Antenatal hydronephrosis and postnatal evaluation: SAQ

This infant has the increased-risk UTD A2-3 antenatal hydronephrosis with the bilateral renal pelvic dilatation exceeding 10 mm at 32 weeks and the dilated thick-walled bladder. The palpable distended bladder and the weak urinary stream in a male infant are the classic presentation of the posterior urethral valves, and the immediate priority was the correct one to pass the urethral catheter to decompress the obstruction. ^[6]

Question 1 (10 marks)

Outline the grading systems for the antenatal hydronephrosis and explain how they guide the timing of the first postnatal ultrasound and the subsequent investigation pathway for this infant. ^[1]

The two grading systems are the Society for Fetal Urology system and the Urinary Tract Dilation classification. The Society for Fetal Urology system grades the hydronephrosis from grade 0 with no dilatation to grade 4 with the parenchymal thinning, with the grade 1 showing the pelvic dilatation only, the grade 2 showing the pelvis and the major calyces, and the grade 3 showing all the calyces with the normal parenchyma. The Urinary Tract Dilation classification, introduced by Nguyen and colleagues in the 2014 multidisciplinary consensus, uses the quantitative anteroposterior renal pelvic diameter with the 28-week gestational cutoff and divides the cases into the low-risk UTD A1 and the increased-risk UTD A2 to A3. ^[1]

The anteroposterior diameter thresholds for the UTD A1 low-risk category are 4 to under 7 mm at 16 to 27 weeks and 7 to under 10 mm at 28 weeks or more. The increased-risk category is assigned by the diameter of 7 mm or more at 16 to 27 weeks or 10 mm or more at 28 weeks or more, or by any of the additional features regardless of the diameter, which include the calyceal dilatation, the abnormal parenchyma, the ureteric dilatation, the abnormal bladder, and the unexplained oligohydramnios. This infant has the dilated bladder and the bilateral diameters exceeding 10 mm at 32 weeks, which places him in the increased-risk category. ^[1]

The timing of the first postnatal ultrasound is the single most testable point. The scan is normally deferred to at least 48 hours and ideally to days 5 to 7 because the neonatal dehydration and the oliguria in the first 48 hours reduce the urine output and underestimate the dilatation, producing the false negative. Wiener and O'Hara established this. The exception is the clinical picture that demands the immediate imaging, and this infant meets the exception because of the bilateral severe hydronephrosis with the dilated bladder and the suspected bladder outlet obstruction. The immediate imaging is performed, the urethral catheter is already in place, and the urgent urology referral is made. ^[4]

The subsequent investigation pathway is the MCUG to confirm the posterior urethral valves and to detect the reflux, performed once the infant is stable with the antibiotic cover. The DMSA scan maps the cortical function and the dysplasia, deferred until the acute situation resolves. The MAG3 renogram assesses the obstruction and the drainage. The prophylactic trimethoprim at 2 mg per kg at night is started to prevent the infection while the investigations are completed. ^[2]