Abnormal renal rotation
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Abnormal renal rotation, also known as renal malrotation, refers to an anatomical variation in the position of the kidneys, in particular to anomalous orientation of the renal hilum. It may occur unilaterally or bilaterally. It is almost always an asymptomatic incidental finding.
Malrotation is rare, perhaps under-reported, and in one autopsy series, a prevalence of ~1 in 2000 was reported 1. It is more common in males.
The renal hilum is normally directed anteromedially. The renal hilum is initially oriented anteriorly, but during its ascent from the pelvis, the kidney rotates 90° along its longitudinal axis to its more typical orientation.
Anomalies in this process can result in 1,2:
incomplete rotation or non-rotation (most common): hilum faces anteriorly, ureters are located laterally
excessive rotation (hyper-rotation): hilum faces posteriorly; renal vessels are located posteriorly
reversed rotation: hilum faces laterally, renal vessels are located anteriorly, ureter is located laterally
sagittal rotation: rotation around its hilum in the sagittal plane, which as yet (c.2015) does not have an embryological explanation; asymptomatic and is virtually always discovered incidentally on imaging 5
on CT urography, the renal pelvis of a non-rotated kidney will project anteriorly 3
malrotation can be demonstrated when the calyces project medially to the renal pelvis
The exact cause of malrotation of the kidneys is unclear, though it has been hypothesized that malrotation may occur if the ureteric bud inserts into an abnormal region of metanephric mesenchyme. Its association with renal ectopia suggests that the process of ascent and rotation are related.
the malrotated kidney is prone to stasis, increasing the risk of urolithiasis
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