Horseshoe kidneys

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The horseshoe kidney is the most common form of renal fusion. It consists of two distinct functioning kidneys on each side of the midline, connected at the lower poles by an isthmus of functioning renal parenchyma or fibrous tissue that crosses the midline of the body.
Horseshoe kidney occurs in a range from 1 out of 400 live births to 1 out of 800 live births and in over 90% of cases the renal fusion is at the lower poles.
The most common associated finding in horseshoe kidney is ureteropelvic junction (UPJ) obstruction, which occurs in up to 35% of patients.. It causes the majority of problems.
The prevalence of stones in the horseshoe kidney ranges from 20-60%. Stone disease is thought to be due to the associated hydronephrosis or UPJ obstruction that causes stasis of urine.
Urinary stasis and stone disease also predispose the horseshoe kidney to infection. Ascending infection from vesicoureteral reflux is another cause of infection in the horseshoe kidney.
Certain cancers are more frequent in the horseshoe kidney : The incidence of renal cell cancer in the horseshoe kidney is no different from that of the normal kidney. But the incidence of Transitional cell cancer, sarcoma, Wilms tumors, carcinoid tumors also is higher in the horseshoe kidney.

Clinical :
Nearly one third of patients with a horseshoe kidney remain asymptomatic, and the horseshoe kidney is an incidental finding during Sonographic examination. Physical examination may reveal a midline lower-abdominal mass.
Symptoms, when present, usually are due to obstruction, stones, or infection. In children, urinary tract infection is the most common presenting symptom.

Ultrasound :
In the horseshoe kidney, the kidneys lie one on each side of the abdomen but their lower poles are fused by a connecting band of renal tissue, or isthmus, which lies anterior to the aorta and inferior vena cava. The kidneys tend to be rotated and lie with their lower poles medially.
The isthmus joining the two kidneys is often hidden by bowel gas and thus diagnosis of horseshoe kidney is easily missed on sonography, though the kidney may be noted to have more vertical axes than normal. The isthmus should be suspected when the axis of the kidney is distorted and the lower poles of the kidneys are hard to image sonographically. It should also be suspected on longitudinal scans of the aorta when an oval hypoechoic mass is seen anterior to the aorta. It appears as a variably thick band of renal tissue (or rarely as a thin fibrous band) extending from both lower poles to connect anterior to the aorta below the level of the inferior mesenteric artery.
The sonographer should be suspicious of a horseshoe kidney when die lower poles of the kidneys cannot be clearly outlined, particularly when both kidneys look a little smaller than expected for age.
Fusion can take other forms, including an L shape, where one kidney lies horizontally across the midline; crossed ectopia, where both kidneys lie on the same side; H-shaped fusion of the hilar regions; and complete fusion to form a 'cake'-shaped solitary kidney.

Differential :
It may be difficult to visualize the isthmus due to bowel gas anterior to it but a horseshoe kidney should always be suspected when the operator is unable to identify the lower poles of the kidneys confidently. When the isthmus can be seen, it is important not to confuse it with other abdominal masses, such as lymphadenopathy. CT is occasionally performed because of this but normally clarifies the findings.

Treatment :
* Medical therapy: The horseshoe kidney is susceptible to medical renal disease. These diseases, if present, are treated as indicated.
* Surgical therapy: Surgical treatment is based on the disease process and standard surgical indications. The anomalous vascular supply to the kidney should be kept at the forefront of the surgeon's mind when planning the surgical approach.
Obstruction of the UPJ is usually treated with open pyeloureteroplasty or ureterocalicostomy.
Kidney stones can be treated with ESWL, endoscopy, or open surgery.

Outcome and prognosis :
The presence of the horseshoe kidney alone does not affect survival. As mentioned previously, the horseshoe kidney does have a higher propensity to become diseased. Therefore, survival is dependent on the disease process that the affected horseshoe kidney may harbor.

Reference :
* 1 Cho JY, Lee YH, Toi A, Macdonald B. Prenatal diagnosis of horseshoe kidney by measurement of the renal pelvic angle. Ultrasound Obstet Gynecol. 2005 Jun;25(6):554-8.
* 2 Kitamura H, Tanaka T, Miyamoto D, Inomata H, Hatakeyama J. Retroperitoneoscopic nephrectomy of a horseshoe kidney with renal-cell carcinoma. J Endourol. 2003 Dec;17(10):907-8.
* 3 Weizer AZ, Silverstein AD, Auge BK, Delvecchio FC, Raj G, Albala DM, Leder R, Preminger GM. Related Articles, Links Determining the incidence of horseshoe kidney from radiographic data at a single institution. J Urol. 2003 Nov;170(5):1722-6.
* 4 Neville H, Ritchey ML, Shamberger RC, Haase G, Perlman S, Yoshioka T. The occurrence of Wilms tumor in horseshoe kidneys: a report from the National Wilms Tumor Study Group (NWTSG). J Pediatr Surg. 2002 Aug;37(8):1134-7.
* 5 Strauss S, Dushnitsky T, Peer A, Manor H, Libson E, Lebensart PD. Sonographic features of horseshoe kidney: review of 34 patients. J Ultrasound Med. 2000 Jan;19(1):27-31.
* 6 Sanghvi KP, Merchant RH, Gondhalekar A, Lulla CP, Mehta AA, Mehta KP. Antenatal diagnosis of congenital renal malformations using ultrasound. J Trop Pediatr. 1998 Aug;44(4):235-40.
* 7 Kapur VK, Sakalkale RP, Samuel KV, Meisheri IV, Bhagwat AD, Ramprasad A, Waingankar VS. Association of extrarenal Wilms' tumor with a horseshoe kidney. J Pediatr Surg. 1998 Jun;33(6):935-7.
* 8 Lampel A, Hohenfellner M, Schultz-Lampel D, Lazica M, Bohnen K, Thurof JW. Urolithiasis in horseshoe kidneys: therapeutic management. Urology. 1996 Feb;47(2):182-6.
* 9 Boullier J, Chehval MJ, Purcell MH. 9 Removal of a multicystic half of a horseshoe kidney: significance of preoperative evaluation in identifying abnormal surgical anatomy. J Pediatr Surg. 1992 Sep;27(9):1244-6.
* 10 Banerjee B, Brett I. Ultrasound diagnosis of horseshoe kidney. Br J Radiol. 1991 Oct;64(766):898-900.
* 11 Morita Y, Kumagai M, Kumagai A, Yamada S, Shinada M. [A report of renal cell carcinoma in a horseshoe kidney] Rinsho Hoshasen. 1990 Sep;35(9):1093-6.
* 12 Mendelson DS, Mitty HA, Janus C, Cohen BA. Horseshoe kidney mimicking adenopathy. Urol Radiol. 1983;5(2):121-2.
* 13 Jenss H, Schulze K, Klott KJ. Horseshoe kidney--is the diagnosis possible by ultrasound? Rofo. 1980 Jul;133(1):71-4.