Non-recurrent Laryngeal Nerve – A Rare Anatomical Anomaly that Increases the Risk of Nerve Injury during Thyroidectomy

Abstract

Introduction

Nonrecurrent laryngeal nerve (NRLN), a rare anatomical variation of recurrent laryngeal nerve, is a branch of the vagus nerve (Morais M, Capela-Costa J, Matos-Lima L, Costa-Maia J (2015) Nonrecurrent Laryngeal Nerve and Associated Anatomical Variations: The Art of Prediction. Eur Thyroid J 4(4):234–238). On the right side, the prevalence of NRLN is 0.3–0.8%, while on the left side, it is extremely rare with a prevalence of 0.004%.

Case-Report

A female in her twenties presented with thyroid swelling for 3 years with an ultrasound neck showing a TIRADS IV lesion in the left thyroid lobe. Contrast-enhanced tomography of the neck reported a lesion in the left thyroid lobe causing mass effect in the form of contralateral deviation of trachea and splaying of bilateral common carotid arteries from its common origin – probability of thyroid neoplasm along with aberrant right subclavian artery with a retroesophageal course was noted. Intraoperatively, the right laryngeal nerve was identified near its entry point in right cricothyroid joint and was traced laterally and was found to be nonrecurrent lying superior to inferior thyroid artery. Total thyroidectomy was done preserving the left recurrent laryngeal nerve and right non recurrent laryngeal nerve.

Conclusion

NRLN should be suspected in cases with vascular anomalies based on preoperative imaging. Meticulous dissection during thyroid surgery for identification of the recurrent laryngeal nerve or NRLN is still considered to be the precise approach to avoid nerve injury.

Introduction

The recurrent Laryngeal nerve (RLN) is a branch of the vagus nerve. The left RLN is found inferior to the aortic arch and posterior to ligamentum arteriosum. The right vagus continues posteriorly to the root of the right lung giving off the right RLN which loops around the right subclavian artery [1]. A non-recurrent laryngeal nerve (NRLN), a rare anatomical variation of recurrent laryngeal nerve (RLN), which is also a branch of vagus nerve, enters the larynx directly from the cervical portion of vagus nerve and provides motor and sensory supply to the larynx [2]. The NRLN was first reported by Stedman in 1823³. The prevalence of NRLN on the right side is 0.3–0.8%, while on left side it is 0.004%¹. The presence of an aberrant subclavian artery with absent brachiocephalic artery is the cause for NRLN on right [3]. Preoperatively, diagnosis of NRLN is difficult, unless an associated vascular anomaly is suspected [4]. According to a study by Ikuo Nagayama, dysphagia and an abnormal shadow of the aortic arch in chest X-ray may indicate towards an aberrant subclavian artery [5]. Intraoperative neuro-monitoring (IONM) in thyroid surgery allows the surgeon to recognize the recurrent laryngeal nerve (ILN), as well as NRLN during surgery [6].

Case Report

A female in her late twenties presented with a swelling on the neck for 3 years which was insidious in onset and gradually progressive in nature with no aggravating or relieving factor. There was no other associated complaints. There was no history suggestive of hypothyroidism or hyperthyroidism.

On examination, the patient had stable vitals. On local examination of neck, a 6 × 3 cm, non-tender, firm, mobile, midline swelling was noted which was irregular in shape with irregular margin with no visible skin changes or pulsations and moved with deglutition, with no movement on protrusion of tongue with no local rise of temperature over it. No bruit on auscultation. On indirect laryngoscopy, normal movement of bilateral vocal cord with adequate glottic chink visualized. Patient was admitted in ENT ward and evaluated thoroughly.

All routine blood investigations were within normal limits. Thyroid profile was done which showed FRT3–0.62 ng/dL, FT4–3.19 pg/mL and TSH – 0.458 uIU/mL. Tg2 was > 486.00 ng/mL (normal range – 1.15–130.77ng/ml).

Ultrasound neck revealed enlarged left lobe of thyroid measuring 2.7 cm in anteroposterior dimension with multiple solid ill-defined isoechoic nodules of varying sizes showing increased vascularity on doppler interrogation noted. A single echogenic focus noted (suggestive of calcification). A reporting of TIRADS – IV (Moderately suspicious) lesion was given. Few subcentimetric lymph nodes were noted in bilateral cervical station II. Fine needle aspiration cytology from the neck swelling revealed features suspicious of follicular variant of papillary carcinoma with Bethesda category V.

Contrast enhanced computed tomography (CECT) of neck was done which revealed heterogeneously enhancing lobulated lesion in the left lobe of thyroid and isthmus measuring 5.1 (cc) x 4.2 (cs) x 2.9 (ap) cm (C6 to D1 vertebral level). The lesion caused mass effect in the form of contralateral deviation of trachea and splaying of bilateral common carotid arteries from its common origin – probability of thyroid neoplasm. Aberrant right subclavian artery was noted with a retroesophageal course. No significant lymphadenopathy was noted.

