Antenatal diagnosis due to bilateral hydronephrosis is the most common presentation. Other fetal ultrasound findings include a distended bladder and a dilated prostatic urethra (“keyhole sign”). The amount of amniotic fluid may or may not be abnormal. Isolated prenatal ultrasound findings are not predictive of long-term outcome. The clinical severity depends on the gestational age of the fetus at onset and the degree of obstruction. Thus, PUVs present as a spectrum.

Other causes of fetal lower urinary tract obstruction (LUTO) include urethral atresia (the other major cause), prune-belly syndrome, bilateral vesicoureteric reflux, and other less common causes of bladder outlet obstruction (BOO).

Another important factor is assessment of the fetal renal function via bladder aspiration, with analysis of urinary parameters. A urine sodium less than 100, urine chloride less than 90, osmolality less than 210 mOsm/L, and β2 microglobulin less than 6 mg/L are normal since fetal urine is hypotonic. Elevated fetal urine electrolytes and β2-microglobulin levels are an indication of irreversible renal dysfunction.

Ruano’s classification of LUTO is widely used:

• Stage I is mild, with anormal amniotic fluid index (AFI), normal fetal renal parenchymal US appearance without dysplasia/cysts, and “good” urinary chemistry on bladder aspiration (if performed). The prognosis is favorable, and no fetal intervention is indicated. However, serial ultrasounds are followed every 2 weeks.

• Stage II has oligohydramnios after 18 weeks’ gestation and hyperechoic kidneys without cystic changes or dysplasia. This group is usually the primary focus of potential fetal interventions.

• Stage III is severe LUTO with oligohydramnios or anhydramnios, echogenic kidneys with or without renal cortical cysts/dysplasia, and poor urine chemistries. Fetal interventions can be considered but observation may be appropriate since clear evidence of benefit from fetal intervention is lacking.

Several fetal therapies are available.

Vesicoamniotic shunts (VASs) have been available for several decades and are the most widely studied fetal intervention. A major randomized trial of VASs, the Percutaneous vesicoamniotic shunting in Lower Urinary Tract Obstruction (PLUTO) study, was reported in 2013. Although not without flaws, it found a higher relative risk of survival in the treated arm at 28 days, 1 year, and 2 years. The overall conclusion was that VAS is likely beneficial in some cases. As with any fetal intervention, there is a risk of fetal loss/morbidity and even maternal complications. Approximately half of the shunts will need to be replaced due to dislodgment/obstruction, and they do not allow normal bladder cycling, which may result in a stiff noncompliant bladder (“valve bladder”). ^, Fetal cystoscopy through the fetal bladder is another intervention. It is more technically difficult than VAS but offers the advantages of being able to confirm the presence of VAS, restore amniotic fluid volume, and allow normal bladder cycling. Complications include persistent urinary fistulas, chorioamnionitis or membrane separation, preterm premature rupture of membranes (PPROMs), fetal urinary ascites, and others. ^, Serial amnioinfusion , usually via repeated percutaneous infusion of warmed isotonic fluid into the amniotic space, has a lower risk of PPROM than VAS or fetal cystoscopy. Although not specific to PUV, there is an ongoing trial of this intervention for bilateral renal agenesis, the Renal Anhydramnios Fetal Therapy (RAFT) trial (ID:NCT03101891).

Despite advances and multiple studies over the past six decades, precise indications for, and the type of, fetal intervention for LUTO from PUV remain unclear. The benefits have been limited, particularly in preventing progression to end-stage renal disease (ESRD). All the techniques carry significant risks and should be applied with caution to those with the highest likelihood of benefit.

The majority of cases are identified either prenatally or shortly after birth. Clinical signs in the newborn may include a distended, palpable bladder, a poor urine stream, and infrequent voiding. A patent urachus may provide a “pop-off” for the BOO. Urinary ascites can be seen due to rupture of the renal fornix in as many as 5%–15% of infants ( Fig. 55.1 ). Abdominal distention, general failure to thrive, or respiratory insufficiency from pulmonary hypoplasia may be present. Late presentations, even in school-age children, are possible, with diurnal enuresis, UTI, voiding dysfunction, an abdominal mass, or renal failure. ^, Laboratory evaluation includes serial electrolytes, BUN, and creatinine (which simply reflects maternal creatinine for the first few days of life).

Posterior urethral valves and ascites. (A) Prenatal ultrasound image demonstrating a perirenal urinoma around the right kidney, which is not hydronephrotic. (B) Prenatal ultrasound image showing ascites and stretched umbilical vessels (arrow) . (C) Plain radiographs of the abdomen in a neonate with a distended abdomen from urinary ascites.

Imaging studies should begin with a renal bladder ultrasound. Findings are typically bilateral hydroureteronephrosis and a thickened distended bladder. Corticomedullary differentiation is a favorable prognostic sign regarding renal function ( Fig. 55.2 ). Echogenic kidneys, subcortical cysts, and the loss of corticomedullary differentiation are unfavorable signs. A voiding cystourethrogram is the definitive study for PUV ( Fig. 55.3 ) The valves appear as a defined lucency in the distal prostatic urethra and the posterior urethra is dilated and elongated. The bladder may be trabeculated with cellules and diverticuli, and bladder neck hypertrophy can be seen. Unilateral VUR is present in 25% and bilateral VUR in 25% of infants with PUV. ^,

This renal sonogram demonstrates a hydronephrotic kidney with intact corticomedullary junction (arrow) in an infant with posterior urethral valves.

These two voiding cystourethrograms show varying degrees of obstruction from posterior urethral valves. In both studies, the location of the valves is marked with an arrow and the posterior urethra is identified with an asterisk. (A) There is no evidence of vesicoureteric reflux. (B) There is massive, bilateral grade V reflux.