This event is not rare and in most cases is not secondary to renal calculi. Although some signs of unilateral renal stasis have been demonstrated in almost 50% of IBD patients, who for various reasons have an intravenous pyelogram , the reported incidence of ureteral obstruction with associated hydronephrosis is 3.1-14.3% in patients with CD [1, 3, 6, 84-86] and 6.8% in those with UC . However, these percentages might be underestimated, considering that this condition is often asymptomatic or paucisymp-tomatic and that not all patients with IBD are examined with a pyelogram.
In patents affected by CD, ureteral obstruction can be due to periureteral fibrosis secondary to retroperitoneal inflammation (Fig. 3), but usually it is the consequence of compression from an abscess . Sometimes it is caused by an enterovesical fistula which has developed near the ureter. Over 70% of cases are right-sided and are associated with ileal or ileo-cecal disease. Left-sided obstruction has been described in 0-30% and it can be secondary to Crohn's jejunity or, more frequently, to sigmoid inflammation (Fig. 4) [2, 84]. Bilateral obstruction suggesting an extensive pelvic abscess is very rare but not impossible .
Ureteral obstruction in the presence of UC is much less frequent and its incidence is equally distributed between the left and right side . Invasive colonic carcinoma  and iatrogenic complication  are the most common causes. The symptoms are often poor or masked by the intestinal disease. There may be a persistent right lower-quadrant pain radiat
Fig. 3a. Mesenteric and retroperitoneal fibrosis due to CD of terminal ileum. b Identification of both ureters ed to the hip or thigh and associated with fever and a palpable mass. Urinalysis is often negative [3, 83]. When positive for infection, an entero-urinary fistula or pyelonephritis should be suspected .
Any of these aspecific signs should alert the doctor and ureteral obstruction should be ruled out because, despite its paucity of symptoms, it can lead to chronic infection of the kidney which may eventually result in nephrectomy [2, 89]. Diagnosis can be usually achieved with an abdominal ultrasound demonstrating dilatation of the ureter and, if present, of the renal pelvis. CT scan is more accurate in identifying whether the cause is an abscess or periureteral fibro-sis and also gives more information about the inflammatory disease. Intravenous pyelography visualises a dilated ureter with a smooth cone-shaped restriction typically located at the level of the pelvic brim (Fig. 4a).
With all these exams, differential diagnosis with calculus obstruction should not be difficult. The
Fig. 4. Left-side ureteral obstruction secondary to Crohn's colitis. a Preoperative intravenous pyelography showing a dilated ureter with smooth cone-shaped restriction at the pelvic brim. b Intraoperative identification of the ureter after mobilization of descending and sigmoid colon. c iv pyelogram 6 months after resection of the affected bowel demonstrating a resolution of ureteral obstruction treatment of this condition has evolved over the last years: older reports recommended routine ureteroly-sis in association with bowel resection [3, 88]. However, this operation is technically difficult and frequently burdened with morbidity like pyonephrosis, ureteral fistula or nephrectomy [84, 85].
It is now evident, that in many cases, the urinary stasis improves or resolves with medical treatment of the underlying bowel disease [84, 85, 90]. Significant obstruction resistant to medical therapy can be initially managed with stenting or percutaneous nephrostomy, but in these cases, surgical resection of the affected bowel and drainage of the associated abscesses should not be delayed and it is generally successful in resolving the obstruction (Fig. 4c) [3,6, 84-86].
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