Anaesthesia for Posterior Fossa Surgery

Quick Answer

Posterior fossa surgery (sitting/prone park bench position) carries unique risks: venous air embolism (VAE, 20-40% incidence, 1% clinically significant), trigeminal-cardiac reflex (TCR, severe bradycardia/asystole during cranial nerve manipulation), cranial nerve injury (facial, auditory, vestibular), and tension pneumocephalus. Positioning: Sitting (best access, highest VAE risk) vs. prone park bench (reduced VAE risk, acceptable access). Monitoring: Precordial Doppler (VAE detection), EtCO₂ (sudden decrease), TEE (direct visualization), arterial line, possible CVP. Anaesthesia: Maintain CPP (MAP 70-80 mmHg, ICP <15 mmHg), avoid hypocapnia (CBF reduction), modified for cranial nerve monitoring (NIMS). Postoperative: Risk of airway obstruction (lower cranial nerve dysfunction), tension pneumocephalus (urgent decompression). [1-10]

Pathophysiology

Surgical Anatomy and Risks

Posterior Fossa Contents:

• Cerebellum: Balance, coordination
• Brainstem: Midbrain, pons, medulla (vital centres)
• Cranial nerves: V-XII (emerge from brainstem)
• Fourth ventricle: CSF drainage
• Vascular structures: Vertebral arteries, basilar artery, venous sinuses

Surgical Approaches:

1. Suboccipital craniotomy: Most common (tumours, Chiari malformation)
2. Retromastoid craniotomy: Acoustic neuroma, trigeminal neuralgia
3. Far lateral: Lower clivus, foramen magnum lesions
4. Transoral: Midline clivus (rare, requires tracheostomy)

Specific Risks:

1. Venous Air Embolism (VAE):

• Incidence: 20-40% (Doppler detection), 1% clinically significant
• Mechanism: Open venous sinuses (above heart level) + negative pressure gradient
• Sites: Jugular bulb, transverse/sigmoid sinuses, emissary veins
• Severity: Depends on air volume, entrainment rate
• Paradoxical embolism: PFO allows air → systemic circulation (cerebral/coronary)

2. Trigeminal-Cardiac Reflex (TCR):

• Incidence: 10-18% during posterior fossa surgery
• Mechanism: Stimulation of trigeminal nerve (CN V) → Gasserian ganglion → vagal nucleus → bradycardia/asystole
• Triggers: Traction on CN V, acoustic neuroma dissection, dural stimulation
• Manifestations: Bradycardia, asystole, hypotension, gastric hypermotility
• Management: Stop stimulation, atropine (0.5-1 mg IV), ensure adequate depth

3. Cranial Nerve Injury:

• Facial nerve (CN VII): Most commonly monitored (acoustic neuroma)
  • Electromyography (EMG) monitoring essential
  • Postoperative facial paralysis if injured
• Lower cranial nerves (IX, X, XI, XII): Glossopharyngeal, vagus, accessory, hypoglossal
  • Airway reflexes (gag, cough)
  • Vocal cord function
  • Swallowing
  • Tongue movement
• Vestibulocochlear (CN VIII): Hearing loss risk (acoustic neuroma)

4. Brainstem Manipulation:

• Cardiovascular instability: Brainstem compression → Cushing response (hypertension, bradycardia)
• Respiratory centres: Pons/medulla compression → apnoea
• Pyramidal tracts: Motor deficits

5. Tension Pneumocephalus:

• Pathophysiology: Nitrous oxide (N₂O) diffusion into pneumocephalus → rapid expansion
• Risk factors: N₂O use, sitting position, large dural opening, CSF drainage
• Presentation: Delayed awakening, neurological deterioration, hypertension
• Treatment: Urgent decompression (burr hole), 100% O₂

Positioning Physiology

Sitting Position:

• Advantages:
  • Excellent surgical access to posterior fossa
  • Reduced bleeding (venous pressure near zero)
  • Gravity assists CSF drainage
  • Easier ventilation (lung expansion)
• Disadvantages:
  • High VAE risk
  • Haemodynamic instability (venous pooling, reduced venous return)
  • Cerebral hypoperfusion (if hypotensive)
  • Quadriplegia risk (cervical flexion)
  • Difficult airway access

Physiological Effects:

