Horseshoe Kidney

Article Author:
Joshua Kirkpatrick
Article Editor:
Stephen Leslie
Updated:
5/14/2019 11:54:27 AM
PubMed Link:
Horseshoe Kidney

Introduction

Horseshoe kidneys are the most common fusion defect of the kidneys, but this still amounts to only about 0.25% of the population [1]. Initially described during autopsies by da Carpi performed in 1522, they are characterized by abnormalities in the position, rotation, and vascular supply of the kidney [2]. Horseshoe kidneys are identified as having functioning renal masses present on both sides of the vertebral column fused together with ureters that remain uncrossed from the renal hilum to the urinary bladder [3]. The isthmus connecting the two renal masses may be positioned in the midline or laterally resulting in asymmetric horseshoe kidney, 70% of which are left dominant, and consists of renal parenchyma in about 80% of cases with the remainder being composed of a fibrous band. In more than 90% of cases, fusion occurs at the lower pole, although fusion may occur at the upper pole in a minority of cases [4][5][6].

Etiology

Despite cases of familial clustering, no clear genetic cause has been described for horseshoe kidneys, although several etiological factors may contribute to their development [7]. These include abnormal migration of nephrogenic cells across the primitive streak, alterations in the intrauterine environment (with teratogenic drugs such as thalidomide, alcohol consumption and glycemic control causing an increase in incidence), and structural factors such as flexion/rotation of the caudal spine and narrowed arterial forks during migration [4][8][9][10]. Traditionally textbooks quote fusion as occurring between weeks four and six of development, although there is some evidence for later fusion, particularly when the isthmus is fibrous rather than renal parenchyma.

Epidemiology

The incidence of horseshoe kidney is approximately 1 in 500 in the normal population with a male preponderance of 2:1 [1][4]. The incidence is higher in those who present to urology clinics (1 in 304), and with some chromosomal disorders. These include Edward syndrome at approximately 67%, Turner syndrome at 14% to 20%, and Down syndrome at about 1% [11][12][13].

History and Physical

Horseshoe kidneys are often asymptomatic, and so are often identified incidentally. The presentation can otherwise be non-specific with one study showing that the two most common presenting complaints of in children with horseshoe kidney are with symptoms of a urinary tract infection or with abdominal pain [14].

The kidneys are normally located in the retroperitoneum between the transverse processes of T12 and L3 with the left kidney slightly more superior than the right. The upper poles are normally positioned slightly medially and posteriorly relative to the lower poles. Horseshoe kidneys are different in three main ways: location, orientation, and vasculature [2]. The horseshoe kidney's ascent is often quoted to be held back by the inferior mesenteric artery at L3 however, the horseshoe kidney can also be found lower in the abdomen and pelvis. During weeks six to eight of development, the renal ascent is coupled with a 90-degree medial rotation. Due to the isthmus, however, horseshoe kidneys experience malrotation, and consequently, the ureters need to either pass over the isthmus or down the anterior surface of the kidneys which can cause urinary drainage problems and stasis [15][16]. Horseshoe kidneys also show a greater variation in the origin and number of renal arteries and veins [4][16][6]. These are largely dependent on where during development ascent has terminated. In one study of 90 horseshoe kidneys, 387 arteries were identified [4]. Despite this, the normal intra-renal vascular segmental pattern remains, and the ligation or division of any of these arteries results in ischemic segmental renal necrosis due to their poor collateral arterial supply [17]. The incidence of renal vein anomalies in horseshoe kidneys is also high (23%)[4].

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 intravenous urography [18]. CT and MRI are the best for demonstrating the anatomy and can detect accessory vasculature and surrounding structures [19][4][19][18]. It is also possible to identify horseshoe kidneys on plain radiography through visualization of the perinephric fat in association with an altered renal axis. The lower poles are positioned more medial than normal and because the kidneys sit lower in the abdomen than expected [18]. Nuclear medicine radionuclide renal scans can be helpful in differentiating true obstruction from passively dilated systems.

