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Introduction
Bladder outlet obstruction (or obstructive uropathy) occurs when urine is unable to flow from the kidneys through the ureters and out of the bladder through the urethra. Decreased flow of urine leads to swelling of the urinary tract, called hydronephrosis. This process of decreased flow of urine through the urinary tract can begin as early as during intrauterine life and it prevents normal development of fetal kidneys and fetal urine. Low levels of fetal urine leads to low amniotic fluid levels and incomplete lung maturation. Older children and adults can also experience bladder outlet obstruction; however, this process is usually reversible and isn't associated with as many poor outcomes as in infants with congenital bladder outlet obstruction.

Congenital Anomalies of the Kidney and Urinary Tract (CAKUT)
Bladder outlet obstruction is included in the spectrum of congenital anomalies of the kidney and urinary tract (CAKUT). CAKUT is common, occurring in 1 out of 1000 live births, and accounts for approximately half of all cases of chronic kidney disease and end-stage renal disease in children

Causes in Children
Bladder outlet obstruction is classified based on where along the urinary tract the obstruction occurs, including upper urinary tract obstruction and lower urinary tract obstruction. Depending on the location of the obstruction, one or both sides of the urinary tract will be involved. In approximately 50% of cases of congenital hydronephrosis, there is no known cause.

Upper urinary tract obstruction
Upper urinary tract obstruction includes the renal pelvis and upper ureters.

Ureteropelvic junction obstruction
Ureteropelvic junction obstruction (UPJ obstruction) is an obstruction at the level of the ureter and renal pelvis. It is the most common cause of hydronephrosis detected in utero. It occurs in approximately 1 in every 1500 live births, is most commonly seen in males, and involves both sides of the urinary tract in approximately 25% of cases.

Lower urinary tract obstruction
Lower urinary tract obstruction involves the lower ureters, urinary bladder and the urethra.

Ureterovesicular junction obstruction
Ureterovesicular junction obstruction (UVJ obstruction) is an obstruction at the level of the ureter and bladder. It accounts for 20% of cases of hydronephrosis detected in utero. It is also most commonly seen in males and involved both sides of the urinary tract in approximately 25% of cases.

Posterior urethral valves
Posterior urethral valves (PUV) is an obstruction at the level of the urethra. It occurs in approximately 1 in every 5000 to 8000 live births and only occurs in males. Since PUV always affects both sides of the urinary tract, patients with posterior urethral valves are at the greatest risk for developing chronic kidney disease and end-stage renal disease due to obstructive uropathy.

Ureterocele
A ureterocele is a cystic dilation of the end of the ureter that can occur in the bladder and/or in the urethra. It occurs in approximately 1 in every 5000 live births, is most commonly seen in females and involves both ides of the urianry tract in approximately half of cases.

Urethral stenosis
Urethral stenosis is a narrowing of the urethra that prevents urine from exiting the bladder.

Prenatal diagnosis
Bladder outlet obstruction can be identified during routine prenatal ultrasonography as dilation of the fetal urinary tract and decreased amniotic fluid levels. If dilation of the fetal urinary tract is suspected during pregnancy, an ultrasound of the infant's kidneys and bladder should be obtained after birth.

Postnatal diagnosis
If patients aren’t diagnosed with dilation of their urinary tract via ultrasound in utero, they can present after birth with vague symptoms such as abdominal pain, blood in their urine or a urinary tract infection.

Associated syndromes
If patients have other congenital anomalies, their bladder outlet obstruction may be recognized during evaluation for their related syndromes. For example, VACTERL association is a constellation of congenital anomalies including vertebral, anal, cardiac, tracheoesophageal, renal and limb defects. Prune belly syndrome (or Eagle-Barrett syndrome) is another group of congenital disorders that involves the kidneys and includes absent abdominal wall musculature, severe urinary tract abnormalities and bilateral undescended testicles.

Dietl crisis
Patients with an undiagnosed ureteropelvic junction obstruction may experience abdominal or flank pain after increased fluid intake, when their bladder is full or when they exercise.