A retroesophageal right subclavian artery may cause compression of the esophagus similar to PRAA in a barium esophagram. This abnormality occurs when the right 7th intersegmental, being pulled forward along the dorsal aorta, fails to reach the right 4th arch before the latter separates from the dorsal aorta.

The surgeon enters the thorax expecting to find a typical vascular ring and discovers the aortic arch on the left side along with the left ligamentum arteriosum. Further dissection reveals the right subclavian artery arising on the dorsal aorta and crossing over the esophagus. 

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This slide shows a lateral view of a post-mortem specimen from a dog with an aorta on the left side and a retro-esophageal right subclavian artery arising from the dorsal aorta and compressing the dorsal aspect of the esophagus.

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This is a post-mortem specimen showing a patent ductus arteriosus (D) and a retroesophageal right subclavian artery (R) coursing dorsally over the esophagus (E). Retroesophageal right or left subclavian arteries can be divided close to the aorta because of collateral circulation to the distal subclavian branches via the vertebral arteries.

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