Antenatal hydronephrosis and postnatal evaluation: Viva

Branch 1: Grading and risk stratification

The candidate should define the antenatal hydronephrosis as the dilatation of the fetal renal collecting system detected on the antenatal ultrasound. A strong candidate describes the two grading systems. The Society for Fetal Urology system, introduced by Fernbach in 1993, grades from 0 to 4 on the basis of the pelvic and the calyceal dilatation and the parenchymal preservation, with the grade 4 defined by the thinning of the renal parenchyma. The Urinary Tract Dilation classification, introduced by Nguyen in the 2014 multidisciplinary consensus, uses the quantitative anteroposterior renal pelvic diameter with the 28-week gestational cutoff and stratifies into the low-risk UTD A1 and the increased-risk UTD A2 to A3. [1]

If the examiner presses on the classification of the stimulus case, the candidate should note that the diameter of 9 mm at 32 weeks, with the normal parenchyma and the normal amniotic fluid, falls within the UTD A1 low-risk range of 7 to under 10 mm at 28 weeks or more, provided there are no additional features. The candidate should state that the presence of any additional feature, such as the calyceal dilatation or the ureteric dilatation or the abnormal parenchyma, would upgrade the case to the increased-risk category regardless of the diameter. [1]

Branch 2: The timing of the first postnatal ultrasound

If asked about the timing, the candidate should state that the first postnatal ultrasound is deferred to at least 48 hours and ideally to days 5 to 7 of life. The rationale, established by Wiener and O'Hara, is that the neonate is relatively dehydrated and oliguric in the first 48 hours, which reduces the urine output, underestimates the renal pelvic dilatation, and produces the false negative. [4]

A strong candidate states the exceptions that demand the immediate imaging. The bilateral severe hydronephrosis, the solitary kidney, and the suspected bladder outlet obstruction are the indications for the immediate postnatal ultrasound in the first hours of life, because the delay worsens the renal dysplasia and the long-term outcome. For the low-risk stimulus case, the deferred timing at days 5 to 7 is appropriate, with the follow-up ultrasound at 4 to 6 weeks if the initial scan is stable. [4]

Branch 3: The investigation pathway and the counselling

If the examiner moves to the investigations, the candidate should describe the sequential image-led pathway. The ultrasound is the first test, the MCUG detects the reflux and the valves, the DMSA maps the cortical function and the scars, and the MAG3 assesses the obstruction and the split function. The candidate should state that the MCUG is reserved for the children with the high pre-test probability of the reflux or the obstruction, because it is invasive and carries the risk of the infection. [3]

A strong candidate discusses the counselling. The family is reassured that the antenatal hydronephrosis is very common and most often harmless, that the ultrasound is done a few days after the birth once the baby is drinking and producing the urine, and that the plan is to find out whether the swelling is the harmless kind that goes away or the kind that needs watching. The majority of the low-risk cases resolve spontaneously, but the surveillance is maintained until the resolution is confirmed, because the chronic kidney disease risk from the underlying anomaly is the long-term concern. [3]