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Horseshoe Kidney

Valerie L Ward, MD
Ramin Khorasani, MD

September 3, 1996

Presentation

A 35-year-old man presented with a history of renal calculi and episodic flank pain.

Imaging Findings

CT of the abdomen and pelvis

Computed tomography (CT) of the abdomen and pelvis shows that the lower poles of both kidneys are directed medially and fused by an isthmus (arrow). The isthmus is just caudal to the origin of the inferior mesenteric artery from the aorta (arrow). There is a left renal calculus (arrow). The bilateral low attenuation masses are most likely renal cysts.

Differential Diagnosis

Embryological renal fusion anomalies include horseshoe kidney, disc or pancake kidney, cake or lump kidney, and crossed renal ectopy with fusion. Horseshoe kidney, the most common fusion anomaly, is characterized by fused lower renal poles. Disc or pancake kidney occurs when the kidneys are fused medially at both the upper and lower poles, forming a ring-like renal mass. Cake or lump kidney has extensive fusion of the two kidneys. In crossed renal ectopy with fusion, the ectopic kidney is on the opposite side from its ureter and is partially fused with the normally located kidney.

Diagnosis

Horseshoe kidney

Discussion

Horseshoe kidney occurs in 1:400 births, and is more common in males. The anomly represents a failure of separation of the embryologic metanephric ridges such that the lower poles of both kidneys are fused by an isthmus and directed medially. In rare instances, the upper poles may be the site of fusion. The isthmus may be renal tissue or fibrous tissue. The isthmus prevents normal renal rotation so that the renal axes are abnormally oriented. Specifically, the lower poles lie more medial than the upper poles, and the renal pelves are more anterior. The isthmus also prevents cephalic migration of the horseshoe kidney above the level of the inferior mesenteric artery. Therefore, a horseshoe kidney is actually an anomaly of renal fusion, malrotation, and ectopia.

The horseshoe kidney characteristically obtains its blood supply from several sources during its incomplete ascent. These sources include the inferior mesenteric artery, aorta, renal arteries and iliac arteries. The isthmus has its own blood supply. Multiple aberrant arteries can cross the ureteropelvic junctions and proximal ureters. These crossing vessels result in obstruction and urinary stasis that lead to infection and calculus formation.

Although many patients with horseshoe kidneys are asymptomatic, important associations to recognize with this anomaly include Wilm's tumor in the isthmus. Also, a horseshoe kidney is associated with genital, anorectal, cardiovascular, and skeletal anomalies. In short, a patient with a horseshoe kidney may come to medical attention for a variety of reasons including nephrolithiasis, infection, and other congenital anomalies.

References

1. Davidson AJ, Hartman DS. Radiology of the kidney and urinary tract. 2nd ed. Philadelphia: Saunders, 1994: 81, 84.

2. Dunnick NR, McCallum RW, Sandler CM. Textbook of uroradiology. Baltimore: Williams & Wilkins, 1991: 17-19.

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