Facial nerve palsy after vestibular schwannoma surgery: dynamic risk-stratification based on continuous EMG-monitoring.

OBJECTIVE

A-trains are a pathological pattern in intraoperative EMG-monitoring. Traintime, a parameter calculated by automated EMG-analysis, quantifies A-train activity. Its extent is associated with the degree of postoperative facial nerve palsy. However, false positive results have been observed. A systematic flaw in automated analysis was hypothesized.

METHODS

Facial nerve EMG-data from 79 patients undergoing vestibular schwannoma surgery were analyzed visually. Automated traintime was compared with these results. The progressive risk for postoperative paresis was calculated with respect to traintime (visual and automated).

RESULTS

Automated analysis identified a small (1.46%), but highly representative fraction of overall A-train activity: Pearson's correlation coefficient between both values was 0.944 (p<0.001). Both were correlated with clinical outcome in a highly significant way (p<0.001) with Spearman's Rho 0.592 (automated) and 0.563 (visual). Progressive risk development was visualized as an inverse sigmoid curve with traintime on a logarithmic scale.

CONCLUSIONS

Automated traintime is a representative and reliable expression for overall A-train activity. As risk-development is complex, rigid thresholds are problematic.

SIGNIFICANCE

Individual risk for postoperative palsy can be estimated on the basis of the calculated curve presented. This approach is of higher practical value than a rigid (and low) threshold.