Intraoperative Nerve Latency Measurements Can Indicate Post-Op Neural Function

What is the normative amplitude and latency of the vagus, the recurrent laryngeal nerve (RLN), and the external branch of superior laryngeal nerve (EBSLN), and how do they indicate post-operative neural function?

Background: In neural monitoring, a visually identified, surgically preserved, structurally intact nerve may not necessarily function normally. Intraoperative nerve monitoring (IONM) during thyroid and parathyroid surgery has gained widespread acceptance as an adjunct to visual nerve identification, which is known to be exceedingly poor. Recognizing latency (and amplitude) measurement variations from normative (pre-dissection) values will alert surgeons to the possibility of an adverse surgical maneuver.

Study design: Quantitative analysis of evoked waveform data performed on both sides in 25 consecutive patients aged 32 to 75 years undergoing thyroid surgery from February to April 2008.

Setting: Division of Surgical Oncology, Department of Surgery, Massachusetts General Hospital, Harvard Medical School, Boston.

Synopsis: During surgery, pulsatile neural stimulus at 4 Hz, duration 100 µs at 1 to 2 mA was used. The mean left vagal latency was 8.14 ms, compared with a mean right vagal latency of 5.47 ms. Mean left RLN latency was 4.19 ms, compared with a mean right RLN latency of 3.73 ms. Pooled RLN mean latency was 3.96 ms, and pooled mean EBSLN latency was 3.56 ms. Pooled mean EBSLN latency was statistically significantly different from left vagal latency and right vagal latency. In ipsilateral comparisons, the mean right vagal latency (5.47 ms) compared to the mean right RLN latency (3.73 ms), and the mean left vagal latency (8.14 ms) compared with the mean left RLN latency (4.19 ms) showed statistical significance. In amplitude, the mean RLN amplitude showed a statistically significant difference to the mean EBSLN amplitude. Limitations included the small number of patients in the study, and the fact that data relate specifically to the authors’ utilized IONM device and setup parameters.

Bottom line: The unique right vagus, left vagus, and RLN latencies are characteristic of the individual nerves and allow the identification of an intact left or right vagus/RLN system.

Citation: Sritharan N, Chase M, Kamani D, Randolph M, Randolph GW. The vagus nerve, recurrent laryngeal nerve, and external branch of the superior laryngeal nerve have unique latencies allowing for intraoperative documentation of intact neural function during thyroid surgery. Laryngoscope. 2015;125:E84-E89.

—Reviewed by Amy Hamaker