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Horseshoe Kidney
Introduction
Horseshoe kidney is the most common congenital renal fusion anomaly, with an incidence of approximately 1 in 500 live births.[1] As first described in autopsy studies by da Carpi in 1522, a horseshoe kidney is characterized by the fusion of the lower poles of the kidneys across the midline. This anomaly is typically associated with vascular, rotational, and positional anatomical abnormalities, typically resulting in a U-shaped unitary renal unit configuration in the midabdomen, positioned lower in position than normal kidneys.[1][2] The isthmus, which connects the 2 renal masses, typically (80%) consists of functioning renal parenchyma with its own blood supply, but it may also be composed of fibrous tissue.[3][4]
The connecting isthmus is located between the lower poles in over 90% of cases, but it may occur elsewhere in a small minority.[3][5][6][7][8] The kidneys are malrotated, with their lower poles fused and their collecting systems facing anteriorly.[2] As they descend to the bladder, the ureters pass anterior to the isthmus, typically located immediately below the inferior mesenteric artery. Abnormal courses of the ureters predispose them to ureteral obstruction and stasis.[2]
Horseshoe kidneys demonstrate a significant variation in the origin and number of renal arteries and veins.[5][6][9] Vascularization is mainly dependent on where the renal ascent is terminated in development. In a study involving 90 horseshoe kidneys, 387 separate arteries were identified.[6] Nevertheless, the regular intrarenal segmental, non-collateral arterial vascular pattern remains, and ligation or division of any renal arteries results in ischemic segmental renal ischemia or necrosis.[10] The incidence of renal vein anomalies in horseshoe kidneys is also high at 23%.[6]
The natural history is generally benign; however, affected individuals are at increased risk for complications such as ureteropelvic junction obstruction, hydronephrosis, nephrolithiasis, urinary tract infections, vesicoureteral reflux, and renal malignancies, such as renal cell carcinoma, transitional cell cancer, and Wilms tumor in children.[3] The overall risk of a renal malignancy in a horseshoe kidney is about 3 to 4 times higher than in the general population.[11][12] There is also a higher risk of traumatic injury to the kidney due to its lower position, anterior location, and lack of protection from the rib cage, although such injuries are rarely reported.[1][13][14][15][16][17][18]
Chronic kidney disease and progression to end-stage renal disease are more common in this population, likely due to the high prevalence of associated genitourinary anomalies and complications.[3][14] The abnormal vasculature and collecting system anatomy can complicate the presentation and management of disease, particularly surgical procedures.[1][13] Extrarenal anomalies, including gastrointestinal and vertebral malformations, are also more frequent in affected individuals, especially in pediatric populations.[19]
Etiology
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Etiology
The etiology of horseshoe kidneys is rooted in abnormal embryologic development, specifically the fusion of the metanephric blastemas at their lower poles before the completion of renal capsule formation.[1][20] This fusion typically occurs between the 4th and 6th weeks of gestation.[1][3][20] As the fused kidneys attempt to ascend from the pelvis to their normal lumbar position, their upward migration is arrested by the inferior mesenteric artery, resulting in the characteristic low anterior position and midline fusion (isthmus) observed in horseshoe kidneys.[3]
The kidneys are typically located in the retroperitoneum between the transverse processes of T12 and L3, with the left kidney slightly superior to the right.[21] The upper poles are positioned slightly medially and posteriorly relative to the lower poles.[21] Horseshoe kidneys are different in three main ways—location, orientation, and vasculature.[2][21]
During development, the kidneys align with their hilums anterior and parallel.[22] As development proceeds, the kidneys normally ascend and separate while rotating 90° medially, with the renal hilums now facing each other across the midline.[22][23][24] When this process is interrupted by the presence of an isthmus between the lower poles of the kidneys, further ascent, migration, and rotation cannot occur.
