Prenatal diagnosis of fetal lower urinary tract obstruction: Fetal...

The three major causes of LUTO are urethral atresia (complete obstruction with no communication from bladder neck through urethra), posterior urethral valves (flap of tissue in proximal urethra), and mid-urethral hypoplasia (significant tapering and narrowing).

Incomplete urethral obstruction leads to progressive oligohydramnios until anhydramnios develops.

Oligohydramnios causes physical deformations including joint contractures, ear flattening, and Potter's phenotype, and leads to pulmonary hypoplasia by interfering with chest expansion.

Obstruction generates pressure to the kidneys causing severe hydronephrosis, collecting system dilation, and progressive renal fibrocystic dysplasia resulting in renal failure after birth.

The San Francisco group's sheep model demonstrated that early ureteral ligation produced fibrocystic dysplasia identical to human LUTO, with earlier and longer obstruction causing greater kidney damage.

Early mid-gestational reversal of obstruction in the sheep model prevented progressive dysplastic changes and preserved kidney function.

Many babies with LUTO have associated anomalies including myelomeningocele, higher frequency of cardiac disease, and genetic syndromes that would preclude benefit from shunting.

A significant portion of LUTO fetuses have chromosomal abnormalities including major trisomies and Klinefelter syndrome.

Female fetuses with large bladder-like structures are usually cloacal abnormalities and do not benefit from shunting due to different pathophysiology.

Renal function evaluation requires complete bladder drainage on several occasions to measure sodium, chloride, calcium, osmolarity, total proteins, and beta-2 microglobulin as markers of tubular and glomerular injury.

Increased kidney echogenicity represents compression of renal parenchyma rather than being a poor prognostic sign per se; following bladder drainage, kidneys re-expand and show more normal echogenicity.

Hydronephrosis pathophysiology involves the collecting system (like a water balloon) compressing the renal parenchyma (like a sponge) against the serosa (like a glass fishbowl), impairing delicate vasculature and causing cell death, fibrosis, and cystic dysplasia.

The presence of cortical cysts indicates irreversible kidney damage and the kidney is not amenable to in utero therapy.

Good prognostic urine values for potential survival with shunting are: sodium <100 mEq/L, chloride <90 mEq/L, osmolarity <210 mOsm, calcium <8 mg/dL, beta-2 microglobulin <6 mg/L, and total protein <40 mg/dL.

Fetuses with values above the prognostic thresholds showed significant fibrotic kidney injury on autopsy, while those below showed very little or early potentially salvageable changes.

The first urine specimen from bladder tap is not predictive or reliable due to degradation products and osmotic gradients; the third or fourth tap after serial drainages has much higher predictive value for detecting kidney injury.

Improving urine values across serial taps (e.g., sodium dropping from poor to good range) indicates an excellent shunting candidate, while worsening values indicate ongoing irreversible damage.

Bladder morphology after drainage correlates with etiology: urethral atresia shows symmetric thick-walled bladder with difficult keyhole; posterior urethral valves show elongated bladder with more proximal thickening; mid-urethral hypoplasia shows 'snowman appearance' with smooth muscle deficiency in upper bladder.

All electrolyte cutoff thresholds are based on urines analyzed between 18 to 22 weeks gestation and cannot be reliably used before 18 weeks or after 22 weeks without adjustment.

The primary goal of fetal intervention is to prevent pulmonary hypoplasia secondary to oligohydramnios; preservation of renal and bladder function are secondary goals.

The PLUTO trial randomized only 31 patients over 4 years (20% of planned 150), showing apparent 3-fold increase in survival with shunting but results were not statistically significant with confidence intervals crossing unity.

All 12 deaths in the PLUTO trial were from pulmonary hypoplasia, suggesting improved survival may relate to decreased lung hypoplasia.

Only 7 of 12 live-born shunted babies in PLUTO were alive at age 2, and only 2 of the shunted survivors had normal renal function.

All 3 conservatively managed survivors in PLUTO had significant renal impairment.

Across 6 long-term outcome studies of shunted LUTO cases, survival rates are consistent at approximately 47-70%, but only 40-50% of survivors have normal renal function and approximately one-third require dialysis or transplant.

Some LUTO cases can resolve spontaneously, as demonstrated by a case where a baby began voiding and refilling amniotic fluid by 19 weeks with recollection by 23 weeks without intervention.

Spontaneous bladder decompression can occur through bladder rupture (usually after drainage, rarely spontaneous) resulting in urinary ascites or perinephric urinoma.

Megacystis-microcolon-intestinal hypoperistalsis syndrome and cloacal dysgenesis are contraindications to shunting intervention.