Continuous vagal nerve stimulation for recurrent laryngeal nerve protection in thyroid surgery.

INTRODUCTION

Newly developed vagal stimulation probes permit continuous intraoperative neuromonitoring of the recurrent laryngeal nerve during thyroid resection. Complete signal loss indicates damage of the nerve. There is no other criterion so far to warn before imminent nerve function impairment.

METHODS

In 100 patients, thyroid resection (188 nerves at risk, 52 thyroidectomies, 21 Dunhill resections, 12 hemithyroidectomies, 5 two-sided subtotal resections) was performed. The vagus electrode V3 was used for continuous stimulation and placed between the carotid artery and the internal jugular vein (V3 electrode; laryngeal adhesive tube electrode; Fa. inomed Medizintechnik GmbH, Teningen, Germany). The signals were recorded via the tube electrode during the complete operation. The signal parameters amplitude, latency and thresholds of nerve conductance were compared at the start of thyroid resection and after completion of thyroid preparation. The changes of these parameters were analyzed.

RESULTS

The latencies (right vagal nerve 4.39 +/- 0.51 ms; left vagal nerve 6.78 +/- 0.75 ms) remained unchanged during the operation. The lower threshold of nerve conduction varied from 0.5 to 2.5 mA, the upper threshold from 1.5 to 5.0 mA. There were no changes between the two measuring points in the majority of cases (lower threshold 92.1%, upper threshold 80.8%). The signal amplitude values were identical in 48% of the cases compared to values at the beginning of operation. A large change in signal amplitude was seen from -58% to +243% after resection. None of the recorded changes of these three parameters were associated with laryngoscopic visible vocal cord disorders. Complete signal loss during operation was documented in 4 cases. Vocal cord palsy was confirmed in 3 cases after operation. In the 4th case, the stimulation signal could be deviated again with diminished amplitude at the end of the operation without vocal cord pareses at laryngoscopy afterwards.

CONCLUSIONS

The parameters signal amplitude, latency and stimulation threshold cannot be used as reliable warning criteria for nerve function impairment during thyroid resection. Loss of signal remains the most important criterion for the surgeon. The coupling of the signal change to operational procedure may be beneficial in difficult thyroid preparation. This gives the surgeon the possibility to react immediately in the case of signal loss.