The ascent of a horseshoe kidney is often described as being held back by the origin of the inferior mesenteric artery at L3; however, horseshoe kidneys can also be found lower in the abdomen and pelvis. The upper poles and calyces now diverge and are more lateral than the lower poles. During 6 to 8 weeks of development, the renal ascent is coupled with a 90° medial rotation.[23][24] Due to the persistent isthmus, however, horseshoe kidneys experience malrotation. Consequently, the ureters should either pass over the isthmus or down the anterior surface of the kidneys, which can cause urinary drainage problems and stasis.[9][25]
Horseshoe kidneys also show a great variation in the origin and number of renal arteries and veins.[5][6][9] These variations are largely dependent on where the renal ascent terminates during development. In a study involving 90 horseshoe kidneys, 387 arteries were identified.[6] Despite this, the normal intrarenal vascular segmental pattern remains, and the ligation or division of any of these arteries results in ischemic segmental renal ischemia or necrosis due to their poor collateral arterial supply.[10] The incidence of renal vein anomalies in horseshoe kidneys is also high at 23%.[6]
A teratogenic component is believed to cause abnormal migration of posterior nephrogenic cells to the inferior medial aspect of the lower poles, which then fuse to form the isthmus across the midline where the 2 kidneys are closest.[1][26][27] Environmental factors contributing to the development of a horseshoe kidney include alterations in the intrauterine environment. Teratogenic drugs such as thalidomide, alcohol consumption, and poor glycemic control cause an increase in incidence. Structural factors include flexion or rotation of the caudal spine and narrowed arterial forks during migration.[1][6][28][29][30][31]
Genetic factors also contribute to the development of horseshoe kidneys, as it is classified among the congenital anomalies of the kidney and urinary tract. Mutations in genes, such as PAX2, HNF1B, Shh, and WT1, as well as disruptions in cell signaling and migration, have been implicated.[1][32][33]
Epidemiology
The incidence of horseshoe kidneys is approximately 1 in 500 or 0.25% in the general population, with a male preponderance of 2:1.[6][13] Horseshoe kidneys are associated with chromosomal disorders, such as Edward syndrome (trisomy 18) at about 67%, Turner syndrome at 20%, and Down syndrome at 1%.[1][34][35][36][37][38][39]
However, chromosomal microarray analyses found that the frequency of abnormal results in fetuses with horseshoe kidneys was no different than that observed in the general population.[40] Horseshoe kidney variants are higher in children as a concomitant presentation in newborns with multiple congenital variants, some of which are not compatible with long-term survival.[1][6][34][35][36]
History and Physical
Horseshoe kidneys are often asymptomatic and are typically identified incidentally. However, when symptoms occur, they are generally nonspecific, with a study showing that the most common presenting complaints in children with a horseshoe kidney include abdominal pain and symptoms of a urinary tract infection (UTI).[16]
Flank pain, hematuria, nephrolithiasis, hydronephrosis, or abdominal discomfort may prompt evaluation in adults. There is also an increased risk of associated congenital anomalies, including genitourinary and extrarenal malformations, particularly in pediatric populations.[14] Horseshoe kidneys are more prone to injury from blunt abdominal trauma due to their anterior, unprotected position, with possible traumatic compression over the lumbar vertebrae, possibly aggravated during a motor vehicle accident due to pressure from the seat belt, dash, and steering wheel.[41][42]
Physical examination may reveal a palpable mass in the lower abdomen in children or thinner patients. Costovertebral angle tenderness may also be present. Horseshoe kidneys are most often identified on imaging performed for unrelated reasons. As many as 85% of patients with horseshoe kidneys have associated anomalies involving the cardiovascular, gastrointestinal, or skeletal systems, and up to two-thirds have associated genitourinary abnormalities, including the following:
- Approximately 4% of male patients have either hypospadias or a cryptorchid testis.[43]
- Approximately 7% of female patients have a bicornate uterus or a septated vagina.[44]
- Kidney cancers are 3 to 4 times more common overall.[11][12]
- Nephrolithiasis is present in about 36% of patients, due to low urinary volume, stasis, hypercalciuria, and hypocitraturia.[45][46][47]
