The normal and abnormal left superior intercostal vein.

A study of 500 normal erect posteroanterior chest radiographs was undertaken to determine the incidence of visualization and size of the left superior intercostal vein in normal individuals. The vein produces a small "nipple" lateral to the aortic knob on 1.4% of normal erect posteroanterior chest films; its diameter can be up to 4.5 mm in normal patients. Dilatation of the vein beyond 4.5 mm is a useful sign of a circulatory abnormality warranting further study. Dilatation may be due to absence of the inferior vena cava, hypoplasia of the left innominate vein, congestive failure, portal hypertension, Budd-Chiari syndrome, or superior or inferior vena caval obstruction. The differential diagnosis of an enlarged left superior intercostal vein includes mediastinal mass, especially lymphadenopathy, and aneurysm of the arch of the aorta.