Occult ureteropelvic junction obstruction presenting as anuria and urinary ascites in an infant with antenatal, unilateral hydronephrosis.

Objective: To discuss an unusual case of occult ureteropelvic junction obstruction.

Case: A premature male infant with severe, unilateral hydronephrosis presented to the emergency room with 20 h of dry diapers. Placement of urethral catheter did not return urine. Imaging revealed persistent hydronephrosis on the previously affected side and new-onset hydronephrosis with perirenal fluid on the contralateral side. Patient was taken to the operating room and forniceal rupture from occult ureteropelvic junction obstruction with urinary ascites was identified. The obstruction was surgically repaired at that time and his postoperative course was uncomplicated.

Discussion: While most agree on postnatal evaluation for some children with antenatal hydronephrosis, there is no consensus as to the timing and frequency of evaluation. The risk of significant obstruction in children with low grades of hydronephrosis is very low, and many are not followed. This is a rare case in which low-grade postnatal hydronephrosis resulted in significant clinical obstruction.