You are in: eMedicine Specialties > Radiology > GENITOURINARY Horseshoe KidneyArticle Last Updated: Aug 28, 2007AUTHOR AND EDITOR INFORMATIONSection 1 of 12
  Author: Abid Irshad, MD, Assistant Professor, Department of Radiology, Medical University of South Carolina Abid Irshad is a member of the following medical societies: Radiological Society of North America and Society of Breast Imaging Coauthor(s): Susan Ackerman, MD, Director of Ultrasound, Assistant Professor, Department of Radiology, Medical University of South Carolina; James Ravenel, MD, Assistant Professor of Radiology, Chief of Thoracic Imaging, Clinical Director of Computed Tomography, Assistant Residency Program Director, Department of Radiology, Medical University of South Carolina Editors: Bernard D Coombs, MB, ChB, PhD, Consulting Staff, Department of Specialist Rehabilitation Services, Hutt Valley District Health Board, New Zealand; Joshua A Becker, MD, Professor, Department of Radiology, New York University School of Medicine; Robert M Krasny, MD, Consulting Staff, Department of Radiology, The Angeles Clinic and Research Institute; Eugene C Lin, MD, Consulting Staff, Department of Radiology, Virginia Mason Medical Center Author and Editor Disclosure Synonyms and related keywords: midline renal fusion, L-shaped kidney, fusion anomaly, fusion anomalies, fused kidneys, symmetric horseshoe kidneys, asymmetric horseshoe kidneys, isthmus, lateral renal fusion, crossed fused ectopia INTRODUCTIONSection 2 of 12
  BackgroundCongenital 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. 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. PathophysiologyThe 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. FrequencyUnited StatesHorseshoe 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. InternationalThe incidence of horseshoe kidney is about 1 case per 400 persons worldwide. Mortality/MorbidityComplications of horseshoe kidney include the following:
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.
SexRenal 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. AgeClinically, 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. AnatomyIn more than 90% of cases, fusion in horseshoe kidney occurs along the lower pole. This region of fusion, called the isthmus, is usually composed of renal parenchymal tissue. However, in many instances, it may consist of fibrous tissue. The isthmus can be wide or narrow, depending on the degree of fusion. The isthmus usually lies anterior to the aorta and inferior vena cava (IVC), and it is posterior to the inferior mesenteric artery. In rare cases, however, the isthmus passes between, or even posterior to, the aorta and IVC. The ureters usually pass anterior to the isthmus, and they may have a high insertion point in the renal pelvis. The renal pelves are usually malrotated and lie anteriorly or laterally. 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. Clinical DetailsClinically, 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 ExaminationIntravenous 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 TechniquesMost 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. DIFFERENTIALSSection 3 of 12
Other Problems to Be ConsideredOn 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. 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. RADIOGRAPHSection 4 of 12
FindingsPlain radiographs may show low-lying renal outlines with an altered renal axis. Usually, the kidneys follow the axis of the psoas muscles, with the lower poles lying at a more lateral position than the upper poles. In horseshoe kidney, this axis is reversed, with the lower poles lying closer to the spine. With plain radiographs alone, however, the degree of confidence in this finding is usually low. IVU usually reveals the classic findings associated with horseshoe kidney. Findings on the initial tomogram may be deceptive because of the exclusion of the anteriorly lying isthmus. Renal axis abnormalities are confirmed, as seen on the plain radiographs. In midline fusion, the kidneys are symmetric, with the lower pole calyces lying closer to or actually overlying the spine. The lower calyces are usually medially rotated, and they may actually lie medial to the ureters. Some degree of malrotation of the kidneys is usually present. A renal pelvis is often extrarenal and large. The degree of malrotation has been associated with the degree of fusion. If the isthmus is narrow, the kidneys are usually less malrotated, with the pelvis lying anteromedially in its near-normal position. In cases of a wide isthmus, the renal pelves lie anteriorly or laterally. Associated UPJ obstruction may be present because of the higher ureteric insertion point, which leads to delayed pelvic emptying. Ureters may have the so-called flower-vase appearance, in which the upper ureters diverge laterally over the isthmus and then converge inferiorly. The L-shaped kidney's lateral fusion anomaly can also be readily appreciated on IVUs. In this anomaly, one kidney has a relatively vertical position, while the other kidney is relatively horizontal. Degree of ConfidenceThe degree of confidence associated with IVU for the diagnosis of horseshoe kidney is high in most cases. Sometimes, ascertaining the diagnosis on the basis of IVU alone is difficult, and further workup—using CT scanning, MRI, or scintigraphy—may be necessary. IVU is also a good modality for discovering associated findings, such as the presence of stones, scarring, and duplex collecting systems. False Positives/NegativesSometimes, malrotated kidneys can be mistaken for horseshoe kidneys. IVU does not aid in the differentiation of a parenchymal isthmus from a fibrous isthmus. CT SCANSection 5 of 12
