Clinical History
The patient presented with chest pain. A PA chest X-ray showed a widened mediastinum with displacement of the trachea to the left. A calcific focus was also seen in that region. A chest CT was performed to exclude a mediastinal mass.
Imaging Findings
CHEST X-RAY:
The right paratracheal stripe is widened, and a curvi-linear calcification is seen at the lateral aspect. The left pulmonary artery is unusually well visible. The distal trachea is slightly deviated to the left. No other abnormalities were seen.
A CT was carried out to exclude an extrapulmonary extrapleural superior mediastinal mass.
CT THORAX:
There is a right sided aortic arch with age-related mural calcification. The descending aorta is found on the right side of the vertebral column before coming to the midline at the level of T11.
The first branch is the left common carotid artery followed by the right common carotid artery. The right vertebral artery takes off directly from the arch soon after, followed by the right subclavian artery. A Kommerell diverticulum is seen just posterior to the oesophagus, giving rise to the left subclavian artery which is slightly compressing the oesophagus.
Discussion
The right aortic arch (RAA) is an anatomic variation which occurs in approximately 0.05% to 0.1% of the population [1]. There are two types of right sided aortic arches.
In type I, also called “mirror-image” type, the first branch is the left innominate artery. This type is often associated with cardiac defects including tetralogy of Fallot, pulmonary atresia with VSD and truncus arteriosus. If there is no associated congenital cyanotic heart disease, type I RAA is usually asymptomatic because no vascular ring is formed [2,3].
Type II RAA is characterised by an aberrant left subclavian artery which arises as a fourth branch from Kommerell’s diverticulum, a vestigial remnant of the distal, embryologic left arch. This type of RAA is usually associated with a normal heart and this is what the patient presented with. The left-sided aberrant SCA and the presence of a left-sided ductus or ligamentum arteriosum result in the formation of a vascular ring. The ring is generally loose and only roughly one fifth of patients have been reported to be symptomatic [2].
Clinical presentation is in a bimodal fashion. During infancy the trachea is more easily compressed and patients typically present with respiratory symptoms such as stridor, wheezing, cyanosis, or recurrent pneumonia when solid foods are introduced. Symptoms may be interpreted as asthma which carries the risk of unnecessary treatment with inhaled steroids [4].
In adults, as the tracheal cartilage becomes more rigid, the oesophagus is more likely to be compressed, resulting in dysphagia [3].
Symptomatic patients may require surgical division of the ligamentum arteriosum to interrupt the vascular ring and achieve decompression [5].
Many patients go undiagnosed for a long time before the true cause of their symptoms is found. Since chest radiography is often the first investigations for the above mentioned symptoms, correct identification of an RAA will allow the radiologist recommend relevant further investigations.
Through careful examination of the chest radiograph it is possible to pick up the essential features of an RAA. These features include absence of the aortic knob and left descending aortic shadow, displacement of the ascending aortic shadow to the right and high positioning of the transverse portion of the aortic arch. Often the left pulmonary artery is unusually well visualised.
Angiography, CT or MRI will generally be needed to demonstrate the exact anatomy.
Awareness of this anatomical variant is important since delayed diagnosis and unnecessary treatment of alternative suspected diagnoses may be harmful to the patient.
Differential Diagnosis List
Right-sided aortic arch with an aberrant left subclavian artery
Dextrally displaced SVC and innominate veins (based on the chest X-ray)
Aortic dilatation (based on the chest X-ray)
Final Diagnosis
Right-sided aortic arch with an aberrant left subclavian artery
