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
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.
