Urinary tract obstructions are a common prenatal diagnosis, and outcomes vary from clinically insignificant to in utero fetal demise. 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 Dr. Michael 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 proven challenging and requires an extensive prenatal 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 lower urinary tract obstructions via fetal cystoscopy. [2, 3]
This article reviews the diagnosis, evaluation, and treatment strategies for fetal urinary tract obstruction.
Fetal urinary tract obstruction
Clinically significant urinary tract obstructions occur with a frequency of 1 in 500 live births and carry a high rate of morbidity and mortality.  With prenatal ultrasonography, urinary tract anomalies can be detected as early as 12-14 weeks’ gestation. They can be stratified into upper and lower urinary tract obstructions.
Common causes of upper urinary tract obstructions include ureteropelvic junction (UPJ) obstruction, ureterovesical junction obstruction, collecting system duplications, multicystic dysplastic kidney, ureterocele/ectopic ureter, and pelvic tumors. As upper urinary tract obstructions are not currently amenable to fetal intervention, the remainder of this chapter focuses on lower urinary tract obstructions.
Posterior urethral valves (PUV) account for 9% of all urinary tract obstructions and more than half of lower urinary tract obstructions.  Other causes of lower urinary tract obstructions include urethral atresia (the second most common cause of lower urinary tract obstructions), anterior urethral valves, meatal stenosis, epispadias, and hypospadias.  Lower urinary tract obstructions are far more common in males and, when identified in females, should raise suspicion for cloacal malformations.
Lower urinary tract obstructions are often diagnosed with prenatal ultrasonography during the first and 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 image below). The presence of a normal volume of amniotic fluid suggests a partial obstruction.
Urinary obstructions can lead to cystic renal dysplasia and impairment of the glomerular and tubular apparatus. In severe cases, oligohydramnios/anhydramnios may ensue, predisposing the fetus to limb abnormalities and pulmonary hypoplasia. Oligohydramnios/anhydramnios associated with urethral obstruction carries a grim prognosis, and, when identified in early- to mid-gestation, mortality approaches 95%.  Prognostic indicators such as bladder aspiration, evaluating refill time, sonographic findings of renal parenchyma, and amniotic fluid volume have proven unreliable. [8, 9, 10]
A retrospective study reviewing different prognostic markers between “poor functioning” kidneys and “good functioning” kidneys based on 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 have identified calcium, β2 -microglobulin, and total protein to be 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 Test Summary).
The first successful fetal decompression operation for hydronephrosis was an open procedure performed in 1981.  Since then, fetoscopic advancements have eliminated the need for open operations. Current management strategies for lower urinary tract obstructions involve ultrasound-guided vesicoamniotic shunt placement under local or regional anesthesia. Amnioinfusion may be required, as oligohydramnios tends to limit visualization. In addition, various groups are beginning to investigate the role of fetoscopic cystoscopy, [2, 3] but the literature is scarce.
The relevant role of fetal surgery for lower urinary tract obstructions was established following experiments using the fetal lamb, suggesting that resolution of lower urinary tract obstructions improved oligohydramnios and decreased the risk of developing pulmonary hypoplasia.  Their work demonstrated that obstructing a single ureter (bilateral obstruction led to fetal demise) resulted in impaired renal function and histological changes compared to an unobstructed kidney. Furthermore, obstructed fetal kidneys had a decreased glomerular filtration rate (GFR) and increased loss of sodium and chloride.  Decompression via end ureterostomy led to improved renal function and fewer histological changes. 
Fetuses with favorable urine electrolyte levels and no indication of renal dysplasia are considered for vesicoamniotic shunting or fetal cystoscopy.
Contraindications for fetal intervention can be either maternal or fetal.
Maternal contraindications include frequent contractions, membrane rupture, short cervix, or uncontrolled comorbidities that may predispose the patient to pre-eclampsia or HELLP syndrome.
Fetal contraindications include multiple fetal anomalies, chromosomal abnormalities, and anatomic restrictions, including an anterior lying placenta, which may prohibit safe access.
Currently, fetal intervention has no role for upper urinary tract obstructions. The algorithm for managing patients with lower urinary tract obstructions is illustrated in the image below.
As fetoscopic cystoscopy is in its infancy, it is not included. Prenatal ultrasonography should first be used to screen for associated anomalies. If no other anomalies are present, fetal karyotyping is recommended. If the patient has a normal male karyotype, the next step is to obtain serial fetal urine samples. If levels of serial electrolytes and β2 -microglobulin and the total protein profile are favorable, vesicoamniotic shunt placement is recommended.
Patients with an untreatable lower urinary tract obstruction tend to have a poor prognosis, and counseling regarding expectations is currently the best practice.
Prenatal ultrasonography is an essential component to the lower urinary tract obstruction algorithm, as anhydramnios poses a challenge to vesicoamniotic shunt deployment, since the catheter requires a fluid pocket for coiling. Amnioinfusion is required in severe cases. Placental location should also be considered, as anterior lying placentas add an extra level of complexity to the procedure and, in some cases, can be prohibitive.
Mortality rates associated with lower urinary tract obstructions vary from 33%-75%, depending on coexisting anomalies and the severity of oligohydramnios.  The presence of early oligohydramnios carries a mortality rate ranging from 45%-80%, [16, 17] likely resulting from pulmonary hypoplasia. Outcome analysis suggests that 25%-30% of patients diagnosed with lower urinary tract obstructions required dialysis or renal transplantation. 
Currently, no randomized control trials have compared the success of prenatal and postnatal therapy for lower urinary tract obstructions. In 1997, a retrospective analysis of 169 successful percutaneous vesicoamniotic shunts placed over a 14-year period calculated the overall survival to be 47%, and 40% of survivors developed end-stage renal disease. 
When comparing vesicoamniotic shunts to fetal cystoscopy, reported survival rates are 40% and 75%, respectively. Postnatal normal renal function was evident in 50% of those who previously underwent vesicoamniotic shunt placement versus 65% who underwent fetal cystoscopy. 
While fetal cystoscopy appears to result in more favorable outcomes than vesicoamniotic shunt placement, only 60 cases have been reported in the literature, and further studies are needed for validation.