Vesicoamnionic shunts are used in selected fetuses with severe bladder-outlet obstruction in which diminished amnionic fluid portends a grim prognosis (Fig. 16-8). Distal obstruction of the urinary tract occurs more often in male fetuses. The most common etiology is posterior urethral valves, followed by urethral atresia and by prune bely syndrome, which is also called Eagle-Barrett syndrome. Sonographic indings include dilation of the bladder and proximal urethra, termed the "keyhole" sign, along with bladder wall thickening (Fig. 10-45, p. 210). Associated oligohydramnios before midpregnancy leads to pulmonary hypoplasia. Unfortunately, postnatal renal function may be poor even when amnionic fluid volume is normal.
Evaluation includes a careful search for concurrent anomalies, which may coexist in 40 percent of cases, and for aneuploidy, which has been reported in 5 to 8 percent of cases (Hayden, 1988; Hobbins, 1984; Mann, 2010). Fetal urine sampled at vesicocentesis may be used to perform genetic studies. As with other structural fetal abnormalities, chromosomal micro array analysis is recommended. Because visualization may be limited due to lack of amnionic fluid, counseling should include the increased likelihood that associated anomalies may be missed sonographically.
Potential candidates are fetuses without other severe anomalies or genetic syndromes and without sonographic features that confer poor prognosis, for example, renal cortical cysts. herapy is generally ofered only if the fetus is male, because in females, the underlying anomaly tends to be even more severe. Serial bladder drainage-vesicocentesis-performed under sonographic guidance at approximately 48-hour intervals is used to evaluate fetal urine electrolyte and protein content. Fetal urine is normally hypotonic due to tubular resorption of sodium and chloride, whereas isotonic urine in the setting of obstruction suggests renal tubular damage. Serial assessment permits classiication of the renal prognosis as good or poor and helps guide candidate selection (Table 16-4).
TABLE 16-4. Fetal Urinary Analyte Values with Bladder Outlet Obstruction
Good or poor prognosis is based on values from serial gestation, using the last specimen obtained.
Data from Mann, 2010.
Shunt placement allows urine to drain from the bladder into the amnionic cavity. When successful, this often prevents pulmonary hypoplasia, however, renal function is not reliably preserved. Before shunting, amnioinfusion of warmed lactated Ringer solution is generally performed to aid catheter placement. Amnioinfusion also aids sonographic evaluation of fetal anatomy. A small trocar and cannula are then inserted into the fetal bladder under sonographic guidance. he shunt is placed as low as possible within the bladder to avoid dislodgement after bladder decompression. A double-pigtail catheter is used. The distal end lies within the fetal bladder, and the proximal end drains into the amnionic cavity.
Complications include displacement of the shunt out of the fetal bladder in up to 40 percent of cases, urinary ascites in about 20 percent, and development of gastroschisis in 10 percent (Freedman, 2000; Mann, 2010). Preterm delivery is common, and neonatal survival rates range from 50 to 90 percent (Biard, 2005; Walsh, 201l). A third of surviving children have required dialysis or renal transplantation, and almost half have respiratory problems (Biard, 2005). In one randomized trial, vesicoamnionic shunt placement was compared with conservative management in 31 cases (Morris, 2013). hose receiving shunts had higher survival rates. However, only two children had normal renal function at age 2 years.