Origin of thyroid arteries in a Kenyan population
Authors: Magoma G, BSc (Anat), H. Saidi BSc (Anat), MBChB, MMed (Surg), FCS (ECSA), FACS, Kaisha WO, MBChB, MMed (Surg). Department of Human Anatomy, University of Nairobi. Corresponding author: Magoma Georgina, P.O. Box 30197 00100, Nairobi
Introduction: The thyroid gland receives blood supply predominantly from paired superior and inferior thyroid arteries. The superior thyroid artery originates from external carotid while the inferior thyroid artery is a branch of thyrocervical trunk. Unusual origins of superior thyroid artery include common carotid and cervical part of internal carotid arteries while those for the inferior thyroid artery are subclavian, common carotid or the vertebral arteries. These origins vary between populations. Knowledge of variant anatomy of these arteries is important in surgical procedures within the neck. This study describes variant origin of the thyroid arteries in a Kenyan population. Materials and Methods: Fifty formalin fixed cadavers from the Depart-ment of Human Anatomy, University of Nairobi were available for the study. Skin incisions were made from the chin to the supra-sternal notch, along the clavicle to the acromion and then to the mastoid process. Skin flaps and platysma muscle were reflected for exposure of the anterior triangle of the neck. The sternocleidomastoid was cut close to its origins on the clavicle and reflected superiorly. The strap muscles were transected and reflected to expose the thyroid gland lying within the visceral fascia. The dissection field was cleaned by blunt and sharp dissection to expose the gland. The two thyroid arter- ies were identified from either pole of the gland and traced to their respective origins
Data Analysis: Data was coded, tabulated and analysed using SPSS 16.0 for windows® (SPSS Inc., Chicago, Illinois) for percentages and frequencies of the observed variations in origin and nerve relations. Results were presented in tables and macrographs.
Results: Superior thyroid artery originated from external carotid and common carotid artery in 74.4% and 25.6 % of cases respectively. The right side displayed a higher frequency of origin from the common carotid artery. The inferior thyroid artery arose from thyrocervical trunk in 87.5% and from subclavian artery in 12.5% of cases. There was a higher incidence of the inferior thyroid artery originating from the subclavian on the left than the right side.
Conclusion: Over 25% of superior thyroid arteries and 11 % of inferior thyroid arteries show variant origins. The common carotid and subclavian arteries were the most frequent variant sites of origin for superior and inferior thyroid arteries respectively. Bilateral asymmetry was observed. Neck surgeons should be aware of these variations during ligation and selective embolisation of thyroid arteries to avoid complications during surgery
The thyroid gland is a highly vascular organ receiving blood mainly from the paired superior (STA), inferior (ITA) thyroid arteries and occasionally thyroidea ima ar-tery (1). The STA is classically described as a branch of the external carotid (EC) artery (2). On the other hand, ITA originates from the thyrocervical trunk (TC) of the subclavian (SC) artery (3).
Variant origins of STA may include the common carotid (CCA) artery either below or at the carotid bifurcation (4), thyrolingual trunk of the EC artery (5) and rarely as originating from cervical part of the internal carotid artery (6). On the other hand ITA may gain origin from the SC (7) CCA (8) and vertebral (VE) arteries (9). These variations in origin display population differ-ences (10). However, reports from African populations including the Kenya are scarce. This variant anatomy is important during clinical and surgical procedures per-formed on the neck such as selective arterial embolisa- tion in tumors of the thyroid gland (11) invasive imag-ing approach for localisation of mediastinal parathyroid adenomas(10), interpretation of angiograms in the neck region (12) and carotid endarterectomy (6). The aim of the present study was therefore to describe the pattern of origin of the thyroid arteries among Kenyans.
