Urinothorax (plural: urinothoraces), also known as urothorax, is a rare cause of pleural effusion due to the accumulation of urine within the pleural space.

Clinical presentation

Patients present with varying degrees of respiratory distress depending on the amount of fluid that has accumulated .


The etiology of urinothorax can be dichotomised as either due to obstructive uropathy or due to trauma (including iatrogenic post-surgical trauma) to the urinary system . Urinothoraces are most often seen alongside urinoma, whereby the urine is thought to traverse the diaphragm into the pleural space . Although the pathophysiology of this remains unclear, there are two leading theories: either urine travels through lymphatic drainage into the pleural space, or retroperitoneal urine moves into the peritoneal cavity and then travels directly into the pleural space via a direct transdiaphragmatic passage along a pressure gradient .


The fluid in a urinothorax is usually a transudate but biochemically, often has a low pH and a high LDH and hence may be misclassified as being exudative as per Light's criteria . However, the most important biochemical feature is the pleural fluid creatinine-to-serum creatinine ratio which is >1, with an average of 1.09–19.80 .

Radiographic features

Plain radiograph

Chest radiographic appearance of a urinothorax is often indistinguishable form that of another cause of pleural effusion . It is reported that urinothorax secondary to obstructive uropathy usually results in bilateral effusions, while those caused by trauma to the urinary tract lead to a unilateral effusion , however, there have been numerous case-reports finding the opposite .


Again, the appearance of urinothorax on chest CT is indistinguishable from another cause of pleural effusion. However, abdominal CT is useful to detect the cause of the urinothorax . Many case reports describe later performing renal scintigraphy to confirm a urine leak .

Treatment and prognosis

The exact management strategy will depend on underlying etiology . However, the urinothorax should be drained if symptomatic and a urology consult sought .