Pneumomediastinum and the aortic nipple: the clinical relevance of the left superior intercostal vein.

This article discusses the appearance of the "aortic nipple" in chest radiography, and reviews the embryology and anatomy of the left superior intercostal vein which causes the appearance of an "aortic nipple." This radiological sign is useful in differentiating certain thoracic pathologies, such as pneumomediastinum, pneumopericardium, and medial pneumothorax. Pneumomediastinum is an encompassing term describing the presence of air in the mediastinum, and may arise from a wide range of pathological conditions. Despite the well-described imaging of pneumomediastinum, it is sometimes difficult to differentiate from other conditions such as pneumopericardium and medial pneumothorax. A separate finding, "aortic nipple" is the radiographic term used to describe the lateral nipple-like projection from the aortic knob present in a small number of individuals. The aortic nipple corresponds to the end-on appearance of the left superior intercostal vein coursing around the aortic knob, and may be mistaken radiologically for lymphadenopathy or a neoplasm. Despite their relative independence, the aortic nipple is defined by new contours in cases of pneumomediastinum, taking on an "inverted aortic nipple" appearance. In this position, the inverted aortic nipple may facilitate radiographic discrimination of pneumomediastinum from similar conditions. This study aims to review the common clinical and radiographic features of both pneumomediastinum and the aortic nipple. The radiologic appearance of the aortic nipple occurring in unison with pneumomediastinum, and its potential role as a tool in the differentiation of pneumomediastinum from similarly presenting conditions will also be described.