Abnormality of renal fusion &/or ascent

Horseshoe kidney (fused lower poles)

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  Isthmus = central bridging tissue

  
  
  
  
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    Isthmus is anterior to aorta

  
  
  
  
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  Often low lying: Isthmus “snags” on aortic vessels during ascent of fused kidneys from pelvis to flank

Ectopic pelvic kidney

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  Failure of embryologic ascent from pelvis to flank

Crossed-fused ectopia

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  Kidneys are fused and mostly in flank

  
  
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  Contralateral fused kidney often small, misshapen, and crosses midline (anterior to aorta)

Associated kidney malrotation common with all renal developmental anomalies

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  Anterior renal pelvis, medial lower pole

Associated obstruction and cystic dysplasia common

Color Doppler shows highly variable blood supply

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  Low, multiple, anteriorly oriented renal arteries

Renal tumor: Mimics cross-fused ectopia

Pelvic mass: Mimics pelvic kidney

More likely to have postnatal hydronephrosis, cystic dysplasia, urolithiasis

Horseshoe kidney present in 1:400 (general population)

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  Associated with monosomy X (Turner syndrome)

  
  
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  VACTERL association

Use color Doppler to find renal arteries

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  Ectopic renal arteries lead to ectopic kidneys

Suspect renal development anomalies when renal pelvis is unusually rotated (anteriorly more often than posteriorly)

Horseshoe kidney may be missed if isthmus is thin