Treatment / Management

Shockwave lithotripsy for nephrolithiasis is less effective in horseshoe kidneys due to problems localizing the energy for pelvic stones and poor stone fragment clearance due to impaired renal drainage [20]. Larger renal stones, those greater than 2.5 cm, or those not allowing ureteroscopic approaches, can be removed via minimally invasive percutaneous surgery [21]. Pre-procedural imaging such as CT is essential during the workup for any surgery required. This is due not only to the highly variable nature of the blood supply but also the association of horseshoe kidneys with having a segment of colon posteriorly and the corresponding increases in risk of incidental bowel injury [22].

Differential Diagnosis

The horseshoe kidney is one form of a renal fusion abnormality. The other two main types are crossed fusion renal ectopia and a fused pelvic kidney. In a crossed renal ectopia both kidneys are positioned on the same side of the body with one ureter crossing the midline to drain into the bladder while in a fused pelvic kidney there is one renal mass which is drained by two ureters that do not cross the midline [23].

Prognosis

An isolated finding of a horseshoe kidney is generally considered benign [3]. Horseshoe kidneys do however have an increase in frequency for some common renal cancers including transitional cell tumors (three to four times more common), Wilms tumor (twice as frequently), and an extremely large increase in very rare tumors such as carcinoid (62 to 82 times) [24][4][25][26][27][28].

Complications

About a third of all patients with horseshoe kidneys remain completely asymptomatic and are often found incidentally during imaging. The intrinsic anatomical defects present within horseshoe kidneys do however predispose individuals to a number of urological sequelae due to the associated ureteric obstruction and impaired urinary drainage [3][18]. Ureteropelvic junction obstruction (UPJ) is the most common abnormality associated with horseshoe kidneys, individuals are also predisposed to hydronephrosis, infection, vesicoureteral reflux [15][16][2]. One study showed that over half of the individuals who are symptomatic had either ureteropelvic junction obstruction or vesicoureteral reflux [14]. A recent meta-analysis suggested that 36% of patients with horseshoe kidney will develop nephrolithiasis at some stage [29]. Due to their ectopic position, horseshoe kidneys are also particularly susceptible to blunt abdominal trauma and can be compressed or fractured against the lumbar vertebrae [30].

Pearls and Other Issues

Symphysiotomy, or division of the fused isthmus, was previously recommended when doing a pyeloplasty in patients with a horseshoe kidney, but this has changed due to the increased risk of infection, fistulas, leakages, and bleeding [31]. It has also been noted that the kidneys return to their original location after such surgery, so symphysiotomy is no longer recommended.

Enhancing Healthcare Team Outcomes

Horseshoe kidneys are the most common fusion defect of the kidneys, but this still amounts to only about 0.25% of the population. Many professionals are not aware of the condition, its evaluation, or treatment. Due to its rarity, this condition is best evaluated and treated by an interprofessional team of specialty trained clinicians and nurses, and radiologists to achieve the best patient outcomes. The nursing urology specialty nurse should assist the clinician in educating the patient and family. [Level V]


