Abnormalities of renal position and fusion predispose to infection, hydronephrosis, stone disease, and, in some instances, neoplasia. Although clinical problems associated with these anomalies present infrequently in urologic practice, an understanding of the deviations from standard urologic techniques required to address them is important.

The ureteral bud branches from the wolffian duct and extends toward the metanephric blastema during the fourth and fifth weeks of gestation. The ureteral bud induces the metanephric blastema to form the functioning kidney. The exact mechanism of renal ascent is not known, but during normal development, the kidneys ascend and rotate. The renal pelvis rotates from its initial anterior position 90 degrees toward the midline until it reaches its final medial position. Migration and rotation occur simultaneously between the fourth and eighth or ninth weeks of gestation. The blood supply to the kidney is derived from successively higher levels of the aorta and its branches during ascent.

The most common anomaly of renal position is malrotation: incomplete rotation of the kidney to its final position. The renal pelvis in a malrotated kidney in general lies anterior to the parenchyma, as opposed to its normal medial location. Simple malrotation of a normally positioned kidney is often an incidental finding. The pyelocaliceal systems of malrotated kidneys are morphologically abnormal, but functionally, they usually drain without impairment. Malrotation occurs occasionally in orthotopically positioned kidneys, and it is commonly observed in ectopic kidneys.

Close approximation of the two proliferating renal blastemas prior to significant ascent is a normal embryologic finding (1). If there is any disturbance of separation of the closely approximated renal blastemas, fusion anomalies of the kidneys may develop.

The most common fusion anomaly is the horseshoe kidney. The horseshoe kidney in general ascends until the upper border of the isthmus is at the level of the inferior mesenteric artery. Horseshoe kidney occurs in 1 in 400 to 1 in 1,800 births (2). There is a male predominance for the condition (3). The fusion in horseshoe kidney almost always occurs at the lower poles; cases of upper pole fusion are recorded rarely (3). The isthmus of the horseshoe kidney lies just below the inferior mesenteric artery at the L4 vertebral level. The blood supply to these kidneys is variable (Fig. 86.1).

Crossed fused ectopia is the second most common fusion anomaly. This abnormality occurs when the developing kidney crosses from one side to the other during its ascent or when the ureteral bud from one side crosses to the contralateral side and induces abnormal development of that metanephric blastema. Crossed ectopia with fusion may occur in a variety of forms (Fig. 86.2). Although crossed ectopia occurs most frequently with fusion, the anomaly may occur without fusion (Fig. 86.3). The diagnosis should be considered in patients with a prenatal diagnosis of solitary kidney or an empty unilateral renal fossa (Fig. 86.4).

An ectopic kidney lies outside of the normal position in the renal fossa. The kidney, in simple ectopia, in general, lies in the ipsilateral retroperitoneal space at a position that is lower than normal (Fig. 86.5). In crossed ectopia, the kidney crosses the midline and is frequently fused to its contralateral mate. The autopsy incidence of renal ectopia is about 1 in 1,000 cases, and the condition often is totally asymptomatic (4). Reviews of renal ectopia show that the left kidney is affected slightly

more frequently than the right. Ectopic kidneys occur bilaterally around 10% of the time, and the most common position of the ectopic kidney is in the pelvis. Pelvic ectopia was reported in about 55% of patients in one series of ectopic kidneys, crossed fused ectopia occurred in 27%, lumbar ectopia occurred in 12%, non-crossed fused ectopia occurred in 5% of patients, and a thoracic kidney was recorded only 1% of the time (4). Rarely, a solitary pelvic kidney occurs. Obstruction or infection in the previously undiagnosed pelvic kidney can be mistaken for other intra-abdominal pathology, such as appendicitis. Furthermore, this kidney suffers the risk of injury during pelvic surgical procedures and on occasion has been reported as an unusual cause of giant hydronephrosis.

Ectopic kidneys are smaller than their orthotopically positioned mates (5). The blood supply to the pelvic kidney, the most common of the ectopic kidneys, is variable. The arterial
supply may arise from the distal aorta or bifurcation, the ipsilateral common iliac, or the hypogastric vessels. In general, the lower the kidney is in its pelvic location, the greater the likelihood that multiple arterial vessels will supply it (6).