- Crossed ectopy = kidney located on the opposite side of the midline from its ureter.
- In 90% of crossed ectopy, there is at least partial fusion of the kidneys (the remainder demonstrate two discrete kidneys on the same side, crossed-unfused ectopy).
Congenital anomalies of the kidneys include a group of so-called fusion anomalies, in which both kidneys are fused together in early embryonic life. Fusion anomalies of the kidneys can generally be placed into 2 categories: (1) horseshoe kidney and its variants and (2) crossed fused ectopia.
Six variations of crossed fusion are possible. In descending order of frequency, they are:
Inferior ectopia, by far the commonest type. The crossing kidney lies inferiorly. The upper pole of the crossed kidney is fused to the lower pole of the resident kidney. Both pelves are directed anteriorly. Sigmoid or S-shaped kidney. The crossed kidney lies inferiorly with itspelvis directed laterally. The pelvis of the superiorly located resident kidney is directed medially. Unilateral lump kidney. The two kidneys are completely fused to form an irregular lump. Both renal pelves are directed anteriorly. Unilateral disc kidney. The kidneys are fused along their medial border. The renal pelvis of the normal kidney is directed anteromedially. The crossing renal pelvis is directed laterally. Unilateral L-shaped kidney. The crossing kidney lies inferiorly and transversely (rare). Superior ectopia. The kidney crosses the midline and lies superior to the resident kidney. Both renal pelves are anteriorly rotated (rare)
- Symptoms of crossed ectopia are rare. Patients generally present in adulthood with urinary tract infection symptoms or calculi thought to be secondary to stasis. Other complications include a high insertion of the ureter into the renal pelvis leading to ureteropelvic junction obstruction, as well as trauma to the inferiorly located kidney. The blood supply to the crossed kidney is usually anomalous. The ureters usually open normally in the bladder and are not ectopic.
- In a crossed fused renal ectopic kidney, complications such as nephrolithiasis, infection, and hydronephrosis approaches 50%.
- Readily detected on conventional urography.
- CT and US very useful.
- On US, identified by characteristic anterior or posterior "notch" between the two fused kidneys.