In nearly one third of cases having horseshoe kidneys the abnormality remains asymptomatic and is discovered during radiological screening carried out for other medical conditions. During physical examinations the condition is often reported as the presence of a mass in the lower abdominal midline. The symptoms appear as a consequence of secondary causes such as obstruction, calculi or infection. In children having horseshoe kidneys, infection is the most common condition that causes symptoms. In most cases, the presenting clinical signs and symptoms are similar to those found in diseases affecting normal kidneys. The symptoms may be vague in some cases. Instead of abdominal pain, feeling of fullness, abdominal distention, nausea and vomiting may predominate. In case of accidental injury such as a car crash, the victims may restrained by the seat belt and kidneys might get compressed between the belt and spine.

Associated abnormalities found in victims having horseshoe kidneys can cause varied symptoms. Approximately half of patients may present with vesicoureteral reflux involving backward flow of urine from bladder to kidneys. On physical examination, patients might show undescended testes, hypospadias, abnormalities of uterus and septate vagina. Patients having associated abnormalities of serious nature, such as those involving cardiovascular, gastrointestinal and skeletal systems can lead to death at birth or early childhood. Such abnormalities are more prevalent in children with horseshoe kidneys found in association with congenital diseases.

• Soft Tissue Mass

  Transverse sonogram of the abdomen demonstrates a soft-tissue isthmus anterior to the spine. Ultrasonogram of a pediatric patient displays a hypoechoic soft-tissue mass anterior to the spine. [emedicine.medscape.com]

  Fluoroscopy On intravenous urography (IVU), the control/scout film will show a soft tissue mass on either side of the midline with a central isthmus. [radiopaedia.org]

  Avoid mistaking of renal tissue anterior to aorta as retroperitoneal tissue; such a situation may arise in case of metastatic nodal enlargement or lymphoma. IVU/IVP: A soft tissue mass is detected on either side of the midline joined by isthmus. [symptoma.com]

• Pallister-Hall Syndrome

  […] a horseshoe kidney Edwards syndrome (trisomy 18): up to 20% have a horseshoe kidney Patau syndrome (trisomy 13) non-aneupliodic anomalies Ellis-van Creveld syndrome 2 Fanconi anemia 1 Goltz syndrome Kabuki syndrome Pallister-Hall syndrome VACTERL association [radiopaedia.org]

  […] up to 7% Down syndrome (trisomy 21) Patau syndrome (trisomy 13) Other syndromes which report incidence of horseshoe kidneys are:Fanconi anemia Ellis-van Creveld syndrome Goltz syndrome Kabuki syndrome Pallister-Hall syndrome VACTERL association Occurrence [symptoma.com]

• Intravenous Administration

  Axial computed tomography (CT) scan obtained through the abdomen after the intravenous administration of contrast material. Fused kidneys are revealed, with a parenchymal isthmus at the lower poles. [emedicine.medscape.com]

If horseshoe kidneys are diagnosed during tests carried out for other purposes, or are suspected, scanning and pathological tests should be carried out to assess the normal structure and function of kidneys. Among the routine tests recommended are determination of baseline renal function to find out creatinine levels and urine culture to detect infection. 

The other investigations carried out are ultrasound, intravenous urogram (IVU)/pyelogram (IVP) and CT scanning of abdomen and pelvis:

Ultrasound:

•  Abnormally rotated and inferior location of horseshoe kidneys may result in poor visualization of inferior pole and the kidneys length. Patients may be scanned with left and right lateral decubitus positions [2]. Avoid mistaking of renal tissue anterior to aorta as retroperitoneal tissue; such a situation may arise in case of metastatic nodal enlargement or lymphoma [2]. 

IVU/IVP:

• A soft tissue mass is detected on either side of the midline joined by isthmus. In contrast with the normal kidneys, the orientation of kidneys shows that the lower pole is closest to the midline. During the intravenous contrast, outlining of pelvicalyceal system may reveal associated complications such as obstruction of ureteropelvic junction (UPJ).