Based on clinical presentation and investigations, patient was diagnosed as a case of thyroid neoplasm and patient underwent pre-anaesthetic check-up and was planned for total thyroidectomy. Under general anaesthesia, patient was taken up for surgery and due to the anomalous course of the subclavian artery as mentioned in CECT neck, we anticipated the possibility of right NRLN. The right RLN was not identified in the usual site, and we identified the nerve near the entry point and traced laterally confirming it as NRLN. Post operative serum calcium level and bilateral vocal cord mobility were normal. Post operative biopsy revealed a multinodular goitre and patient was discharged on postoperative day seven. In one year follow up, patient is asymptomatic with no clinical signs of recurrent disease.

Discussion

The course of RLN varies from person to person which increase the risk of nerve injury during thyroidectomy. The RLN is mostly noted in the tracheoesophageal groove (50–77%), followed by the paratracheal area (17–40%), and the paraesophageal area (6%) or within thyroid parenchyma (4%).¹ NRLN, on the other hand, enters the larynx directly from the vagus nerve without recurring and is often an incidental finding during thyroid surgery [2]. NRLN is extremely rare on the left side with a prevalence of 0.004%, while on the right side, the prevalence is 0.3–0.8%¹. According to a study done by Sofia Guerreiro et al., the NRLN is associated with subclavian artery abnormalities [7]. It arises from an anomaly in the fourth aortic arch [5]. In case of absence of the right fourth aortic arch, the right RLN can freely move upward, and then from the cervical level of vagus, the NRLN arises directly in a horizontal direction and enters the larynx. Due to this, the NRLN is almost always seen on the right side as was seen in our case. Left-side cases are associated with an aortic arch to the right side and dextrocardia, a left subclavian artery, and an absent arterial ligament on the left side [8].

NRLN if seen on left side, is most commonly associated with situs inversus [7]. The brachiocephalic trunk and aberrant retroesophageal course of the right subclavian artery (arteria lusoria) can be seen on CECT scan of the chest [9]. In a meta-analysis by Brandon Michael Henry et al., it was found that 86.7% of right NRLN patients had an aberrant subclavian artery [10].

In preoperative imaging, it is difficult to visualize the RLN, and hence neither is the NRLN visualized on imaging. Thus, a surrogate indicator for identification of NRLN is an arteria lusoria [11]. According to a paper by Marina Morais et al., in order to avoid damage to the nerve during thyroid surgery, meticulous dissection based on the anatomical landmarks should be done [2]. Barium swallow scan if done for dysphagia lusoria can help to diagnose this aberrant vessel; however, this is not a routine investigation for patients undergoing thyroidectomy [11]. CECT of the neck is considered as a routine investigation preoperatively for thyroid malignancies in some centres, wherein presence of an aberrant subclavian artery and the absent brachiocephalic artery may establish a diagnosis of NRLN [8]. In our case, CECT neck was done to look at extrathyroidal extension of the disease as ultrasound and FNAC reports indicated towards malignancy.

According to a study by Er-li Gao et al., the risk of injury to the NRLN (12.9%) in thyroidectomy is higher than RLN (1.8%)⁸. According to a study by R Forde et al., during thyroid surgery, the surgeon should perform meticulous dissection to look for the right RLN as a whitish structure lying vertically near to or in the tracheoesophageal groove. However, the right RLN is lesser found in the tracheoesophageal groove compared to the left side because of its anatomical relation to the ITA and oesophagus, therefore one must undertake a proper dissection techniques in the fascial plane after mobilization of the gland, between the carotid artery and the tracheoesophageal groove. If the RLN is not seen after blunt dissection, a NRLN should be suspected which is recognized as a transverse whitish structures which runs into the larynx close to the inferior cornu of the thyroid cartilage [4].

Three courses of non-recurrent laryngeal nerve are described in the English literature. In type 1 the nerve runs with the superior vascular pedicle of the thyroid gland. In type 2a, it runs above the trunk of the inferior thyroid artery. In type 2b, it runs below the trunk or between the branches of the inferior thyroid artery [12]. In our case it was a type 2a.

Conclusion

The NRLN which was first reported by Stedman in 1823. On the right side, it is caused by the absence of the brachiocephalic artery and the presence of “arteria lusoria”. NRLN should be suspected in cases with vascular anomalies based on preoperative imaging and intraoperatively by nerve monitoring, although it is not routinely recommended to undergo these investigations for all patients because of its extremely rare incidence. Meticulous dissection during thyroid surgery for identification of the recurrent laryngeal nerve or NRLN is still considered to be the precise approach to avoid nerve injury.

Fig. 1.

“A and B shows anterior and lateral views of the neck with swelling”

Fig. 2A.

Contrast enhanced computed tomography (CECT) showing the appearance of right sided Aberrant Subclavian artery from the aorta

Fig. 2B and 2 C: shows the subsequent course of the right aberrant subclavian artery behind the esophagus

Fig. 3.

Intraoperative image showing non-recurrent laryngeal nerve on the right side (black arrow), and the ligated inferior thyroid artery (white arrow)