Horseshoe Kidney 

Updated: Feb 25, 2020
Author: Nicholas N Tadros, MD, MCR; Chief Editor: Bradley Fields Schwartz, DO, FACS 

Overview

Practice Essentials

The horseshoe kidney is the most common type of renal fusion anomaly. It consists of two distinct functioning kidneys on each side of the midline, connected at the lower poles (or rarely at the upper poles) by an isthmus of functioning renal parenchyma or fibrous tissue that crosses the midline of the body.[1]

A urogram protocol computed tomography (CT scanning of the abdomen and pelvis, with and without intravenous contrast and with delayed images) is the best initial radiologic study to determine anatomy and relative renal function (see Workup). Horseshoe kidneys are susceptible to medical renal disease. These diseases, if present, are treated as indicated.

For patient education information, see What Is Horseshoe Kidney?

Relevant Anatomy

Horseshoe kidneys may be found at any location along the path of normal renal ascent from the pelvis to the mid abdomen. The kidneys may be lower than normal because the isthmus is tethered by the inferior mesenteric artery during renal ascent. The isthmus usually lies anterior to the great vessels, at the level of the third to fifth lumbar vertebra. Rarely, it is posterior to these vessels or runs between them.

The vascular supply is variable and originates from the aorta, the iliac arteries, and the inferior mesenteric artery. Bilateral single renal hilar arteries occur in 30% of cases, and various combinations of single and multiple renal hilar and isthmus vessels are seen in 70% of cases. The isthmus of the kidney may not have a separate blood supply or, in 65% of cases, is supplied by a single vessel from the aorta. The blood supply to the isthmus may arise from the common iliac or inferior mesenteric arteries.

The collecting system has a characteristic appearance on intravenous urography or CT delayed phase because of an incomplete inward rotation of the renal pelvis, which cauces it to face anteriorly. The axis of the collecting system is deviated inward at the lower poles because of the lower pole's connection with the isthmus. The ureter may have a high insertion point into the renal pelvis and may cross anteriorly over the isthmus as it descends to the bladder,;; which can cause drainage problems. Rarely does the collecting system cross the isthmus to the contralateral kidney.

Pathophysiology

By itself, the horseshoe kidney does not produce symptoms. However, by virtue of its embryogenesis and abnormal anatomy, it is predisposed to a higher incidence of disease than the normal kidney. The variable blood supply, presence of the isthmus, high insertion point of the ureter, and abnormal course of the ureter all contribute to these problems. Because of these embryogenic and anatomical factors, the rates of hydronephrosis, stone formation, infection, and certain cancers are higher, resulting in a diseased horseshoe kidney (see image below).

Excretory urogram shows a horseshoe kidney with le Excretory urogram shows a horseshoe kidney with left hydronephrosis.

The most common associated finding in horseshoe kidney is ureteropelvic junction (UPJ) obstruction, which occurs in up to 35% of patients and is the cause of many of the problems associated with a horseshoe kidney. Obstruction is classically due to the high insertion of the ureter into the renal pelvis. The crossing of the ureter over the isthmus may also contribute to obstruction. Nonobstructive dilatation can be distinguished from obstructive dilatation using diuresis radioisotope renal scans.

The prevalence of stones in the horseshoe kidney ranges from 20-60%[2]  with a recent meta-analysis putting the prevalence toward the middle of that range, at 36%.[3] Stone disease is thought to be due to the associated hydronephrosis or UPJ obstruction that causes urinary stasis, which hinders stone passage. Metabolic factors, as in the normal population, have also been suggested as contributing to stone formation in these patients. The orientation of the calyces also impairs drainage, resulting in stasis. These kidneys appear dilated or abnormal on most imaging studies, although the radionuclide scans are generally accepted as being diagnostic.

Urinary stasis and stone disease also predispose the horseshoe kidney to infection, which occurs in 27-41% of patients.[4] Ascending infection from vesicoureteral reflux is another cause of infection in the horseshoe kidney.

