Introduction
The aorta is the largest artery in the body carrying oxygenated blood from the left side of the heart to the rest of the body. It gives off important branches that supply various regions. The arch of the aorta is the second and curved middle portion of the aorta which gives it the characteristic cane shape and is usually present towards the left side. The aortic arch is the bridge between the ascending and descending aorta; conventionally it crosses the left main bronchus and descends on the left side of the spine. It gives rise to three important branches responsible for blood supply to the head, neck, and thoracic regions:
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The brachiocephalic trunk which further divides to form the right subclavian artery and the right common carotid artery.
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The left common carotid artery.
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The left subclavian artery.
Sometimes a variation in normal development can result in a right-sided aortic arch. It is a very rare presentation occurring in less than 0.1% of the population. This condition was first described by Fioratti and Aglietti way back in 1763.
What Are the Types of Right-Sided Aortic Arches?
Right-sided aortic arches can be classified into three types depending on their presentation and branching. These include:
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Type I: Right-sided aortic arch with mirror image branching - It is a very common presentation of the right-sided aortic arch. It occurs due to an interruption of the dorsal portion of the left arch that lies between the left subclavian artery and the descending aorta, accompanied by regression of the right ductus arteriosus in the hypothetical double aortic arch. This type is characterized by the left brachiocephalic trunk arising first, and then the right common carotid artery and right subclavian artery. Type I is known to be associated with cyanotic congenital heart diseases, such as tetralogy of Fallot, transposition of the great arteries, truncus arteriosus, tricuspid atresia, etc.
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Type II: Right-sided aortic arch with aberrant left subclavian artery is also a common presentation, accounting for about 39.5% of all the reported right-sided arches. It is associated with a Kommerell diverticulum. This variant is attributed to an interruption of the dorsal segment of the left arch between the left common carotid and left subclavian arteries and the regression of the right ductus arteriosus in the hypothetical double aortic arch. The order in which the branches arise is as follows - the left common carotid artery, the right common carotid artery, the right subclavian artery, and then the left subclavian artery. The descending aorta may either be on the right side or in close proximity to the midline. It is mostly asymptomatic which is why many of the reported cases are incidental findings during routine examinations or diagnostic work-ups for unrelated reasons. It is seldom associated with other cardiovascular disorders.
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Type III: Right-sided aortic arch with isolation of the left subclavian artery - It is the rarest of the three forms of right-sided aortic arches. It occurs due to interruption of the left arch at two different levels, one level is between the left common carotid and left subclavian arteries and the other level is distal to the attachment of the left ductus. It can sometimes be associated with congenital subclavian steal syndrome or vertebrobasilar insufficiency. Co-occurring congenital heart diseases are rare.
What Results in a Right-Sided Aortic Arch With an Aberrant Left Subclavian Artery?
A deviation from the normal development of the aorta results in this anatomic abnormality.
The embryological origin of the various different anomalies of the aortic arch is based on the theory of the hypothetic double aortic arch, which was put forth by Edwards et al. The aortic arches develop from the aortic sac.In the hypothetic double aortic arch theory, the ascending aorta is divided into two right aortic arches and two left aortic arches, which surround each side of the trachea and esophagus and connect together to form descending aorta. A common carotid artery and subclavian artery are derived from each aortic arch. Each side has a ductus connected to the aortic arch and pulmonary artery. Normally, the regression occurs between the origin of the right subclavian artery and descending aorta in the right aortic arch. The regression also includes the right ductus. Thus, the normal left-sided aortic arch is formed. When there is persistence of the right fourth aortic arch and involution of the left aortic arch, a right sided aortic arch results. Generally the left subclavian artery originates directly from the arch of the aorta, but in a type II right-sided aortic arch, it arises from a conical dilation called the Kommerell’s diverticulum which is a remnant of the aortic arch.
What Are the Symptoms of a Right-Sided Aortic Arch With an Aberrant Left Subclavian Artery?
