Parapelvic Renal Cyst
A parapelvic cyst sits in or projects into the renal sinus and characteristically mimics hydronephrosis on ultrasound — the key skill is using CT urography/delayed images to show it does not communicate with or dilate the collecting system.
The big picture
Parapelvic (renal sinus) cysts lie in or extend into the renal sinus, the fat-filled space around the collecting system. Because they sit where the pelvis and calyces are, on ultrasound they can look exactly like a dilated, obstructed collecting system — multiple anechoic spaces clustered centrally.
Central cystic 'dilatation' that does not fill with contrast and leaves the calyces undilated on delayed images is a parapelvic cyst, not hydronephrosis — don't treat it as obstruction.
Red flags
Consider a parapelvic cyst — confirm with CT urography/delayed images before treating obstruction.
Prove obstruction first — a parapelvic cyst does not need drainage.
If genuinely obstructed, manage as obstruction — but prove it.
Symptom sorter
The common presentation.
Diagnostic algorithm
Each step answers one question. Tap to expand.
Treatment ladder
Recognise the benign mimic and reassure; intervene only for proven symptomatic disease or genuine obstruction.
Complications
- Rarely true obstruction, infection, or mass effect
- Unnecessary intervention from misdiagnosis as hydronephrosis
- Confirm with delayed contrast imaging before any drainage/stenting
- Reassure; treat only proven symptomatic/obstructive cases
Summary tables
Parapelvic cyst vs hydronephrosis
| Feature | Parapelvic cyst | Hydronephrosis |
|---|---|---|
| Location | Renal sinus cyst | Dilated collecting system |
| Delayed contrast filling | Does NOT fill | Collecting system fills |
| Calyces | Not dilated | Dilated |
| Action | Reassure | Assess/relieve obstruction |
Memory hooks
Parapelvic cyst = renal sinus mimic of hydronephrosis.
Delayed contrast images settle it: cyst doesn't fill; calyces not dilated.
Benign — usually reassure.
Don't drain a 'dilated' pelvis that's really a cyst.
Board traps
Parapelvic cyst mimics hydronephrosis on ultrasound.
Delayed contrast images distinguish them.
Don't treat it as obstruction without proof.
Clinical cases
An ultrasound reports 'moderate hydronephrosis' in an asymptomatic patient with normal renal function. CT urography shows the collecting system fills normally with contrast and is not dilated, while a central cystic space does not fill.
What is the diagnosis and management?
A patient with central renal cystic spaces has flank pain and a rising creatinine; a diuretic renogram shows genuine obstruction.
How does management differ here?