Management of Stones in Horseshoe Kidneys

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Management of Stones in Horseshoe Kidneys

Chandra Shekhar Biyani & Adrian D. Joyce

Department of Urology, St James’ University Hospital, Leeds, UK

Introduction

Renal fusion anomalies can be broadly divided into two distinct types: horseshoe kidneys and cross‐fused ectopia. Horseshoe kidneys are the commonest type of renal anomaly and have been reported to have an incidence of 1 in 400–700 live births in both autopsy and radiographic data [1, 2]. There is a male preponderance for this condition with a male‐to‐female ratio of 2:1. No genetic predisposition is known and horseshoe kidneys are frequently diagnosed incidentally and in isolation for adult patients, but may be part of a syndrome in the pediatric population, where it is estimated that 23% of patients with horseshoe kidney have other severe associated malformations, which may be gastro‐ intestinal, musculoskeletal, neurological, or most commonly urogenital anomalies [3]. There are several syndromes that are commonly linked to horseshoe kidneys including Down’s syndrome, Turner’s syndrome, and Trisomy 18.

Development

Normal kidney development occurs at the fourth week of gestation from the fusion of the ureteric bud and metanephric blastema. The kidney is originally formed in the pelvic location and ascends with the growth of the body, in particular the sacral and lumbar regions during fourth to ninth weeks of gestation. Primitive renal vessels that originate from the pelvic aorta degenerate as the vascular supplies gradually arise from the proximal aorta. As the kidneys ascend through the arterial fork, they may be pushed together too closely and become fused at the lower poles. A more recent theory suggests that fusion may be the result of abnormal migration of the posterior nephrogenic cells which then fuse with the lower poles of the kidneys, forming an isthmus [4]. Notochord is necessary for correct positioning of the metanephric kidney and the axial sonic hedgehog (ASH) gene signal is essential step for kidney positioning along the mediolateral axis. Kidney fusion may occur with disruption of notochord or the ASH gene [5]. The ascent of the fused lower pole kidneys terminates just below the inferior mesenteric artery and oftens lie between the L1 and L4 vertebral levels (Figures 67.1 and 67.2).

Image described by caption. — **Figure 67.1** Coronal view of the horseshoe kidney; three‐dimensional reconstruction with a calculus in the left moiety.

Horseshoe kidneys are located anterior to the aorta and inferior vena cava. The fused lower poles of horseshoe kidneys are bridged by either functional renal tissue or fibrous tissue. As a consequence of the fusion at the lower poles, the renal pelvis fails to rotate medially and consequently remains ventrally located. This results in the ureter exiting the renal pelvis ventrally and more superiorly, and the longitudinal renal axis converges medially [6]. Matusmoto et al. [7] classified horseshoe kidneys into five types: Type A (a), fused at the upper poles; Type A (b), fused at the lower pole; Type B (a), fused by fibrous tissue; Type B (b), fused directly; and Type B (c), fused by mediators. Graves [8] proposed a classification system (Table 67.1) with six basic patterns of arterial supply for each horseshoe kidney segment.

Table 67.1 Arterial vascular pattern of horseshoe kidneys.

Type 1	The arterial supply may follow the normal pattern with a single renal artery supplying the upper, middle, and lower segments.
Type 2	Upper and middle segments may be supplied by a single renal artery with a branch from the aorta entering each lower segment.
Type 3	The arteries to the lower segment arise from the aorta by a common trunk and upper and middle segments are supplied by a single renal artery on either side.
Type 4	All three segments are supplied by separate arteries arising from the aorta.
Type 5	The isthmus may be supplied by arteries arising above or below the isthmus; these may be unilateral or bilateral and may originate from the aorta, independently, or by a common trunk.
Type 6	The fused lower segments may be supplied by arterial branches from the common iliac artery or rarely from the internal iliac artery (hypogastric) or middle sacral artery.

A horseshoe kidney by itself is asymptomatic; however, due to the anatomy of high ureteric insertion, the presence of the isthmus, and the ventral orientation of the renal pelvis such kidneys are more prone to certain complications which include recurrent infections [9], renal stones, pelviureteric junction obstruction, vesicoureteric reflux [10], and renal tumor.

Urolithiasis

Urolithiasis is the commonest complication related to horseshoe kidney and is estimated to be 21–60% [11]. This is likely to be a consequence of impaired urinary drainage leading to stasis and infection predisposing to calculi formation. The treatment of urolithiasis poses a unique challenge due to the variable vascular and renal anatomy and the predisposition to multiple and large staghorn‐like calculi [3]. The abnormal orientation of the renal pelvicalyceal system further increases the potential complexity for stone management (Table 67.2).

Table 67.2 Endourological considerations in horseshoe kidney.

Anatomic variations	Effect of variations	Therapeutic adjustments
Incomplete rotation	Medially directed calyces	May require more medial percutaneous puncture
Abnormally oriented calyces	Acute angle at infundibulum	Difficult or incomplete access to collecting system
Lack of ascent	Low‐lying kidney	Lower puncture may be necessary
Lack of usual pararenal support	Mobile kidney	Percutaneous puncture more demanding
Renal pelvis anterior	Increase distance from skin	Long instruments necessary
Arteriovenous variations	Single renal artery in 30%	Vascular hazards

Minimally invasive intervention for the treatment of urolithiasis in a horseshoe kidney is now widely accepted with good stone‐free results and low complication rates [12, 13]. However, the clinical decision making process must balance the likelihood of stone‐free rates against the intervention risk and potential for complications.

Extracorporeal shock‐wave lithotripsy

Extracorporeal shock‐wave lithotripsy (ESWL) has revolutionalized the treatment of renal stone disease. ESWL has been advocated as the primary treatment modality for stones less than 20 mm [11]. Sheir et al. retrospectively reviewed 198 patients with anomalous kidneys and showed that ESWL is a safe and reliable method of treatment with a stone‐free rate of 72% [14]. This is comparable with the stone‐free rate of a solitary stone, in a normal kidney, at an average of 72.8%; however, the reported range for stone‐free rates in horseshoe kidneys is 33–79% [14, 15].

The anatomy and orientation of a horseshoe kidney presents a challenge to the use of ESWL as the renal pelvis is ventromedially orientated and approximately a third may have a concomitant ureteropelvic junction obstruction. Locating these calculi can be difficult as the vertebrae may act as an obstacle and in some patients there may be associated vertebral anomalies. The skin‐to‐stone distance may also be increased, resulting in difficulty in positioning focal point (Figure 67.2). Prone position and “blast path” may need to be used to target these awkwardly positioned calculi [16]. A better stone‐free result can also be achieved when a second‐generation lithotripter is used [14]. Concerns have also been raised, due to the high ureteric insertion of the ureter, as to whether fragments post‐ESWL are able to pass spontaneously. The most important predictor for failure of ESWL or significant residual calculi needing retreatment is location, stone density, size, and number. Various studies have reported a stone‐free rate of 39–66% (Table 67.3). Ancillary procedures may need to be used pre‐ESWL such as percutaneous nephrostomy, double‐J ureteric stents, or ureteral catheterization and these should be decided on a case‐by‐case basis. It is important to emphasize that there have been no prospective studies comparing the different treatment modalities in this group of patients but ESWL may be beneficial in patients with small calculi.