**5. Morphological characteristics**

The literature presents two main classifications of the aberrant right subclavian artery. According to Neuhauser's threefold classification, the first type of arteria lusoria crosses the posterior wall of the esophagus, and this is observed in more than 80% of the cases. In the second type, this artery passes between the trachea and the esophagus (15% of the cases), and in the third type, it crosses the midline of the body ahead of the trachea (in 5% of the cases) [23].

In contrast, the Adachi and Williams classification recognizes four basic morphological types. Type I/G is characterized by an aberrant right subclavian artery arising from the arch of the aorta as the final branch. Type II/CG is similar to the first type, but an additional left vertebral artery arises from the arch of the aorta. In type III/H, three arteries arise from the arch of the aorta: as the first common trunk of the common carotid arteries (truncus bicaroticus), as the left subclavian artery, and as the last aberrant right subclavian artery. In type IV/N, the aberrant left subclavian artery arises from the right-sided arch of the aorta as the final branch [31].

The most common vascular anomalies coexisting with an aberrant right subclavian artery (arteria lusoria) were found to be truncus bicaroticus (19–29%), Kommerell's diverticulum (15– 60%), aneurysm (just after the origin of arteria lusoria) (13%), and right-sided aortic arch (9%) [15]. Klinkhamer regards truncus bicaroticus is a precondition for tracheal–esophageal compression and the development of clinical symptoms. Under these circumstances, the truncus bicaroticus holds the trachea from the front, and the aberrant right subclavian artery compresses the esophagus from behind [22].

In 1936, Kommerell published the first radiological findings of the route of the aortic arch as an aortic diverticulum (Kommerell's diverticulum), which was identified as being located at the origin of an aberrant subclavian artery [4] (**Figure 3** and **4**). Kommerell's diverticulum is usually found incidentally on a chest roentgenogram and is often misdiagnosed as a mediastinal tumor [29]. Kommerell's diverticulum is a normal broadening of the proximal origin of the aberrant right subclavian artery from the aortic arch and is most frequently present in patients with a right aortic arch and an aberrant left subclavian artery [32]. However, Kommerell's diverticulum is not analogous with an aneurysm: The primary indication for surgical repair of Kommerell's diverticulum is a diameter larger than 50 mm and the presence of clinical symptoms.

The identification of arteria lusoria should alert the radiologist and surgeon that a nonrecurrent inferior laryngeal nerve (NRILN) is present and that an anticipating surgical technique should be performed to reduce the risk of neural injury. Due to its anatomical position, an NRILN is not only at risk of being damaged during thyroidectomy, but also during such other surgical procedures as neck dissection, parathyroidectomy, and carotid endarterectomy [30, 33].
