How do the orbicularis oculi and adductor pollicis compare for neuromuscular blockade monitoring?

Comparison of Orbicularis Oculi vs Adductor Pollicis for Neuromuscular Blockade Monitoring

The adductor pollicis (ulnar nerve stimulation) is the preferred monitoring site for neuromuscular blockade, while the orbicularis oculi should only be used when the hand is inaccessible, with mandatory reversion to ulnar nerve monitoring before extubation due to a five-fold increased risk of residual paralysis with facial nerve monitoring 1.

Key Differences Between the Two Sites

Timing Characteristics

The orbicularis oculi demonstrates:

• Faster onset time - significantly shorter than adductor pollicis 2, 3
• Faster recovery - reaches TOF ratio 0.80 earlier than adductor pollicis 3
• This creates a critical monitoring gap where the orbicularis oculi appears recovered while the adductor pollicis (and respiratory muscles) remain significantly blocked

Accuracy and Reliability

Critical limitation: The orbicularis oculi systematically underestimates residual blockade 3. When acceleromyography is used at the orbicularis oculi:

• Pronounced variations exist between the two sites during both onset and recovery
• It is impossible to obtain a reasonable estimate of adductor pollicis blockade from orbicularis oculi measurements 3
• Substantial risk of overlooking residual paralysis exists 3

Clinical Implications for Intubation

When monitoring for intubation timing 2:

• Orbicularis oculi: Allows faster intubation but associated with unacceptable incidence of inadequate intubating conditions (only 32% excellent conditions with 0.6 mg/kg rocuronium)
• Adductor pollicis: Provides excellent intubating conditions most frequently (87% with same dose) but requires longer delay before intubation

Relationship to Respiratory Muscles

Both monitoring sites differ from the actual target - the respiratory muscles (diaphragm and chest wall) 4. However:

• The adductor pollicis more closely approximates respiratory muscle recovery
• The orbicularis oculi recovers significantly faster than respiratory muscles, creating a dangerous false sense of adequate recovery

Guideline-Based Recommendations

Primary monitoring site: The ulnar nerve (adductor pollicis) is designated as "the most useful site for neuromuscular monitoring" 1.

When to use orbicularis oculi: Only when thumb movement is impeded (e.g., hand inaccessible during surgery) 1.

Critical safety requirement: If facial nerve monitoring is used, the risk of residual paralysis is five times greater and it is best to revert to ulnar nerve stimulation at the end of surgery 1.

Special Populations

Myasthenic patients: The relationship between these muscles is reversed 5:

• In healthy patients: orbicularis oculi shows less blockade than adductor pollicis
• In myasthenic patients: orbicularis oculi shows greater blockade than adductor pollicis (96% vs 62% with 0.01 mg/kg vecuronium)
• Monitoring at orbicularis oculi may overestimate blockade in myasthenic patients 5
• For myasthenic patients, orbicularis oculi monitoring is actually recommended to avoid persistent blockade, as it represents the most sensitive muscle

Common Pitfalls to Avoid

1. Never rely solely on orbicularis oculi for extubation decisions - always confirm with adductor pollicis monitoring before awakening and extubation 1

2. Acceleromyography limitations - when using acceleromyography devices at the adductor pollicis, readings may be unreliable if thumb movement is impeded; consider electromyography or compressomyography alternatives 1

3. Patient factors affecting accuracy 4:

   • Temperature (hypothermia reduces accuracy)
   • Peripheral edema (may require ultrasound to locate ulnar nerve)
   • Diaphoresis and skin resistance

4. The "monitoring gap" - absence of fade on peripheral nerve stimulator only indicates TOF ratio ≥0.4, not the required ≥0.9 for safe extubation 1

Practical Algorithm

For routine cases:

• Apply quantitative monitor to ulnar nerve (adductor pollicis) after induction but before neuromuscular blockade
• Maintain monitoring throughout surgery
• Document TOF ratio >0.9 at adductor pollicis before extubation

When hand is inaccessible:

• Use orbicularis oculi or alternative sites (tibial nerve, TOF-Cuff) during surgery
• Mandatory: Switch to ulnar nerve monitoring before emergence
• Confirm TOF ratio >0.9 at adductor pollicis before extubation

If unable to access ulnar nerve at end of surgery:

• Use electromyography devices rather than acceleromyography
• Consider compressomyography (TOF-Cuff)
• Exercise extreme caution and consider additional reversal