Intermittent hydronephrosis: a unique feature of ureteropelvic junction obstruction caused by a crossing renal vessel.

Obstruction at the ureteropelvic junction (UPJ) is usually intrinsic. Recently, however, 13 (11%) of approximately 120 children with UPJ obstruction were found to have extrinsic obstruction caused by a lower-pole renal vessel. In each case, the obstruction was confirmed at the time of surgery. Most of these patients had intermittent pain. Excretory urograms did not show obstruction between episodes of pain, but when a urogram was obtained during an episode, marked obstruction was noted. Contrast agent trapped in a segment of proximal ureter suggests the presence of an extrinsic obstruction. Properly timed renal imaging, therefore, can identify the urinary tract as the source of the symptoms.