FindingsContrast-enhanced CT scanning has a high degree of accuracy in defining the structural abnormalities of horseshoe kidney, including the degree and site of fusion, the degree of malrotation, associated renal parenchymal changes (eg, scarring, cystic disease), and collecting system abnormalities (eg, duplex system, hydronephrosis). It can also be used to differentiate a parenchymal isthmus from a fibrous isthmus and to show the relation of the isthmus to surrounding structures. Although routine CT scanning may show the variant arterial supply, this is better defined with CT angiography scanning with 3-dimensional reconstruction and volume rendering. In cases of neoplasm associated with horseshoe kidney, the use of 3-dimensional multisection helical CT scanning also has been advocated, because it further clarifies the structural details. Degree of ConfidenceThe degree of confidence associated with the use of CT scanning for diagnosing horseshoe kidney is high. However, associated stone disease may be missed with contrast-enhanced CT scanning alone, and nonenhanced CT scanning, ultrasonography, or plain radiography may be required. MRISection 6 of 12
FindingsMRI has an advantage in depicting structural details because of its ability to permit multiplanar imaging, but it is more costly than other examinations. However, an added advantage may be obtained by using MR angiography to delineate the vascular anatomy. MRI is probably the best modality to use in evaluating the extent of renal tumors associated with horseshoe kidney. Degree of ConfidenceThe degree of confidence connected with the use of MRI for the diagnosis of horseshoe kidney, as well as for the purpose of defining associated structural findings, is high. However, associated small stones may be missed on MRIs. ULTRASOUNDSection 7 of 12
FindingsUltrasonography can be useful for diagnosing horseshoe kidney. To establish the diagnosis, the most important ultrasonographic findings are the presence of the isthmus and its continuity with the lower poles. Other features, such as malrotation and an altered renal axis, may be difficult to assess with ultrasonography. In cases in which the isthmus is composed of only a thin, fibrous band, this midline soft tissue may not be seen. Various findings, such as a curved configuration of the lower poles, elongation of the lower poles, and poorly defined lower poles, suggest the presence of horseshoe kidney. Other associated findings, such as stones, hydronephrosis, and cortical scarring, are reliably depicted on sonograms. Ultrasonography has also been useful in the diagnosis of horseshoe kidney in utero. Degree of ConfidenceThe degree of confidence depends on the visualization of the isthmus and the proof of its continuity with the lower poles. In many patients, especially those with a large body habitus, overlying bowel gas makes the acquisition of adequate scans difficult for technical reasons. In cases in which the continuity of the poles with the isthmus cannot be clearly demonstrated, the degree of confidence is low. False Positives/NegativesOccasionally, a midline soft-tissue mass over the spine (eg, a lymphomatous mass) may be mistaken for an isthmus, especially if it extends laterally to the kidneys. If a horseshoe kidney has a thin, fibrous isthmus, ultrasonography may produce a false-negative result. NUCLEAR MEDICINESection 8 of 12
FindingsScintigraphy best demonstrates the fusion if the isthmus consists of functioning parenchymal tissue, because this imaging modality depends not only on the structure of the tissue but also on the function of the tissue. Technetium-99m (99mTc)–labeled dimercaptosuccinic acid (DMSA) can be used to define the fused segments, as well as the altered axis of both kidneys. Many reports of horseshoe kidney exist. This condition is incidentally diagnosed on bone scans, 99mTc-labeled red blood cell studies, or other nuclear medicine studies obtained for reasons other than the evaluation of horseshoe kidney. The use of mercaptoacetyltriglycine (MAG-3) with diuresis is helpful in differentiating nonobstructed parts from obstructed parts of the collecting systems. Degree of ConfidenceHorseshoe kidney can be confidently diagnosed with scintigraphy, which reveals the functioning parenchymal isthmus. False Positives/NegativesOccasionally, the hot isthmus of a horseshoe kidney can be mistaken for spinal metastatic disease. ANGIOGRAPHYSection 9 of 12
FindingsAngiography is not normally performed to diagnose horseshoe kidney, but it is performed to evaluate the vascular anatomy and its variations in a presurgical setting. Angiograms may show 1, 2, or 3 renal arteries in either of the fused kidneys and can reveal large variations in the blood supply of the isthmus, as discussed in Anatomy. In cases of associated renal tumors, angiography is used to evaluate tumor vascularity. Angiography is occasionally performed to check renal artery stenosis in hypertensive patients who have horseshoe kidney. INTERVENTIONSection 10 of 12
Image-guided percutaneous nephrostomy can be performed to relieve hydronephrosis that is associated with horseshoe kidney. Percutaneous stone removal also may be achieved with image guidance. MULTIMEDIASection 11 of 12
REFERENCESSection 12 of 12
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