eMedicine Specialties > Radiology > Genitourinary

Horseshoe Kidney

Author: Abid Irshad, MD, Assistant Professor, Department of Radiology, Medical University of South Carolina
Coauthor(s): Susan Ackerman, MD, Director of Ultrasound, Associate Professor, Department of Radiology, Medical University of South Carolina; James G Ravenel, MD, Associate Professor of Radiology, Vice Chair for Education, Chief of Thoracic Imaging, Department of Radiology, Medical University of South Carolina
Contributor Information and Disclosures

Updated: Mar 1, 2010

Introduction

Background

Congenital anomalies of the kidneys include a group of so-called fusion anomalies, in which both kidneys are fused together in early embryonic life. Fusion anomalies of the kidneys can generally be placed into 2 categories: (1) horseshoe kidney and its variants and (2) crossed fused ectopia. Horseshoe kidney is probably the most common fusion anomaly.

The term horseshoe kidney refers to the appearance of the fused kidney, which results from fusion at one pole (see some examples in the images below). In more than 90% of cases, fusion occurs along the lower pole. Technically, the term horseshoe kidney is reserved for cases in which most of each kidney lies on one side of the spine. It includes symmetric horseshoe kidney (midline fusion) or asymmetric horseshoe kidney (L-shaped kidney). In the latter, the fused part, or isthmus, lies slightly lateral to the midline (lateral fusion). Horseshoe kidney is generally differentiated from crossed fused ectopia, in which both fused kidneys lie on one side of the spine, and the ureter of the crossed kidney crosses the midline to enter the bladder.

Glodny et al examined the radiologic findings of horseshoe kidneys and crossed fused ectopias in 209 patients to assess the frequency and clinical significance of associated anomalies and diseases. CT scanning was the most reliable imaging modality for both horseshoe kidneys and crossed fusion ectopias, but individual cases with complex anatomic configurations required special examination strategies. Crossed fused ectopias differed anatomically from horseshoe kidneys in having a lower position, greater axial rotation, smaller pelvic width, more caudal origin, and fewer vessels. Children had higher rates of malformations than adults.¹

In a retrospective analysis, Zumsteg et al reported on laparoscopic heminephrectomy for benign anomalies, as compared with traditional open surgical removal. Of 11 patients who underwent heminephrectomy, 8 had duplicated collecting systems, 2 had horseshoe kidneys, and 1 had a cross-fused renal ectopic kidney. There were no intraoperative complications, and all presenting symptoms resolved with preservation of renal function. There were, however, 3 major and 2 minor postoperative complications, such as 50% loss of remaining ipsilateral renal parenchyma, postoperative neuralgia and anejaculation, urinoma, and partial wound separation.²

Pathophysiology

The development of the normal kidney depends on the union of ureteric buds from the mesonephric ducts with the nephrogenic cords in the embryo. The union is believed to occur around the fourth gestational week, normally occurring at the level of the first or second sacral vertebral segment. Subsequent straightening of the hind end of the embryo, along with differential growth of the developing pelvic structures, leads to the ascent of both kidneys to their normal dorsolumbar regions between the fourth and ninth weeks of gestation.

The abnormal fusion probably occurs at the 5- to 12-mm embryonic stage, when the kidneys are in the true pelvis and the renal capsule has not yet matured. One embryologic explanation regarding midline fusion is that, at that stage, abnormal variation in growth, ventral flexion of the hind end of the embryo, or other variations in the growth of pelvic structures may bring the metanephric blastemas (developing kidneys) abnormally close together for a longer period; this proximity can lead to fusion.

The explanation for lateral fusion is that, during early embryonic life, lateral flexion of the lumbosacral spine may push one of the developing kidneys toward the midline. This positioning can lead to asymmetric fusion. In later embryonic life, the ascent of the fused kidney is hindered by the inferior mesenteric artery, with the isthmus of the horseshoe kidney becoming trapped under it. Consequently, the horseshoe kidney always lies at a position that is lower than normal. However, whether the cause is abnormal fusion, abnormal migration of the posterior nephrogenic areas, or another teratogenic factor is not yet clear.

Frequency

United States

Horseshoe kidney is the most common renal fusion anomaly, the incidence being about 1 case per 400 persons worldwide. As reported, it can occur in a single member of a set of identical twins and in both members. At present, no clear evidence of a hereditary trait exists.

International

The incidence of horseshoe kidney is about 1 case per 400 persons worldwide.

Mortality/Morbidity

Complications of horseshoe kidney include the following:

• Ureteropelvic junction (UPJ) obstruction is a common complication, possibly because of the high insertion of the ureter.
• Recurrent infections occur because of urine stasis and associated vesicoureteric reflux.
• Recurrent stone formation related to UPJ obstruction or infection may occur.
• An increased risk of trauma to the isthmus exists because of its position anterior to the spine.
• Horseshoe kidney may pose problems for surgeons during abdominal surgery for other abdominal problems.
• Evidence indicates that an increased incidence of certain renal tumors is associated with horseshoe kidney.

Horseshoe kidney may occur as an isolated anomaly or in association with other congenital anomalies. The morbidity and mortality rates largely depend on whether it is associated with other anomalies.

