Measure the renal pelvis in the transverse plane and grade the calyces, look at the ureter and bladder, then decide between pelviureteric junction obstruction (ureter not seen) and vesicoureteric reflux or a lower obstruction (ureter dilated) — the next test differs.
Orient first
- Antenatally detected hydronephrosis is common; most resolves. Postnatal ultrasound after the first few days of life avoids underestimating dilatation from neonatal dehydration.
- The urinary tract dilatation (UTD) classification uses the anteroposterior renal pelvic diameter, calyceal dilatation, parenchymal thickness and appearance, ureter and bladder — verify the current thresholds.
- A dilated ureter moves the question to the ureterovesical junction or the bladder: reflux, a primary megaureter, a ureterocele or posterior urethral valves in a boy.
Acquire the study
- Supine and prone curvilinear imaging of both kidneys and a full bladder.
The manoeuvre
- Transverse plane at the renal hilum: anteroposterior diameter of the renal pelvis in mm.
- Central and peripheral calyces: dilated or not; parenchymal thickness and echogenicity; corticomedullary differentiation.
- Ureter: visible behind the bladder or along its course means it is dilated — measure it.
- Bladder: wall thickness, ureterocele, and in a boy the posterior urethra (keyhole sign) on a perineal view.
- The contralateral kidney and a duplex system (two pelvicalyceal systems).
What confirms it
- A graded dilatation with the level of the problem named: pelviureteric junction, ureterovesical junction, bladder or urethra.
What licenses you to exclude it
- A normal postnatal ultrasound after the first days of life makes significant obstruction unlikely; it does not exclude reflux.
The classic misread
- Scanning on day one of life, when physiological oliguria can hide significant dilatation.