Retroesophageal right subclavian artery: a case report and review of the literature

Background: Variations of vessels arising from the aortic arch are numerous. One of the common anatomical variations is the right subclavian artery originating as the last branch of the aortic arch. This is a report of a case of an adult male cadaver with a retroesophageal right subclavian artery. Objective: To highlight the significance of a retroesophageal right subclavian artery, especially its clinical and surgical im -plications. Method: Is a report of a case of an anomalous vessel found during routine student dissection of the chest region in a male cadaver. Result: The retroesophageal subclavian artery was seen originating as the last branch from the postero-lateral aspect of the thoracic aorta at the vertebral level T4. The heart was normal with no other vascular variations seen in this region. Conclusion: Anatomists and pathologists mainly encounter a retroesophageal right subclavian artery by chance and is usually described as asymptomatic, but several clinical conditions have been associated with its occurrence. This is a clear example of when knowledge of an anatomical variation is helpful in clinical practice. nostic and surgical procedures in the thorax and neck. This present study describes a case of RRESA in our specimen and discusses the anatomy and embryology


Introduction
Various vascular anomalies usually occur as a result of failure in the congenital development of the primordial aortic arch 1 .In about 80% of individuals, 3 branches arise from the aortic arch; the brachiocephalic trunk, the left subclavian artery, and the left common carotid artery.Adachi first described this branching pattern as type A 2 .Another 11% of reported cases exhibit Adachi's type B pattern, which consists of a common trunk for the left common carotid artery and the brachiocephalic artery.This branching pattern results in only 2 trunks originating from the aortic arch.The third most common pattern, type C, is characterized by the vertebral artery originating proximally to the left subclavian artery as a 4 th branch of the aortic arch.Numerous other variations of the branching pattern of the aortic arch are found in less than 1% of cases 3 .In type G, Ad-achi described the pattern, in which the right subclavian artery was found as the last branch of the aortic arch.He found the frequency of the type G at 0.2% out of 516 Japanese cadavers.
The subclavian arteries may vary in their origin, course and length.A right retroesophageal subclavian artery (RRESA) is a frequent defect of the embryological aortic arches.It is the most common significant aortic arch anomaly with a reported incidence between 0.4 to 1.8% of the population 4,5 .This variation is due to interruption of the fourth right aortic arch between the notches for the common carotid artery and subclavian artery while the left fourth arch remains intact 6 .A regression of the proximal portion of the right subclavian artery occurs and the retroesophageal aortic arch persists 7 .The retroesophageal right subclavian artery is usually described as not producing symptoms, because its discovery is usually coincidental.Nevertheless, it may be the site of formation of atherosclerotic plaque, inflammatory lesions or aneurysm 6 .The seriousness of the RRESA aneurysm is associated with the high risk of clot-related events, tracheal or venous compression and rupture 8,9 .The anatomic and morphologic variations of the aortic arch and its branches are significant for diagnostic and surgical procedures in the thorax and neck.This present study describes a case of RRESA in our specimen and discusses the anatomy and embryology of this case, the first report of its kind from Uganda, and the likely clinical and surgical implications.

Case report
During routine students' dissection teaching of the retropharyngeal space to medical students in the Anatomy dissection laboratory of Mbarara University of Science and Technology, an abnormal vessel with an unusual origin and course was found originating from the right postero-lateral aspect of the descending aorta at the level of the fourth thoracic vertebra.It measured about 10mm in diameter at its origin.The specimen was an African, adult male formalin-fixed cadaver and there were no records as to the exact age and cause of death.The variant vessel passed antero-laterally to the right, posterior to the esophagus and trachea, to the right axilla allowing it to be more accurately described as a retroesophageal right subclavian artery (Figure 1).
The vessel left an impression of its course on the right lung.Further dissection revealed that this vessel was a retroesophageal right subclavian artery (Figure 2), consistent with the Adachi type G classification.
The right vertebral and internal thoracic arteries originate normally from this vessel.No other aortic arch or vascular variations were noted in this region.The aortic arch appeared of normal length and width, located above the heart.Four branches originated from the aortic arch, namely from right to left; the right common carotid, the left common carotid, the left subclavian and right subclavian arteries respectively.The abdominal viscera were all normal.

Case discussion
During the fourth and fifth week of embryological development, aortic arches arise from the aortic sac ventral to the pharynx, and terminate in the right and left dorsal aortae 10 .Usually, six pairs of aortic arches are formed, and during the sixth to eighth weeks of development, the primitive aortic arch pattern is transformed into the adult arterial arrangement 11 .
In the normal development of the right subclavian artery, the proximal part of the artery is formed from the fourth embryonic aortic arch and part of the right dorsal aorta from the fourth aortic arch to the seventh inter-segmental artery 11 .In the case of an abnormal or-igin of the right subclavian artery, it occurs when the right fourth aortic arch and the right dorsal aorta disappear cranial to the seventh intersegmental artery 12 .As a result, the retroesophageal right subclavian artery forms from the seventh intersegmental artery distally, and the distal part of the right dorsal aorta proximally 12 .With further development, differential growth shifts the origin of the retroesophageal right subclavian artery and the left subclavian artery cranially 11 .The fact that the stem of the anomalous subclavian artery is derived from part of the right dorsal aorta explains the retroesophageal course that this artery takes as it passes to the upper limb 13 .
In the adult, an abnormal origin of the subclavian artery is usually from the arch of the aorta, just distal to the origin of the left one 14 .Other variations of the branches of the aortic arch are shown in Figure 3.Our specimen represented a right retroesophageal artery, which, by definition is a right subclavian artery that arises from the distal portion of the aortic arch as its last branch (Figure 3e).This condition is possibly explained by the persistence of the embryonic right aorta and the obliteration of the fourth aortic arch of the right side 15 .Other branches of the arch were normally placed.
Much as RRESA is usually described as asymptomatic 16 , several clinical conditions have been associated with its occurrence.Symptoms from a right retroesophageal artery may include inequality of upper extremity pulses, dysphagia and dyspnoea.Dysphagia lusoria may occur as a clinical finding, due to the compression of the esophagus between the right common carotid artery and the trachea anterior to it and the right subclavian artery posterior to it 17 .Acute ischemia of the right upper limb due to thrombosis of an aberrant subclavian artery was described 16,18 .Akers Jr et al. 18 described a case of non-occlusive and presumably non-atherosclerotic thrombosis in a 35-year-old woman that led to an extensive distal embolization requiring amputation of the right hand.Boas et al. 16 reported a case of acute ischemia of the right upper extremity due to an extensive thrombosis of a RRESA in a 79-year-old woman that was successfully treated by extensive thrombectomy of the arteries in the upper extremity and right subclavian-to-carotid artery transposition.Other rare but clinically important complications include tracheoesophageal fistulas and arterioesophageal fistulas.The presence of a variant subclavian artery should be considered especially when ischemia of the upper limb is severe and occlusion is located at the shoulder level.

Conclusion
This is a clear example of when knowledge of an anatomical variation is helpful in clinical practice.This case report therefore becomes imperative as it shows that the possibility of occurrence of a retroesophageal right subclavian artery is still in the population.