I. Etiology
The exact cause of renal cross transposition is not yet clear. Wilmer believes that it may be due to an abnormality in the umbilical artery, which produces a pressure that obstructs the migration of the kidneys towards the head side, causing them to turn towards the side with less resistance. Potter and others believe it is because the ureteric bud wandered to the opposite side. Cook and Stephens speculate that an abnormal transposition occurs in the tail side of the embryo during development, causing abnormal positioning of the cloaca and Wolffian duct structures. It may be that one side of the ureteric bud crosses the midline into the opposite mesonephric rudiment, or that the kidneys and their ureters transpose to the opposite side during the process of kidney ascent. The fusion of metanephric tissue occurs either before or simultaneously with its upward migration and before it is still within the true pelvis, or it may occur in the later stage of its ascent. The extent of fusion depends on the proximity of the developing renal primordia to each other.
Second, pathogenesis
The occurrence of fusion in crossing ectopic kidneys is related to the time of contact between the two kidneys. Ectopic kidneys are often located below the normal kidney, and generally the rise of the two kidneys is synchronous, but the ectopic kidney lags behind the normal kidney, which may be due to a time lag in crossing to the opposite side. Therefore, it is usually the upper pole of the ectopic kidney and the lower pole of the normal kidney that fuse together. When the normal kidney rises to its own position or is blocked by retroperitoneal structures, the fusion kidney stops rising. The final shape of the fused kidney depends on the degree of fusion and self-rotation. Once the two kidneys fuse, they no longer rotate, so the position of the two kidneys suggests the time of occurrence of congenital defects. A more anterior pelvis suggests an earlier fusion. If the pelvis is in a normal position, it suggests that fusion occurs after complete rotation. 90% of ectopic kidneys usually fuse with the normal kidney. If the two kidneys do not fuse, the kidney that has not occurred ectopically usually remains in its normal position, while the ectopic kidney is located below it, with the pelvis facing forward. The two kidneys are separated and each is enveloped by its own capsule. When the two kidneys do not fuse, the normal kidney's ureter enters the ipsilateral bladder, while the ectopic kidney's ureter crosses the midline to enter the contralateral bladder. In solitary kidney crossing ectopia, the kidney is often located in the opposite renal fossa, with a lower position, at the level of the first to third lumbar vertebrae, often poorly rotated. When the kidney is located in the pelvis, it only rises to the lower lumbar level, almost without rotation. The pelvis faces forward, and the ureter crosses over the sacral level 2 to enter the contralateral bladder, and the opposite ureter is often absent or only leaves a degenerated remnant. In bilateral crossing ectopia, the shape of the two kidneys is normal, and the ureters cross the midline at a lower lumbar level to enter the contralateral bladder. The fusion has various different types, and the blood supply of each kidney also varies greatly, with one or more fusions possible.
Among various fused anomalies, the ureters are mostly normal, and the bladder trigone is also normal. Only when the solitary kidney is ectopic does the opposite ureteral orifice degenerate or fail to form. Occasionally, the ectopic kidney may also have an ectopic ureteral orifice on the normal side. Bladder ureteral reflux often occurs in ectopic kidneys. In children, the most common complications of solitary kidney ectopia include the following: about 50% of patients have skeletal system abnormalities, about 40% of patients have abnormalities of the reproductive system, such as concealed penis in males and vaginal atresia in females, in addition, there are defects in the cardiovascular system, etc.
