The costodiaphragmatic recess, also called the costophrenic recess or phrenicocostal sinus, is a potential space in the pleural cavity, at the posterior-most tips of the cavity, located at the junction of the costal pleura and diaphragmatic pleura (in the costophrenic angle). It measures approximately 5 cm vertically and extends from the eighth to the tenth rib along the mid-axillary line.
Chest X-ray of a 30-year-old healthy man, with the costodiaphragmatic recess label in red ellipse
Each costophrenic angle can normally be seen as on chest x-ray as a sharply-pointed, downward indentation (dark) between each hemi-diaphragm (white) and the adjacent chest wall (white). A small portion of each lung normally reaches into the costophrenic angle. The normal angle usually measures thirty degrees.
With pleural effusion, fluid often builds up in the costophrenic angle (due to gravity). This can push the lung upwards, resulting in "blunting" of the costophrenic angle. The posterior angle is the deepest. Obtuse angulation is sign of disease.
Chest x-ray is the first test done to confirm the presence of pleural fluid. The lateral upright chest x-ray should be examined when a pleural effusion is suspected. In an upright x-ray, 75 mL of fluid blunts the posterior costophrenic angle. Blunting of the lateral costophrenic angle usually requires about 175 mL but may take as much as 500 mL. Larger pleural effusions opacify portions of the hemithorax and may cause mediastinal shift; effusions > 4 L may cause complete opacification of the hemithorax and mediastinal shift to the contralateral side.