Horseshoe kidneys are the most common type of renal fusion anomaly. They render the kidneys susceptible to trauma and are an independent risk factor for the development of renal calculi and transitional cell carcinoma of the renal pelvis.
Horseshoe kidneys are found in approximately 1 in 400-500 adults and are more frequently encountered in males (M:F 2:1) . The vast majority of cases are sporadic, except for those associated with genetic syndromes (see below) .
Horseshoe kidneys are, in themselves, asymptomatic and thus they are usually identified incidentally. They are however prone to a number of complications as a result of poor drainage, which may lead to clinical presentation. These complications include:
- hydronephrosis, secondary to pelviureteric junction obstruction
- renal calculi: up to 60% of patients
- increased susceptibility to trauma
- infection and pyeloureteritis cystica
- increased incidence of malignancy
- renovascular hypertension
A horseshoe kidney is formed by fusion across the midline of two distinct functioning kidneys, one on each side of the midline. They are connected by an isthmus of either functioning renal parenchyma or fibrous tissue. In the vast majority of cases, the fusion is between the lower poles (90%) . In the remainder, the superior, or both the superior and inferior poles are fused. This latter configuration is referred to as a sigmoid kidney .
The normal ascent of the kidneys allows the organs to take their place in the abdomen below the adrenal glands. However, with a horseshoe kidney, ascent into the abdomen is restricted by the inferior mesenteric artery (IMA) which hooks over the isthmus. Hence horseshoe kidneys are low lying.
As a result of this fusion the inferior pole of each kidney point medially which is the reverse of the normal renal axis. The ureters leave the kidneys and pass anterior to the isthmus, which is typically located immediately below the inferior mesenteric artery.
Also due to the halted ascent, renal vascular anomalies are common: usually, multiple renal arteries arise from the distal aorta or iliac arteries; this is important when these patients undergo any procedure, particularly a renal angiogram.
Horseshoe kidneys are frequently associated with both genitourinary and non-genitourinary malformations, and are also seen as part of a number of syndromes :
- chromosomal/aneuploidic anomalies
- non-aneuploidic anomalies
Unless aware of the typical appearances of a horseshoe kidney, the abnormally rotated and inferiorly located kidney results in poor visualization of the inferior pole and underestimation of the length. This is especially the case if the patient is scanned prone, and is an additional argument for scanning patients supine with left and right decubitus positions .
Alternatively, the renal tissue located anterior the aorta may be mistaken for retroperitoneal tissue, such as may be seen in lymphoma or metastatic nodal enlargement .
On intravenous urography (IVU), the control/scout film will show a soft tissue mass on either side of the midline with a central isthmus. The kidneys are also oriented with the lower pole closest to the midline, which is the reverse of normal. Following intravenous contrast, the orientation of the pelvicalyceal system is clearly outlined and may illustrate associated complications such as a PUJ obstruction.
CT and MRI
Both CT and MRI demonstrate renal tissue of normal imaging appearance, but with abnormal configuration. Enhancement is normal, and excretory phase imaging may be used to assess the collecting system.
Treatment and prognosis
Horseshoe kidneys in themselves do not require any treatment, and patients have a normal life expectancy. It is, however, important to recognize their presence prior to abdominal surgery or renal intervention for one of their many complications (see above).
When visualized with cross-sectional imaging (CT or MRI) there is essentially no differential. On ultrasound, care must be taken to not mistaken a horseshoe kidney for a midline retroperitoneal mass, or to underestimate the length of the kidney.
Other entities to be aware of, from purely a nomenclature point of view include:
- renal cell carcinoma remains the most common tumor in a horseshoe kidney, although this does not appear to occur at increased rates to the general population .