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INTRODUCTION
Aberrant right subclavian artery (ARSA) is a rare anatomical variant of the origin of the right subclavian artery. ARSAs usually originate from the posterior aspect of the aorta and have a retroesophageal course to the right thoracic outlet. A prevalence of 0.16% to 2% of ARSA has been reported [4,5,15,16,26,28,33]. Anatomical variations of the aortic arch and great vessels have historically been well documented in postmortem studies. A better understanding of this variability has been attained by the recent addition to the literature of relatively large studies based on radiological data from unselected living populations [4,5,15,16,26,27,33].
We sought to determine the frequency and course of ARSA evaluated with computed tomography (CT) angiography at a single institution in Korea. We reviewed the clinical importance of ARSAs.
MATERIALS AND METHODS
After local ethics committee approval of Pohang SM Christianity Hospital was obtained for the present study (approval No. PSMCHIRB-17-116-C), images obtained by CT angiography were analyzed for the presence of ARSA. The CT angiographic studies were performed for a variety of clinical reasons, including symptoms of cerebral ischemia, hemorrhagic contusion, intracerebral hemorrhage, headache, dizziness, sensory change, and routine checkup. We excluded CT angiography performed on non-Korean patients or obtained outside hospitals. Patients younger than 18 years old were excluded. All images were evaluated by a single neurosurgeon.
CT angiography of the intracranial and extracranial vessels was performed in 3460 patients (1908 female, 1552 male; 59.10±15.02 years) between March 1, 2014 and November 30, 2015 and the results were analyzed. An Aquilion Prime 160-slice CT scanner (Toshiba Medical Systems, Tokyo, Japan) was used for 2499 patients, and an Aquilion CXL edition 128-slice CT scanner (Toshiba Medical Systems) was used in 961 patients. After the acquisition of nonenhanced CT data, contrast-enhanced CT angiography was performed. The parameters for the CT angiographic acquisition were as follows : 100 kVp, 225 mA, field of view 220 mm, detector collimation 80×0.5 mm, table speed 25.5 mm/rotation, gantry rotation speed 0.75 s/rotation, reconstructed section thickness 0.5 mm, and reconstruction increment 0.3 mm for the Aquilion Prime, and 120 kVp, 250 mA, field of view 240 mm, detector collimation 64×0.5 mm, table speed 20.5 mm/rotation, gantry rotation speed 0.5 s/rotation, reconstructed section thickness 0.5 mm, and reconstruction increment 0.5 mm for the Aquilion CXL. The scan range extended from 2 cm below the aortic arch to a point 1 cm above the level of the lateral ventricles.
The results of CT angiography were analyzed using the following methods. A total of 100 mL of iopamidol (Pamiray 370; Dongkook Pharmaceuticals, Seoul, Korea) was administered intravenously using a power injector at 4.0 mL/s via an 18-gauge catheter positioned in a peripheral vein, and the scan delay adapted individually using a bolus-tracking technique. For the bolus tracking, first, a single nonenhanced low-dose scan at the level of the upper neck was obtained. With the start of contrast material administration, repeated low-dose monitoring scans were obtained every second. When the contrast medium was first seen in the common carotid artery (CCA), the CT angiography was triggered automatically without any delay. The data were transferred to a personal computer and three-dimensional (3D) reconstructions of the images were obtained using commercially available software (Vitrea 2; Vital Images, Minnetonka, MN, USA). From these data, 3D CT angiography images were reconstructed using a volume-rendering technique. A series of 17 projection images at every 20° around the cephalocaudal axis were generated and then transferred to a picture archiving and communication system.
The origin of the ARSA, course of the vessel, possible inadvertent ARSA puncture site during subclavian vein catheterization, Kommerell diverticula, and associated vascular anomalies were evaluated. A Kommerell diverticulum in ARSA was defined as a widening of the base of the subclavian artery to >1.5 times the size of the distal subclavian artery [12].
