Facial nerve prognostication in vestibular schwannoma surgery: the concept of percent maximum and its predictability.

OBJECTIVES/HYPOTHESIS: To evaluate percent maximum as an intraoperative facial nerve measurement for the long-term prognostication of vestibular schwannoma surgery.

STUDY

DESIGN

Prospective cohort study.

METHODS

Evoked amplitude responses to varying levels of stimulus intensity at the nerve root were compared to their supramaximal responses (Mmax) as a percentage, that is, percent maximum. Response charts were constructed for each of the levels of stimulus intensity between 0.05 to 0.3 mA, vis-à-vis facial nerve outcome at 1 year, to establish sensitivities, specificities, and positive predictive values. Logistic regression analyses were used to determine the impact of sex, age, tumor size, and historically defined response parameter on outcomes.

RESULTS

Seventy-eight patients who underwent vestibular schwannoma surgeries between 2005 and 2010 were studied. The positive predictive value (PPV) of a good facial nerve outcome, defined as House-Brackmann (HB) I-II, increases with percent maximum responses. A 90% PPV could be established when the response amplitude was 50% or greater compared to Mmax. Long-term prognostication appeared best at a higher stimulus level of 0.3 mA. Age and sex did not have an impact on outcome, but tumor size did; with each centimeter increase in tumor size, patients were 105% more likely to have a poor outcome (HB III-VI). If the response parameter "≥240 μV at 0.05 mA" was not present, there was a trend toward poor outcome.

CONCLUSIONS

Percent maximum is a valid intraoperative monitoring measure to prognosticate long-term facial nerve outcome. It should be considered a complementary method of monitoring when evoked responses do not conform to conventional predictors.

LEVEL OF EVIDENCE: 4.