Renal Ectopia and Renal Fusion in Patients Requiring Abdominal Aortic Operations

Embryology

Three renal precursors, called the pronephros, the mesonephros, and the metanephros, develop from the mesoderm during early gestation. The first two of these early structures regress, but around the 28th day of gestation, the metanephros differentiates into the fetal kidney, which forms in the sacral region. In this primitive structure, a pair of ureteric buds develops and interacts with the metanephric mesenchyme to form a pair of fetal kidneys. During this process, the collecting system develops from each ureteric bud, and the nephrons develop from the metanephric mesenchyme through reciprocal interaction. Between the 6th and 9th weeks of gestation, the two kidneys rotate and ascend from the pelvis to their usual anatomic position in the right and left side of the lumbar region, just below the adrenal glands. As the kidneys ascend, they receive their blood supply through a series of paired arteries arising from the primitive aorta, which sequentially disappear as the kidneys migrate. The result is usually a single pair of renal arteries, one for each kidney. Each renal segment is usually supplied by a single end-arterial branch originating from the main renal artery, but between 15% and 30% of kidneys have more than one renal artery.

If the embryonic kidneys fuse during development, normal rotation and ascent are interrupted, leading to a variety of anomalies ranging from positional disorders alone to a combination of fusion, ectopia, and malrotation problems. Although isolated renal artery anomalies are usually easily handled by the experienced vascular surgeon, renal parenchymal developmental abnormalities are less common and are often associated with anomalies of the collecting system as well as the vasculature. The combination can pose a technical challenge to the vascular surgeon at the time of aortic reconstruction, especially during repair of abdominal aortic aneurysm (AAA), and may be particularly difficult to manage if it is encountered unexpectedly, because adequate exposure may be difficult through the traditional anterior transperitoneal approach to the infrarenal aorta. The risk of injury to the urinary collecting system may be increased because of the anterior location of the renal mass, and ligation of anomalous renal arteries can lead to renal infarction. For these reasons, preoperative imaging, when feasible, is valuable at the outset to allow the preoperative detection of associated renal anomalies and to plan the most advantageous choice of incision and patient positioning.

Anatomic Subgroups of Renal Ectopia

Developmental anomalies of the kidneys and upper urinary tracts may be categorized according to those leading to problems of number, ascent, rotation, or form and fusion of the renal parenchymal mass ( Box 1 ). All may be associated with abnormalities in the renal vasculature as well as in the collecting system, but the problems with renal form and fusion are typically the most troublesome for the vascular surgeon during AAA repair. Anomalies of renal ascent or rotation alone rarely seriously interfere with traditional approaches to aortic reconstruction. In contrast, anomalies of fusion are usually associated with varying degrees of ectopia and malrotation, which can require modification of standard surgical techniques.