Evolution of nerve injury with unexpected EMG signal recovery in thyroid surgery using continuous intraoperative neuromonitoring.

BACKGROUND: Intermittent intraoperative neuromonitoring cannot prevent preparative surgical damage or predict imminent recurrent laryngeal nerve (RLN) damage with subsequent development of loss of electromyogram (EMG) signal during thyroid surgery. In case the nerve is stressed, i.e., from traction near the ligament of Berry, the nerve injury is only detected after it has occurred, not allowing the surgeon to correct the mechanical maneuver and salvage nerve function intraoperatively.
METHODS: The unusual clinical scenario of sacrifice of a tumor-infiltrated RLN was used to study real-time evolution of RLN injury caused by mechanical distention. The ipsilateral vagus nerve (VN) was continuously stimulated with a new stimulation probe, and changes in EMG response were correlated with the varying levels of stretch and traction.
RESULTS: Mechanical traction induced an intermittent depression of EMG amplitudes as a sign of impaired propagation of axon potentials or synaptic transmission. Prolonged mechanical stress caused a long-lasting depression of EMG response. When the mechanical distention was relieved, neurotransmission was gradually restored, with reappearance of singular muscle depolarization of full magnitude interspersed between the barely detectable deflections in the EMG recording. These responses of full amplitude appeared with increasing frequency, until the regular continuous EMG pattern was completely restored.
CONCLUSIONS: Only continuous VN stimulation serves to detect early changes in EMG response that indicate imminent danger to RLN functional integrity and alerts the surgeon to immediately correctable surgical actions, thus possibly preventing nerve damage or transforming damage into a reversible event.