costophrenic angle

ANATOMY: The costophrenic recess (sulcus) is the potential space where the costal parietal pleura reflects onto the diaphragmatic parietal pleura. It is the most inferior and dependent part of the pleural cavity. On PA chest X-ray, the lateral costophrenic angles appear as sharp, acute (~30-degree) angles at the lung bases. On lateral film, the posterior costophrenic angles are deeper and lower, so they blunt first with small effusions (~50-75 mL). Thoracentesis is performed above the rib (to avoid the neurovascular bundle at the inferior rib border) typically at the midscapular line, 8th-9th intercostal space, to avoid entering the costophrenic recess and injuring the diaphragm, liver, or spleen.

CLINICAL SIGNIFICANCE: - BLUNTED (obliterated) CPA: pleural effusion (most common; transudate vs exudate by Light criteria), hemothorax, empyema, pleural thickening or fibrosis (e.g., prior infection, asbestos-related disease), pleural adhesions/scarring. Effusion typically has a meniscus sign (concave upward, higher laterally). Lateral decubitus film layering >1 cm confirms free-flowing fluid; ultrasound is the most sensitive. - DEEP SULCUS SIGN: abnormally deep, lucent CPA on a supine film, indicating pneumothorax (air collects anteromedially and basally when supine). Seen in trauma patients. CT is more sensitive. - FLATTENED/ELEVATED diaphragm with an altered CPA: hyperinflation in COPD/emphysema (flattened hemidiaphragms, increased AP diameter, sometimes a deepened or irregular CPA), or subpulmonic effusion, or phrenic nerve paralysis. - Loss of the sharp angle can also be seen with lower lobe consolidation or atelectasis contiguous with the diaphragm (silhouette sign of the hemidiaphragm). - Heart failure: bilateral blunted CPAs (often right > left) with Kerley B lines, cephalization, and cardiomegaly. - Trauma: FAST/eFAST examines the costophrenic recess for hemothorax (the "spine sign" on ultrasound, where the vertebral column becomes visible above the diaphragm when fluid is present).

HIGH-YIELD ASSOCIATIONS / BOARD PEARLS: - Pleural reflections: lung lower border crosses the 6th rib (midclavicular), 8th rib (midaxillary), and 10th rib (paravertebral); pleural lower border crosses the 8th, 10th, and 12th ribs respectively. The difference between these is the costodiaphragmatic recess, which lung expands into during deep inspiration. - Exudate vs transudate: Light criteria (pleural protein/serum protein >0.5, pleural LDH/serum LDH >0.6, pleural LDH >2/3 upper limit of normal serum). - Most common causes of effusion: transudate-CHF; exudate-pneumonia (parapneumonic), malignancy, TB, PE. - Order of imaging sensitivity for effusion: ultrasound/CT > lateral decubitus > lateral > PA. - Tension pneumothorax is a clinical diagnosis. Do not delay needle decompression to obtain imaging.