Certain cancers are more common in the horseshoe kidney.[5] This is thought to be due to teratogenic factors present at birth and the susceptibility of the diseased horseshoe kidney to certain cancers. Renal cell carcinoma is the most common renal cancer in a horseshoe kidney, accounting for 45% of all tumors.[6, 7] The incidence of renal cell cancer in the horseshoe kidney is no different from that of the normal kidney.

Transitional cell cancer and sarcoma account for 20% and 7% of tumors, respectively. The relative risk of transitional cell carcinoma in the horseshoe kidney is 3- to 4-fold higher than in a normal kidney. This is thought to be due to chronic obstruction, stones, and/or infection in the affected kidneys.

The incidence of both Wilms and carcinoid tumors is also higher in the horseshoe kidney. Examination of these tumors may provide an insight into the development and embryogenesis of the horseshoe kidney and the predilection of these two tumors to form in the horseshoe kidney.

Wilms tumor accounts for 28% of malignant lesions. The relative risk of Wilms tumor is increased 2-fold. Interestingly, half of these arise from the isthmus.

Renal carcinoids are rare, with only 32 reported cases. Of the 32 cases, 5 of these renal carcinoids arose in a horseshoe kidney.[5, 8] The relative risk of a carcinoid tumor in a patient with a horseshoe kidney is 62 times that found in the normal population. Of the 5 reported carcinoid tumors reported in patients with horseshoe kidneys, 3 have originated in or have involved the isthmus. The location of these tumors in the isthmus may be explained by the embryogenesis involving abnormal migration of posterior nephrogenic cells, leading to the formation of the isthmus. This is a teratogenic event, which may explain this increased incidence of tumor within the isthmus. This theory may also explain the greater incidence of Wilms tumor in the isthmus, as well. When compared with carcinoid tumor arising in a normal kidney, those that arise in a horseshoe kidney follow a more benign course.

Etiology

Two theories regarding the embryogenesis of the horseshoe kidney have been proposed. The classic teaching of mechanical fusion holds that the horseshoe kidney is formed during organogenesis, when the inferior poles of these early kidneys touch, fusing in the lower midline. The theory of mechanical fusion is valid for horseshoe kidneys with a fibrous isthmus.[9]

Alternatively, more recent studies postulate that the abnormal fusion of tissue associated with the parenchymatous isthmus of some horseshoe kidneys is the result of a teratogenic event involving the abnormal migration of posterior nephrogenic cells, which then coalesce to form the isthmus.[10, 9] This teratogenic event may also be responsible for the increased incidence of related congenital anomalies and of certain neoplasias, such as Wilms tumor and carcinoid tumor associated with the isthmus of the horseshoe kidney.

Epidemiology

Horseshoe kidneys occur in 1 per 400-800 live births. The true incidence probably lies somewhere between these two extremes. Horseshoe kidney is twice as common in males as in females. No genetic determinant is currently known, although horseshoe kidneys have been reported in identical twins and in siblings within the same family.

Prognosis

The horseshoe kidney does not complicate pregnancy or delivery. Importantly, it should be noted that the presence of a horseshoe kidney alone does not affect survival. However, because the horseshoe kidney does have a higher propensity to become diseased, longevity depends on the disease process that the affected horseshoe kidney may harbor or develop.

 

Presentation

History

Nearly one third of patients with a horseshoe kidney remain asymptomatic, and the horseshoe kidney is an incidental finding during radiological examination. Physical examination may reveal a midline lower-abdominal mass.

Symptoms, when present, are usually due to obstruction, stones, or infection. In children, urinary tract infection is the most common presenting symptom. Clinical signs of disease are similar to those that would be found in a normal kidney. However, symptoms may be vague. Instead of flank pain, abdominal pain, and gastrointestinal symptoms such as nausea, abdominal distension and fullness may predominate. The Rovsing sign, consisting of abdominal pain, nausea, and vomiting with hyperextension of the spine, is rare.

The horseshoe kidney may be at higher risk from blunt abdominal trauma because it is unprotected by the rib cage and may be compressed or fractured across the lumbar vertebral column by an abdominal blow. This can occur during a car crash when the victim is restrained by a seat belt— the kidney is compressed between the belt and the spine.

