Facial nerve monitoring by monopolar low constant current stimulation during acoustic neurinoma surgery.

Electrophysiological characteristics of monopolar low constant current stimulation, and evoked facial muscle responses to such stimulation, were evaluated in 34 cases of acoustic neurinoma. Our study, using stimulus parameters of 0.1-ms-duration pulse wave with 0.5-0.6 mA intensity, revealed that extent of spreading current depends on the current intensity, being about 1 mm from the electrode with 0.5-0.6 mA, and insulation of the electrode prevented the current from spreading through the cerebrospinal fluid, giving a reliable amount of current to the tissue. Evoked facial muscle responses to facial nerve stimulation in the internal auditory meatus and over the pons were analyzed and revealed the following: (1) The responses to stimulation in the internal auditory meatus showed no amplitude attenuation throughout the operation. (2) The responses to stimulation over the pons gradually decreased in amplitude as tumor dissection from the nerve proceeded, especially in large tumors. (3) Anatomical continuity of the nerve was obtained in 100% of small, 91.7% of medium, and 88.2% of large tumors. Our data show that monopolar low constant current stimulation is effective in obtaining the precise anatomical orientation of the facial nerve during tumor removal.