Renal Fusion
Development of the definitive adult kidney (metanephros) begins when the
two ureteric buds invade the paired masses of metanephric mesenchyme (see Plate
2-2). Through a process known as branching morphogenesis, which depends on
reciprocal signals between each ureteric bud and its associated mass of
metanephric mesen- chyme, the ureteric buds give rise to the ureters, renal
pelves, calices, and collecting ducts, whereas the meta- nephric mesenchyme
gives rise to nephrons.
Throughout this process, the two
kidneys undergo separate but simultaneous development. As they undergo
structural maturation, they also ascend in position (see Plate 2-5) from the
sacral end of the fetus to the lumbar retroperitoneum.
Renal fusion can occur secondary to
abnormalities in renal ascent, as in crossed renal ectopia (see Plate 2-6), or
vice versa. In the former case, the superior pole of the crossed kidney ends up
situated near the inferior pole of the normally positioned kidney, leading to
fusion. In the latter case, a primary fusion event occurs, which then results
in ectopia.
Horseshoe Kidney
Horseshoe kidney, the most common type
of renal fusion, occurs when an isthmus consisting of either fibrous tissue or
functioning renal parenchyma connects the two kidneys in the midline. The
overall incidence of this abnormality is estimated to about 1 : 600, with males
affected twice as often as females. The horseshoe kidney is especially common
in patients with chromosomal disorders, such as trisomy 18 and Turner syndrome.
The horseshoe kidney is thought to
result from fusion of the two metanephroi during the sixth week of development,
while they are still near one another at the sacral end of the fetus. It is
believed that abnormal lateral flexion of the embryo may dislocate one kidney
more medially, approximating it near the contralateral kidney and causing a
fusion event. Ascent of the fused horseshoe kidney is prematurely terminated
when the isthmus reaches the level of the inferior mesenteric artery, beyond
which it is unable to cross.
The horseshoe kidney is typically
situated in the lower lumbar region, below the normal position of the mature
kidneys. The isthmus almost always connects the lower poles of the two fused
kidneys, although in rare cases it may join the upper poles instead. The
isthmus is usually situated anterior to the aorta and the inferior vena cava
but may rarely be situated between these vessels or posterior to them both.
Both renal pelves are usually oriented ventrally or ventromedially, secondary
to a failure of rotation. The ureters insert normally into the bladder but are
prone to reflux. In about 10% of patients, ureteral duplication is seen (see
Plate 2-23). The renal vasculature is variable. The upper poles of each kidney
are usually perfused by one or more ipsilateral branches of the aorta, whereas
the lower poles and isthmus may receive their own branches from the aorta,
iliac, or sacral arteries.
A horseshoe kidney rarely causes
symptoms and is typically an incidental finding. A minority of patients,
however, develop ureteropelvic junction obstructions, nephrolithiasis, or
urinary tract infections. These com plications may result from the abnormally
high ureteropelvic junction or kinking of the ureters as they cross over the
fused isthmus. In addition, some patients may experience traumatic injury to
the isthmus due to its midline position anterior to the spine. A smaller subset
of patients may present during childhood with Wilms tumor, as horseshoe kidney
increases the risk.
A small subset of patients with
horseshoe kidney have concomitant abnormalities in other organ systems.
Associated genital abnormalities include hypospadias and undescended testes in
males, or vaginal septation and bicornuate uterus in females. Other associated
abnormalities include neural tube defects and cardiac ventriculoseptal defects.
Lump/Cake Kidney
The “lump” or “cake” kidney is a renal
fusion variant in which there is complete merging of the two kidneys, such that
two separate masses are no longer distinguishable. This anomaly reflects a very
early, complete fusion of the metanephroi. The symptoms, risks, and treatment o
tions are largely the same as for horseshoe kidney.
