Horseshoe Kidney

Horseshoe kidney is the commonest renal fusion anomaly, found in about 1 in 400 to 600 people. Males are affected twice as often as females. The lower poles (95 percent) or, rarely, the upper poles of the two kidneys are joined across the midline by a parenchymal or fibrous isthmus.

The isthmus lies anterior to the aorta and inferior vena cava, just below the origin of the inferior mesenteric artery, which stops the ascent of the kidneys. The kidneys are therefore low lying and malrotated (the renal pelves face anteriorly). The ureters cross anterior to the isthmus and enter the pelvis high, so drainage is impaired.

Blood supply is variable, with multiple renal arteries from the aorta, iliac arteries and inferior mesenteric artery in about 30 percent.

It is associated with Turner syndrome (about 7 percent of girls with Turner syndrome), trisomies 18 and 21, vertebral and cardiac anomalies, hypospadias and cryptorchidism, anorectal and neural tube defects.

Complications:

• Ureteropelvic junction obstruction (about 30 percent) and hydronephrosis.
• Stones (20 percent).
• Infection and vesicoureteral reflux.
• Trauma (the isthmus lies over the spine).
• Malignancy: increased risk of Wilms tumour (about 2-fold), renal carcinoid tumour, and transitional cell carcinoma.

• Usually asymptomatic (about 70 percent); found incidentally on imaging or at autopsy.
• Flank or abdominal pain (often a dull ache), pain on hyperextension of the spine (Rovsing sign, from compression of the isthmus), nausea.
• Recurrent urinary tract infection, haematuria, renal colic from stones.
• A palpable midline abdominal mass in thin patients, sometimes pulsatile.
• Hydronephrosis from pelviureteric obstruction, with antenatal detection by ultrasound.
• Presentation with hypertension (rare).
• Associated anomalies noted on examination (Turner features, cardiac murmur, hypospadias).

• Ultrasound: kidneys lower than normal with a midline isthmus anterior to the great vessels, lower poles pointing medially; hydronephrosis, stones and cysts.
• CT with contrast (including CT angiography or CT urography) and MR urography define the isthmus, the vascular supply (important before surgery) and the collecting system.
• Nuclear renography with diuretic (MAG3 with furosemide 1 mg/kg) to evaluate obstruction and split function.
• Intravenous urography shows low lying kidneys with the calyces pointing medially ("flower vase" sign), medial orientation of the lower pole axis, and ureters passing anteriorly.
• Voiding cystourethrogram if reflux is suspected.
• Karyotype in girls with features of Turner syndrome; screening for associated cardiac and skeletal anomalies in children.
• Urine culture, renal function, and stone analysis.

• Asymptomatic horseshoe kidney without obstruction: no treatment; reassurance, regular follow up with blood pressure, urinalysis, renal function and renal ultrasound every 1 to 2 years, and prompt treatment of infections.
• Antibiotic prophylaxis for children with hydronephrosis or reflux (trimethoprim 2 mg/kg at night).
• Ureteropelvic junction obstruction with symptoms, decline in function, or recurrent infection: dismembered pyeloplasty (open, laparoscopic or robotic), usually Anderson-Hynes, with lower pole calyx exposure.
• Division of the isthmus (isthmusectomy) and anterior transposition of the pelvis are added in some cases. Function is improved in 70 to 90 percent.
• Endopyelotomy has lower success rates (about 50 percent) because of the high ureteral insertion.
• Stones: the kidneys' abnormal anatomy and slow drainage make stone clearance difficult.
• Extracorporeal shock wave lithotripsy is used for stones below 1.5 to 2 cm, with a lower success rate (the stone free rate about 50 to 60 percent). The patient may need to be positioned prone to bring the stone into focus.
• Ureteroscopy with laser lithotripsy (holmium, 0.6 to 1.0 J at 6 to 10 Hz) for ureteral and pelvic stones, using a flexible scope.
• Percutaneous nephrolithotomy for large stones (above 2 cm), with a puncture through a posterior calyx. Use CT-guided access, as the kidney is more posterior and lower.
• Prophylaxis of stone recurrence: hydration (urine volume above 2.5 L a day), dietary measures, and urine metabolic evaluation.
• Surgery on the isthmus is required only for symptoms or in tumour surgery; it is not routinely divided. Nephrectomy for tumour with partial nephrectomy as possible; Wilms tumour is managed with the standard approach (chemotherapy with vincristine and actinomycin D, and surgery).
• Trauma: horseshoe kidney is more vulnerable to blunt injury (the isthmus over the vertebrae). Non-operative management with bed rest, imaging and monitoring is the rule, as in other renal injuries; angioembolisation for bleeding.
• Surgery in the region (aortic aneurysm repair, retroperitoneal surgery) requires preoperative imaging because of anomalous renal arteries and the isthmus. Kidneys must be preserved.
• Kidney transplantation: the donor with a horseshoe kidney can be a source of two organs if separated at the isthmus, with careful vascular reconstruction.
• Prognosis is generally good, and life expectancy is normal in the absence of complications. Counsel patients to seek care promptly for infection or stone symptoms.