Surgical anatomy of the external branch of the superior laryngeal nerve: a systematic review and meta-analysis

Purpose To provide a comprehensive evidence-based assessment of the anatomical characteristics of the external branch of the superior laryngeal nerve (EBSLN). Materials and methods A thorough systematic search was performed on the major electronic databases PubMed, EMBASE, Cochrane library, and ScienceDirect to identify eligible studies. Data were extracted and pooled into a meta-analysis. The primary outcomes were the EBSLN identification rate (total number of EBSLN identified divided by the total number of dissected hemilarynges) and the prevalence of various EBSLN types. Results A total of 56 studies (n = 13,444 hemilarynges) were included. The overall pooled EBSLN identification rate was 89.24% (95% CI 85.49-92.49). This rate was higher for cadaveric (95.00%; 95% CI 89.73-99.35) compared to that reported in intraoperative studies (86.99%; 95% CI 82.37-91.01). Significantly higher identification rates were reported for studies in which intraoperative nerve monitoring was used (95.90%; 95% CI 94.30-97.25) compared to those which only relied on direct visual identification of the EBSLN (76.56%; 95% CI 69.34-83.08). Overall, Cernea type IIa (nerves crossing the superior thyroid artery less than 1 cm above the upper edge of the superior thyroid pole) and Friedman type 1 (nerves running their entire course superficial to the inferior pharyngeal constrictor) were the most prevalent (41.84%; 95% CI 33.28-48.08 and 50%; 95% CI 29.90-65.62, respectively). The combined prevalence of Cernea IIa and IIb (nerves crossing the superior thyroid artery below the upper edge of the superior thyroid pole) was higher in intraoperative studies compared to that in cadaveric studies (64.3% vs 49.4%). The EBSLN coursed medial to the superior thyroid artery in 70.98% (95% CI 55.14-84.68) of all cases. Conclusion The use of intraoperative nerve monitoring improves EBSLN identification rates. In light of the highly variable anatomical patterns displayed by the EBSLN, thorough pre-operative knowledge of its anatomy can be crucial in minimizing incidences of its iatrogenic injury.