Associated genitourinary anomalies in horseshoe kidney are common and occur in as many as two thirds of patients. Vesicoureteral reflux is present in approximately half the patients. Ureteral duplication is present in 10%. Hypospadias or undescended testis is present in 4% of males. Bicornuate uterus or septate vagina is present in 7% of females.

Other anomalies may be present in other organ systems as well. In autopsy series, these abnormalities are more prevalent in children because some of the congenital anomalies associated with horseshoe kidney are incompatible with long-term survival. Therefore, the incidence of other anomalies is greater in those who die at birth or early infancy than in those who reach adulthood. These coexisting abnormalities in the cardiovascular, gastrointestinal, and skeletal systems occur in up to 85% of patients. These include ventriculoseptal defects, hemivertebrae with scoliosis, myelomeningocele, and colobomata of the iris. Horseshoe kidney is associated with known genetic syndromes, including Turner syndrome, oral-cranial-digital syndrome, and trisomy 18 (Edwards syndrome).

Autosomal-dominant polycystic kidney disease has also been found in the horseshoe kidney, with 20 such cases reported in the literature.

 

DDx

Diagnostic Considerations

On ultrasound care must be taken to not mistaken a horseshoe kidney for a midline retroperitoneal mass.  On CT or MRI, the diagnosis is usually clear.

Differential Diagnoses

  • Crossed fused renal ectopia

  • Fused pelvic kidney

  • Pancake kidney

 

Workup

Laboratory Studies

Once a horseshoe kidney is diagnosed or suspected, further laboratory and imaging evaluation should be performed to assess the status of the kidneys and to look for treatable causes of renal pathology, as follows:

  • Urinalysis with urine culture should be performed. Abnormalities of the urine sediment should be evaluated as clinically indicated. Infection should be treated.

  • Serum chemistry with creatinine is recommended to determine baseline renal function.

Imaging Studies

Urogram protocol computed tomography (CT scanning of the abdomen and pelvis, with and without intravenous contrast) is the best initial radiologic study to determine anatomy and relative renal function. See CT scan below. Abdominal and pelvic CT scanning or renal ultrasonography is helpful to screen for the presence of stones, masses, or hydronephrosis.

This CT scan demonstrates the isthmus of a horsesh This CT scan demonstrates the isthmus of a horseshoe kidney. Note the uptake of contrast in the isthmus.

 

Other Tests

Further studies are performed as indicated and tailored to the clinical situation. These include the following:

  • Dedicated CT scanning of the kidneys with and without intravenous contrast
  • Diuresis renal scanning to assess renal function and drainage
  • Voiding cystourethrography to rule out concomitant vesicoureteral reflux

The frequency of vesicoureteral reflux in association with horseshoe kidney warrants voiding cystourethrography in all pediatric patients.

 

Treatment

Medical Therapy

Horseshoe kidneys are susceptible to medical renal disease. These diseases, if present, are treated as indicated. A metabolic evaluation should be performed because metabolic causes for kidney stone disease are no less common in the patient with horseshoe kidney than in the general population with kidney stone disease.

Any identified metabolic abnormality should be treated. Metabolic evaluation includes a 24-hour stone risk assessment and serum studies, including calcium, uric acid, and phosphorous.

Surgical Therapy

Surgical treatment is based on the disease process and standard surgical indications. The anomalous vascular supply to the kidney should be kept at the forefront of the surgeon's mind when planning the surgical approach. Because of the anomalous and variable vascular supply to the horseshoe kidney, authorities recommend preoperative arteriography to delineate the vascular anatomy in addition to the routine diagnostic procedures. Accessory and aberrant arteries to the parenchyma and the tumor are the rule. Generally, a midline abdominal incision provides access to both sides of the horseshoe kidney and vessels, robotic-assisted laparoscopy is becoming increasingly utilized as well.