CT and MRI:

• Though scans may appear normal, the configuration of kidneys is abnormal. An assessment of collecting system can be made by imaging during the excretory phase.  

• Pyuria

  In another review of 30 caseS (5) twice as many patients were male; 22 cases presented with abdominolumbar pain, 12 with hematuria and 2 with pyuria. [fetalultrasound.com]

• Pelvis Ultrasound Abnormal

  The other investigations carried out are ultrasound, intravenous urogram (IVU)/pyelogram (IVP) and CT scanning of abdomen and pelvis: Ultrasound: Abnormally rotated and inferior location of horseshoe kidneys may result in poor visualization of inferior [symptoma.com]

Presence of horseshoe kidneys as such does not warrant any treatment and the patient can enjoy a normal life. Treatment is required based on secondary complications due to renal calculi, infection, UPJ obstruction, cancer and patient's condition. Surgery is recommended if other means of treatment are inadequate, and it depends upon the associated pathology of the individual case.

• Following metabolic evaluation by serum studies, including calcium, phosphorus and uric acid tests, and the existing condition found is adequately treated.
• Surgical procedures depend upon the case. Available vascular supply to kidneys is the foremost consideration for surgical procedures. In general, midline abdominal incision allows access to both sides of kidneys and their vessels.
• UPJ obstruction is treated by performing open pyeloureteroloplasty or ureterocalicostomy. Recently, success has been achieved by robotic pyeloplasty using the da Vinci surgical system [10] [11]. Another approach, endopyelotomy is however inferior to open or laparoscopic techniques [12].
• Extracorporeal shock wave lithotripsy (ESWL) may be used as a noninvasive technique to remove renal calculi. In cases where pyeloplasty is used, calculi can be concomitantly by using nephroscope [13].
• Horseshoe kidneys complicate surgery of abdominal aortic aneurism, an enlarged area of lower part of aorta, in association with the management of anomalies found in common renal artery.
• Periodic ultrasonography is recommended in children to detect Wilms tumor early. It will also be necessary at regular intervals in adults, especially those with hydronephrosis.
• Horseshoe kidneys can be used for transplant purposes en block to either a single recipient or divided among two recipients. However, dividing isthmus increases the risk for urinary fistula.  

The presence of horseshoe kidneys is not a disease as such and therefore any adverse effect on health depends upon the presence of concurrent predisposing factors discussed under pathophysiology. Affected persons may not need any treatment and have a normal life expectancy. The condition also does not necessarily affect normal pregnancy and delivery. In cases where complications exist due to UPJ obstruction, kidney calculi, infection and cancer, the progress of disease will be highly variable depending upon the severity of abnormality and the patient's condition and the follow-up and treatment. 

The formation of horseshoe kidneys is caused by two possible mechanisms: abnormality during organogenesis and teratogenetic effects in the formation of kidneys. In the first mechanism, during the early development, the inferior poles of kidneys touch each other and fuse in the lower midline. The formation of horseshoe kidneys, as a consequence of mechanical fusion, is held true when a fibrous isthmus joining the two kidneys is present. The more recent studies favor teratogenicity as the basis for the condition.

In the second mechanism, due to some teratogenetic effects, the posterior nephrotic cells in developing kidneys undergo abnormal movement and come in contact with each other, and then coalesce, leading to the formation of isthmus. The theory explains an increased incidence of congenital defects and neoplasms, such as Wilms tumor and carcinoid tomor found in association with the isthmus of horseshoe kidneys. Association of horseshoe kidneys also occurs with genitourinary malformations and chromosomal abnormalities involving their numbers (aneuploidy). 

