Embryology
Horseshoe kidney is the most common renal fusion abnormality of the genitourinary tract, characterized by the anatomic abnormalities of vascular changes, ectopia, and malrotation. It is composed of two distinct functional renal moieties, joined in the midline by an isthmus that varies from a fibrous band to a parenchymatous isthmus with associated calyces. Embryologically, there are two theories about horseshoe kidney development. One, it is thought to result from abnormal migration of the metanephric blastema across the midline during weeks 4–6 of development. The inferior poles then fuse to form the isthmus, and the ascent of the kidney is arrested by the inferior mesenteric artery, preventing normal rotation of the kidney and presenting each renal pelvis anteriorly at a lower lumbar position ( Fig. 16.1 ). Rarely, the isthmus can connect at the upper poles, and an inverted horseshoe kidney can occur. It has also been postulated that horseshoe kidney can result as a teratogenic event involving the abnormal migration of posterior nephrogenic cells that form the isthmus, which could explain the increased incidence of malignancies including Wilms tumor and carcinoid.
Vascular Anatomy
Horseshoe kidney can be classified according to the morphologic appearance of the fusion. The U-shape is formed by the medial fusion of the kidneys in symmetrical position on each side of the vertebrae. Lateral fusion between a vertical and horizontal kidney results in an L-shape with the isthmus lateral to the midline. Horseshoe kidneys are located anywhere along the expected path of renal ascent from the pelvis to the mid-abdomen but is commonly found lower than normal, and the isthmus usually lies anterior to the great vessels, at the level of the third to fifth lumbar vertebrae. The vascular anatomy is variable and can arise from the aorta, the iliac arteries, or the inferior mesenteric artery. A classification system with six basic patterns of arterial supply for each horseshoe segment has been proposed with a single artery supplying every segment. Normal hilar arterial anatomy can be seen in about one-third of cases, with variable arterial anatomy present in greater than two-thirds of cases. The isthmus may have a separate blood supply arising from the aorta, iliac, or inferior mesenteric artery. The renal pelvis lies anteriorly and the calyceal system is oriented medially ( Fig. 16.2 ).
Indications for Surgery
The indications for surgery in the horseshoe kidney are based on the disease process and are similar to indications for anatomically normal kidneys. The incidence of malignancy in patients with horseshoe kidney is the same as in the general population, although there is a twofold increased incidence of Wilms tumor and transitional cell carcinoma of the renal pelvis. Ureteropelvic junction (UPJ) obstruction and vesicoureteral reflux (VUR) are common findings in the horseshoe kidney, as is urolithiasis.
Nephrolithiasis occurs more commonly in the horseshoe kidney, at an incidence of 21% to over 60%. It is postulated that the horseshoe kidney is more susceptible to stone formation because of several factors. The calyceal anatomy, anteriorly located renal pelvis, abnormal course of the ureter over the isthmus, and the occasional high-inserting ureter all favor urinary stasis, leading to increased risk for stone formation.
Shock-Wave Lithotripsy
Shock-wave lithotripsy (SWL) is the preferred first-line management of small stones in the horseshoe kidney. The patient is placed in the supine position and the stone is localized at the focal point.
If the stone cannot be localized, because of body habitus or projection of the calculus over the spine, it may be helpful to place the patient in the prone position. Insertion of a retrograde catheter with instillation of radiopaque contrast may aid in stone localization in difficult cases. Additionally, dependent drainage in the horseshoe kidney may impair the passage of stone fragments and necessitate retrograde placement of a ureteral stent to aid in stone passage. The presence of untreated obstruction or hydronephrosis precludes the use of SWL.
Percutaneous Approach and Ureteroscopy
Nephrolithotomy
The operating table should be configured to permit C-arm fluoroscopy in more than one plane. The patient is initially positioned in the dorsal lithotomy position for cystoscopy and insertion of an open-ended ureteral catheter or ureteral access sheath into the renal pelvis to allow the use of air or contrast to opacify the collecting system to aid in the identification and targeting of the involved calyces. Direct vision of the targeted calyx with a flexible ureteroscope can also be utilized to confirm needle placement into the appropriate location. A Foley catheter is placed at the end of the cystoscopic portion. The patient is then positioned prone on the operating table with the aid of a stretcher. Chest rolls are placed, if the table is not equipped with such. The extremities and torso are padded. The flank is prepared and draped with a nephrostomy drape. It is also acceptable to start the operation with the patient in the prone position on a split-leg operative table without retrograde access first. In those cases, the genitals are prepped into the field in order to facilitate access to the lower urinary tract.
