Kidneys form at sacral level and ascend to L1 by term, renal pelvis initially directed anteriorly but rotates 90 degrees medially as it ascends.

Abnormal ascent and rotation of metanephric blastema after induction by the ureteric bud.

Isthmus or midline junctional zone of a horseshoe kidney may contain functioning renal tissue or fibrotic nonfunctional tissue.

Fusion at lower pole 90% and upper pole 10%.

Horseshoe kidney incidence is 1 in 400 births, most common fusion anomaly and is more common in males.

Crossed fused ectopic kidney is less common.

Associated urological abnormalities include;

Vesicoureteral reflux 20-50%

Contralateral renal dysplasia

Cryptorchidism

Hypospadias

Syndromes with renal ectopia:

Turner syndrome

VACTERL

Trisomy 18, E

Complications:

Obstruction-UPJ- 35%

Stone formation- 20-60%

Injury

Infection- 27-40%

Increased risk of Wilm,Carcinoid tumors,Renal cell carcinoma, Transitional carcinoma and sarcoma.

One third of patients with horseshoe kidney are asymptomatic throughout life.

TAKE HOME MESSAGE: The lower pole ureter of crossed fused ectopic kidney inserts into trigone on contralateral side.