Anatomical and radiological study of the internal branch of the superior laryngeal nerve and its importance for neck surgical approaches: a cadaveric study

Abstract

The internal branch of the superior laryngeal nerve (ibSLN) can be injured during anterior cervical surgeries. This can carry significant comorbidities, such as risk of aspiration pneumonia due to the loss of integrity of the cough reflex. Thus, anatomical knowledge is crucial, and awareness of potential danger zones can help surgeons more easily recognize key structures to avoid harming them. This study used anatomical dissection in formalin-embalmed adult cadavers to expose the ibSLN, marking it afterward with a radiopaque wire and proceeding with X-ray imaging. Then, we analyzed the nerve's path in relation to the vertebral levels and measured the distance between the ibSLN's various points to the median plane. The results showed that the ibSLN originated at C2 through C3's level and perforated the thyrohyoid membrane (TM) at the level of C3-C4 in almost all cases (95.8%). The median distance from the origin of the ibSLN to the median plane was 3.03 cm; the median distance between the point where the nerve pierced the TM and the median plane was 2.05 cm. These measurements were close to the values of the distance between the most lateral point of the nerve and the median plane (3.19 cm) and the same as the distance between the most medial point of the nerve and the median plane (2.05 cm), respectively, thus suggesting and overlap between those landmarks. We also found a tendency for the nerve to have its most lateral point at lower vertebral levels on the left side compared to the right side (x2=0.04). In addition, for higher measurements of C3's height, as well as of the thyroid cartilage's (TC) height, the ibSLN emerged at lower vertebral levels (p=0.003 and p<0.001, respectively). Men also had lower levels of origin of the ibSLN (Fisher=0.05). Surgeons should pay additional attention when operating on vertebral levels above C4, as to avoid patient harm by injury to the ibSLN. Extra care should also be taken in neck interventions on men and taller people, considering the nerve's lower origin. The differences in laterality should be taken into account when choosing surgical approaches to higher vertebral levels. An adequate understanding of the normal anatomy of the neck is vital in several medical specialties, so as to accurately perform both a broad range of diagnoses and procedures.