Ureteropelvic junction obstruction

Ureteropelvic junction (UPJ) obstruction is usually treated with open pyeloureteroplasty or ureterocalicostomy. With the advent of and surgeon familiarization with laparoscopic techniques, the preferred approach has become laparoscopic dismembered pyeloplasty. Recently, robotic dismembered pyeloplasty using the da Vinci surgical system has become more common.[11, 12, 13] Endopyelotomy is an option but yields success rates inferior to those of open or laparoscopic techniques, as it is more difficult to perform in a horseshoe kidney.[14]

In bilateral UPJ obstruction, the midline transperitoneal incision provides access to both sides of the horseshoe kidney and the vessels. Symphysiotomy (division of the isthmus) is controversial; it was recommended routinely after pyeloplasty to improve drainage but it is associated with an increased risk of hemorrhage, fistula, and renal infarction. In addition, after division of the isthmus, the kidneys, because of their abnormal vasculature, return to their original position. Therefore, symphysiotomy is rarely, if ever, indicated in conjunction with pyeloplasty.

Kidney stones

Kidney stones can be treated with extracorporeal shockwave lithotripsy (ESWL),[15]  endoscopy, or open surgery. If pyeloplasty is to be performed, stones can be removed concomitantly with a flexible nephroscope and stone basket.[16]

Treatment decisions and indications are similar to those for a normal kidney. The presence of obstruction or hydronephrosis precludes treatment of kidney stones using ESWL.[17]  Concomitant percutaneous nephrostolithotomy with endopyelotomy has been used successfully in the treatment of stones with obstruction. Alternatively, successful retrograde therapy of the UPJ with an Acucise cutting-balloon dilator followed by ESWL has also been performed.

A retrospective review of patients with horseshoe kidney found that symptomatic urolithiasis can be managed safely by a variety of endoscopic approaches, including percutaneous nephrolithotomy (PNL), ureteroscopy, and ESWL, but need for secondary procedures and recurrence were common.[18] In a retrospective analysis of the outcomes of PNL for stone removal, the success rate after a single PNL session was 66.7%, increasing to 90.7% with additional treatments.[19]

Kidney tumors

Interruption of the blood supply of the tissue to be resected, guided by the angiographic findings, is the first step. This prevents significant hemorrhage during dissection and tissue removal. In cancer surgery, the isthmus usually needs to be divided to gain access to the tumor and surrounding lymph nodes.

Abdominal aneurysmectomy

Problems may arise when operating on the great vessels (eg, with abdominal aortic aneurysm repair). Horseshoe kidney complicates aortic aneurysm surgery in 1 out of 200 cases.[20] The most important aspect of abdominal aneurysm repair in association with horseshoe kidney is appropriate surgical management of the common renal artery anomalies.[21]  Survival in these patients is predicated on preservation of renal function. Renal artery continuity can be established via branch grafts or reimplantation into the aortic graft. The approach can be midline-abdominal or retroperitoneal through a low-left thoracoabdominal incision. Successful endoluminal transfemoral repair via stent placement has also been described.

Kidney transplantation

Horseshoe kidneys can be used for transplantation.[22]  They can be transplanted into a single recipient en bloc or can be divided and transplanted into two individuals. Dividing the isthmus can increase the risk of urinary fistula.

The donor’s medical history must be obtained to preclude complications such as hydronephrosis, renal calculi, and urinary tract infection. The decision to transplant a horseshoe kidney en bloc depends on renal isthmus morphology and vascular anatomy, as well as the medical and functional status of the kidney.

Long-Term Monitoring

Despite the increased incidence of Wilms tumor in children with horseshoe kidney and carcinoid tumor in adults with horseshoe kidney, no recommendations regarding periodic surveillance to detect occult malignancies have been made.

Performing periodic renal ultrasonography in children with horseshoe kidney seems prudent for early detection of Wilms tumor. Periodic renal ultrasonography in the adult may also be prudent, especially in patients with hydronephrosis. Routine ultrasound can also be used to monitor for kidney stone formation or growth.  All patients with horseshoe kidneys and stones should undergo 24-hour urine tests for kidney stone prophylaxis. Otherwise, surveillance should be performed as indicated based on the clinical situation.