Acceleromyography of the orbicularis oculi muscle I: significance of the electrode position.
BACKGROUND
This study aimed to establish the best position of the stimulating electrodes for facial nerve stimulation during acceleromyographic monitoring from the orbital part of the orbicularis oculi muscle.


METHODS
In 59 anaesthetised patients, an acceleration transducer was positioned over the middle of the eyebrow. In 47 patients, supramaximal train-of-four (TOF) stimulation was established for four electrode pairs with different positions along the facial nerve and behind the ear. The electrode pair with the lowest, the intermediate, and the highest supramaximal stimulating current based on first response (T1) in TOF was found in each patient. A possible response from direct muscle stimulation was evaluated during complete vecuronium block. In 12 patients, supramaximal stimulation was established using a stimulation sequence different from the one used in the first part of the study.


RESULTS
The best positions for the stimulating electrodes were either with both electrodes just lateral to the eye, or with one electrode lateral to the eye and one in front of the ear. In most patients, supramaximal stimulation was obtained at these positions at 20-60 mA. However, in 10-21% of the patients, supramaximal stimulation could not be obtained. During complete block, 1-4 responses with a twitch height of 3-11% were recorded in 80% of the patients.


CONCLUSIONS
The best placements of the stimulation electrodes are either just lateral to the eye or along the zygomatic arch. However, supramaximal stimulation may often not be obtainable, and activation of other facial muscles may be a confounding factor that may impede correct evaluation of the degree of neuromuscular block.
D000328:Adult; D000368:Aged; D004558:Electric Stimulation / Q000379:methods; D004566:Electrodes; D004576:Electromyography / Q000379:methods*; D005143:Eyelids / Q000294:innervation*; D005154:Facial Nerve / Q000502:physiology; D006801:Humans; D008875:Middle Aged; D016343:Monitoring, Intraoperative; D019148:Neuromuscular Blockade; D009469:Neuromuscular Junction / Q000502:physiology; D009435:Synaptic Transmission