Arteria lusoria.

Although variation in number and arrangement of the great vessels of the aortic arch are not rare, there are, nevertheless only a few developmental disorders which are of clinical significance. In this latter group, the arteria lusoria is an interesting variation from the radiologists' point of view. The aberrant right subclavian artery (a.r.s.a.) arises on the left side of the midline as the 4th branch of the aortic arch. From the aorta it runs behind the esophagus to reach the right upper extremity. Crossing the mediastinum obliquely, it makes an indentation on the posterior wall of the esophagus and may cause difficulty in swallowing, or, in childhood when the tracheal wall is less rigid, respiratory symptoms can develop. The radial pulse may be weaker on the right arm.