Background

• Kidneys in the fetus can usually be identified on ultrasonography after 16 weeks gestational age.
• Single umbilical artery may be associated with anomalies, among them renal.
• Oligohydramnios may be a consequence of renal abnormality and poor urinary output by the fetus.
• Many of the aneuploidies and syndromes are associated with renal anomaly.
• Dilatation of the collecting system can involve calyces, renal pelvis and ureter(s).
• Dilatation may be due to obstruction, reflux, dysplasia.
• The definition of hydronephrosis varies between authors of publications.
• A normal antenatal scan rules out neither reflux nor the possibility of obstruction.

Information

Anatomy does not equal physiology; it is common for the fetal renal collecting system to fluctuate in size during an antenatal scan. We also know that most babies and children ‘outgrow’ reflux. However it would be prudent to identify those newborns who are at risk of renal infection because of reflux and those who have significant obstruction. Because screening ultrasonography is generally done at 18 weeks, a ‘normal’ scan at this gestational age does not rule out subsequent, severe dilatation associated with obstruction. Therefore, any comment as to normal or abnormal prenatal ultrasonography has to include the gestational age at which the scans were obtained.

If one uses a transverse pelvic measurement of ≥ 4mm at any time during intrauterine life, as a definition of dilatation, 13% of those neonates would be expected to have primary vesicoureteric reflux in neonatal life (based on Christchurch study of primarily Caucasian babies). There is no difference in the prevalence of reflux between fetuses who have anywhere from 4-9mm of measured renal pelvic diameter. Although unproven, it is likely that this same prevalence of reflux also occurs amongst those with 0-3mm pelvic measurement. As mentioned, obstructive hydronephrosis (e.g pelviureteric junction obstruction), may become apparent only later in pregnancy or after birth. A ‘normal’ scan rules out neither future obstruction nor ongoing reflux.

Because of the difficulties of screening for urinary tract abnormality in the prenatal population, the following guidelines are given as a compromise solution.

Indications for Scan

1. Remember:  Any antenatally detected renal tract abnormality needs to be confirmed with postnatal imaging.
2. Postnatal renal ultrasonography can be requested at any time if there is concern regarding pulmonary hypoplasia, other anomalies, a renal mass etc.

Antenatal Findings

The table below is to guide referral patterns rather than to state definitive neonatal clinical management.

Note:  PUT clinic = Paediatric Urinary Tract Clinic.  This is a multidisciplinary clinic with paediatric nephrologists, urology, and paediatric surgical services.  There is close liaison with the Radiology service.

References