Intraoperative facial nerve monitoring in posterior fossa surgery: prognostic value.

A retrospective study was done of 43 patients operated on for acoustic neuroma by a translabyrinthine approach with intraoperative facial nerve monitoring. Direct stimulation of the facial nerve at the level of the cerebellopontine angle (CPA) and the internal auditory canal (IAC) permits the calculation of a R ratio that has a predictive value for postoperative facial function: R = R'/R'', where R' is the ratio of the minimal intensity (I) of stimulation capable of inducing a response after direct stimulation of the facial nerve at the level of the CPA over the electromyographic response amplitude (A): R' = I(CPA)/A(CPA), R'' being equal to the ratio of these same measurements after stimulation at the IAC: R'' = I(IAC)/A(IAC). Statistical analysis has shown that an R ratio </= 2 has a favorable prognostic value on facial mobility whereas an R ratio > 2 permits the supposition of an alteration of facial function. This prognostic value is more precise if the R ratio is co-related to the macroscopic aspect of the facial nerve at the end of surgery.