Several conditions, involving chromosomal abnormalities (aneuploidy), which are associated with the incidence of horseshoe condition are:

• Edward syndrome (trisomy 18); horseshoe kidney, up to 20%
• Turner syndrome (x chrome monosomy); horseshoe kidney incidence up to 7%
• Down syndrome (trisomy 21)
• Patau syndrome (trisomy 13)

Other syndromes which report incidence of horseshoe kidneys are:
Fanconi anemia [1]

• Ellis-van Creveld syndrome [2]
• Goltz syndrome 
• Kabuki syndrome 
• Pallister-Hall syndrome 
• VACTERL association

Occurrence of horseshoe kidney is a rare condition. It is found in association with several types of syndromes involving genetic defects and chromosomal abnormalities and their incidence is 1 to 4 cases in 1000 persons. The incidence is higher in males as compared to females, in the ratio of 2:1 [1][2][3]. Majority of cases involving horseshoe kidneys are sporadic cases but others are found in association with several genetic and chromosomal disorders [3].

Horseshoe kidneys are the result of fusion between metanephric buds, formed during the fourth and eighth week of embryogenesis, and blockade of their cephalic migration and normal rotation [4]. Abnormal migration of posterior nephrogenic cells is considered as the cause for the joining and fusion of two developing kidneys [5]. The degree of fusion can vary from fusion by normal parenchyma just at the lower poles of kidneys to more extensive fusion or connection by a fibrosed tissue band. Fusion of kidneys results into their failure to ascend and rotate in the normal position.

In about 35% of patients having horseshoe kidneys, obstruction of ureteropelvic junction (UPJ) is a common feature. The obstruction is caused due to high insertion point of ureters into the renal pelvis, which funnels urine into the ureters. Ureters crossing over the isthmus also contribute to the obstruction of free flow of urine through ureters. Presence of atypical anatomical features like variable blood supply, abnormal course of ureters, isthmus joining two kidneys, ureteropelvic junction obstruction, and ureterovesical reflux together contributes to problems in proper functioning [6]. As a result, the metabolism of kidneys and their proper functioning is affected leading to formation of calculi, infection, hydronephrosis and certain cancers which individually or together lead to severe disease of kidneys.

Formation of stones or calculi occurs in 20-60% of patients having horseshoe kidneys. A high incidence of renal calculi is believed to be caused by urinary stasis consequent to UPJ obstruction and hydronephrosis. Obstruction also hinders free passage for calculi along with the urine naturally. Presence of calculi predisposes kidneys to infection in about 27-41% of cases. Kidney infection is also caused by vesicoureteral reflux, which causes backward flow of urine from the bladder to kidneys.

Genetic and teratogenicity factors present in patients having horseshoe kidneys increase their susceptibility to high incidence of certain cancers [7]. Out of the various types of renal cancers found in patients having horseshoe kidneys, renal cell carcinoma accounts 45% of cases [8] [9]. Other common cancers present in patients having horseshoe kidneys are Wilms and carcinoid tumors.

Being a congenital abnormality, horseshoe kidneys cannot be prevented from happening. The abnormality is found in association with several genetic syndromes and complications arising as a result of calculi, infection, UPJ obstruction, crash or injury. Prevention and management of complications is required by suitable therapeutic and surgical procedures. A healthy lifestyle is necessary for good prognosis of the condition.

This condition as such is not a disease but an anomaly that occurs by birth and therefore it is required to be managed by suitable interventions depending upon the condition of the patient. The condition is part of a larger group of condition, termed as Congenital Anomalies of the Kidney and Urinary (CAKUT), caused as a result of disruption in normal migration of embryonic kidneys during the process of fetal development.

The occurrence of horseshoe kidneys in a population is quite common. The condition is caused by the fusion of two distinct functioning kidneys, one lying on each side of the midline but connected at the lower pole of both the kidneys by a functioning parenchyma or fibrous tissue forming a narrow strip or isthmus. The condition may exist in association with other urogenital or renal vessel anomalies which may lead to severity of the condition. Some of the severe conditions that may arise due to the horseshoe anomaly are obstruction of uteropelvic junction, vesicoureteral reflux and renal cystic dysplasia. While the kidneys are joined through the isthmus, the ureters enter the bladder on each side separately and drain well in the urinary bladder.

