Optimal current intensity for supramaximal stimulation during electroneurography for facial palsy

To determine the optimal current intensity for supramaximal stimulation during electroneurography (ENoG) for facial palsy.

Forty patients with unilateral facial palsy (32 Bell's palsy, 7 Ramsay Hunt syndrome, and 1 temporal bone fracture) were enrolled. All patients were initially treated with intravenous steroid injections and examined using ENoG. Compound muscle action potentials (CMAPs) of the orbicularis oris muscle were measured on the paralyzed and healthy sides. Stimulation current intensity was varied every 5 mA from 20 mA to 50 mA using two recording methods (the midline and standard methods). The CMAPs of both sides were monitored to see whether they would saturate under the high current intensity stimulation or not.

No obvious saturation of CMAPs was observed in either side with the midline or standard methods. Statistically, a current of 35 mA and above in the healthy side, and 30 mA and above in the paralytic side, resulted in no difference to each side when using the midline recording method. On the other hand, a current of 35 mA and above in the healthy side, and 25 mA and above in the paralytic side, resulted in no difference to each side when using the standard recording method.

These results indicate that a current intensity of at least 35 mA is required to achieve supramaximal stimulation on the healthy side in a patient with unilateral facial nerve palsy. Clinically, for simplicity or standardization purposes, if the same current intensity is introduced bilaterally for ENoG measurements, adopting 40 mA (35 mA plus 10-20%) stimulation would be appropriate for supramaximal stimulation, while being cognizant of the potential effects of artifacts from other muscles.