IS Case 475: Horseshoe kidney

Meena Moorthy, MD, MBA

University of Rochester


Imaging Sciences URMC 2010
Publication Date: 2010-08-30

History

Patient is a 7-year-old male who had been hiking with his family when he was struck in the abdomen by a falling boulder. The child was seen at an outside hospital, where the work up included a CT of the abdomen and pelvis. No acute injuries were found, however it was noted that the patient had a horseshoe kidney, with a calcific density overlying the lower pole of the left moiety. Because of these findings, the patient was seen by a urologist and an ultrasound was requested to determine the exact location of the calcified focus (intrarenal or extrarenal).

Of note, the patient was a full term infant, of an uncomplicated pregnancy. Past medical history was significant only for asthma and eczema. There was no history of any urinary symptoms.

Findings

On CT exam of the abdomen and pelvis there was a horseshoe kidney with the isthmus crossing anterior to L3. There was no evidence of hydronephrosis or hydroureter. There was an irregular, calcific density just to the left of midline at the level of the isthmus; it was not clear if this is within or adjacent to the renal parenchyma.

Retroperitoneal ultrasound done as follow-up again showed a horseshoe kidney, with no evidence of obstruction. There was no dilatation of the collecting system. The calcification seen on CT was now seen to lie posterior to the isthmus, and not in the renal parenchyma.

Diagnosis

Horseshoe kidney

Discussion

A horseshoe kidney is the result of the failure of the primitive nephrogenic cell masses to separate during development. It is the most common type of renal fusion, and occurs in almost 1/600 births.

In a horseshoe kidney, there is fusion of the lower poles across the midline. The connected area is referred to as the isthmus, and may consist of functional renal tissue or or fibrous tissue; the isthmus generally lies anterior to the great vessels. Horseshoe kidneys are located slightly inferiorly; this is because renal ascent is interrupted as the isthmus encounters the IMA in midline. There can be a variable number of ureters, which exit the kidney ventrally (rather than ventromedially). Vasculature can also be abnormal, with a variable number of renal arteries which are ectopic in origin.

Many patients with horseshoe kidney are asymptomatic and it is discovered incidentally. However, patients are at an increased risk for infection and injury from mild trauma. Hydronephrosis and renal calculi are also increased in patients with horseshoe kidney. This is secondary to the anterior orientation of the pelvicalyceal system, which results in a relative obstruction to the outflow of urine.

Horseshoe kidneys are readily diagnosed on CT. The diagnosis is harder by US, however this modality is often used for follow up in patients with a horseshoe kidney.

References

  1. Blickman JG, Parker BR, Barnes PD. Pediatric Radiology: The Requisites, 3rd ed. Mosby, 2009; 127-8.
  2. Donnelly LF. Pediatric Imaging: The Fundamentals. Saunders, 2009;133-4.

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