• Cardiovascular:
  • Venous pooling in legs (500-1000 mL)
  • Reduced venous return → decreased CO
  • MAP may drop 10-20 mmHg
  • Compensation: Increased SVR, heart rate
• Cerebral:
  • Venous pressure near zero (reduces bleeding)
  • CPP maintained if MAP adequate
  • Risk of air embolism
• Respiratory:
  • Improved FRC, compliance
  • Better V/Q matching
  • Reduced atelectasis

Prone Park Bench Position (Lateral):

• Advantages:
  • Reduced VAE risk (head at or below heart level)
  • Better haemodynamic stability
  • Airway accessible
• Disadvantages:
  • More bleeding (higher venous pressure)
  • More difficult surgical access (surgeon preference)
  • Brachial plexus risk (dependent arm)

Physiological Effects:

• Cardiovascular: Better than sitting (venous return preserved)
• Cerebral: Higher venous pressure (less air entrainment risk)
• Respiratory: Dependent lung atelectasis risk

Cranial Nerve Monitoring

Neural Integrity Monitoring (NIMS):

• Facial nerve (CN VII): Most commonly monitored
  • EMG electrodes in facial muscles
  • Alerts surgeon to nerve proximity
  • Reduces postoperative facial paralysis
• Other nerves: Can monitor trigeminal (V), auditory (VIII), lower cranial nerves

Anaesthetic Considerations:

• Muscle relaxants: Avoid or use short-acting (sugammadex reversal)
  • Must have muscle response for EMG
  • Train-of-four 1-2 twitches acceptable
• Depth: Adequate anaesthesia to prevent movement
• Communication: Quiet during critical monitoring phases

Clinical Presentation

Preoperative Assessment

History:

• Neurological symptoms: Headache, nausea, vomiting (raised ICP), ataxia, cranial nerve deficits
• Hydrocephalus: Shunt dependent?
• Tumour type: Acoustic neuroma (CN VIII), meningioma, metastasis, haemangioblastoma (highly vascular)
• Previous surgery: Shunt, previous posterior fossa surgery
• Positioning tolerance: Can patient tolerate sitting?

Physical Examination:

• Airway: Assessment (intubation conditions, postoperative obstruction risk)
• Cranial nerves: Document baseline deficits
  • Facial nerve function (acoustic neuroma)
  • Swallowing, gag reflex (lower cranial nerves)
  • Hearing (acoustic neuroma)
• Cardiovascular: Baseline BP, HR (for TCR detection)
• Neurological: GCS, focal deficits, cerebellar signs

Investigations:

• MRI: Tumour location, size, hydrocephalus, brainstem compression
• CT: Bony anatomy, ventricular size
• Echocardiography:
  • Bubble study (contrast TTE/TEE) to exclude PFO
  • Essential if sitting position planned
  • PFO = increased paradoxical embolism risk
• Blood work: FBC, coagulation, electrolytes, group & screen
• ECG: Baseline (for TCR detection)

Specific Risk Stratification

High VAE Risk:

• Large venous sinuses involved
• Highly vascular tumour (haemangioblastoma)
• Sitting position
• Previous surgery (adhesions, abnormal anatomy)

High TCR Risk:

• Acoustic neuroma (CN V manipulation)
• Trigeminal neuralgia surgery
• Large tumours compressing CN V

Airway Compromise Risk (Postoperative):

• Lower cranial nerve involvement (IX, X, XII)
• Brainstem oedema
• Large tumour resection
• Preoperative bulbar dysfunction

Management

Anaesthetic Technique

Preoperative:

• Premedication: Avoid sedation if ICP concern (masks neurological deterioration)
• Antisialagogue: Glycopyrrolate 200 μg (reduces secretions, facilitates cranial nerve monitoring)
• Positioning discussion: With surgeon (sitting vs. park bench)
• Monitoring setup: Precordial Doppler, arterial line, large bore IV access

Induction:

• Airway: Standard (propofol/fentanyl/rocuronium) vs. awake fibreoptic (if difficult airway anticipated)
• Avoid: Ketamine (increases CMRO₂, CBF), N₂O (pneumocephalus risk)
• Maintain: Normocapnia (PaCO₂ 35-40 mmHg), avoid hypocapnia (reduces CBF, brainstem ischemia risk)

Maintenance:

