Case of the Month: December, 2012, Page 5

Answer: C. There are no clips or other evidence of post-surgical change in the expected location of the right kidney. Instead, there is a congenital fusion of the kidneys, specifically cross-fused renal ectopia, complicated by obstructive uropathy. The small calcification in the right side of the pelvis near the bladder is a phlebolith, and the upper pole moiety, which connects to the bladder on the right side, is not hydronephrotic. Instead, there is a faint calcification present within the proximal ureter of the lower pole moiety (red arrow) resulting in moderate to severe hydronephrosis and stranding of the lower pole moiety.

Discussion: Cross-fused renal ectopia complicated by obstructive uropathy.

Although the patient had indeed suffered a previous gun-shot injury, his left kidney had not been removed—his kidneys were congenitally fused and located in the right retroperitoneum.  

Cross-fused renal ectopia results from abnormal ascent and rotation of metanephric blastema after induction by the ureteric bud.  The kidneys form in utero at sacral level and ascend to L1 by term. The renal pelvis initially directed anteriorly but rotates 90 degrees medially as it ascends. In some instances, there is a failure of separation of the two pelvic kidneys, resulting in one of several congenital malformations, such as:

  • Pelvic Kidney
  • Subdiaphragmatic kidney
  • Crossed fused renal ectopia
  • Horseshoe kidney

Cross-fused renal ectopia is relatively rare, with incidence estimated at 1 in 1000-7500. By comparison, a horseshoe kidney (where the lower poles of the kidneys are fused by a fibrous or parenchymal band) is much more common, 1 in 450.

The exact inciting factors for this anomaly are unknown. More than half of patients with cross-fused renal ectopia and a third of patients with horseshoe kidney have other urogenital, gastrointestinal, cardiopulmonary, skeletal, or chromosomal anomalies. Studies have shown geographic "hot spots" where this occurs, suggesting either a common exposure to teratogenic factors, or a hereditary condition with variable penetrance. Interestingly, all types of congenital renal anomalies are more common in males (1:2 ratio).

Renal function is normal, and prognosis following this diagnosis is excellent. In fact many patients remain asymptomatic throughout life if no complications occur. However, malpositioned kidneys are more susceptible to trauma, iatrogenic injury, obstruction, and infection. Obstructive uropathy is more common in these patients.  There is reportedly a slightly increased risk of Wilms and carcinoid tumors in patients with horseshoe kidneys.

Case Conclusion:

Following the diagnosis, the patient underwent cystoscopy and retrograde pyelogram. The operative report describes a complete obstruction at the proximal ureter of the lower pole moiety, likely secondary to a urinary calculus with debris seen within the urine proximally. The other ureter was also explored with retrograde pyelogram and no evidence of obstruction noted. These findings correspond well with the findings on CT.  A ureteral stent was placed in the lower pole moiety and the patient was put on antibiotics. He endorsed complete resolution of his symptoms following the procedure.


  • Glodny et al. Kidney fusion anomalies revisited: clinical and radiological analysis of 209 cases of crossed fused ectopia and horseshoe kidney. BJU Int. 2009 Jan;103(2):224-35.
  • Statdx.