Renal Ascent And Ectopia
The adult kidneys are positioned in the lumbar retroperitoneum; however, their development begins in the sacral region of the fetus, where the paired metanephroi appear during the ﬁfth week of development. Their change in position reﬂects a process known as renal ascent, which occurs during the sixth to ninth weeks of gestation. Although its exact mechanism is not well understood, it likely reﬂects rapid growth of the sacral end of the fetus, which leads to a change in the relative position of the kidneys.
As they ascend, the kidneys are vascularized by a series of transient branches from the dorsal aorta. In most individuals, all but the ﬁnal pair of arterial branches degenerate, leaving one major renal artery extending to each kidney. In some individuals, however, the earlier branches of the aorta may fail to degenerate, resulting in aberrant persistence of an extrahilar (polar) artery. (This condition is so common that it is considered a normal anatomic variant, rather than a congenital defect, and is thus described in more detail in the section on normal renal vasculature. See Plate 1-12.)
Renal ectopia results from abnormalities during the process of ascent. If a kidney fails to ascend at all, it is known as a pelvic kidney. If it undergoes incomplete ascent, it is known as a lumbar kidney. If it ascends too far and reaches the thorax, it is known as a thoracic kidney. Finally, if a kidney ascends to the contralateral side, it is known as a crossed ectopic kidney.
Pelvic Kidney. Pelvic kidney is the most common form of renal ectopia, with an estimated incidence of 1 in 2,200 to 3,000. A common view is that this pattern of ectopia represents persistence of fetal vasculature, which prevents the normal ascent of the kidney. Other possible causes, however, encompass intrinsic defects in the ureteric bed or metanephric mesenchyme.
The vessels that supply a pelvic kidney typically arise from the iliac vessels or the most inferior portion of the abdominal aorta. The ureter is short and often prone to reﬂux. The hilum may be directed ventrally, rather than medially, because of a failure of normal rotation (see Plate 2-7).
Most patients with pelvic kidneys are asymptomatic, and the abnormality is either incidentally noted or never discovered. A subset of individuals, however, may become symptomatic secondary to the development of an upper urinary tract obstruction, nephrolithiasis, or urinary tract infection. These sequelae occur if malrotation results in high insertion of the ureter or a vessel crossing the collecting system, since these can both cause urinary stasis and outﬂow obstruction.
Thus, patients with pelvic kidneys may occasionally have abdominal pain, hematuria, or a palpable abdominal mass. The pelvic kidney is then detected on further workup with ultrasound or computed tomography (CT). The treatment strategies for nephrolithiasis and ureteropelvic junction obstructions in patients with pelvic kidneys are largely the same as those used for patients with normally positioned kidneys; however, the abnormal course of the ureter may make ureteroscopy difﬁcult, and there is a risk of damaging abnormally positioned vessels and nerves.
A pelvic kidney is more susceptible to injury from blunt trauma than a normally positioned kidney because the latter is (1) surrounded by a large, protective cushion of perinephric and retroperitoneal fat, (2) protected posteriorly by the ribs, and (3) located at a safe distance from the anterior abdominal wall and narrow pelvis. As a result, patients known to have pelvic kidneys should be encouraged to wear appropriate protection if they participate in contact sports.
Thoracic Kidney. Thoracic kidney is the rarest form of all renal ectopias, with an estimated incidence of 1 in 13,000 according to one autopsy series. Unlike pelvic kidneys, thoracic kidneys appear to be more common in males. An ectopic thoracic kidney may be located predominantly above or below the diaphragm. In either case, the intrathoracic portion passes through the lum- bocostal triangle (foramen of Bochdalek) and is covered by a thin membrane of the diaphragm. As a result, the kidney does not reside within the pleural space; however, the adjacent region of the lung may be hypoplastic. Thoracic kidneys are more commonly seen on the left side, possibly because the liver blocks excessive ascent of the right kidney.
It is unclear why thoracic kidneys occur, but two possibilities include delayed closure of the diaphragm, as well as excessive and accelerated renal ascent.
The vessels that supply a thoracic kidney usually arise from the abdominal aorta at a higher position than normal. The ureter is appropriately increased in length and inserts normally into the bladder. Renal rotation is usually complete, and thus the renal pelvis has a normal medial orientation. Both the ureter and renal vessels pass through the lumbocostal triangle as they course from the kidney to the abdomen. The associated adrenal gland typically remains in its normal position but has been documented in some cases to be associated with the ectopic kidney.
Most thoracic kidneys are asymptomatic, causing neither respiratory nor urinary symptoms. Thus this abnormality often goes undetected unless a patient undergoes imaging for another unrelated reason.
Crossed Renal Ectopia. Crossed ectopia of the kidney is an uncommon condition in which one or both kidneys are found on the contralateral side of the abdomen. The “crossing” of a kidney is evidenced by the path of its associated ureter, which crosses the midline to insert into the opposite side of the bladder.
The embryologic basis for crossed renal ectopia is not known. It has been speculated that during renal development, the ureteric bud may cross the midline to enter the contralateral metanephric mesenchyme. Others have suggested that abnormally positioned vessels, such as the umbilical artery, may obstruct the normal path of an ascending kidney, which then takes the path of least resistance to the contralateral side. Teratogens or genetic factors may also play a role.
The crossed kidney generally lies caudal to the normally positioned kidney and has a ventrally oriented renal pelvis. In 90% of cases, the crossed kidney is fused with the inferior pole of the normally positioned kidney. In about 10% of cases, the two kidneys remain separate and distinct. The renal artery of the ectopic kidney may originate from the iliac artery or from either the lateral or anterior surfaces of the abdominal aorta.
Like pelvic kidneys, crossed ectopic kidneys are usually incidental, asymptomatic ﬁndings but may rarely occu with abdominal pain, hematuria, or other symptoms.