The procedure begins with the injection of contrast medium. The calyx is then targeted with an 18-gauge nephrostomy needle using the Bull’s eye or triangulation techniques. It is important to obtain upper pole puncture because of the high-riding insertion of the ureter into the renal pelvises and to decrease the risk of bowel perforation. The access tract is more posterior and vertical when targeting the upper pole calyx in a horseshoe kidney ( Fig. 16.3 ). This compensates for the incomplete rotation and lower position of the kidney. Dilation is done with Amplatz-type fascial dilators up to 30 French or with balloon dilators such as the NephroMax High Pressure Balloon Dilatation Catheter. The 30-French Amplatz sheath is left in place. Nephroscopy is performed with a 24-French rigid nephroscope, and ultrasonic or pneumatic lithotripsy is used for stone fragmentation. New modalities such as the dual ultrasonic lithotripter (Cyberwand) can provide additional suctioning to improve stone clearance. Residual fragments are extracted with a rigid two-prone grasping forceps via the working port of the nephroscope. Extra long nephroscopes and Amplatz sheaths may be required because of the deeper renal pelvis and its more medial location within the abdomen. After completing the procedure, a 20–24-French nephrostomy tube may be placed. Nephrostomy tubes can be removed 24–48 hours after the surgery depending on stone-free status; an antegrade or retrograde second-look nephroscopic evaluation may be required if significant stone burden remain.
Ureteroscopy
Cystourethroscopy is performed to obtain access to the ureteral orifice. A guidewire, preferably one with a hydrophilic tip (Sensor), is placed with the aid of an open-ended ureteral catheter, and advanced into the renal pelvis. Fluoroscopy aided by retrograde pyelography obtained by injecting radiopaque contrast media through the open-ended catheter is used to guide proper placement. A second safety guidewire is placed alongside the first wire in similar fashion. A dual-lumen ureteral catheter can be used to facilitate placement of the safety guidewire. A ureteral access sheath is placed over one of the wires and advanced to immediately below the UPJ if feasible. The safety wire remains outside of the access sheath to maintain collecting system access. Renoscopy can then be performed using a flexible ureterscope through the access sheath. The lithotripsy or extraction of stones or treatment of tumors can then proceed as with a normal kidney.
Laparoscopic Approach
Laparoscopy can be applied for the excision of renal masses, pyeloplasty, renal cyst unroofing. The general approach is the same for the exposure of the lesion. Preoperative contrast-enhanced imaging such as computed tomography (CT) or magnetic resonance imaging (MRI) should be done to aid in delineating the anomalous vasculature, particularly the vessels supplying the isthmus. In particular, it may be useful to obtain CT or MR angiograms to clearly delineate the vascular pattern as this is crucial for horseshoe kidney surgery. Specifically for pyeloplasty cases, MR urography can be used to simultaneously evaluate the renal function and anatomy.
Access and Exposure
The transabdominal approach is preferred, although the retroperitoneal approach has been described. A Foley catheter is inserted and the patient is positioned in a semilateral decubitus position by rotating the operative side up by about 45 degrees axially. Pneumoperitoneum is initiated. The authors prefer the use of a valveless trocar system such as the AirSeal (SurgiQuest) as this facilitates in maintaining pneumoperitoneum during introduction and removal of instruments and also decrease cautery smoke and condensation on the lens. Three or four trocars are inserted as illustrated ( Fig. 16.4A ). An 11- or 12-mm port is placed at the paraumbilical position for the camera. Two 5- or 10-mm working ports are placed. Bipolar scissors, or any energy device of the surgeon’s choosing, are used to take down Toldt’s line and release the colorenal attachments. Gerota’s fascia is then opened and dissected to expose the area of interest.
