Horseshoe kidneys are the most common fusion defect of the kidneys, but this still amounts to only about 0.25% of the population . 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 . 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 . 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 .
Despite cases of familial clustering, no clear genetic cause has been described for horseshoe kidneys, although several etiological factors may contribute to their development . 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 . 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.
The incidence of horseshoe kidney is approximately 1 in 500 in the normal population with a male preponderance of 2:1 . 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% .
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 .
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 . 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 . Horseshoe kidneys also show a greater variation in the origin and number of renal arteries and veins . These are largely dependent on where during development ascent has terminated. In one study of 90 horseshoe kidneys, 387 arteries were identified . 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 . The incidence of renal vein anomalies in horseshoe kidneys is also high (23%).
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 . CT and MRI are the best for demonstrating the anatomy and can detect accessory vasculature and surrounding structures . 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 . Nuclear medicine radionuclide renal scans can be helpful in differentiating true obstruction from passively dilated systems.
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 . Larger renal stones, those greater than 2.5 cm, or those not allowing ureteroscopic approaches, can be removed via minimally invasive percutaneous surgery . 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 .
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 .
An isolated finding of a horseshoe kidney is generally considered benign . 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) .
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 . Ureteropelvic junction obstruction (UPJ) is the most common abnormality associated with horseshoe kidneys, individuals are also predisposed to hydronephrosis, infection, vesicoureteral reflux . One study showed that over half of the individuals who are symptomatic had either ureteropelvic junction obstruction or vesicoureteral reflux . A recent meta-analysis suggested that 36% of patients with horseshoe kidney will develop nephrolithiasis at some stage . 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 .
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 . It has also been noted that the kidneys return to their original location after such surgery, so symphysiotomy is no longer recommended.
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