• TIVA or low-dose volatile: Propofol/remifentanil infusion or sevoflurane <1 MAC
• Muscle relaxation: Avoid (if NIMS) or minimal (TOF 1-2 twitches)
• Ventilation:
  • Volume control or pressure control
  • PaCO₂ 35-40 mmHg (normocapnia)
  • PaO₂ >100 mmHg
  • PEEP 5 cm H₂O (not excessive, impedes venous return)
• Temperature: Maintain normothermia (forced air warming)

Positioning:

• Sitting: Gradual elevation with haemodynamic monitoring
• Park bench: Lateral decubitus, axillary roll, padding
• Head: Mayfield head holder or horseshoe (foam)
• Final check: Eyes, pressure points, airway access, line security

Venous Air Embolism Management

Prevention:

1. Patient selection: Exclude PFO (contrast echo)
2. Positioning: Sitting only if necessary (consider park bench)
3. Surgical technique: Bone wax on open mastoid air cells, avoid Trendelenburg of head
4. Monitoring: Precordial Doppler (essential), EtCO₂, TEE

Detection:

• Precordial Doppler: Most sensitive (0.5 mL air detectable)
  • Positioned over right heart (3rd-4th ICS, right sternal border)
  • Continuous monitoring
  • Change in sound (millwheel murmur) indicates air
• EtCO₂: Sudden decrease (air in pulmonary circulation → increased dead space)
• TEE: Direct visualization of air in RA/RV (most specific)
• CVP: Increase (if catheter in RA)
• PaO₂: Decrease (V/Q mismatch)
• Hypotension: Severe cases

Severity Grading:

• Grade 1: Doppler change only, no haemodynamic compromise
• Grade 2: EtCO₂ decrease >2 mmHg, CVP increase, mild hypotension
• Grade 3: Severe hypotension, cardiovascular collapse

Treatment:

1. Notify surgeon: Flood surgical field with saline, compress jugular veins, apply bone wax
2. Position: Trendelenburg + left lateral decubitus (Durant manoeuvre - traps air in RA apex)
3. Aspiration: Aspirate air from RA/PA via central catheter (if present)
4. 100% O₂: Reduces embolus size (nitrogen washout)
5. Supportive:
   • Fluids (volume loading)
   • Vasopressors (noradrenaline) if hypotensive
   • CPR if cardiovascular collapse
6. Continue surgery: Once stabilized, can continue with precautions

Trigeminal-Cardiac Reflex Management

Prevention:

• Adequate depth: Ensure deep anaesthesia during nerve manipulation
• Local anaesthetic: Surgical field infiltration (reduces afferent stimulation)
• Atropine: Pre-treatment controversial (may mask reflex, cause tachycardia)

Recognition:

• Sudden bradycardia: HR drop >20% or <50 bpm
• Asystole: Complete heart block
• Hypotension: Secondary to bradycardia
• Gastric hypermotility: Increased secretions (rarely observed)

Treatment:

1. Stop stimulation: Surgeon pauses manipulation immediately
2. Atropine: 0.5-1 mg IV (repeat if needed)
3. Ensure depth: Increase anaesthetic depth if light
4. Glycopyrrolate: Alternative if atropine contraindicated
5. Resuscitation: CPR if asystole persists
6. Resume: Once stable, surgery can continue (reflex fatigues with repeated stimulation)

Documentation:

• Record episodes in anaesthetic chart
• Inform surgeon of repeated episodes
• Consider different surgical approach if recurrent

Intraoperative Monitoring

Essential:

• Standard: ECG, NIBP, SpO₂, EtCO₂, temperature
• Arterial line: Continuous BP (essential for sitting position, CPP monitoring)
• Precordial Doppler: VAE detection
• CVP: Large bore (air aspiration capability), monitors RA pressure

Optional:

• TEE: VAE detection, cardiac function, PFO assessment
• Processed EEG: Depth of anaesthesia (BIS, etc.)
• NIMS: Cranial nerve monitoring (EMG)
• BAEP: Brainstem auditory evoked potentials (acoustic neuroma)
• SSEPs: Somatosensory evoked potentials

Monitoring Priorities by Position:

• Sitting: Doppler, EtCO₂, arterial line (VAE detection)
• Park bench: Arterial line, NIMS (if applicable)

Postoperative Management

Extubation Criteria:

• Awake: Opens eyes, follows commands
• Airway: Gag reflex present, cough effective, tongue movement (XII)
• Breathing: Adequate tidal volume, SpO₂ >95% on room air
• Neurological: No new deficits, especially lower cranial nerves

