A46-21 Silent Leak, Surprising Effusion: A Case of Delayed-Onset Urinothorax From Chronic Ureteropelvic Junction Obstruction in a Solitary Kidney

Introduction A urinothorax is a rare pleural effusion of extra-vascular origin (PEEVO) caused by urine leakage into the pleural space, usually following acute genitourinary obstruction or trauma. It is classically transudative and non-loculated, but atypical presentations may obscure the diagnosis. Here, we report a case of delayed-onset, loculated urinothorax secondary to chronic unilateral obstruction in a patient with a solitary kidney. Case Presentation A 76-year-old man with a history of urothelial carcinoma requiring right nephrectomy presented with acute encephalopathy and anuria. Computed tomography (CT) of chest, abdomen and pelvis revealed a moderate loculated left pleural effusion, moderate but stable left hydronephrosis with a new abrupt transition at the ureteropelvic junction, and ascites with an abnormal nodular soft tissue component within both lower abdominal quadrants. This left-sided finding specifically involved the left paracolic gutter. Laboratory studies showed serum creatinine 6.83 mg/dL and pleural fluid creatinine 7.5 mg/dL (pleural/serum ratio 1.1), with protein <2 g/dL, LDH 118 U/L, and glucose 138 mg/dL, consistent with a transudative effusion. Fluid cytology and cultures were negative. A pigtail catheter drained the effusion, while a nephrostomy tube was placed to relieve obstruction. Follow-up imaging confirmed resolution of hydronephrosis and no recurrence of the effusion. The patient’s renal function recovered fully, and he was discharged without dialysis dependence. Discussion The diagnosis of urinothorax was supported by the pleural fluid-to-serum creatinine ratio, transudative profile, ipsilateral hydronephrosis, and resolution after targeted intervention. Although urinothoraces usually develop acutely, this case illustrates an unusually delayed-onset presentation in the setting of chronic obstruction. It is also worth noting that loculated effusions are rare in cases of urinothoraces. This may be reflective of the chronicity or mild chemical irritation rather than infection, as cultures were negative and pleural fluid was transudative. Recognition within the PEEVO framework is crucial to distinguish urinothorax from more common exudative etiologies. Conclusion Delayed urinothoraces are rare but potentially reversible. The diagnosis requires integration of biochemical, radiographic, and clinical data, and management depends on multidisciplinary collaboration between pulmonology, urology, and interventional radiology for simultaneous pleural drainage and relief of obstruction, if present. Awareness of this condition, even in cases of stable hydronephrosis, can facilitate timely treatment and prevent recurrent or permanent renal injury.