The horseshoe condition, in general, is identified fructuously during radiological examination carried out for other purposes. In most cases the condition exists without any apparent symptoms but in some cases it predisposes to complications in the functioning of kidneys, leading to more complex diseases such as hydronephrosis, pyelonephritis or nephrolithiasis. Complications due to obstruction arise mostly due to secondary conditions and can be treated by suitable interventions like pyeloplasty, which involves surgical reconstruction of renal pelvis to decompress kidneys.

The term horseshoe kidneys are used because the two kidneys are fused at the lower end during their development. In about one third of cases having horseshoe condition, the abnormalities of genitourinary, cardiovascular, central nervous system are present. Some common abnormalities found are:

• Hydronephrosis 
• Presence of calculi leading to obstruction of urinary tract
• Polycystic kidney disease
• Wilms tumor in early childhood 
• Renal cancer
• Congenital syndromes 

The condition as such does not need any treatment except in cases where other anomalies or complications are found. The common symptoms of patients with horseshoe kidneys are abdominal pain, renal calculi and urinary tract infections. Presence of a vesicoureteral reflux, mass in abdomen and blood in urine are present in association with horseshoe kidneys. As there is no cure for the condition, based on the symptoms of the individual condition of the case and symptoms, suitable therapeutic or surgical procedures may be adapted.

1. Tischkowitz MD, Hodgson SV. Fanconi anaemia. J. Med. Genet. 2003;40 (1): 1-10.
2. Nahm AM, Ritz E. Horseshoe kidney. Nephrol. Dial. Transplant. 1999;14 (11): 2740-1. doi:10.1093/ndt/14.11.2740.
3. Kumar P, Burton BK. Congenital Malformations, Evidence-Based Evaluation and Management. McGraw-Hill Professional; 2007. ISBN:0071471898.
4. Kehagias DT, Gouliamos AD, Vlahos LJ. Horseshoe kidney associated with anomalous inferior vena cava. Eur Radiol. 1999;9:935.
5. Lee CT, Hilton S, Russo P. Renal mass within a horseshoe kidney: preoperative evaluation with three-dimensional helical computed tomography. Urology 2001;57:168.
6. Bradshaw A, Donnelly LF, Foreman JW. Thrombocytopenia and absent radii (TAR) syndrome associated with horseshoe kidney. Pediatr Nephrol. 2000;14:29.
7. Litwinowicz R, Szpor J, Janus G, Worek M, Okon K. Primary carcinoid tumour in horseshoe kidney. Pol J Pathol. 2011; 62(1):72-4.
8. Palmer BW, Strom K, Wong C. Hand-assisted laparoscopic nephroureterectomy with cystoscopic en-bloc excision of the distal ureter and bladder cuff and isthmusectomy in a horseshoe kidney for invasive urothelial carcinoma of the renal pelvis. JSLS. 2011;15(3):412-4.
9. Husillos-Alonso A, Bueno-Chomón G, Lledó-García E, Subirá-Ríos D, Ramón-Botella E, Hernández-Fernández C. First percutaneous computed tomography-guided radiofrequency ablation of renal tumor in horseshoe kidney. Urology. 2011; 78(2):466-8.
10. Mufarrij PW, Woods M, Shah OD, et al. Robotic dismembered pyeloplasty: a 6-year, multi-institutional experience. J Urol. 2008; 180(4):1391-6.
11. Etafy M, Pick D, Said S, Hsueh T, Kerbl D, Mucksavage P. Robotic pyeloplasty: the University of California-Irvine experience. J Urol. 2011; 185(6):2196-200.
12. Bellman GC, Yamaguchi R. Special considerations in endopyelotomy in a horseshoe kidney. Urology. 1996; 47(4):582-5; discussion 585-6.
13. Atug F, Castle EP, Burgess SV, et al. Concomitant management of renal calculi and pelvi-ureteric junction obstruction with robotic laparoscopic surgery. BJU Int. 2005; 96(9):1365-8.