Finding any abnormality on a kidney scan can be concerning, but most renal cysts are benign sacs that rarely cause problems. Parapelvic cysts are a specific type of simple renal cyst, generally not dangerous, and often found incidentally during unrelated imaging. Attention is only required in rare circumstances when their unique location leads to complications by interfering with the kidney’s plumbing system.
What Exactly Is a Parapelvic Cyst?
A parapelvic cyst is a fluid-filled sac located within the renal sinus, the central cavity of the kidney. This sinus contains the renal pelvis, major blood vessels, and lymphatic structures. The term “parapelvic” means “next to the pelvis,” referring to its close proximity to the renal pelvis, the funnel-like structure that collects urine before it passes to the ureter.
Parapelvic cysts differ from simple renal cortical cysts, which are far more common and reside on the outer surface of the kidney. The parapelvic cyst’s location near the kidney’s drainage system is what makes it a potential concern, unlike cortical cysts which typically cause no issues.
The term is sometimes used interchangeably with “peripelvic cysts.” Regardless of the exact origin, the defining characteristic is the cyst’s location deep inside the kidney’s center, near the collecting system and major blood vessels. They are simple cysts, lacking complex features like solid components or thick walls that would suggest malignancy.
Assessing the Potential for Harm
A parapelvic cyst becomes problematic by growing large enough to exert pressure on surrounding structures. Situated in the restricted space of the renal sinus, expansion threatens the kidney’s internal plumbing. The most common complication is the physical compression of the renal pelvis or the calyces, which collect urine from the kidney tissue.
If the cyst compresses the collecting system, it impedes urine flow out of the kidney, causing hydronephrosis. This obstruction causes urine to back up and swell the kidney, potentially leading to progressive damage over time. The severity of harm relates directly to the cyst’s size and its impact on urine drainage.
Another issue arises from the cyst’s proximity to the renal blood vessels, specifically the arteries. Compression of these vessels can sometimes trigger the activation of the renin-angiotensin-aldosterone system. This hormonal pathway regulates blood pressure, and its activation can lead to renin-mediated arterial hypertension.
Less frequent risks include developing a secondary infection within the stagnant urine or the cyst itself, which can lead to pyelonephritis. Hematuria, or blood in the urine, can also occur if the cyst ruptures or causes local irritation.
Symptoms and Methods of Detection
The majority of parapelvic cysts remain small and asymptomatic, often discovered incidentally during imaging for other medical reasons. When a cyst grows large enough to cause complications, a person may begin to experience noticeable symptoms linked to urinary tract obstruction or vessel compression.
The most common symptom prompting investigation is dull, persistent flank or back pain, resulting from increased pressure due to hydronephrosis. Other signs of complication include recurrent urinary tract infections, signaling poor drainage, and hematuria (blood in the urine). New onset of high blood pressure may also indicate a large, compressive parapelvic cyst.
Initial detection often occurs via a standard abdominal ultrasound, which reveals a fluid-filled mass in the kidney’s center. However, the cyst’s appearance can closely mimic hydronephrosis, making it difficult to distinguish between a cyst and an enlarged, urine-filled renal pelvis.
To confirm the diagnosis and rule out actual obstruction, a computed tomography (CT) scan is often necessary. On a CT scan with intravenous contrast dye, the contrast fills the renal pelvis but does not enter the cystic mass. This confirms the mass is a non-communicating cyst rather than a dilation of the urine-collecting system.
Monitoring and Intervention Strategies
For most patients with an asymptomatic parapelvic cyst, the standard management is conservative observation. This involves regular follow-up imaging, typically with ultrasound or CT, every six to twelve months to monitor for changes in size or characteristics.
Intervention is reserved for cysts that become symptomatic or cause measurable functional damage. Treatment criteria include persistent flank pain, documented hydronephrosis causing a decline in kidney function, or a confirmed infection. The goal is to decompress the cyst and relieve pressure on surrounding structures.
The least invasive procedure is percutaneous aspiration, where a needle is guided by imaging to drain the fluid. To reduce recurrence, a sclerosing agent, such as ethanol, is often injected into the empty cavity to cause the walls to scar and collapse. The cyst may still re-accumulate fluid, sometimes requiring repeated aspirations.
If the cyst is large, complex, or recurs, a more definitive surgical procedure may be considered. Laparoscopic deroofing is a minimally invasive technique where the cyst is surgically opened, allowing the fluid to drain permanently. Surgery is effective but is reserved for patients whose symptoms or kidney function decline warrants the greater risk.