Crossed Fused Renal Ectopia

Crossed fused renal ectopia is a congenital anomaly in which both kidneys lie on the same side of the body. The ureter of the ectopic kidney crosses the midline to insert into the bladder in its normal position on the opposite side. The two kidneys are joined (fused) in 85 to 90 percent.

Incidence is about 1 in 1,000 to 7,500, with a male predominance of 2:1 and a left-to-right ectopia ratio of about 3:2.

It arises from deviation of the ureteric bud, which gets drawn across the midline by an abnormally positioned metanephric blastema or due to a vascular abnormality (an arterial hypothesis).

Types (McDonald and McClellan):

• inferior crossed ectopia (the ectopic kidney lies below the normal one; the commonest, 85 percent)
• sigmoid or S-shaped kidney
• lump or cake kidney
• L-shaped kidney
• disc kidney
• superior ectopia

The ectopic kidney is typically malrotated and has an anomalous blood supply from the aorta, iliac or even the inferior mesenteric arteries.

It is associated with:

• anorectal malformations
• cardiovascular defects (in 30 percent)
• skeletal anomalies (scoliosis, hemivertebrae)
• genital anomalies (undescended testis, hypospadias, septate vagina, unicornuate uterus)
• VACTERL association

Complications: hydronephrosis from ureteropelvic junction obstruction or vesicoureteral reflux, stones, infection and rarely tumour.

• Asymptomatic in 70 to 90 percent; found incidentally on imaging for other reasons.
• Abdominal or flank pain, a palpable abdominal mass, or recurrent urinary tract infection and haematuria.
• Hydronephrosis, stones and renal colic; hypertension in some.
• Antenatal diagnosis of an abnormally placed kidney, or an absent kidney in the contralateral renal fossa.
• Associated anomalies (anorectal malformation, cardiac, skeletal and genital abnormalities) identified at birth.

• Ultrasound: absence of a kidney in the renal fossa on one side with two kidneys on the contralateral side, a mass in the pelvis or lower abdomen, and hydronephrosis.
• CT or MR urography: demonstrates fusion, pelvis orientation, vascular supply and the ureter crossing the midline to its normal insertion.
• DMSA and MAG3 renography: function and drainage.
• Voiding cystourethrogram for reflux; cystoscopy shows a normal ureteric orifice position, and the ureteric orifice of the ectopic kidney enters on the contralateral side of the trigone.
• Screening for associated anomalies: spine X-ray, echocardiography, and a genital examination (and pelvic ultrasound in girls).
• Urine culture, blood pressure and renal function.

• Asymptomatic crossed fused ectopia: no treatment; follow up with ultrasound, blood pressure and renal function every 1 to 2 years; prompt treatment of infections; avoid contact sports with unprotected kidney in some cases.
• Hydronephrosis or ureteropelvic junction obstruction: pyeloplasty (dismembered, laparoscopic or open) with reduction of the dilated pelvis, performed when there is documented obstruction (reduced split function, differential function below 40 percent, growing hydronephrosis, symptoms or infections) after MAG3 with diuretic.
• Vesicoureteral reflux: antibiotic prophylaxis (trimethoprim 2 mg/kg at night) and endoscopic injection or ureteral reimplantation for breakthrough infections.
• Stones: shock wave lithotripsy, ureteroscopy with laser lithotripsy or percutaneous nephrolithotomy, taking account of the ectopic location and anomalous vascularity (CT angiography beforehand); prevention with fluids and metabolic evaluation.
• Tumours (Wilms tumour, renal cell carcinoma): nephron sparing surgery or nephrectomy of the affected moiety, with care to preserve the vascular supply to the other.
• Surgical operations in the abdomen and pelvis, such as aortic surgery or caesarean delivery, require preoperative imaging, since the ectopic kidney may lie in the surgical field and has aberrant vessels.
• Genetic counselling and screening of relatives is not routinely needed; family studies show a low recurrence risk.
• Prognosis is good with function preserved; lifetime follow up is based on symptoms and the presence of complications.