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Abnormal Kidney Location
Introduction
There are three types of abnormalities of renal location: simple renal ectopia (e.g., pelvic kidney), crossed renal ectopia, and horseshoe kidney. They are collectively termed renal ectopia . During normal development, the metanephros migrates from the pelvis to the abdomen, and rotates about its longitudinal axis. Abnormalities of renal location may occur because of failure of ascent of the metanephric buds or because of fusion before ascent. Although renal ectopia is relatively benign, it is associated with an increased risk of complications, including vesicoureteral reflux (VUR), ureteropelvic junction (UPJ) obstruction, infection, and renal calculi. Affected individuals are usually asymptomatic. If a prenatal diagnosis is not made, affected individuals may not receive medical attention until complications arise. Improvements in ultrasound (US) technology have made it easier to identify abnormalities of renal location; however, these anomalies are subtle, and an index of suspicion is required.
Disorder
Definition
Pelvic kidney is also called simple renal ectopia and refers to a kidney that is on the correct side but has failed to migrate upward. A pelvic kidney is located opposite the sacrum and below the aortic bifurcation.
Crossed renal ectopia occurs when both kidneys are located on the same side of the spine. In most cases, the kidney on the incorrect side is located below the normal kidney and is fused with it (termed crossed-fused ectopia ). The ureter of the displaced kidney crosses the midline and inserts into the bladder in the normal location.
Horseshoe kidney is a fusion of the lower poles of the kidneys, with an isthmus that crosses the midline, usually anterior to the great vessels, and just below the inferior mesenteric artery—preventing the horseshoe kidney from migrating upward to the normal location.
Prevalence and Epidemiology
Pelvic kidney occurs in 1 : 2000 to 1 : 3000 births. It is slightly more common in males, and a left-sided predominance has been reported. Pelvic kidney is bilateral in 12% of cases.
The prevalence of crossed renal ectopia is 1 : 7500 individuals. It is twice as common in males, and the left kidney is more often the one that is crossed, meaning that both kidneys are commonly found on the right side. In approximately 75% of cases, the kidneys are fused (i.e., crossed-fused ectopia).
Horseshoe kidney is the most common renal ectopia, with a reported prevalence of 1 : 400 individuals. It is also twice as common in males.
All forms of renal ectopia are associated with other renal abnormalities. Hydronephrosis has been reported in more than 50%; VUR and UPJ obstruction each occur in approximately 20% (see Chapter 12 ). Renal calculi have been reported in 20% of cases of horseshoe kidney, postulated to be caused by a combination of stasis and infection.
Renal ectopia is also associated with a high prevalence of anomalies of other organ systems. Reproductive tract abnormalities have been reported in approximately 25%, musculoskeletal anomalies in 25%, and cardiac malformations in 10%. Horseshoe kidney is further associated with chromosomal abnormalities, including Turner syndrome and trisomies 21, 18, and 13.
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