Antenatal Hydronephrosis and Pelviureteric Junction Obstruction
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Direct answer
Antenatal hydronephrosis is dilatation of the renal pelvis and calyces seen on prenatal ultrasound, and pelviureteric junction obstruction is its commonest cause of obstructive hydronephrosis in infancy. Severity is graded by the Society for Fetal Urology system, which guides how closely the child is followed. Most cases are mild and settle, but a MAG3 renogram showing obstruction with reduced split renal function prompts pyeloplasty.
What you must remember
- Definition: an anteroposterior renal pelvic diameter above about 4 mm in the second trimester, with thresholds varying by gestation.
- SFU grading: grades 0 to 4 based on pelvic dilatation, calyceal dilatation and parenchymal thickness; grades 3 and 4 carry higher risk.
- Other causes: vesicoureteric reflux, multicystic dysplastic kidney and, in boys, posterior urethral valves.
- PUJ obstruction: the commonest obstructive cause in infancy, often from a high insertion or crossing vessel.
- MAG3 renogram: with diuretic, a half-time under about 10 minutes suggests no obstruction, over about 20 minutes suggests obstruction, and values between are indeterminate.
- Split renal function: differential function on MAG3 or DMSA is the key outcome measure; below about 40 per cent favours pyeloplasty.
- Pyeloplasty: Anderson-Hynes dismembered pyeloplasty is the standard open repair; minimally invasive approaches are used in many centres.
- Antibiotic prophylaxis: commonly used for higher-grade hydronephrosis while awaiting assessment.
A grading approach using the SFU system
A newborn boy has a 6 mm anteroposterior pelvic diameter on a postnatal scan. The grade and the renogram guide whether he needs observation, prophylaxis or surgery.
| SFU grade | Renal pelvis | Calyces and parenchyma |
|---|---|---|
| 1 | Mild dilatation | No calyceal dilatation |
| 2 | Moderate dilatation | Few calyces visible, parenchyma normal |
| 3 | Marked dilatation | Dilated calyces, parenchyma preserved |
| 4 | Marked dilatation | Dilated calyces, thin parenchyma |
Lower grades are usually observed with serial ultrasound, while grades 3 and 4 need a functional study. The renogram then decides whether obstruction is present and whether split function justifies repair.
Where the exam tries to catch you
Hydronephrosis questions usually test grading, the renogram and when surgery is justified.
- Trap: operating on every mild antenatal hydronephrosis. Correct: most mild cases are observed with serial ultrasound, since many resolve.
- Trap: deciding on pyeloplasty from ultrasound alone. Correct: a diuretic MAG3 renogram defines obstruction and split function before surgery.
- Trap: confusing PUJ obstruction with vesicoureteric reflux. Correct: reflux is shown on MCU, while PUJ obstruction causes pelvic dilatation without bladder reflux.
- Trap: judging severity by pelvic diameter alone. Correct: the SFU grade and split function together give a truer picture of risk.
Frequently asked questions
Which renal scan confirms obstruction in pelviureteric junction obstruction?
A diuretic MAG3 renogram confirms obstruction in pelviureteric junction obstruction. A half-time above about 20 minutes after furosemide suggests obstruction, while a half-time under about 10 minutes suggests no obstruction.
What are the indications for pyeloplasty in antenatal hydronephrosis?
Pyeloplasty is indicated for pelviureteric junction obstruction with reduced split renal function, usually below about 40 per cent, or with symptoms, progressive dilatation or recurrent infection. The decision rests on the renogram and serial imaging together.
What does SFU grade 4 hydronephrosis indicate?
SFU grade 4 hydronephrosis indicates marked pelvic and calyceal dilatation with thinning of the renal parenchyma. It is a high-risk finding that requires functional assessment and urology follow-up, since parenchymal loss can be irreversible.
Which conditions other than pelviureteric junction obstruction cause antenatal hydronephrosis?
Vesicoureteric reflux, multicystic dysplastic kidney and posterior urethral valves in boys also cause antenatal hydronephrosis. Their workup differs, so the cause must be identified with micturating cystourethrography and renal imaging.
Does mild antenatal hydronephrosis always need surgery?
Mild antenatal hydronephrosis does not always need surgery. Most lower-grade cases are observed with serial ultrasound, and surgery is considered only when obstruction, reduced function or progressive dilatation is shown.