References

[1] Schiappacasse G,Aguirre J,Soffia P,Silva CS,Zilleruelo N, CT findings of the main pathological conditions associated with horseshoe kidneys. The British journal of radiology. 2015 Jan;     [PubMed PMID: 25375751]
[2] Natsis K,Piagkou M,Skotsimara A,Protogerou V,Tsitouridis I,Skandalakis P, Horseshoe kidney: a review of anatomy and pathology. Surgical and radiologic anatomy : SRA. 2014 Aug     [PubMed PMID: 24178305]
[3] Cook WA,Stephens FD, Fused kidneys: morphologic study and theory of embryogenesis. Birth defects original article series. 1977     [PubMed PMID: 588702]
[4] Glodny B,Petersen J,Hofmann KJ,Schenk C,Herwig R,Trieb T,Koppelstaetter C,Steingruber I,Rehder P, Kidney fusion anomalies revisited: clinical and radiological analysis of 209 cases of crossed fused ectopia and horseshoe kidney. BJU international. 2009 Jan     [PubMed PMID: 18710445]
[5] Stroosma OB,Scheltinga MR,Stubenitsky BM,Kootstra G, Horseshoe kidney transplantation: an overview. Clinical transplantation. 2000 Dec     [PubMed PMID: 11127302]
[6] Crawford ES,Coselli JS,Safi HJ,Martin TD,Pool JL, The impact of renal fusion and ectopia on aortic surgery. Journal of vascular surgery. 1988 Oct     [PubMed PMID: 3172375]
[7] David RS, Horseshoe kidney: a report of one family. British medical journal. 1974 Dec 7     [PubMed PMID: 4434143]
[8] Solhaug MJ,Bolger PM,Jose PA, The developing kidney and environmental toxins. Pediatrics. 2004 Apr     [PubMed PMID: 15060203]
[9] Friedland GW,de Vries P, Renal ectopia and fusion. Embryologic Basis. Urology. 1975 May     [PubMed PMID: 1129903]
[10]     [PubMed PMID: 8875880]
[11]     [PubMed PMID: 23088440]
[12]     [PubMed PMID: 11498234]
[13] Bhattarai B,Kulkarni AH,Rao ST,Mairpadi A, Anesthetic consideration in downs syndrome--a review. Nepal Medical College journal : NMCJ. 2008 Sep     [PubMed PMID: 19253867]
[14]     [PubMed PMID: 11992090]
[15] Lallas CD,Pak RW,Pagnani C,Hubosky SG,Yanke BV,Keeley FX,Bagley DH, The minimally invasive management of ureteropelvic junction obstruction in horseshoe kidneys. World journal of urology. 2011 Feb     [PubMed PMID: 20204377]
[16] GLENN JF, Analysis of 51 patients with horseshoe kidney. The New England journal of medicine. 1959 Oct 1     [PubMed PMID: 13828436]
[17] O'Hara PJ,Hakaim AG,Hertzer NR,Krajewski LP,Cox GS,Beven EG, Surgical management of aortic aneurysm and coexistent horseshoe kidney: review of a 31-year experience. Journal of vascular surgery. 1993 May     [PubMed PMID: 8487363]
[18]     [PubMed PMID: 18477115]
[19]     [PubMed PMID: 11164171]
[20] Stein RJ,Desai MM, Management of urolithiasis in the congenitally abnormal kidney (horseshoe and ectopic). Current opinion in urology. 2007 Mar     [PubMed PMID: 17285023]
[21] Rais-Bahrami S,Friedlander JI,Duty BD,Okeke Z,Smith AD, Difficulties with access in percutaneous renal surgery. Therapeutic advances in urology. 2011 Apr     [PubMed PMID: 21869906]
[22]     [PubMed PMID: 4009801]
[23]     [PubMed PMID: 9326964]
[24] Neville H,Ritchey ML,Shamberger RC,Haase G,Perlman S,Yoshioka T, The occurrence of Wilms tumor in horseshoe kidneys: a report from the National Wilms Tumor Study Group (NWTSG). Journal of pediatric surgery. 2002 Aug     [PubMed PMID: 12149688]
[25] Rubio Briones J,Regalado Pareja R,Sánchez Martín F,Chéchile Toniolo G,Huguet Pérez J,Villavicencio Mavrich H, Incidence of tumoural pathology in horseshoe kidneys. European urology. 1998     [PubMed PMID: 9519360]
[26] Huang EY,Mascarenhas L,Mahour GH, Wilms' tumor and horseshoe kidneys: a case report and review of the literature. Journal of pediatric surgery. 2004 Feb     [PubMed PMID: 14966742]
[27] Kölln CP,Boatman DL,Schmidt JD,Flocks RH, Horseshoe kidney: a review of 105 patients. The Journal of urology. 1972 Feb     [PubMed PMID: 5061443]
[28] Pitts WR Jr,Muecke EC, Horseshoe kidneys: a 40-year experience. The Journal of urology. 1975 Jun     [PubMed PMID: 1152146]
[29] Pawar AS,Thongprayoon C,Cheungpasitporn W,Sakhuja A,Mao MA,Erickson SB, Incidence and characteristics of kidney stones in patients with horseshoe kidney: A systematic review and meta-analysis. Urology annals. 2018 Jan-Mar     [PubMed PMID: 29416282]
[30] Chopra P,St-Vil D,Yazbeck S, Blunt renal trauma-blessing in disguise? Journal of pediatric surgery. 2002 May     [PubMed PMID: 11987100]
[31] Boatman DL,Kölln CP,Flocks RH, Congenital anomalies associated with horseshoe kidney. The Journal of urology. 1972 Feb     [PubMed PMID: 5061444]