External laryngeal nerve in thyroid surgery: recognition and surgical implications

Injury to the external laryngeal branch of the superior laryngeal nerve during thyroid surgery is not uncommon. Most surgeons tend to avoid rather than expose and identify the external laryngeal nerve (ELN). The aim of the present study was to analyse the frequency and types of ELN crossing the avascular space in relationship to the structures to the upper pole of the thyroid and related thyroid pathology.

One hundred and fifty-one consecutive patients who had thyroid surgery from February 1998 to February 1999 and who met the required preoperative criteria were included into this prospective study. The frequency and type of ELN was documented as it crossed the potential avascular space.

A total of 202 ELN (92.7%) nerves were identified during thyroid surgery. The frequency of the ELN documented crossing the avascular space were: type 1 nerve, 35 (17.3%); type 2a, 113 (56%); and type 2b, 54 (26.7%). Sixteen (7.3%) ELN were not seen despite an extensive search; seven (3.2%) were cases of secondary thyroid surgery and six (2.8%) were cases of malignant related pathology. In 78 cases the goitres weighed more than 100 g and the frequencies of ELN distribution seen were: type 1, six (7.7%); type 2a, 32 (41%); and type 2b, 40 (51.3%). There was a considerably higher frequency of type 2b ELN (51.3%) in large goitres of more than 100 g.

The various technical problems in preserving the ELN need to be considered. Recognition of the potential avascular space aids the exposure and preservation of ELN. As such every attempt should be made to ensure safe dissection in order to reduce morbidity related to thyroid surgery.