- Ureteral duplications are found in 10% of cases.[43][48]
- Ureteropelvic junction obstructions may be found in up to 33% of patients.[43][49][50]
- Vesicoureteral reflux is present in about 50% of patients with horseshoe kidneys.[18][51]
Evaluation
Horseshoe kidneys can be identified using most abdominal imaging modalities. The diagnosis of a horseshoe kidney is most commonly made using either ultrasound or computed tomography (CT) scans.[52] Magnetic resonance imaging and CT are the most effective modalities for clearly demonstrating the anatomy, detecting accessory vasculature, and visualizing surrounding structures, which may be affected.[6][52][53]
Optimally, a CT urogram—abdomen and pelvis CT imaging performed without and then with intravenous (IV) contrast—should be performed, allowing for the identification of urolithiasis, urinary blockages, and ureteropelvic junction obstructions. Magnetic resonance imaging examinations can be used where ionizing radiation should be avoided or in patients who cannot tolerate standard IV contrast media.
Horseshoe kidneys may occasionally be identified on plain radiography through visualization of the perinephric fat in association with an altered renal axis. The lower poles are more medial and closer together than normal, with the kidneys sitting lower in the abdomen and the upper poles wider apart.[52]
Nuclear medicine radionuclide renal scans can help differentiate actual blockages from passively dilated systems and assist in the diagnosis of ureteropelvic junction obstructions.[54][55] Voiding cystourethrograms can be used to identify vesicoureteral reflux, which is more common in patients with horseshoe kidneys than in the general population.[51] In rare cases of horseshoe kidney and associated polycystic kidney disease, genetic testing (PKD1 (16p13.3) and PKD2 (4q21)) becomes a key component of the evaluation.[56]
Treatment / Management
Most patients with horseshoe kidneys are asymptomatic and do not require intervention. However, regular monitoring is recommended, as these individuals are at an increased risk of complications such as UTIs, nephrolithiasis, hydronephrosis, and chronic kidney disease. Management should be individualized based on the presence and type of complications.[1][14][57][58] Medical management includes regular surveillance of blood pressure, urinalysis for proteinuria, and assessment of renal function, given the increased risk of chronic kidney disease and end-stage renal disease in this population.[58][59]
Nephrolithiasis
Nephrolithiasis is managed according to stone size and location. Shock wave lithotripsy or ureteroscopy is preferred for small stones, whereas percutaneous nephrolithotomy is indicated for larger calculi or those refractory to shock wave lithotripsy and ureteroscopy; open surgery is reserved for cases with significant anatomic abnormalities and cases refractory to minimally invasive treatment.[60][61][62][63][64][65](B3)
Shockwave lithotripsy is less effective in horseshoe kidneys due to difficulty localizing the energy for pelvic stones and poor stone fragment clearance due to impaired renal drainage.[66] Larger renal stones, those greater than 2.5 cm, or those that do not allow ureteroscopic approaches, can be removed through minimally invasive percutaneous or mini-percutaneous surgery.[67][68](B3)
Guidelines recommend that patients with horseshoe kidneys who develop stones undergo a 24-hour urine analysis for stone prevention and receive appropriate prophylactic treatment.[62][69][70][71][72][73][74] Successful prevention requires a high level of patient compliance on a long-term basis; however, all patients with horseshoe kidneys who develop stones should be offered nephrolithiasis preventive testing, as surgical stone treatment is often far more complex than in anatomically normal patients with urolithiasis.[58][59](B3)
Urinary Tract Infections
UTIs should be managed with standard culture-specific antibiotic therapy, and evaluation for underlying structural abnormalities is warranted, especially in cases of recurrent infections.[1][14][58][59]
Surgical and Interventional Management
Surgical and interventional management is reserved for symptomatic or complicated cases. Pre-procedural imaging, such as CT scans, is essential during the workup for any procedure due to the horseshoe kidney's highly variable vascular anatomy, including multiple renal arteries frequently arising from the aorta or iliac vessels, and common aberrant veins.[75] This complex vasculature increases the risk of intraoperative complications and should be carefully delineated before surgical intervention.