RESULTS
Seventeen in 3460 patients had ARSA. All ARSAs in 17 patients (17/3460=0.49%, 13 female, four male; age range, 22–78 years) originated from the posterior aspect of the aortic arch and traveled along a retroesophageal course to the right thoracic outlet. Characteristics of all 17 ARSAs are presented in Table 1. ARSAs had two different origins from the aortic arch. Two ARSAs (cases 6 and 15) had an inferoposterior origin from the aortic arch (Fig. 1); the other 15 ARSAs had an origin from a superoposterior direction (Fig. 2). Subsequently, the ARSAs had three different courses between the thoracic vertebra body and esophagus. One ARSA (case 7) had a horizontal course anterior to the third thoracic vertebral body. Two ARSAs (cases 8 and 15) had a horizontal course anterior to the vertebrae and ascended to the right side of the vertebrae (Fig. 1). The other 14 ARSAs had an oblique course in the anterior thoracic vertebrae (one thoracic vertebral level in one patient [case 12], two thoracic vertebral levels in 10, and three thoracic vertebral levels in three [cases 1, 2, and 6]) (Fig. 2). Nine of 17 cases were confirmed to have a Kommerell diverticulum and only one (case 10) of 17 patients presented with dysphagia.
All 17 ARSAs were posterior to the right subclavian vein. Possible inadvertent ARSA puncture sites during subclavian vein catheterization in all 17 ARSAs were in the medial third of the clavicle. In this area, the ARSA and subclavian vein were located in close proximity (Fig. 3).
Associated vascular anomalies included two aortic origins of the left vertebral artery, one accessory middle cerebral artery, two C2 segmental arteries, two fenestrations of the basilar artery, and two fenestrations of A1. Both CCA had a common stem from the aortic arch in 10 cases and a separate origin from aortic arch in seven cases.
DISCUSSION
Embryogenic mechanism of ARSA
During human embryogenesis, the aortic arches appear in the 4th week of fetal development. Normally, six aortic arch pairs appear. In the normal development of the right subclavian artery, the proximal part of the artery is formed from the most caudal portion of the right dorsal aorta, while the distal portion is formed by the 7th right intersegmental artery [20]. Departure from the normal embryologic developmental pattern of the primitive aortas and aortic arches results in the formation of an ARSA. The 4th vascular arch involutes along with the right dorsal aorta, while the 7th intersegmental artery remains attached to the descending aorta. This persistent intersegmental artery becomes an ARSA formed from the distal aortic root and the 7th intersegmental artery [20,28].
Shortening of the left distal aortic root brings the origin of the ARSA just distal to the normal left subclavian artery [22]. Because the persisting right aortic arch forms the root of the ARSA, the artery often has a broad base, which is referred to as a Kommerell diverticulum [19]. The stem of the anomalous subclavian artery is derived from part of the right dorsal aorta; this explains the retroesophageal course that this artery takes as it passes to the right upper limb [30,32].
Ten of 17 patients with ARSA in this study had common stem of both CCAs. This vessel anomaly is similar with bovine type aortic arch. Bovine type aortic arch refers to both a shared origin of the left CCA and innominate artery, and an origin of the left CCA from the innominate artery [21]. A bovine arch is thought to result from slow growth or regression of the ventral aortic roots between the 3rd and 4th aortic arches, resulting in fusion of the left CCA and innominate artery [1,21,25,29]. The involution of the 4th aortic arch and the right dorsal aorta result in ARSA. Fourth aortic arch and right dorsal aorta are located in close position from ventral aortic roots between the 3rd and 4th aortic arches. In our opinion, slow growth or regression of the ventral aortic roots between the 3rd and 4th aortic arches in ARSA development may result to common stem of both CCAs. The embryologic development of an ARSA and common stem of both CCAs is shown in Fig. 4.
Clinical significance
In 1794, Bayford [3] described a 62-year-old woman who died after years of dysphagia. On autopsy, an ARSA compressing the esophagus was identified. Previous studies have reported a prevalence of 0.16% to 2% of ARSA [4,5,15,16,25,28,33]; the rate was 0.49% (17/3460) in the present study. ARSA is a rare anatomical variant of the origin of the right subclavian artery. ARSA originates from the left half of the body, and in its course to the right arm, it usually (85%) crosses the midline behind the esophagus, and is thus known as retroesophageal ARSA [2,28]. Furthermore, it may run between the trachea and the esophagus or in front of the trachea [6]. Based on autopsy studies and retrospective analysis of patients’ symptoms during life, Janssen et al. [17] found that a substantial proportion (60–70%) of patients remain symptom-free during their lifetime. Coughing, dysphagia, thoracic pain, and Horner syndrome may develop during aging. It is unknown why dysphagia may develop in the older patient. Various mechanisms are proposed : 1) increased rigidity of the vessel wall or the esophagus itself; 2) aneurysm formation, especially in the presence of a Kommerell diverticulum; and 3) elongation of the aorta [18]. Only one of the 17 patients in the present study presented with dysphagia, which improved after conservative treatment.
