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Findings: CT of the abdomen and pelvis with IV contrast shows axial images of a displaced left kidney located right of mid line at the level of the lower pole of the right kidney. The hilum of the left kidney faces anteriorly and the left ureter crosses mid line back to the left to insert on the bladder trigone. The right kidney and ureter are normal.
Diagnosis: Crossed non-fused renal ectopia
Discussion: The kidney develops from 3 sets of successively appearing structures. The first set, pronephros regresses completely by the 5th week. The second set, mesonephros, regresses almost completely forming only the epididymis, vas deferens, seminal vesicles, and ejaculatory ducts in the male and only the mesosalpinx in the female. The metanephros develops in the 5th week and forms the kidney. The metanephros forms from the ureteric bud and nephrogenic blastema. The ureteric bud invaginates into the nephrogenic blastema with subsequent formation of the collecting system by the ureteric bud and development of the kidney and Bowman's capsule from the nephrogenic blastema. The kidneys then begin to "ascend" by the elongation of the lumbar and sacral regions and concomitant medial rotation. Failure of the primitive embryonal structures to develop results in aplasia or hypoplasia. Splitting of the ureteric bud results in kidney and ureter duplication. Failure or incomplete ascent results in kidney ectopia.
During elongation a ureteral bud may cross the midline and meet the contralateral metanephros. The total renal mass becomes located on one side of the abdomen. The distal ureter inserts into the trigone on the side of origin. Four types of crossed ectopia have been described: cross fused ectopia, crossed ectopia without fusion, solitary crossed ectopia, and bilaterally crossed ectopia without fusion. Most patients have a normal trigone with only 3% found to have ectopic trigones. VUR is associated with the ectopic kidney. A minority of patients have UTI and renal calculi related to the ectopia. The most common associated anomaly is an imperforate anus, 4%. Most patients are asymptomatic and have normal life expectancy.
References:
Principles of Genitourinary Radiology. Barbaric, ZL. 1994.
Pediatric Radiology: The Requisites. Blickman JG, 1994.
Campbells Urology, 5th edition. Walsh, Gittes, Perlmutter, Stamey.
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