Additionally, pre-procedural imaging is essential to visualize adjacent colon segments to reduce the risk of incidental bowel injury.[75] Computerized 3-dimensional image modeling can help dramatically improve surgical decision-making, facilitate preoperative treatment planning, assist in tailoring the surgical approach, and help develop a surgical strategy in complex anatomical cases, such as horseshoe kidneys, that require surgical intervention.[76][77][78][79][80](B3)
Indications for Surgical Intervention
Indications for surgical intervention include symptomatic ureteropelvic junction obstruction, recurrent or complicated UTIs, ureterolithiasis or nephrolithiasis not amenable to conservative management, significant symptomatic vesicoureteral reflux with cortical thinning, or a nonfunctioning renal moiety.[51][58][81] Treatment options for ureteropelvic junction obstruction include percutaneous endopyelotomy, laparoscopic or robotic pyeloplasty, or open pyeloplasty, based on anatomical considerations and surgical expertise.[59]
Significant or symptomatic hydronephrosis may require surgical correction. Total or partial nephrectomy is considered for nonfunctioning, malignant, or severely diseased renal moieties.[82] Historically, surgical division of the isthmus was frequently performed, but this is no longer routinely recommended due to the increased risk of bleeding, fistula formation, infection, and leakages.[83][84] The kidneys ultimately return to their original location regardless.(B3)
The use of perpendicular, alternating, biplanar fluoroscopy has been described to assist in the percutaneous approach to stones in patients with a horseshoe kidney. The technique allows for better visualization of stone position and the complex renal anatomy during percutaneous surgery without requiring patient repositioning or standard fluoroscopy, reducing patient radiation exposure and shortening overall operating time.[83]
Managing renal cancers in patients with a horseshoe kidney is essentially the same as in normal individuals, except surgical procedures are far more complex due to the varied vascular anatomy, unusual renal location, and other factors. Traditionally, the surgical approach has been open surgery. Laparoscopic and robotic techniques can also be used, although these procedures are more complex than in anatomically normal kidneys.[84][85] Regardless of their number, all renal arteries in horseshoe kidneys are end-arteries, and damage to them results in renal ischemia.(B3)
Intraperitoneal laparoscopic and robotic techniques for heminephrectomy, pyelolithotomy, and pyeloplasty in patients with a horseshoe kidney have been successfully performed, demonstrating good outcomes and minimal complications in experienced centers.[86][87][88][89][90][91][92][93][94][95][96][97][98](B3)
Differential Diagnosis
The horseshoe kidney is 1 form of renal fusion abnormality. The other 2 main types are crossed-fusion renal ectopia and a fused pelvic kidney.
In crossed renal ectopia, both kidneys are positioned on the same side of the body, with 1 ureter crossing the midline to drain into the bladder. In contrast, in a fused pelvic kidney, there is a single renal mass that is drained by 2 ureters that do not cross the midline.[94] Failure of renal ascent results in a kidney located in the pelvis, which may be mistaken for a horseshoe kidney, especially if both kidneys are low-lying. However, there is no fusion between the kidneys, the renal axes are not divergent, and the vascular supply is often anomalous.[99][100]
Autosomal dominant polycystic kidney disease presents as bilateral renal enlargement with multiple cysts that can mimic the mass effect and abnormal renal contour of a horseshoe kidney, but cystic changes and family history help distinguish the 2 entities.[56][101][102][101]
In children, a renal mass may represent Wilms tumor rather than a congenital anomaly.[103] The National Comprehensive Cancer Network recommends thorough imaging and laboratory evaluation to differentiate Wilms tumor from benign renal conditions and other masses.[104]
Prognosis
Approximately one-third of all patients with horseshoe kidneys remain completely asymptomatic, and the renal anomalies are often found incidentally during imaging. An isolated finding of an incidentally discovered, asymptomatic horseshoe kidney is generally considered benign and requires no treatment.[51] Symptomatic patients may require specific therapy based on their disorder, circumstances, and findings.
