Urinary tract obstructions are a common antenatal diagnosis, and outcomes range from clinically insignificant to in-utero fetal demise (IUFD). Many of these patients survive to birth but develop end-stage renal failure, requiring renal replacement therapy and, ultimately, kidney transplantation.
A role for fetal intervention was originally suggested in the 1980s when Harrison’s group demonstrated that urinary diversion could hinder the progression of renal parenchymal damage.  However, determining which patients would benefit from urine diversion has proved challenging and requires an extensive antenatal evaluation.
Vesicoamniotic shunts can be a temporizing measure and provide a survival advantage in a select cohort of fetuses with urinary tract obstruction. In addition, various groups are investigating approaches to treat obstruction of the lower urinary tract via fetal cystoscopy. [2, 3]
This article reviews the diagnosis, evaluation, and treatment strategies for fetal urinary tract obstruction.
Clinically significant urinary tract obstructions occur with a frequency of 1 in 500 live births and are associated with high morbidity and mortality.  With antenatal ultrasonography, urinary tract anomalies can be detected as early as 12-14 weeks’ gestation. These anomalies can be stratified into upper urinary tract obstruction and lower urinary tract obstruction (LUTO).
Common causes of upper urinary tract obstruction include ureteropelvic junction (UPJ) obstruction, ureterovesical junction obstruction, collecting system duplications, multicystic dysplastic kidney, ureterocele/ectopic ureter, and pelvic tumors. Because upper urinary tract obstruction is not currently amenable to fetal intervention, the remainder of this article focuses on LUTO.
Posterior urethral valves (PUVs) account for 9% of all urinary tract obstructions and more than 50% of LUTOs.  Other causes of LUTO include urethral atresia (the second most common cause), anterior urethral valves, meatal stenosis, epispadias, and hypospadias.  LUTOs are far more common in males; when identified in females,they should raise suspicion for cloacal malformations.
LUTO is often diagnosed with antenatal ultrasonography during the first or second trimester. Findings frequently include megacystis, bilateral hydronephrosis, pyelocaliectasis, and oligohydramnios/anhydramnios. The hallmark sonographic finding of PUV is the "keyhole sign," which results from bladder and urethral distention proximal to the valve (see the image below). The presence of a normal volume of amniotic fluid suggests a partial obstruction.
Classic keyhole sign seen in patients with posterior urethra valves
Urinary obstructions can lead to cystic renal dysplasia and impairment of the glomerular and tubular apparatus. In severe cases, oligohydramnios or anhydramnios may ensue, predisposing the fetus to limb abnormalities and pulmonary hypoplasia. Oligohydramnios/anhydramnios associated with urethral obstruction carries a grim prognosis, and when it is identified in the early to middle stages of gestation, mortality approaches 95%.  Prognostic indicators such as bladder aspiration, evaluation of refill time, sonographic findings of renal parenchyma, and amniotic fluid volume have proved unreliable. [8, 9, 10]
A retrospective study reviewing different prognostic markers between “poor functioning” kidneys and “good functioning” kidneys on the basis of autopsy or biopsy pathology found that “poor functioning” kidneys had decreased amniotic fluid volume, urine output of less than 2 mL/hr, a urine sodium concentration of more than 100 mEq/L, urine chloride levels of greater than 90 mEq/L, and a urine osmolality level greater than 210 mOsm/L. 
Further studies identified calcium, β2 -microglobulin, and total protein as being of prognostic importance.  Repeated fetal urine aspirations, trending osmolarity, and sodium and chloride levels may be of use in deciding which fetuses would benefit from in-utero intervention (see Laboratory Medicine).
The first successful fetal decompression operation for hydronephrosis was an open procedure performed in 1981.  Since then, advances in fetoscopy have eliminated the need for open operations. Current management of LUTO involves ultrasound-guided vesicoamniotic shunt placement under local anesthesia. Amnioinfusion may be required, in that oligohydramnios tends to limit visualization. In addition, various groups have begun to investigate the role of fetoscopic cystoscopy. [3, 2]
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