What’s the incidental finding on this CT scan?
Vol.17, No.02, February 2009

A 51-year-old man presented to the emergency department with intermittent left lower quadrant abdominal pain and low-grade fever. Blood results revealed mild leukocytosis. An enhanced CT scan was performed which revealed mild proximal sigmoid diverticulitis (not shown on images above). In addition, the images showed an incidental finding that perplexed the ER physician on duty, warranting a call to the radiologist on call. What did this specialist find?

The most likely diagnosis for this incidental finding is:

a) retroperitoneal adenopathy, likely lymphoma
b) ‘wandering spleen’
c) mesenteric carcinoid tumour
d) congenital ‘horseshoe kidney’
e) internal small bowel hernia

Solution to Analyze This!

The most likely diagnosis for the incidental finding on the enhanced CT scan is:

d) congenital ‘horseshoe kidney’

The ‘abnormality’ shown (thick arrows in Figures 1 & 2) is a discrete solid enhancing (hyperdense) soft tissue structure, which lies just anterior to the lower abdominal aorta in the deep retroperitoneal space. There are no regional inflammatory changes. The mass itself exhibits two central areas of hypodensity extending to the anterior surface (which represent the renal sinus/collecting system regions of the two renal moieties; the two ureters were present and unremarkable but are not evident on the images shown). Note the “horseshoe”-shaped appearance of the lesion on the coronal CT scan, Fig. 2.

‘Horseshoe kidney,’ a form of congenital renal fusion, develops when the primitive renal buds fuse together early in embryonic development. 90% of these cases fuse at the lower poles. As the developing kidneys rise into their orthotopic positions in the upper flank regions, the ascent of the fused renal isthmus is restricted by the inferior mesenteric artery axis and vein. As such, a horseshoe kidney variant is always noted lower than expected. Horseshoe kidney is generally differentiated from ‘crossed fused ectopia,’ the other main form of congenital renal fusion anomaly, in which both fused kidneys lie on one side of the spine, and the ureter of the crossed kidney crosses back across the midline to enter the bladder in its normal UVJ position. The incidence of horseshoe kidney is about 1 in 400 persons worldwide, with a 2:1 male-to-female ratio.

The majority of horseshoe kidneys are isolated findings; however, there are reported associations with other GI tract anomalies including malrotation, imperforate anus and Meckel’s diverticulum, as well as CNS abnormalities such as neural tube defects, skeletal system anomalies such as congenital hip dysplasia and clubfoot, cardiovascular findings including VSD, and even chromosomal aneuploidy including Turner’s syndrome and Trisomy 18 (Edward’s syndrome).

Although many horseshoe kidneys are discovered incidentally, complications can occur and may include:

• Ureteropelvic junction (UPJ) obstruction, possibly because of the high insertion of the ureters in relation to the rotated renal pelves

• Recurrent infections due to urine stasis and/or associated vesicoureteric reflux

• Recurrent stone formation related to UPJ obstruction or infection

• An increased risk of trauma to the isthmus due to its position anterior to the spinal column

• Undiagnosed horseshoe kidneys may result in unexpected problems for surgeons during abdominal surgery for other abdominal problems

• Reported mild increased incidence of certain renal tumours associated with horseshoe kidney (transitional cell carcinoma, Wilm’s tumour)

The horseshoe kidney in the case presented is unremarkable with no complications evident.

Of the differential diagnosis provided, most can be fairly easily excluded. The incidental lesion (horseshoe kidney) is an oblong enhancing soft tissue structure and if you go back and review the images you may see the bilateral renal sinus moieties present in the central portion of each renal half. The appearance is not that of adenopathy, ruling out choice #1. A ‘wandering spleen’ refers to a rare congenital anomaly in which the splenic ligament and vascular pedicle are unusually elongated, allowing the spleen to shift in position within the abdomen (and even pelvis), depending on the patient’s position. Although a possibility in this case on the limited images shown, the normal spleen was indeed present in the orthotopic position in the left upper quadrant and the two renal sinus appearances in the horseshoe kidney mass also make this diagnosis much less likely. A mesenteric carcinoid tumour typically presents as an irregular stellate mass lesion with regional cicatricial reaction. The appearance shown is not in keeping with a carcinoid lesion. Since the incidental lesion shown (the horseshoe kidney) is a solid enhancing soft tissue structure, the diagnosis of internal small bowel hernia should not be entertained.


  1. Wessam B et al Am J Roentgenol Sep 2007;189:S29-S31.
  2. Perry J et al. Am J Roentgenol Mar 2006;186:718-28.
  3. Cassart M et al. Am J Roentgenol Mar 2004;182:689-95.

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Clinical challenge image
Figure 1: Axial enhanced CT image at level
Clinical challenge image
Figure 2: Coronal image of same CT exam of mid-abdomen, showing abnormality
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