A 1 week-old boy is evaluated in the hospital. The patient has been admitted due to urosepsis and is receiving antibiotic therapy. The mother received no prenatal care; the neonate was born at term via spontaneous vaginal delivery. Vital signs are normal. Examination shows lower abdominal distension with normal bowel sounds. The penis and scrotum appear normal. Renal ultrasonography and voiding cystourethrography reveal a diffusely thickened bladder wall with bilateral vesicoureteral reflux and hydronephrosis. Which of the following is the most likely cause of this patient's findings?
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This patient has a diffusely thickened bladder wall with bilateral vesicoureteral reflux and hydronephrosis, findings consistent with bladder outlet obstruction. In newborn boys, the most common cause is posterior urethral valves (PUVs).
In utero, the urethra normally begins as a solid structure that canalizes to form its characteristic tubular structure. PUVs are likely due to incomplete canalization and are defined by a persistent urogenital membrane that obstructs the posterior urethra. The bladder outlet obstruction, which may be partial or complete, causes increased pressure in the proximal urinary tract, leading to the following:
Bladder distension with an increased contractile force required to overcome the obstruction, resulting in detrusor muscle hypertrophy and bladder wall thickening
Bilateral secondary vesicoureteral reflux due to higher pressure in the bladder relative to the ureters and hydronephrosis due to retrograde urine flow
Diagnosis is often made via prenatal ultrasound, which may show oligohydramnios if the obstruction is severe. PUVs diagnosed after birth often present with lower abdominal distension, weak urinary stream (if the obstruction is partial), or urinary tract infection (UTI) due to urinary stasis and reflux.
(Choice A) Primary vesicoureteral reflux is due to a congenitally short intravesical ureter that fails to fully compress during micturition, causing retrograde urine flow. Ureteral dilation, hydronephrosis, and UTIs may develop, but the bladder is distal to the defect and would not be distended or have a thickened wall.
(Choice B) Failed obliteration of the allantois leads to a patent urachus, or a connection between the bladder and umbilicus. The risk for UTI is increased, but drainage of urine through the umbilicus is typical and urinary obstruction is not seen.
(Choices C and E) Fusion of the bilateral metanephros (metanephric blastema) causes horseshoe kidney. This condition increases the risk for UTIs, as well as hydronephrosis due to potential ureteropelvic junction (UPJ) obstruction, but the kidneys would be fused on imaging. Moreover, the bladder (distal to the defect) would not be distended. Similarly, isolated UPJ stenosis can cause hydronephrosis but not bladder distension.
Educational objective:
Bladder outlet obstruction leads to increased pressure proximally and findings of bladder distension and wall thickening, secondary vesicoureteral reflux, ureteral dilation, and hydronephrosis. The most common cause in newborn boys is posterior urethral valves due to a persistent urogenital membrane.