• Horseshoe kidney with other congenital anomalies
  • About one third of cases of horseshoe kidney are associated with other congenital anomalies, which include anomalies of the urogenital, gastrointestinal, neurologic, and skeletal systems, as well as some chromosomal abnormalities.
  • Some of these anomalies are not compatible with life, whereas others have a minor degree of morbidity.
  • Depending on the severity of the associated anomalies, outcomes vary, in more severe cases, from stillbirth to early neonatal death.
  • When patients have minor associated anomalies, they may live relatively healthy lives.
  • In patients with other anomalies, the horseshoe kidney itself is usually not the major cause of morbidity or mortality.
• Horseshoe kidney as an isolated entity
  • About one third of patients with horseshoe kidney remain asymptomatic.
  • Horseshoe kidney is often an incidental finding at autopsy.
  • The cause of morbidity in symptomatic patients is most commonly related to recurrent urinary tract infection (UTI) and stones, which are frequent.
  • No significant decrease in the survival rate has been established in association with this isolated anomaly.³

Sex

Renal fusion anomalies occur predominantly in males. The male-to-female ratio is approximately 2:1 for horseshoe kidney and 6:1 for crossed fused ectopia.

Age

Clinically, this congenital anomaly is diagnosed in individuals of all ages; horseshoe kidney is found prenatally, as well as in the elderly. However, because of its association with other congenital anomalies, horseshoe kidney is more commonly diagnosed in children.

Anatomy

In the midline fusion anomaly, the kidneys are symmetric, with each of the lower poles of the kidneys converging toward the midline. In the lateral fusion anomaly, one kidney is more vertical, while the other kidney is more horizontal; the isthmus lies slightly toward one side. In rare cases, the upper poles fuse, reversing the horseshoe appearance. In another rare event, the upper and lower poles will fuse, producing a ringlike mass, a condition termed disc kidney, doughnut kidney, or pancake kidney.

Blood vessel variations occur in horseshoe kidney. In about 30% of cases, blood is supplied to each kidney by 1 renal artery. In other instances, 1 or both kidneys are supplied by 2 or 3 renal arteries. The blood supply to the isthmus also varies. It may come from the renal artery, or it may arise directly from the aorta, above or below the isthmus. Occasionally, the blood supply arises from the common iliac, the external iliac, or the inferior mesenteric arteries.

Presentation

Clinically, horseshoe kidney can be divided into 2 groups: horseshoe kidney with associated anomalies and isolated horseshoe kidney with no associated anomalies.

Horseshoe kidney with associated anomalies

About one third of the patients with horseshoe kidney have associated anomalies. These include multisystem abnormalities, such as urogenital anomalies (eg, UPJ obstruction, vesicoureteric reflux, ureteral duplication, hypospadias, undescended testis, ectopic ureter, retrocaval ureter, bicornuate and/or septate uterus).

Gastrointestinal (GI) abnormalities include anorectal malformations, such as imperforate anus, malrotation, and Meckel diverticulum. Central nervous system (CNS) anomalies, such as neural tube defects, may be seen. Skeletal anomalies include rib defects, clubfoot, or congenital hip dislocation. Cardiovascular abnormalities, such as a ventricular septal defect (VSD), occur in some patients.

Horseshoe kidney has also been found in association with some chromosomal abnormalities, such as Turner syndrome and trisomy 18.

The clinical course largely depends on the nature of the anomalies, because horseshoe kidney itself is relatively asymptomatic.

Isolated horseshoe kidney

In the pediatric clinical setting, about 90% of patients are asymptomatic, and the most common presentation is UTI.

When symptoms are present, they are usually related to hydronephrosis, infection, stone formation, or hematuria. The most common symptom is vague abdominal pain, which may radiate to the back. Occasionally, nausea and vomiting may be reported. Also, the so-called Rovsing sign (nausea, vomiting, and abdominal pain with hyperextension of the spine) may be positive in some patients.

A small percentage of patients may have a palpable lump in the abdomen.

Horseshoe kidney has been reported to be associated with increased risk for renal neoplasms, such as Wilms tumors, renal carcinoids, and transitional cell carcinoma.

Preferred Examination

Intravenous urography (IVU), computed tomography (CT) scanning, magnetic resonance imaging (MRI), and scintigraphy depict horseshoe kidney with a high degree of accuracy. For the purpose of diagnosis, IVU is usually the first-line investigation, followed by CT scanning or scintigraphy in cases with doubtful findings. Ultrasonography also is helpful, but it may have some technical limitations (which are discussed in the next section).

Although MRI accurately reveals the anatomy associated with horseshoe kidney, it is not generally used for diagnosis because of its high cost. MR angiography provides additional information about the vascular anatomy. A voiding cystourethrogram is usually required to evaluate associated vesicoureteric reflux. A diuretic renal scintigram is helpful in differentiating obstructed and nonobstructed dilated collecting systems. Angiography is usually reserved for presurgical planning to fully evaluate the arterial supply pattern.

CT angiography scanning with 3-dimensional reconstruction also may reveal the vascular anatomy and collecting system for presurgical planning.

Limitations of Techniques

Most of the time, IVU cannot be used to differentiate between a fibrous isthmus and a parenchymal isthmus. Also, in many cases, the diagnosis of a horseshoe kidney is difficult to make on the basis of only IVU findings. In these instances, CT scanning or scintigraphy may be helpful.

Ultrasonography sometimes has technical limitations, especially in patients with a large body habitus, in whom visualization of the isthmus may be difficult. Also, horseshoe kidney may be missed on routine abdominal scans unless particular attention is paid to ruling out this condition.

Differential Diagnoses

Other Problems to Be Considered

On images, the main differential diagnosis of horseshoe kidney includes another fusion anomaly, crossed fused ectopia. However, in this latter condition, the fused kidneys lie on the same side of the spine, and the ureter of the crossed kidney crosses the midline to enter the bladder. However, this differentiation may not always be possible.⁴

On IVUs, a malrotated or an ectopic kidney may sometimes be confused with horseshoe kidney.

Gibbous deformity of the spine may alter the renal axis, which may then resemble horseshoe kidney.

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