Patients with horseshoe kidneys have a higher incidence of ureteropelvic junction obstructions—the most common complication—followed by hydronephrosis, nephrolithiasis, UTIs, and vesicoureteral reflux than the general population.[3]
Complications
The intrinsic anatomical and genetic defects associated with horseshoe kidneys predispose individuals with this anomaly to several urological sequelae due to the associated ureteric obstruction, impaired urinary drainage, and other factors.[52][105]
Approximately two-thirds of patients develop associated complications, including hydronephrosis, nephrolithiasis, ureteropelvic junction obstructions, UTIs, and vesicoureteral reflux, which is higher than in the general population.[2][9][25][55][51][61][62] Ureteropelvic junction obstruction is the most common complication, affecting up to one-third of patients.[49][55]
Horseshoe kidneys also have an increased frequency of common kidney cancers, including transitional and renal cell malignancies (each is 3-4 times more common), Wilms tumor (twice as frequently), and a huge increase in the incidence of some sporadic cancerous neoplasms, such as carcinoid (62-82 times).[6][45][58][106][107][108][109][110] Renal cell cancer is the most common kidney malignancy found in patients with horseshoe kidneys and accounts for 50% of all renal neoplasms in these patients.[110][111][112]
Primary renal lymphoma in association with a horseshoe kidney has also been reported and should be included among the possible neoplasms that patients may develop.[113] A renal biopsy should be performed when imaging provides atypical findings, a renal mass, or when the outcome can impact treatment and clinical decision-making.[113]
The risk and difficulty of renal biopsies in patients with horseshoe kidneys are increased due to structural abnormalities and anatomical variances. Still, renal biopsies can be accomplished through precise positioning with abdominal ultrasonographic or CT image guidance.[114]
Over 50% of individuals with horseshoe kidneys who become symptomatic have either ureteropelvic junction obstruction or vesicoureteral reflux.[59]
Meta-analysis suggests that 36% of patients with a horseshoe kidney develop nephrolithiasis at some stage.[45]
Due to their ectopic position, horseshoe kidneys are also particularly susceptible to blunt abdominal trauma, as they can be fractured or compressed against the lumbar vertebrae.[1][13][14][15][16][17][18]
Deterrence and Patient Education
Patient education regarding horseshoe kidney focuses on the monitoring and prevention of complications through lifestyle modifications and ongoing patient counseling. Patients should be educated about their increased risk for UTIs, nephrolithiasis, ureteropelvic junction obstruction, and, in rare cases, chronic kidney disease and malignancy.[1][14][19][57]
Key prevention strategies include maintaining adequate hydration, performing 24-hour urine testing to reduce stone formation risk, promptly recognizing and treating UTIs, and conducting regular follow-up with urinalysis and renal function monitoring to detect early signs of proteinuria, hypertension, or renal scarring.[1][14]
Patients should be counseled to avoid activities that increase the risk of abdominal trauma, given the abnormal position and aberrant vascular supply of the horseshoe kidney.[57] Lifestyle modifications such as a balanced diet low in salt and animal protein may help reduce stone risk, and blood pressure should be monitored regularly. Education should also address the importance of adherence to follow-up imaging and laboratory assessments, as early detection of complications is critical for preserving renal function.[1][14][19][57]
Children and adults should be evaluated for associated genitourinary and extrarenal anomalies, and families should be informed about the potential for syndromic associations, especially in pediatric cases.[1][19]
Pearls and Other Issues
Symphysiotomy, or division of the fused isthmus, was previously recommended during pyeloplasty in patients with a horseshoe kidney. However, this practice has changed due to the increased risk of infection, fistulas, leakages, and bleeding.[115] Notably, the kidneys return to their original location after surgery, so symphysiotomy is no longer routinely recommended.