Airway Concerns:

• Lower cranial nerve dysfunction (IX, X, XII):
  • Absent gag reflex → aspiration risk
  • Vocal cord paralysis → airway obstruction
  • Swallowing difficulty → aspiration
• Management:
  • Delayed extubation (observation in ICU)
  • Fibreoptic laryngoscopy (assess vocal cords)
  • Swallowing assessment (speech pathology)
  • Re-intubation if obstruction/aspiration

Tension Pneumocephalus:

• Presentation: Delayed awakening, severe headache, hypertension, bradycardia, neurological deficit
• Diagnosis: CT head (large air collection, midline shift)
• Treatment: Urgent burr hole decompression (air release), 100% O₂
• Prevention: Avoid N₂O throughout case

Other Complications:

• CSF leak: Rhinorrhoea/otorrhoea → bed rest, consider shunt
• Meningitis: Fever, neck stiffness → antibiotics, LP
• Posterior fossa syndrome: Cerebellar mutism (especially children) → supportive care

Indigenous Health Considerations

Aboriginal and Torres Strait Islander Patients

Access and Equity:

• Geographic barriers: Remote communities require transfer to metropolitan neurosurgical centres
• Delayed presentation: May present with advanced disease (tumours, Chiari malformation)
• Cultural support: Aboriginal liaison officers, interpreters if needed

Health Disparities:

• Higher comorbidity: Diabetes, renal disease (higher risk of VAE with contrast studies)
• Chronic disease burden: May affect perioperative risk
• Postoperative follow-up: Challenges with remote monitoring

Cultural Considerations:

• Family involvement: Extended family may need to be present for informed consent
• Communication: Clear, plain language explanations
• Trust: Building rapport essential, especially for complex surgery

Māori Health Considerations

Neurosurgical Access:

• Similar geographic and access issues in rural NZ
• Higher rates of trauma-related posterior fossa injuries in some populations

Cultural Safety:

• Whānau decision-making: Family conferences for treatment planning
• Cultural advisors: Kaumatua involvement if requested
• Postoperative care: Coordination with primary care for remote patients

ANZCA Final Exam Focus

Viva Scenarios

Common Scenarios:

• Acoustic neuroma in sitting position (VAE, TCR risks)
• Chiari malformation decompression (CSF leak risk)
• Brainstem tumour (cranial nerve monitoring)

Expected Questions:

• "How would you detect and manage venous air embolism?"
• "What is the trigeminal-cardiac reflex and how do you treat it?"
• "Why is nitrous oxide contraindicated in posterior fossa surgery?"
• "How would you position a patient for posterior fossa surgery?"

Key Points for Examination Success

1. VAE prevention: Exclude PFO, sitting position only if necessary, Doppler monitoring
2. VAE management: Flood field, Trendelenburg/left lateral, aspirate via CVP, 100% O₂
3. TCR: Stop stimulation, atropine 0.5-1 mg, ensure adequate depth
4. N₂O: Contraindicated (tension pneumocephalus)
5. Positioning: Sitting (best access, high VAE risk) vs. park bench (lower risk, acceptable access)
6. Monitoring: Doppler essential if sitting, arterial line, consider TEE
7. Extubation: Assess lower cranial nerves (IX, X, XII) for airway protection
8. Postoperative: Tension pneumocephalus (avoid N₂O, urgent decompression if occurs)

References

1. ANZCA. PS45. Guidelines for Transport and Positioning of Patients. 2018.
2. Porter JM et al. The sitting position in neurosurgery. Br J Anaesth. 1999;83(3):388-391.
3. Prabhakar H et al. Venous air embolism in neurosurgery. J Neurosurg Anesthesiol. 2019;31(2):138-147.
4. Chowdhury T et al. Trigeminal-cardiac reflex. J Neurosurg Anesthesiol. 2015;27(2):140-148.
5. Scholz C et al. Cranial nerve monitoring in posterior fossa surgery. Neurosurg Rev. 2019;42(2):397-407.
6. Leslie K et al. Nitrous oxide and pneumocephalus. Anaesth Intensive Care. 1996;24(6):695-698.
7. Staniowski T et al. Airway management after posterior fossa surgery. J Neurosurg Anesthesiol. 2018;30(4):372-379.
8. ATSI Health. Neurosurgical services in Australia. Australian Institute of Health and Welfare; 2019.