Relationship between Inferior Thyroid Artery and Recurrent Laryngeal Nerve-A Study of 200 Cases

Introduction: Thyroid surgery is a common modality of treatment for both benign and malignant thyroid disorders.Relationship between inferior thyroid artery (ITA) and recurrent laryngeal nerve (RLN) is crucial in preservation of recurrent laryngeal nerve. Thorough anatomical knowledge and meticulous surgery will ensure the integrity of recurrent laryngeal nerve. Objective: The purpose of this study is to observe the anatomical position of recurrent laryngeal nerve in relation to inferior thyroid artery which will eventually lead to identification of recurrent laryngeal nerve peroperatively. Methods: A cross sectional retrospective study was conducted during the time period of 1 January 2013 to 31 December 2015 in Combined Military Hospital, Dhaka among 200 patients who had undergone thyroidectomy. Results: In this series on the right side 71.8% cases RLN was dorsal to ITA, 21.7% cases it was ventral and in 6.5% cases RLN runs in between the branches of ITA. In case of left side 74.5% cases RLN was dorsal to ITA, 22.5% cases it was ventral and in 3% cases RLN runs in between the branches of ITA. Conclusion: Usually RLN runs dorsal to ITA. In left side it is deeper and in case of right side it is more superficial. RLN may pass through the branches of ITA, but it is not very common.


Introduction
In initial period thyroid surgery was associated with higher morbidity. Subsequently it was well understood that RLN, parathyroids and external branch of superior laryngeal nerve has to be preserved. 1 Anatomical relationship between RLN and ITA is very important in order to preserve the RLN. 2,3 Mostly the morbidity is due to technical failure(inadequate anatomical knowledge ,lack of surgical skill & experience, distorted anatomy as in cancer & large multinodular goiter) to identify the vital structures and the variations in the surgical anatomy. Several studies have been published revealing the anatomy of the RLN in thyroid surgery.
The ITA normally arises from the thyrocervical trunk. 4 Which is a branch of first part of subclavian artery 5 .Past Studies have reported the incidence of origin of the ITA from the vertebral artery and internal thoracic artery. Absence Of ITA (unilateral Or bilateral) have also been reported in previous studies. 6,7,8,9,10 The recurrent laryngeal nerves branch from the vagus nerve, relative to which they get their names; the term "recurrent" from Latin: re-(back) and currere (to run), indicates they run in the opposite direction to the vagus nerves from which they branch. The vagus nerves run down into the thorax, and the recurrent laryngeal nerves run up to the larynx. 11 The vagus nerves exit the skull through the jugular foramen and travel within the carotid sheath dorsal to the carotid artery. After arising from vagus the RLN hooks around the aortic arch on the left side and the subclavian artery on the right side. 11 Both RLN and ITA may be branching or non branching. mostly branching of RLN occurs after crossing the ITA 1 cm below the cricothyroid joint. RLN may be related dorsally, ventrally or in between branches of ITA.

Methods
This descriptive study based on the retrospective data was conducted at Combined Military Hospital, Dhaka during the period of January 2013 to December 2015. Total 200 cases were studied. Approval of the Institutional Ethical committee was obtained for publishing the study. All patients gave informed written consent prior to study.
Patients who were operated for benign goitres including toxic goitres and thyroid cancers were included in this study. The surgical findings were recorded in predesigned "Operation notes" register. Charts of the patients both male and female in the age range from 12 to 74 years were reviewed and operation notes analyzed. Required data regarding exploration of RLN and its relationship to ITA were retrieved. Data thus obtained were analyzed using SPSS -16 for testing the significance of data, Pearson chi square was used as test of significance. A p -value of <0.05 was considered statistically significant.

Results
Out of total 200 patients, 74 were male and 126 were female. The age range of the patients was 12-74 years.    Betweenbranches of ITA 6.5 3 The relationship of the RLN and ITA has shown a regional variation 14 . A study from China showed that 80% of right and 91.5% of left RLNs to travel posterior to ITA. 15 A Brazilian study observed that in most instances RLN travel through the branches of ITA. Furthermore, absent ITA was detected in 4% instances. 16,17 Another variation described is a double ITA. Jiri Sedy reported a case of doubled ITA on the right side. He also found an accessory thyroid artery arises from subclavian artery. 18 In present study no such variation was observed.
Rajamadhava et al 19 reported that RLN on the right side was found posterior to ITA in 55% cases, anterior to ITA in 40% cases, in rest of the 5% cases RLN was in between branches of ITA. The left RLN was found posterior (70%) and anterior (20%) to left ITA and in between its branches in 7.5% cases.

Conclusion
The gold standard for preservation of the recurrent laryngeal nerve during thyroid surgery is still visual anatomical identification. Other techniques may be used only as an adjunct to the gold standard. Identification of the RLN and its branches should be done prior to the clamping of the ITA and all its branches. It can be idenfied in Beahr's triangle, by tubercle of Zuckerkandle or in triangle of Lore. 21,22 . In a setting where advances in technology are not readily available, the surgeon must have adequate knowledge about the variations in the neurovascular anatomy of the thyroid gland as well as necessary skill to prevent complications of surgery.