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 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.