The shape of the thyroid influences the surgical anatomy of the external branch of the superior laryngeal nerve in the treatment of thyroid carcinoma

Abstract Background Patients with a Cernea type 2b external branch of the superior laryngeal nerve (EBSLN) have the highest risk of EBSLN injury during thyroidectomy. Therefore, evaluating the probability of the Cernea type 2 anatomical variant preoperatively is beneficial to avoid EBSLN injury. In this study, we aimed to assess the effect of gland shape on the relationship of the EBSLN to the superior pole of the thyroid considering the Cernea classification. Methods A retrospective descriptive study of 154 patients (89 unilateral lobectomy and 65 bilateral total thyroidectomy) with 219 neck sides was conducted. The EBSLN was meticulously visualized and functionally identified during dissection of the thyroid upper pole and categorized according to the Cernea classification. The relative upper distance (RUD) and the ratio of the thyroid length to the relative upper distance (TL/RUD) were used to describe the shape of the thyroid lobes. Factors influencing the surgical anatomy of the EBSLN, described by the Cernea classification, and the factors influencing the presence of a type 2b EBSLN were evaluated. Results Out of the 219 sides, the EBSLN was identified in 211, and the nerve identification rate was 96.35%. Among the 211 EBSLNs identified, 20 (9.478%) were type 1, 87 (41.23%) were type 2A, and 104 (49.29%) were type 2B. The mean RUD of resected thyroid lobes was 24.75 ± 2.05 mm (range 21–29), 25.99 ± 2.01 (range 20–31) and 27.88 ± 2.04 mm (range 21–35) in types 1, 2a and 2b, respectively. The comparison between the three groups was statistically significant (p < 0.001). The neck length/thyroid length (NL/TL) and TL/RUD also showed statistically significant changes in the Cernea classification. In multinomial logistic regression analysis, RUD was the only independent factor associated with the Cernea type 2b nerve course. Conclusion The shape of the thyroid lobe described by the RUD and TL/RUD can influence the relative localization of the EBSLN and the degree of Cernea classification. A longer RUD increased the likelihood of a type 2b EBSLN, which is at highest risk during thyroidectomy.