Non-communicating cysts of varying size with no identifiable renal pelvis or normal parenchyma is MCDK; cysts that connect to a central dilated pelvis are hydronephrosis — the distinction decides surgery.
Orient first
- MCDK is non-functioning and usually involutes; the contralateral kidney is what matters.
- Contralateral vesicoureteric reflux or PUJ obstruction is common.
- MAG3 scintigraphy confirms no function when doubt remains.
Acquire the study
- Renal and bladder ultrasound in transverse and sagittal planes; follow-up scans for involution and contralateral compensatory growth.
The manoeuvre
- Affected kidney: cysts of varying size in mm, whether they communicate, a central dominant cyst or not.
- Parenchyma between cysts: absent or echogenic dysplastic tissue.
- Contralateral kidney: length in cm for age (compensatory hypertrophy), pelvic dilatation.
- Bladder and ureters: dilated ureter, ureterocele.
- Follow-up: involution of the MCDK.
What confirms it
- Non-communicating cysts without a renal pelvis or normal parenchyma; absent function on scintigraphy.
What licenses you to exclude it
- Cysts converging on a dilated central pelvis with preserved parenchyma mean hydronephrosis, not MCDK.
The classic misread
- Calling severe hydronephrosis MCDK because the calyces look like cysts.
- Missing contralateral abnormality.