The horseshoe kidney is probably the most common of all renal fusion anomalies.
– Horseshoe kidney occurs in 0.25% of the population, or about 1 in 400 persons
– Horseshoe kidneys have been reported in identical twins and among several siblings within the same family
– abnormality occurs between the 4th and 6th week of gestation, after the ureteral bud has entered the renal blastema
– in view of the ultimate spatial configuration of the horseshoe kidney, the entrance of the ureteral bud had to have taken place before rotation and considerably before renal ascent ensued.
– Boyden (1931) postulated that at the 14-mm stage (4.5 weeks), the developing metanephric masses lie close to one another; any disturbance in this relationship might result in joining at their inferior poles. A slight alteration in the position of the umbilical or common iliac artery could change the orientation of the migrating kidneys, leading to contact and fusion. It has been postulated that an abnormality in the formation of the tail of the embryo or another pelvic organ could account for the fusion process.
– Domenech-Mateu and Gonzales-Compta (1988) , after studying a 16-mm human embryo, suggested that posterior nephrogenic cells migrate abnormally to form an isthmus or connection between the two developing kidneys to create the horseshoe shape.
– Whatever the actual mechanism responsible for horseshoe kidney formation, the joining occurs before the kidneys have rotated on their long axis.
– In its mature form, the pelves and ureters of the horseshoe kidney are usually anteriorly placed, crossing ventrally to the isthmus.
– Very rarely, the pelves are anteromedial, suggesting that fusion occurred somewhat later, after some rotation had taken place.
– In addition, migration is usually incomplete, with the kidneys lying lower in the abdomen than normal.
– It is presumed that the inferior mesenteric artery prevents full ascent by obstructing the movement of the isthmus.
– In 95% of patients, the kidneys join at the lower pole
– in a small number, an isthmus connects both upper poles instead
– Generally, the isthmus is bulky and consists of parenchymatous tissue with its own bloodsupply. Occasionally it is just a flimsy midline structure composed of fibrous tissue that tends to draw the renal masses close together.
– isthmus is located adjacent to the L3 or L4 vertebra just below the origin of the inferior mesenteric artery from the aorta. As a result, the paired kidneys tend to be somewhat lower than normal in the retroperitoneum. In some instances, the anomalous kidneys are very low, anterior to the sacral promontory or even in the true pelvis behind the bladder. The isthmus most often lies anterior to the aorta and vena cava, but it is not unheard of for it to pass between the inferior vena cava and the aorta or even behind both great vessels.
– The calyces, normal in number, are atypical in orientation. Because the kidney fails to rotate, the calyces point posteriorly, and the axis of each pelvis remains in the vertical or obliquely lateral plane (on a line drawn from lower to upper poles). The lowermost calyces extend caudally or even medially to drain the isthmus and may overlie the vertebral column.
– The ureter may insert high on the renal pelvis and lie laterally, probably as the result of incomplete renal rotation. It courses downward and has a characteristic bend as it crosses over and anterior to the isthmus, a deviation that is proportionate to the thickness of the midline structure. Despite upper ureteral angulation, the lower ureter usually enters the bladder normally and rarely is ectopic.
– blood supply to the horseshoe kidney is variable
– In 30% of cases, it consists of one renal artery to each kidney, but it may be atypical with duplicate or even triplicate renal arteries supplying one or both kidneys.
– blood supply to the isthmus and lower poles is also variable. The isthmus and adjacent parenchymal masses may receive a branch from each main renal artery, or they may have their own arterial supply from the aorta originating either above or below the level of the isthmus. Not infrequently this area is supplied by branches from the inferior mesenteric, common or external iliac, or sacral arteries
– Three cases of retrocaval ureter and isthmus have been reported
Associated Anomalies: increased occurrence of other genitourinary anomalies in patients with a horseshoe kidney.
– Hypospadias and undescended testes each occurred in 4% of males
– bicornuate uterus or septate vagina or both were noted in 7% of the females.
– Duplication of the ureter occurs in 10% of patients; in some cases this has been associated with an ectopic ureterocele.
– Vesicoureteral reflux has been noted in more than half of affected individuals
– UPJ dilation has been seen in over 20%, but on diuretic nuclear scanning less than 20% have an obstructive pattern
– Cystic disease, including multicystic dysplasia in one half (the upper pole) of one side
– adult polycystic kidney disease, has been reported
– DMSA scanning in 22 patients revealed asymmetrical function in 63% ( Kao et al, 2003 ).
– Because stone formation has been seen commonly in these patients, evaluation in 37 revealed altered calcium, oxalate, uric acid, and citrate excretion in 50%, suggesting an underlying metabolic etiology ( Raj et al, 2004 ). Thus, stone formation may not be due to just delayed drainage from the anatomic abnormality and other causes should be considered.
– UPJ obstruction causing significant hydronephrosis occurs in as many as one third of individuals
– The high insertion of the ureter into the renal pelvis, its abnormal course anterior to the isthmus, and the anomalous blood supply to the kidney may individually or collectively contribute to this obstruction.
