Renal Rotation And Malrotation
During their normal process of ascent (see Plate 2-5),
the kidneys undergo 90 degrees of medial rotation, such that the renal pelvis
is reoriented from its original ventral position to a final medial position. The
mechanism of normal renal rotation is unknown but has been hypothesized to
represent asymmetric branching of the ureteric bud in the metanephric
mesenchyme. With an increased number of ventrolateral rather than dorsomedial
ureteric bud branches, the metanephric mesenchyme would preferentially
differentiate in a manner that could cause the appearance of rotation.
Renal Malrotation
Renal malrotation is a rare phenomenon that may occur
either in isolation or, more commonly, in combination with renal ectopia (see
Plates 2-5 and 2-6). The true incidence is not well-characterized but, based on
previous reports, is likely in the range of 1 : 500 to 1 : 1500 with an
increased propensity among males.
It is unclear if renal malrotation represents abnormalities
in the asymmetric branching process thought to underpin normal rotation, or
whether it results from other factors. For example, it has been hypothesized
that malrotation may occur if the ureteric bud inserts into an abnormal region
of the metanephric mesenchyme. The association with renal ectopia suggests that
certain processes may interrupt both normal ascent and rotation, or that ascent
itself is important in some way for the normal process of rotation to occur.
In most cases of malrotation, the kidney fails to rotate
at all, leaving the renal pelvis facing ventrally. Less frequently, the kidney
may be only partially rotated, excessively rotated, or rotated in the wrong
direction. Because the renal vessels are not believed to be responsible for
malrotation, but rather twist around the kidneys as they rotate, their course
offers a clue into the direction and degree of malrotation. For example, a
kidney with a laterally directed renal pelvis may have undergone either 270
degrees of medial rotation or 90 degrees of lateral rotation. Likewise, a
kidney with an ventrally directed renal pelvis may have undergone either no
rotation at all or 365 degrees of rotation. In these cases, the path of the
renal vessels allows one to make the distinction, as shown in the plate.
In addition to its association with ectopy, renal malrotation
is usually associated with abnormalities in renal structure. For example, fetal
lobulations are typically prominent over the gross surface. In addition, the
renal pelvis is usually encased with an abnormally thick amount of fibrous
tissue.
In most cases, malrotated kidneys cause no symptoms and
are discovered only as incidental findings. In rare cases, however, patients may
experience symptoms of upper tract obstruction, nephrolithiasis, or urinary tract
infection. These occur if there is urinary stasis or outflow obstruction
secondary to the fibrous encasing of the renal pelvis, a high insertion of the
ureteropelvic junction, or obstruction of the renal pelvis by an overlying renal
vessel. Such symptoms usually consist of nonspecific abdominal, flank, or back
pain, and/or hematuria. The malrotation is then discovered on radiographic
imaging of the abdomen. It is important to rule out the presence of a pelvic
mass, which can rotate and displace the kidney from its normal position.
Most malrotated kidneys do not require definitive
treatment. If significant symptoms persist, however, or if significant
hydronephrosis is present, surgical repair of the renal pelvis and/or
ureteropelvic junction may benecessary.
