Horseshoe Kidney

Horseshoe kidney is the most common congenital renal fusion anomaly, with an incidence of approximately 1 in 500 live births. As first described in autopsy studies by da Carpi in 1522, a horseshoe kidney is characterized by the fusion of the lower poles of the kidneys across the midline. This anomaly is typically associated with vascular, rotational, and positional anatomical abnormalities, typically resulting in a U-shaped unitary renal unit configuration in the midabdomen, positioned lower in position than normal kidneys. The isthmus, which connects the 2 renal masses, typically (80%) consists of functioning renal parenchyma with its own blood supply, but it may also be composed of fibrous tissue. The connecting isthmus is located between the lower poles in over 90% of cases, but it may occur elsewhere in a small minority. The kidneys are malrotated, with their lower poles fused and their collecting systems facing anteriorly. As they descend to the bladder, the ureters pass anterior to the isthmus, typically located immediately below the inferior mesenteric artery. Abnormal courses of the ureters predispose them to ureteral obstruction and stasis.  Horseshoe kidneys demonstrate a significant variation in the origin and number of renal arteries and veins. Vascularization is mainly dependent on where the renal ascent is terminated in development. In a study involving 90 horseshoe kidneys, 387 separate arteries were identified.Nevertheless, the regular intrarenal segmental, non-collateral arterial vascular pattern remains, and ligation or division of any renal arteries results in ischemic segmental renal ischemia or necrosis. The incidence of renal vein anomalies in horseshoe kidneys is also high at 23%. The natural history is generally benign; however, affected individuals are at increased risk for complications such as ureteropelvic junction obstruction, hydronephrosis, nephrolithiasis, urinary tract infections, vesicoureteral reflux, and renal malignancies, such as renal cell carcinoma, transitional cell cancer, and Wilms tumor in children. The overall risk of a renal malignancy in a horseshoe kidney is about 3 to 4 times higher than in the general population. There is also a higher risk of traumatic injury to the kidney due to its lower position, anterior location, and lack of protection from the rib cage, although such injuries are rarely reported. Chronic kidney disease and progression to end-stage renal disease are more common in this population, likely due to the high prevalence of associated genitourinary anomalies and complications. The abnormal vasculature and collecting system anatomy can complicate the presentation and management of disease, particularly surgical procedures. Extrarenal anomalies, including gastrointestinal and vertebral malformations, are also more frequent in affected individuals, especially in pediatric populations.