In crossed fused renal ectopia, both the kidneys are fused and situated on one side of the body. It is a rare congenital anomaly with an incidence of approximately 1:1300 to 1:7500 [1]. Although the exact incidence is difficult to be estimated, as it does not cause symptoms in most of the patients. Males are more commonly affected, and mostly ectopic left kidney crosses to the right side of the abdomen [2].
McDonald and McClellan classified crossed fused renal ectopia into six types based upon orientation of fused kidney and ureter [3, 4] (Fig. 5). In descending order of occurrence, they are:
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Unilateral fused kidney inferior ectopic where the lower pole of orthotopic kidney fuses with the upper pole of the crossed ectopic kidney.
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Sigmoid or S-shaped kidney where normal side kidney is present superiorly and the direction of pelvic is medial, the kidney which crosses positioned inferiorly with direction of pelvic toward the lateral side. As kidneys fuse after complete rotation on the vertical axis, both renal pelves lie in correct orientation.
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Unilateral lump kidney where fusion occurs over a wide margin, both renal pelves anteriorly directed and placed more inferiorly.
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L-Shaped or tandem kidney in which the crossed kidney lies inferiorly and transversely, fused with the lower pole of the normal kidney.
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Unilateral disk kidney in which kidneys fuse along medial borders and
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Unilateral fused kidney superior ectopia, the ectopic kidney lies superiorly and its lower pole fuses with the upper pole of the normal kidney. The renal pelvis of both kidneys lies anteriorly.
As per the above classification, corresponding ureters of these kidneys drain down orthotopically into the bladder. In the majority of the patients, diagnosis is almost an incidental finding, but 30% of these patients present with on and off flank pain, dysuria, hematuria, and fever due to obstruction or infection [5]. Pelviureteric junction obstruction, hydronephrosis, and nephrolithiasis may be associated with this condition [6]. Other associated anomalies are imperforate anus (4%), skeletal deformities (4%), and cardiovascular septal anomalies [7].
USG is a good modality to demonstrate the presence of fused renal ectopia. The sonographic findings usually suggest an absent kidney in the contralateral renal fossa or pelvis and fused kidneys on the ipsilateral side. Contrast-enhanced CT with delayed film is useful for better appreciation of vascular supply, anatomy of draining ureter which is required for proper surgical management. If there are no associated anomalies and complications, then the prognosis of the crossed fused renal ectopia is very good [8].
The exact embryological basis of crossed fused renal ectopia has not been clearly understood till date. Normally, kidney develops from the ureteric bud and the metanephric blastema. The ureteric bud originates from the Wolffian duct, and the metanephric blastema is mesodermal in origin; both of them grow toward each and fuse to form kidney and pelvicalyceal system. The most commonly accepted theories of the development of crossed fused ectopia are ureteral theory and mechanical theory [2]. According to ureteric theory, it is assumed that excessive bending along with embryonal caudal end rotation prevents fusion of the ipsilateral metanephric blastema and ureteric bud and it turns toward the opposite side. Because of the stimulation of the mesonephric blastema twice by normal migrated ureteric bud, two kidneys are formed on one side [2, 9]. According to mechanical theory, an abnormally positioned umbilical artery is responsible for the abnormal ascent of the developing kidney, which migrates toward the opposite side due to the least resistance in the pathway [2, 4].
However, this theory does not fit in our case as there was solitary ureter for both kidneys which was draining in opposite side of ectopia. So embryology of ectopia needs to be re-evaluated.
