The kidney pelvis collects urine from the kidney’s filtering units before it travels down the ureter to the bladder. While most people have a renal pelvis that is tucked entirely within the kidney’s fatty center, some individuals have an anatomical variation where the pelvis extends partially outside the kidney tissue. This variant is known as an extrarenal pelvis (ERP) and is found in up to 10% of the population. Because this structure is positioned differently, it often appears enlarged on medical imaging, leading to concern over whether this finding is a sign of danger. This article clarifies what an extrarenal pelvis is and distinguishes it from serious kidney conditions.
Understanding the Extrarenal Pelvis (ERP)
The standard renal pelvis, sometimes called an intrarenal pelvis, is largely enclosed by the kidney’s protective tissue and fat, which naturally limits its ability to expand. This confined arrangement means the pelvis is not typically visible as a large fluid-filled sac on an ultrasound unless there is a true collection of excess fluid. The extrarenal pelvis, by contrast, projects beyond the kidney’s central sinus and into the surrounding space. This outward projection removes the surrounding restrictive tissue, allowing the pelvis to be naturally larger and more distensible.
Because of its exposed location, the extrarenal pelvis appears as a prominent, fluid-filled mass just outside the kidney on imaging scans. This appearance can easily be misinterpreted as a dilation caused by a blockage, which is why it often raises suspicion initially. However, the ERP is simply a structural difference and is considered a normal, benign anatomical variant.
Differentiating ERP from Urinary Obstruction
The main reason an extrarenal pelvis is a concern is its visual similarity to hydronephrosis, a condition where the kidney swells due to a blockage of urine flow. The key difference between the two is that an ERP does not involve a urinary obstruction, meaning urine flows freely and normally down the ureter. In a true obstruction, the blockage causes urine to back up, leading to increased pressure that dilates the entire collecting system, including the minor and major calyces inside the kidney.
This pressure buildup makes hydronephrosis dangerous, as it can compress the kidney’s filtering tissue, known as the renal parenchyma, eventually causing thinning and permanent damage. A distinguishing feature of a benign extrarenal pelvis is the absence of this destructive back pressure. On imaging, an ERP shows a large pelvis but does not exhibit the characteristic signs of obstruction, such as dilated calyces, thinning of the kidney’s outer cortex, or dilation of the ureter. Most individuals with an extrarenal pelvis are entirely asymptomatic and have normal kidney function. In contrast, a urinary obstruction often causes symptoms such as flank pain, infection, or a decline in overall kidney function.
Diagnosis and Long-Term Monitoring
An extrarenal pelvis is most commonly discovered incidentally when a person undergoes an abdominal ultrasound or CT scan for an unrelated reason. The initial imaging may flag the enlarged pelvis as suspicious for hydronephrosis, prompting doctors to seek further confirmation. Because the visual appearance can be misleading, specialized tests are often performed to confirm that the enlarged pelvis is not obstructed and that kidney function is preserved.
To definitively rule out a blockage, a dynamic imaging test such as renal scintigraphy, often using a MAG-3 scan, may be ordered. This nuclear medicine test uses a small amount of radioactive tracer injected into the bloodstream to measure how quickly the kidney filters and drains the tracer, providing a direct assessment of urine flow. If the kidney drains the tracer rapidly and efficiently, it confirms the finding is an extrarenal pelvis and not a functional obstruction.
For an extrarenal pelvis that is confirmed to be non-obstructive, no specific intervention or treatment is necessary. The standard management is typically watchful waiting, which may involve periodic follow-up ultrasounds to ensure the size remains stable over time. Long-term monitoring also includes routine checks of renal function and urinalysis to detect any signs of infection or a change in status. If the size remains stable, the kidney tissue appears healthy, and function tests remain normal, the extrarenal pelvis is considered a stable, benign anatomical variant.