Sixty percent of ARSA are associated with a Kommerell diverticulum. This dilatation at the origin of the ARSA is the embryologic remnant [19]. In the present study, nine of 17 cases had a Kommerell diverticulum.
If the ARSA comes in contact with the trachea, this may cause dyspnea, whereas if it is found in front of the trachea, this may cause catastrophic complications during tracheostomy [6]. Desvant et al. [7] reported that aneurysmal disruption of a retroesophageal ARSA in patients with spinal deformities should be kept in mind as a potential cause of tracheostomy bleeding in patients requiring long-term follow-up.
A fistula between the esophagus and an ARSA remains an exceptional event that has mostly been described in association with prolonged nasogastric intubation [13,24]. The abnormal anatomic proximity to the esophagus or trachea likely renders the ARSA vulnerable to extrinsic compression and pressure necrosis by indwelling nasogastric or endotracheal tubes [9,10]. The sequence of events may involve occlusion and thrombosis of the vasa vasorum, leading to vessel wall infarction and eventual wall dissolution. Moreover, ischemia and bacterial invasion of the vessel wall have been suggested as important etiologic factors [8]. Diagnostic difficulty in patients with massive hemorrhage secondary to a fistula between an ARSA and the esophagus results in the high mortality of patients with this complication [23]. To avoid the devastating consequences of an ARSA-esophageal fistula, we reiterate the warning of Miller et al. [24] to avoid prolonged nasogastric and endotracheal intubation in patients known to have an ARSA or other vascular ring anomaly; we recommend early gastrostomy, early tracheostomy, or early removal of nasogastric intubation or tracheal intubation.
ARSAs have a course close to the thoracic vertebrae. When performing an anterior approach to the upper thoracic vertebrae, the area at the level above the aortic arch in the space between the esophagus and the spine is usually regarded as a safe area for dissection of upper thoracic vertebrae. If surgeons do not consider the ARSA, pedicle screw insertion into a thoracic vertebra via a posterior approach or thoracic vertebral body removal using an anterior approach may result in uncontrollable bleeding. Thorpe et al. [31] reported that ARSA bleeding in a case of debridement of T2 for osteomyelitis resulted in a complication and eventual mortality. All 17 ARSAs in the present study were located in the anterior region of the first to fourth thoracic bodies. When planning an operative approach to the upper thoracic region, surgeons should be aware of anatomic variants of ARSAs [31].
A transradial approach has been used in diagnostic angiography and interventional treatment. A diagnosis of ARSA should be suspected if the guide wire repeatedly enters the descending aorta from the right subclavian artery rather than the ascending aorta during aortography via the right radial artery. In this case, catheterization of the ascending aorta may be difficult or even impossible because of the angular course of the ARSA to the ascending aorta [18] as in this study. Because the success rate of the right transradial approach in the setting of an ARSA is only 60%, with an additional potential risk of dissection as in the patient reported by Huang et al. [11], arch anomaly should be considered and included in preintervention planning. In this case, we recommend angiography via the left radial artery or using a transfemoral approach.
Jahnke et al. [14] reported that inadvertent puncture of an ARSA during right subclavian vein catheterization may lead to a potentially fatal complication. Several different skin entry sites are described in the literature. Some practitioners consider that the preferred entry site is 1 cm caudal to the junction of the medial and middle thirds of the clavicle. Other practitioners prefer to enter the skin inferior to the clavicle at the deltopectoral groove, or the point just lateral to the midclavicular line along the inferior surface of the clavicle. We consider that the close proximity of the subclavian vein and ARSA at the medial third of clavicle may risk unintentional puncture of the ARSA during subclavian catheterization. Ultrasound-guided puncture for central venous lines should be used wherever possible, and is especially encouraged when an ARSA has been detected.
CONCLUSION
ARSA is rare variation of the branch from the aortic arch. Clinicians should be aware of the anatomy and clinical importance of an ARSA. In particular, to avoid long-term use of a nasogastric tube and the devastating consequences of an ARSA-esophageal fistula, it is important to be aware ARSA, as well as preoperative identification of ARSA before surgical approaches to upper thoracic vertebrae in order to avoid complications and effect proper treatment. In patients with a known ARSA, a right transradial approach for aortography or cerebral angiography should be changed to a left radial artery or transfemoral approach.
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