If the adductor pollicis is the preferred muscle for assessing neuromuscular blocker recovery, why is the orbicularis oculi also evaluated?

Why Monitor Orbicularis Oculi When Adductor Pollicis is the Gold Standard?

The orbicularis oculi is monitored to predict the earliest safe time for tracheal intubation because it recovers from neuromuscular blockade faster than the adductor pollicis and more closely reflects laryngeal muscle relaxation, while the adductor pollicis remains the mandatory site for assessing adequate recovery before extubation. 1, 2

The Fundamental Difference: Onset vs Recovery Monitoring

Different muscle groups have varying sensitivities to neuromuscular blocking agents due to differences in nicotinic acetylcholine receptor density and regional blood flow. 3, 1

Orbicularis Oculi for Intubation Timing

• The orbicularis oculi (facial nerve stimulation) recovers from blockade faster than the adductor pollicis, making it useful for determining when intubation conditions are optimal during rapid sequence induction. 2
• Facial muscles more closely reflect laryngeal muscle relaxation compared to peripheral hand muscles. 2
• Monitoring the corrugator supercilii (another facial muscle) can determine the earliest time for tracheal intubation as it reflects laryngeal relaxation better than the adductor pollicis. 2

Adductor Pollicis for Recovery Assessment

• The ulnar nerve with thumb adduction (adductor pollicis) is the most reliable monitoring site and the only validated site for assessing adequate recovery before extubation. 3, 1
• The adductor pollicis is the last muscle to recover from neuromuscular blockade, providing a safety margin—if this muscle has recovered, you can be confident that respiratory and pharyngeal muscles have also recovered. 4, 2
• A TOF ratio ≥0.9 must be documented at the adductor pollicis before extubation to prevent residual paralysis complications. 3, 1

The Critical Pitfall: Facial Monitoring Increases Risk Five-Fold

• Facial nerve monitoring increases the risk of residual paralysis five-fold compared to ulnar nerve monitoring—you must revert to ulnar nerve monitoring at surgery's end. 1
• Response to peripheral nerve stimulation differs significantly between the adductor pollicis, orbicularis oculi, and respiratory muscles (chest wall/diaphragm) due to varying nicotinic receptor density. 3, 1
• A patient with a TOF of 0/4 at one site may still have cough response or intrinsic respiratory effort depending on which nerve is being stimulated. 3

Practical Clinical Algorithm

During Induction (Determining Intubation Timing):

• Monitor facial muscles (orbicularis oculi or corrugator supercilii) if rapid assessment of intubation readiness is needed. 2
• These muscles reach adequate blockade faster and better reflect laryngeal conditions. 2

During Maintenance:

• If the surgical site requires specific depth of blockade, monitor muscles that best reflect NMB at that site. 2
• Absent twitches at the adductor pollicis using train-of-four stimulation do not preclude intraoperative activity of more resistant muscles like the diaphragm. 4

During Recovery (Mandatory):

• Switch to ulnar nerve/adductor pollicis monitoring before considering extubation. 1, 2
• Document TOF ratio ≥0.9 at the adductor pollicis using quantitative monitoring (acceleromyography or electromyography). 3, 1
• Clinical assessment alone (head lift, hand grip) has only 10-30% sensitivity and cannot exclude residual blockade. 1

Patient-Specific Factors Affecting Monitoring Site Choice

• Peripheral edema may obscure external landmarks at the adductor pollicis—palpation to identify the ulnar artery or ultrasound guidance may be necessary to locate the ulnar nerve. 3
• If thumb movement is impeded (hand inaccessible during surgery), acceleromyography readings become unreliable. 1
• Patient temperature, diaphoresis, and skin resistance all influence TOF monitoring accuracy at both sites. 3

The Bottom Line for Clinical Practice

The orbicularis oculi serves a specific, limited role in determining optimal intubation timing during induction, but the adductor pollicis is non-negotiable for recovery assessment. 3, 1, 2 Using facial monitoring alone at the end of surgery dramatically increases the risk of inadequate reversal, which is associated with higher 24-hour postoperative morbidity and mortality, critical respiratory events, postoperative pneumonia, pharyngeal dysfunction with aspiration risk, and delayed recovery room discharge. 1