Report of an incidental finding of a congenital intradiaphragmatic cyst in an adult and a critical review of the literature.

An incidental chest x-ray finding of an oval soft tissue opacity in the right costophrenic recess in a 55-year-old man prompted further investigation by unenhanced CT which demonstrated a 3.9 cm diameter lesion of fluid density intimately related to the lateral aspect of the right hemidiaphragm and right lobe of the liver. It was not possible on CT to determine whether this was intrapulmonary, an unusual exophytic hepatic simple cyst, or a diaphragmatic cystic abnormality. Subsequent ultrasound, however, allowed the diagnosis of a diaphragmatic mesothelial cyst by confirming an anechoic, thin-walled cystic structure separate from the liver capsule, from which it moved independently, with a fixed relationship to the diaphragm and splitting of muscular fibres of the right hemidiaphragm around its periphery. This case serves to highlight a rare congenital abnormality of the hemidiaphragm as a cause of a pulmonary nodule visible on a chest radiograph and to illustrate the usefulness of ultrasound in confirming the diagnosis. It also allows a critical review of the literature on intradiaphragmatic mesothelial cysts in adults and children.