– Often asx
– The clinical features from a diseased kidney are often vague and nonspecific.
– The anomalies, therefore, may not be suspected until a renal ultrasound or an excretory urogram is obtained.
– Prenatal ultrasonography is detecting horseshoe kidneys before birth
– The classic radiologic features are easily recognized and a diagnosis is readily made
When symptoms are present, however, they are related to hydronephrosis, infection, or calculus formation.
– most common symptom that reflects these conditions is vague abdominal pain that may radiate to the lower lumbar region.
– Gastrointestinal complaints may be present as well.
– The so-called Rovsing sign—abdominal pain, nausea, and vomiting on hyperextension of the spine—has been infrequently observed.
– Signs and symptoms of urinary tract infection occur in 30% of patients, and calculi have been noted in 20% to 80%
– Five to 10 percent of horseshoe kidneys are detected after palpation of an abdominal mass
– horseshoe kidney may occur more often in patients with other serious congenital anomalies. The organ systems most commonly affected include the skeletal, cardiovascular (primarily ventriculoseptal defects), and central nervous systems.
– Horseshoe kidney is found in 3% of children with neural tube defects. Anorectal malformations are frequently encountered in these patients. Horseshoe kidney may also be seen in 20% of patients with trisomy 18 and as many as 60% of females with Turner’s syndrome and in patients with Townes-Brock syndrome who have a SALL1 transcription factor defect
– The abnormality has been discovered clinically in all age groups ranging from fetal life to 80 years, but in autopsy series it is more prevalent in children
Findings that suggest a horseshoe kidney singly or collectively include the following:
– kidneys that are somewhat low lying and close to the vertebral column;
– a vertical or outward axis, so that a line drawn through the midplane of each kidney bisects the midline inferiorly;
– a continuation of the outer border of the lower pole of each kidney toward and across the midline;
– the characteristic orientation of the collecting system, which is directly posterior to each renal pelvis, with the lowermost calyx pointing caudally or even medially; and the high insertion of the ureter into the pelvis as well as the anteriorly displaced upper ureter that appears to drape over a midline mass
– However, obstruction from either a calculus or a UPJ stricture may obscure the radiologic picture
– Other studies, such as retrograde pyelography or CT, may be necessary to confirm the diagnosis.
The horseshoe kidney, even though it produces no symptoms, is frequently found in association with other congenital anomalies.
Genitourinary Anomalies Associated with Horseshoe Kidneys
– Undescended testes4%
– Bicornuate uterus7%
– Septate vagina7%
– Urinary Collecting System
– Ureteral duplication10%
– Ureteropelvic junction obstruction20%
– Vesicoureteral reflux50%
Renal Parenchymal Abnormalities
– Multicystic dysplasia1%
– Autosomal recessive polycystic kidney1%
– Metabolic Derangements in Patients with Stones
– Hypercalciuria, hyperoxaluria, hypocitraturia, hypouricuria50%
– As with other fusion anomalies, it is found more commonly in males by a slightly greater than a 2:1 margin
Glenn (1959) observed patients with horseshoe kidneys for an average of 10 years after discovery and found that almost 60% of these remained asymptomatic.
– Only 13% had persistent urinary infection or pain, and 17% developed recurrent calculi.
– In Glenn’s series, no patients benefited from division of the isthmus for relief of pain
– Often a horseshoe kidney is found incidentally, and it is rarely a cause for mortality
– Renal carcinoma has been reported within a horseshoe kidney in 123 patients; more than half of these cancers were renal cell carcinoma and included two cases of bilateral tumors.
– However, renal pelvic tumors and Wilms’ tumor each accounted for 25% of the total.
– Overall, 41 of 8617 (0.48%) Wilms’ tumors in the National Wilms’ Tumor study occurred in horseshoe kidneys, mostly on the left side, rarely in the isthmus, and practically all with favorable histology.
– This incidence of Wilms’ tumor in horseshoe kidneys is more than twice that expected in the general population.
– It has been suggested that the increased occurrence of chronic infection, obstruction, and stone formation may be instrumental in producing a higher than expected incidence of renal pelvic tumors in this group
– Because it is located above the pelvic inlet, a horseshoe kidney should not adversely affect pregnancy or delivery.
– Glomerulocystic disease has been reported in children before 1 year of age but does not appear to be related specifically to the horseshoe anomaly
– The North American Pediatric Renal Transplant Cooperative Study and the Department of Health and Human Services 2000 Annual Report (U.S. Department of Health and Human Services, U.S. Scientific Registry of Transplant Recipients and the Organ Procurement and Transplantation Network, 2000 ) each failed to reveal any patient with a horseshoe kidney receiving a renal transplant.
– A worldwide review from the Netherlands noted that 23 whole and 57 split horseshoe kidneys have been transplanted into 120 patients with only a 7% initial failure rate (4.3%) for en bloc and a 13.4% failure rate for split transplants and an overall graft survival rate of 80% at 5 years.