The subcapsular C sign: a rare radiographic finding associated with neonatal urinary ascites

An infant with urinary ascites owing to obstructing posterior urethral valves had a subcapsular urinoma. The subcapsular urinary collection was documented surgically and resulted in a rare radiographic finding, the subcapsular C sign that is easily differentiated from the P sign of extracapsular urinary extravasation. The solute concentration gradients in the urine, subcapsular urinoma, ascites and serum plus the absence of a renal capsule urine leak suggested that the renal capsule and perirenal fascia functioned as a semipermeable membrane, permitting movement of solvent and solute into the peritoneal cavity where equilibrium with the spleen occurred.