Renal transplantation can be successfully performed using donors with horseshoe kidneys.[116][117][118][119][120][121] There is no consensus on separating the isthmus, but several techniques have been described and used successfully.[117]
Enhancing Healthcare Team Outcomes
Patients with horseshoe kidneys are at an increased risk of ureteropelvic junction obstructions, nephrolithiasis, vesicoureteral reflux, UTIs, transitional cell cancers, and malignant renal tumors. Most cases are discovered serendipitously during imaging for unrelated problems, often with ultrasound. A CT urogram—abdomen and pelvis CT imaging performed without and then with IV contrast—is the basis for a definitive diagnosis and initial evaluation of the horseshoe kidney anatomy, as well as identification of any obstructions or calculi.
Judicious use of diuretic renal scans and voiding cystourethrograms helps identify ureteropelvic junction obstructions and vesicoureteral reflux that may not be clear from the CT scans alone. 24-hour urine testing and prophylactic therapy for patients with horseshoe kidneys who develop urinary calculi is recommended. Percutaneous, ureteroscopic, laparoscopic, and robotic surgery can now be safely performed on horseshoe kidneys, but may be technically challenging and require additional planning.
Horseshoe kidneys are the most common congenital renal fusion defect, but they affect only about 0.25% of the population. Many professionals are unaware of the condition, its evaluation, recommended monitoring, or treatment. Due to its rarity, optimal management of horseshoe kidneys requires a coordinated, multidisciplinary approach to improve team performance, safety, outcomes, and patient care.
Clinicians must provide an early, accurate diagnosis, risk stratification, and individualized surveillance plans, recognizing the increased risk of complications, such as obstruction, nephrolithiasis, infection, and chronic kidney disease. Advanced practitioners and nurses play a critical role in patient education, early identification of complications, and reinforcement of adherence to follow-up and lifestyle modifications, such as hydration and infection prevention.[1][14][57]
Pharmacists are essential for medication reconciliation, optimizing antibiotic stewardship in UTIs, and counseling on nephrotoxic drug avoidance. Given the unique vascular and anatomic considerations in horseshoe kidneys, radiologists and urologists must collaborate closely to interpret complex imaging and plan interventions.[1][122] Surgeons must be skilled in minimally invasive and open techniques, with preoperative planning that accounts for aberrant vasculature and renal anatomy to minimize intraoperative risk.[1][81]
Effective interprofessional communication is necessary for timely information sharing, especially regarding imaging findings, laboratory results, and perioperative planning. Regular multidisciplinary case reviews and care conferences can facilitate consensus on management strategies for complex cases.[122] Ethical considerations include respecting patient autonomy, providing clear information about risks and benefits of surveillance versus intervention, and ensuring equitable access to specialized care.
Care coordination is vital, particularly for pediatric patients or those with associated anomalies, to ensure seamless transitions between specialties and continuity of care.[57] The healthcare team must maintain up-to-date knowledge of evolving management strategies and foster a culture of safety, vigilance, and shared decision-making to optimize outcomes for patients with horseshoe kidneys.
Media
(Click Image to Enlarge)
Horseshoe Kidney on Computed Tomography. This axial computed tomography image shows a horseshoe kidney, a congenital anomaly where the lower poles of both kidneys are fused, forming a U-shaped structure. The red outline highlights the fused renal tissue crossing the midline anterior to the vertebral column.
Contributed by S Bhimji, MD
(Click Image to Enlarge)
Horseshoe Kidney. Shutterstock
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