Aberrant origin of the inferior thyroid artery from the common carotid artery: a rare anatomical variation

Introduction

The thyroid gland is an endocrine cervical gland richly vascularized by major arterial and venous networks. Arterial supply comprises 4 pedicles, 2 superior thyroid arteries originating from the external carotid arteries, usually as their 1st branch and 2 inferior thyroid arteries originating from the thyrocervical trunk (TCT), branch of the subclavian arteries (SCA). There is one accessory middle thyroid artery called thyroidea ima, which when present, originates from the brachiocephalic trunk or directly from the aortic arch. Good knowledge of thyroid blood supply is imperative for surgeons, as it helps to avoid hemorrhagic complications and nerve injuries for patients (1). We report a case of an inferior thyroid artery (ITA) taking its origin directly from the common carotid artery (CCA).

Case presentation

A 46-year-old male patient without any specific past history was referred to our department after discovering a large substernal goiter on a cervico-thoraco-abdominal CT (Figures 1,2) performed for investigation of retrosternal oppression. This goiter was almost invisible in the cervical region but enclosed mediastinal vessels and the trachea and extended into the anterior upper mediastinum, explaining the patients’ symptoms of retrosternal discomfort and oppression.

Figure 1 Cervico-thoracic CT scanner showing substernal goiter, sagittal view.

Figure 2 Cervico-thoracic CT scanner showing substernal goiter, frontal view.

Total thyroidectomy through collar neck incision and complementary mediastinoscopy were performed for extraction of right latero-tracheal and substernal part of the goiter; Surgery was performed using continuous neuromonitoring (NIM^© 3.0 Medtronic) of the recurrent laryngeal nerves (RLN). During right lobectomy and after section of the middle thyroid veins, an arterial trunk originating directly from the right CCA was discovered (Figure 3). This artery moved towards the lower third of the right thyroid lobe where it divided into 3 branches. After identification and preservation of the RLN, which ran posterior to the ITA, arterial branches were controlled meticulously using harmonic scalpel.

Figure 3 Intra-operative images of the right inferior thyroid artery (white arrow) originating from the right common carotid artery (white dot).

The contralateral ITA originated from the left TCT and the superior thyroid arteries from the external carotid arteries, as expected. No other anatomical abnormalities of the two CCA trunks were visualized. The substernal goiter was entirely removed through mediastinoscopy and the patient left the hospital 2 days after his surgery without any postoperative complications (drain was removed 24 hours after surgery, calcium and parathyroid hormone serum levels were normal and no voice changes were noted).

Discussion

The ITA originates from the TCT, branch of the SCA. It follows a curved route anterior to the medial border of the anterior scalene muscle behind the prevertebral fascia and turns posteriorly to the CCA and the internal jugular vein, then passing the anterior surface of the longus coli muscle to reach the thyroid. It then divides into several branches at the level of the middle third of the lateral side of the thyroid lobe. Usually, there are no arterial branches from the CCA before it divides into external and internal carotid artery at the top edge of the thyroid cartilage (2).

The ITA has variable relationship with the RLN, nerve that is important for vocal cords mobility and breathing. Hence, visualization of the anatomical relationship of the RLN with branches of the ITA is essential for its preservation. It can be deep or superficial, or even passing in between arterial branches. Therefore, knowledge of its anatomy requires special attention (3).

Very few cases have been published about this specific type of anatomical abnormalities of the ITA. For Toni et al., the ITA can originate from the TCT and less frequently from the SCA directly, but origin from the CCA remains extremely rare, no matter the ethnicity of patients (4). In their report, it originated from the TCT in 90% of cases, in 10% from the SCA, in 0.6% from vertebral artery and only in 0.2% from the CCA. It is therefore a rare anatomical variation (5). Only two cases of an ITA originating from the CCA in a series of over 8,000 thyroidectomies were reported in Italy (6). From a surgical perspective, this anomaly exposes to two major risks: the first one is hemorrhagic by injuring this artery when on aberrant course; the second risk is increased injury to the RLN. This might be due to the fact that when an ITA directly originates from the CCA, goiters develop in a more retro-vascular position such as present in our patient. Thus, the goiters tend to push the RLN anteriorly, exposing it to per-operative injuries (6).

Our patient presented with a unilateral variation. In literature, a similar case affecting both sides was reported and according to the authors, it would be unreasonable to presume that the thyroid blood supply cannot have other origins than from the SCA or the CCA; those variations might be explained by anatomical changes occurring between the 29^th day and the 7^th week of embryological development of the aortic arches and the synchronous descent of the thyroid gland (7).

Conclusions

Anatomical variations of the arterial pedicles of the thyroid gland are usually discovered incidentally during operative procedures. They must be known by thyroid surgeons, especially by the trainees in order to avoid major complications such as bleeding and nerve injuries.

Acknowledgements

None.

Footnote

Conflicts of Interest: The authors have no conflicts of interest to declare.

Informed Consent: Written informed consent was obtained from the patient for publication of this case report and any accompanying images. A copy of the written consent is available.

References

1. Jianu AM, Motoc A, Mihai AL, et al. An anatomical study of the thyroid arteries anastomoses. Rom J Morphol Embryol 2009;50:97-101. [PubMed]
2. Windisch G. Unusual thyroid gland vascularization. J Craniofac Surg 2004;15:862-5. [Crossref] [PubMed]
3. Ray B, Pugazhandhi B, D'Souza AS, et al. Analysis of the arterial anatomical variations of thyroid gland: anatomic guide for surgical neck dissection. Bratisl Lek Listy 2012;113:669-72. [PubMed]
4. Toni R, Casa CD, Castorina S, et al. A meta-analysis of inferior thyroid artery variations in different human ethnic groups and their clinical implications. Ann Anat 2005;187:371-85. [Crossref] [PubMed]
5. Toni R, Della Casa C, Mosca S, et al. Anthropological variations in the anatomy of the human thyroid arteries. Thyroid 2003;13:183-92. [Crossref] [PubMed]
6. Toniato A, Bernante P, Tamagnini P, et al. Aberrant inferior thyroid artery: two cases. Surgery 2003;133:599. [Crossref] [PubMed]
7. Mariolis-Sapsakos T, Kalles V, Papapanagiotou I, et al. Bilateral aberrant origin of the inferior thyroid artery from the common carotid artery. Surg Radiol Anat 2014;36:295-7. [Crossref] [PubMed]