PROXIMAL-TO-DISTAL FACIAL AMPLITUDE RATIOS AS PREDICTORS OF FACIAL-NERVE FUNCTION AFTER ACOUSTIC NEUROMA EXCISION

Electrophysiological studies (for example, electroneuronography, nerve action potentials, absolute amplitudes of the muscle compound action potentials, and stimulation thresholds) do not accurately predict faci al nerve function after the excision of acoustic neuromas. To eliminat e individual nerve variability, the authors measured the ratio of the amplitudes of muscle compound action potentials produced by stimulatin g the facial nerve at the brainstem proximally and at the internal aud itory meatus near the transverse crest distally after total tumor exci sion in 20 patients. The mean tumor size was 36 mm. The facial nerves were anatomically intact in all patients after tumor excision. The fol low-up period ranged from 14 to 28 months. Facial nerve outcome was de termined by a modified House-Brackmann grading scale. Initial facial n erve function was measured at Days 4 to 7 postoperatively, and final f unction was the grade at last follow up. The following results were ob tained: all patients with proximal-to-distal amplitude ratios greater than 2:3 had Grade III or better initial function and Grade I final fa cial nerve function; 90% of patients with amplitude ratios between 1:3 and 2:3 had Grade III or worst initial facial nerve function, and 100 % of these patients had Grade III or better final facial nerve functio n; all patients with amplitude ratios less than 1:3 had Grade IV or wo rse initial and final facial nerve function. The authors conclude that the proximal-to-distal amplitude ratios after acoustic neuroma excisi on can accurately predict postoperative facial nerve function.