To determine the pattern of origin of thyroid arteries and their nerve relations in a Kenyan population
Materials and Methods
Fifty formalin fixed cadavers obtained from the Depart-ment of Human Anatomy, University of Nairobi were available for the study. Dry cadaveric specimen that were difficult to dissect and those that were macerated by the students before data collection were excluded from the study. Skin incisions were made from the chin to the supra-sternal notch, along the clavicle to the acromion and from the medial end of the clavicle to the mastoid. Skin flaps and platysma were reflected for exposure of the anterior triangle of the neck. The sternocleidomas-toid were cut close to their origins on the clavicle and reflected superiorly. The strap muscles were transected and reflected to expose the thyroid gland lying within the visceral fascia. The dissection field was cleaned by blunt and sharp dissection to expose the gland. The thy-roid arteries were identified and traced to their respective origins. STA was considered a branch of the CCA if it originated anywhere along its course before the bifurca-tion. Branches that arose from the EC directly or from a common trunk as the lingual artery were both consid-ered as a branch from the EC. The ITA was considered a branch of the TC when it was associated with transverse cervical, suprascapular or superficial cervical arteries. Therefore an ITA separated from these canonical vessels was not considered a branch of the TC.
Macrographs of thyroid vessels that exhibited variations in origin were taken using a digital Kodak Easy Share V103® camera.
Data Analysis and Presentation
Data obtained was coded, tabulated and analysed using SPSS 16.0 for windows® (SPSS Inc., Chicago, Illinois) for percentages and frequencies of the observed varia-tions in origin. Results are presented in tables and mac-rographs.
Superior and inferior thyroid arteries were present in all the cases and approached the superior (STA) and inferi-or poles (ITA) of the thyroid gland. The STA divided into an anterior and posterior branch that anastomosed with branches of the ITA. Division of the ITA was variable but commonly it divided into a medial and lateral branch.
Pattern of Origin of Superior and Inferior Thyroid arteries
Eighty two STA (41 right and 41 left) were studied. On the right side, 30 (73.2%) arose from the external carotid while 11 (26.8%) originated from the common carotid arteries. On the left side, 31 (75.6%) were branches of external carotid while 10 (24.4%) had their origin from the common carotid artery. Cumulatively, 61 out of 82 (74.4%) superior thyroid arteries originated from ex-ternal carotid artery. The remaining 21 (25.6%) were branches from the common carotid artery. (Figure 1A, B) Seventy two inferior thyroid arteries (38 right and 34 left) were analysed in the current study. Out of 38 arter-ies dissected on the right side, 34 (89.5%) arose from the thyrocervical trunk while 4 (10.5%) originated from the subclavian artery. On the left side out of 34, 29 (85.3%) were branches of the thyrocervical while 5 (14.7%) had their origin from the subclavian artery. Cumulatively, of the 72 ITA, 63 (87.5%) were branches of the thyrocervi-cal while the remaining 9 (12.5%) originated from the subclavian artery (Figure 2A, B)
The STA and ITA were present in all the specimen stud-ied. This is inconsistent with literature reports (Ada-chi 1928; Toni et. al, 2003; Toni et. al, 2005) that have shown these arteries to be absent in a few cases.
Superior thyroid artery
Superior thyroid artery is usually a branch of the EC ar-tery (1,10). Observations of the current study, however, reveal that STA originated from EC in only 74.4% of cas-es. This is higher than 46.7% reported in German (13) and 58.4% in Japanese (14) studies (Table 2).
This difference is probably attributable to the sample size or methodolgy. For example, Adachi (14) reported from a sample size of 310 and Toni et. al, (10) used in vivo angiography while in the current study the sample size of 82 arteries and gross dissection were used. It is possible however that actual population differences in anatomical variations exist. Superior thyroid arteries originating from the CCA are liable to inadvertent injury during carotid endarterectomy and when carrying out selective arterial embolisation in the superior thyroid pedicle (11) Accordingly, in view of the high prevalence of the variant origin in the Kenyan population, due dili-gence should be exercised during thyroid surgery to pre-vent inadvertent damage to this artery.
The STA was more frequently a branch of the com-mon carotid on the right side than on the left. This dif-fers with what has been observed in other populations where the STA was more often a branch of CCA on the left side (table 3). Bilateral asymmetry even on the same individual should therefore be expected during surgery around the area of the superior thyroid pedicle.
Variations in origin of STA are as a result of asymmetries during development of the aortic arch and