An extrarenal pelvis is a renal pelvis that protrudes from the familiar, bean-shaped indentation in the middle of the kidney. "Bilateral" indicates that both pelvises in both kidneys exhibit this anatomical variation.
The renal pelvis itself is an anatomical structure normally found inside the indentation of the kidney. It is a large tubular funnel into which drains smaller tubular funnels called calyxes. The renal pelvis channels waste fluid from the kidneys to the ureter, which leads to the bladder.
It contains a loosely folded mucus membrane that allows it to expand when there is an excess of urine to be channeled from one organ to the other. When this happens, the renal pelvis distends and protrudes, which can be mistaken for an extrarenal pelvis on occasion.
An extrarenal pelvis is a normal variation from the usual anatomy, and it does not indicate a state of disease. However, renal pelvises do not usually protrude in such a manner, and it may appear as if there is a blockage in the renal pelvis that is preventing urine from emptying, which is a serious medical condition.
An extrarenal pelvis can be distinguished from this potentially life-threatening problem by its presence regardless of how filled the other organs are.