The majority of the affected individuals are asymptomatic.
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However, it causes symptoms when conjoint with other vascular abnormalities (vascular ring formation) which lead to compression of the trachea and/or esophagus. It is most likely to experience symptoms during childhood following which it may be repaired.
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In adults, the symptoms may show up due to atherosclerotic changes (deposition of fat, cholesterol, and other substances on the walls of the blood vessel which can cause narrowing and obstructed blood flow). Compression of the adjacent structures causes shortness of breath (dyspnea), difficulty in swallowing (dysphagia), and choking spells resulting from dilatation of the kommerell’s diverticulum.
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Other presenting findings may include arm claudication, numbness and pain in the left upper limb on exercising, chronic cough, changes in voice, dysphagia with regurgitation, etc.
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Subclavian steal syndrome in which blood flow is diverted from its normal target, symptoms of cerebral insufficiency, etc may also occur.
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Some serious complications may include aortic dissection that presents with chest pain. The aberrant subclavian artery is most commonly situated posterior to the esophagus. It can also be present between the esophagus and trachea, or in front of the trachea applying pressure on the trachea, esophagus, or both.
How Is the Condition Diagnosed?
Prenatal diagnosis (before birth) is possible. Diagnosis is based on radiological investigation.
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Chest X-Ray: It may reveal signs pointing to a right aortic arch with aberrant left subclavian artery such as deviation of the tracheal axis such that it lies medially or to the left; the descending thoracic aorta and the aortic knob are present on the right side. There is a bowing of the trachea to the left at the level of the right aortic arch. The right aortic arch is seen as a mass projecting in the right paratracheal area.
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Barium Enema: It enables the identification of abnormalities in the esophagus. It helps to affirm the diagnosis and even specifies the type of anomaly in some cases. However, the advent of MRI has obscured the use of barium enema in such conditions.
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CT Angiography: Chest is a combination of computed tomography with the injection of a dye that has very high sensitivity and specificity (98 to 100%). It enables complete evaluation of the aortic morphology along with the identification of any congenital abnormalities that may be present. It accurately images the position of the aortic arch and the relation of the supra-aortic trunks to the trachea and the esophagus.
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MRI (Magnetic Resonance Imaging): It permits functional assessment in addition to anatomical evaluation of all the parts of the aorta. Technically, the sequences in T1 spin echo, three-dimensional gradient echo sequences with gadolinium injection, and cine-MRI sequences are the basic sequences.
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Bronchoscopy or Upper Gastrointestinal Tract (Ugi) Endoscopy: It helps identify the region of compression of the trachea and esophagus. It can identify tracheomalacia.
What Is the Treatment?
The treatment of the right aortic arch with an aberrant left subclavian artery depends on the clinical presentation of the condition.
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Asymptomatic cases are not actively treated, radio clinical monitoring of these individuals is deemed sufficient.
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In patients suffering from compressive symptoms surgical intervention may be required. Evaluation of the arch anatomy prior to the surgery is important to decide on the surgical approach. Surgical decompression procedures like thoracotomy were performed traditionally. With the advent of new technology contemporary practitioners have adopted a hybrid approach involving thoracic endovascular aortic repair with plugs. Endovascular intervention is an advanced and less invasive technique compared to open surgery, it involves making incisions near the hips to gain access to blood vessels.
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If the right aortic arch with aberrant left subclavian artery is associated with other disorders then appropriate treatment is required for the co-occurring diseases.
Conclusion
Right-sided aortic arch is a rare anatomical variant present in 0.1% of the adult population. It may be associated with an aberrant left subclavian artery; it is mostly asymptomatic but sometimes it causes compressive symptoms like dyspnea or dysphagia. Various imaging techniques like an MRI, CT scan, etc. help in the diagnosis of this condition. Treatment depends on the symptoms, asymptomatic cases require monitoring, individuals with compressive symptoms may require surgical decompression procedures to relieve pressure on the adjacent structures.
