- Author: Robert C Allen, Jr, MD, FACS; Chief Editor: Bradley Fields Schwartz, DO, FACS more...
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 by an isthmus of functioning renal parenchyma or fibrous tissue that crosses the midline of the body.
Horseshoe kidney occurs 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 known, although it has been reported in identical twins and in siblings within the same family.
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. 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. 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.
By itself, the horseshoe kidney does not produce symptoms. However, by virtue of its embryogenesis and 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, and abnormal course of the ureters 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).
The most common associated finding in horseshoe kidney is ureteropelvic junction (UPJ) obstruction, which occurs in up to 35% of patients. It causes most problems. Obstruction is 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 must be distinguished from obstructive dilatation using diuresis radioisotope renal scans.
The prevalence of stones in the horseshoe kidney ranges from 20-60%. 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. Ascending infection from vesicoureteral reflux is another cause of infection in the horseshoe kidney.
Certain cancers are more common in the horseshoe kidney. 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 horseshoe kidney, accounting for 45% of tumors.[2, 3] 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 increased 3- to 4-fold. 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. 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. 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. When compared with carcinoid tumor arising in a normal kidney, those that arise in a horseshoe kidney follow a more benign course.
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 predisposed to 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 occur in other organ systems. In autopsy series, these abnormalities are more prevalent in children because 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 trisomies 18 and E.
Autosomal-dominant polycystic kidney disease has also been found in the horseshoe kidney, with 20 such cases reported in the literature.
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 because of an incomplete inward rotation of the renal pelvis, which faces anterior. 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. Rarely does the collecting system cross the isthmus to the contralateral kidney.
In the presence of UPJ obstruction, symphysiotomy (division of the isthmus) was once recommended routinely after pyeloplasty to improve drainage. However, this procedure is associated with an increased risk of hemorrhage, fistula, and renal infarction. Also, because of their abnormal vasculature, the kidneys return to their original position after division of the isthmus. Because of this, symphysiotomy is rarely, if ever, indicated in conjunction with pyeloplasty.
Litwinowicz R, Szpor J, Janus G, Worek M, Okon K. Primary carcinoid tumour in horseshoe kidney. Pol J Pathol. 2011 Mar. 62(1):72-4. [Medline].
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 Jul-Sep. 15(3):412-4. [Medline]. [Full Text].
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 Aug. 78(2):466-8. [Medline].
Ray AA, Ghiculete D, D'A Honey RJ, Pace KT. Shockwave lithotripsy in patients with horseshoe kidney: determinants of success. J Endourol. 2011 Mar. 25(3):487-93. [Medline].
Mufarrij PW, Woods M, Shah OD, Palese MA, Berger AD, Thomas R, et al. Robotic dismembered pyeloplasty: a 6-year, multi-institutional experience. J Urol. 2008 Oct. 180(4):1391-6. [Medline].
Etafy M, Pick D, Said S, Hsueh T, Kerbl D, Mucksavage P. Robotic pyeloplasty: the University of California-Irvine experience. J Urol. 2011 Jun. 185(6):2196-200. [Medline].
Bellman GC, Yamaguchi R. Special considerations in endopyelotomy in a horseshoe kidney. Urology. 1996 Apr. 47(4):582-5; discussion 585-6. [Medline].
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 Dec. 96(9):1365-8. [Medline].
Al-Tawheed AR, Al-Awadi KA, Kehinde EO, et al. Treatment of calculi in kidneys with congenital anomalies: an assessment of the efficacy of lithotripsy. Urol Res. 2006 Oct. 34(5):291-8. [Medline].
Bauer SB, Perlmutter AD, Retik AB. Anomalies of the Upper Urinary Tract. Walsh PC, Retik AB, Vaughan ED Jr, Wein AJ, eds. Campbell's Urology. 6th ed. Philadelphia, Pa: WB Saunders; 1992. Vol 2: 1376-81.
Bégin LR, Guy L, Jacobson SA, et al. Renal carcinoid and horseshoe kidney: a frequent association of two rare entities--a case report and review of the literature. J Surg Oncol. 1998 Jun. 68(2):113-9. [Medline].
Chammas M Jr, Feuillu B, Coissard A, et al. Laparoscopic robotic-assisted management of pelvi-ureteric junction obstruction in patients with horseshoe kidneys: technique and 1-year follow-up. BJU Int. 2006 Mar. 97(3):579-83. [Medline].
Davidovic LB, Kostic DM, Jakovljevic NS, et al. Abdominal aortic surgery and horseshoe kidney. Ann Vasc Surg. 2004 Nov. 18(6):725-8. [Medline].
Dewan PA, Clark S, Condron S, et al. Point of technique: Ureterocalycostomy in the management of pelvi-ureteric junction obstruction in the horseshoe kidney. BJU Int. 1999 Aug. 84(3):366-8. [Medline].
Doménech-Mateu JM, Gonzalez-Compta X. Horseshoe kidney: a new theory on its embryogenesis based on the study of a 16-mm human embryo. Anat Rec. 1988 Dec. 222(4):408-17. [Medline].
Ferko A, Krajina A, Jon B, et al. Juxtarenal aortic aneurysm associated with a horseshoe kidney. Transfemoral endoluminal repair. Arch Surg. 1997 Mar. 132(3):316-7. [Medline].
Gleason PE, Kramer SA. Ectopic Kidneys and Renal Fusion Anomalies. AUA Update Series. 1995; Lesson 33. XIV:268-71.
Hohenfellner M, Schultz-Lampel D, Lampel A, et al. Tumor in the horseshoe kidney: clinical implications and review of embryogenesis. J Urol. 1992 Apr. 147(4):1098-102. [Medline].
Krishnan B, Truong LD, Saleh G, et al. Horseshoe kidney is associated with an increased relative risk of primary renal carcinoid tumor. J Urol. 1997 Jun. 157(6):2059-66. [Medline].
Lampel A, Hohenfellner M, Schultz-Lampel D, et al. Urolithiasis in horseshoe kidneys: therapeutic management. Urology. 1996 Feb. 47(2):182-6. [Medline].
Murphy JT, Borman KR, Dawidson I. Renal autotransplantation after horseshoe kidney injury: a case report and literature review. J Trauma. 1996 May. 40(5):840-4. [Medline].
O'Hara PJ, Hakaim AG, Hertzer NR, et al. Surgical management of aortic aneurysm and coexistent horseshoe kidney: review of a 31-year experience. J Vasc Surg. 1993 May. 17(5):940-7. [Medline].
Schuster T, Dietz HG, Schütz S. Anderson-Hynes pyeloplasty in horseshoe kidney in children: is it effective without symphysiotomy?. Pediatr Surg Int. 1999. 15(3-4):230-3. [Medline].
Snow BW. Ectopic Kidneys and Renal Fusion Anomalies. AUA Update Series. 1987; Lesson 6. VI:2-5.
Stroosma OB, Schurink GW, Smits JM, et al. Transplanting horseshoe kidneys: a worldwide survey. J Urol. 2001 Dec. 166(6):2039-42. [Medline].
Viola D, Anagnostou T, Thompson TJ, et al. Sixteen years of experience with stone management in horseshoe kidneys. Urol Int. 2007. 78(3):214-8. [Medline].
Yohannes P, Smith AD. The endourological management of complications associated with horseshoe kidney. J Urol. 2002 Jul. 168(1):5-8. [Medline].