prone), pressure sores, brachial plexus injury (lateral).

• Venous Air Embolism (VAE): Critical concern.

  • Incidence: Up to 76% in sitting position.
  • Pathophysiology: Air via open non-collapsible dural venous sinuses above heart.
  • Detection:
    • Gold Standard: TEE.
    • Standard Non-invasive: Precordial Doppler.
    • Signs: Sudden ↓ ETCO₂, ↓ SpO₂, ↑ ETN₂, hypotension, arrhythmias; "mill-wheel" murmur (late).

  ⭐ Paradoxical air embolism (PAE) via PFO can cause stroke/MI.

VAE Management Algorithm

Wake-Up & Watch Out - Recovery Roadmap

• Emergence: Smooth; avoid coughing/straining (↑ICP).
  • Awake extubation for neuro-assessment. Criteria: GCS 15, TV > 5 ml/kg, TOF > 0.9.
  • Control BP surge (e.g., labetalol, lignocaine).
• Post-Op Care (PACU/Neuro-ICU):
  • Neuro checks: GCS, pupils, motor function.
  • Respiratory: airway, SpO2. Monitor for depression.
  • Hemodynamics: maintain CPP; avoid BP extremes.
  • Pain control (multimodal). PONV prophylaxis.
• ⚠️ Key Complications:
  • Brainstem dysfunction (apnea, CN palsies).
  • Posterior fossa bleed (↓GCS, new deficits).
  • CSF leak, pneumocephalus.
  • Delayed awakening.

⭐ Posterior fossa hematoma can cause rapid neurological deterioration and requires immediate surgical intervention.

High‑Yield Points - ⚡ Biggest Takeaways

• Sitting position: High risk of Venous Air Embolism (VAE); precordial Doppler for detection.
• Brainstem manipulation: Can cause bradycardia, hypotension, or apnea.
• Intraoperative neurophysiological monitoring (IONM) is crucial for cranial nerve preservation.
• Anesthetic goals: Stable CPP, avoid ↑ICP; TIVA or low-dose volatiles preferred.
• Key VAE management: 100% O2, flood field, Trendelenburg, aspirate via CVC.
• Post-op concerns: CSF leak, pneumocephalus, cranial nerve palsies.
• Avoid N2O due to VAE and pneumocephalus risk (expansion).