kidney – ectopic
the three embryonic kidneys are the pronephros, mesonephros and metanephros. the first two are important in inducing the development of the third. the ureteral bud grows toward the metanephros and induces differentiation of the nephrogenic blastema into the renal parenchyma. the ampulla (growing ureteral bud) undergoes multiple divisions to induce the nephron development to establish a communication. this process continues until the 32-36 week of gestation. the ureteral bud gives rise to the renal pelvis, calyces, papillae, and collecting tubules, but the metanephric blastema gives rise to the nephron. the differentiation, elongation and maturation of the nephron continues even after birth. during the 7th week the kidneys move over the ridges formed by the umbilical arteries and by the ninth week they rise above the aortic bifurcation. a rotation occurs during the ascent so that the renal pelves which have been forward become directed toward the midline. this occurs by the 10th week. if the migration or rotation of the kidney are disturbed, the embryonic anteroposterior rotation is maintained. the metanephros are originally located in the fetal pelvis. the rapid growth of the embryo results in kidney displacement until they reach their normal position in the lumbar fossae. abnormalities in this process of ascent result in ectopic kidneys. a pelvic kidney is one that has failed to ascend out of the pelvis. the lower the kidney, the less it will have rotated. in a crossed renal ectopic it has been proposed that one kidney is located on the contralateral side and the ureter crosses the median plane. frequently the ectopic and normal kidney are united. rarely, the kidneys are displaced into the thorax. the length of the ureter depends upon the kidney placement.
although the diagnosis is usually made after 20 weeks of pregnancy, the use of high frequency transvaginal ultrasound may allow for earlier diagnosis. by 10 weeks gestation, the kidneys have ascended to their final position. pelvic kidneys are usually located opposite the sacrum and below the aortic bifurcation. this diagnosis is not usually accompanied by other anomalies, but hydronephrosis has been occasionally reported. however, all fetal structures should be evaluated. if there are no other anomalies, renal function is usually normal and obstetric intervention is not indicated. the clinical evidence suggests an incidence of approximately 1/12,000. in crossed renal ectopia, the ectopic kidney can be abnormal large and bi-lobed. reflux or other obstructive uropathy may also be noted. this form of ectopic kidney is also likely to be associated with congenital heart defects or gastrointestinal anomalies.
ectopic kidney may be mistaken for unilateral renal agenesis if efforts are not made to identify a second kidney above the lumbar plane. fetal adrenals are relatively large and kidney-shaped and may be mistaken for kidneys. the fetal bowel may be found near the renal fossa and mistaken for a kidney. it can be difficult to observe the kidneys due to lack of normal echogenicity in the second trimester. colour flow doppler studies may help differentiate kidney from adrenals or bowel. a thoracic ectopic kidney may be mistaken for other tumors or masses in the thorax.
usually kidneys are normal size and shape
usually unilateral; can be bilateral, fused or crossed: if unilateral, can be right or left sided.
if crossed, kidney can be large and bilobed.
pelvic kidney(s) located opposite the sacrum and below the aortic bifurcation
ectopic kidney – rarely located in thorax
normal fetal bladder
amniotic fluid volume normal
- acro-renal, siegler type
- acro-renal; dieker type
- charge association
- fanconi’s pancytopenia
- murcs association
- smith-lemli-opitz type ii
- translocation trisomy 19
- trap (twin reversed arterial perfusion)
- trisomy 18
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, p55
crawfurd md’a in: the genetics of renal tract disorders oxford u press: new york/london, p544
crawfurd md’a in: the genetics of renal tract disorders oxford u press: new york/london, p443
stevenson r, hall t, goodman r in: human malformations and related anomalies, vol ii oxford u press: new york/london, p546,547