description
the permanent kidneys begin to develop around the 5th week of gestation. there are 2 cell sources for this organ, the metanephric diverticulum or ureteric bud, and the metanephric mesoderm or metanephrogenic blastema. both are of mesodermal origin. the ureteric bud forms from a dorsal outgrowth from the mesonephric duct near its entry to the cloaca. this supplies tissue for the ureter, renal pelvis, calyces and collecting tubules. the ureteric bud expands; the stalk becomes the ureter and the cranial end becomes the renal pelvis. duplications of the upper part of the ureter and renal pelvis result from division of the ureteric bud. the extent of the duplication depends on how completely the diverticulum divides. incomplete division leads to a divided kidney with a bifid ureter. complete division results in a double kidney with a bifid ureter or separate extra ureter. a supernumerary kidney probably results from the formation of two ureteric buds.
diagnosis
duplication anomalies may be unilateral or bilateral and may involve the renal pelves, and/or ureter(s). supernumerary kidneys can be seen as separate structures of normal shape, and normal or smaller in size. a double kidney may involve duplication of one pole or duplicated renal pelves. duplication of ureters may be difficult to visualise unless hydronephrosis or duplication of a dilated renal pelvis and/or proximal ureter is seen. a duplicated kidney can be in an unusual location and may involve a pelvic or horseshoe kidney. in the presence of a ureterocele a duplication anomaly should be suspected.
differential diagnosis
renal tumours or adrenal tumours produce a mass which may mimic additional kidney structures; solid tumours often have a different pattern of echogenicity. renal or adrenal cysts are fluid-filled, not solid, extra renal structures.
sonographic features
supernumerary” kidney will present as three (or more) separate kidneys, usually of normal shape; may be smaller than normal and in an unusual location “double” kidney will present with duplication of part of the kidney, usually the renal pelvis. may appear as a large, misshapen renal structure amniotic fluid volume normal there may be duplicated or partially duplicated ureters a ureterocele (cystic structure seen in full bladder) may be seen especially involving the upper pole of a double ureter.”
associated syndromes
- brachio-oto-renal syndrome
- congenital hydronephrosis
- diabetes
- trisomy 13
- turner
references
moore kl, perstad tvn in: the developing hum, clinically oriented embryology, 5th ed w. b. saunders: philadelphia, p 265-275
warkany j in: congenital malformations, notes and comments mosby year book: chicago, p1061-1064
crawfurd md’a in: the genetics of renal tract disorders oxford u press: new york/london, p45, p42
crawfurd md’a in: the genetics of renal tract disorders oxford u press: new york/london, p554, p562
stevenson r, hall t, goodman r in: human malformations and related anomalies, vol ii oxford u press: new york/london, p541