Posterior urethral valves

Posterior urethral valve, or PUV, is a congenital disorder in boys where the posterior urethra, which is the section of the urethra nearest the bladder, is obstructed by membranous folds, or flaps of tissue. These flaps of tissue are collectively referred to as a congenital obstructive posterior urethral membrane, or COPUM.

This blockage means urine can’t easily flow out, which leads to a backup of urine which can cause kidney problems, as well as less amniotic fluid which can cause respiratory problems.

Although the cause of PUV isn’t completely understood, it’s thought that normal development of the male urethra is disrupted between weeks 9 and 14 of gestation.

Normal development involves the Wolffian duct integrating with the posterior urethra, which results in thin mucosal folds called plicae colliculi.

It’s thought that PUV might result from abnormal integration of the wolffian duct, resulting in large plicae colliculi that fuse anteriorly, making it more difficult for urine to flow through.

When that urine can’t easily flow out because of increased resistance from an obstruction, the intravesical pressure, or bladder pressure, starts to creep up.

Holding urine under higher pressure leads to bladder wall hypertrophy and collagen deposition, both of which thicken the bladder wall. This thickening makes the bladder less compliant, meaning that small increases in urine volume causes large increases in bladder pressure, which makes the problem even worse.

That high-pressure urine has nowhere to go but up to the ureters and eventually to the kidneys, causing hydronephrosis, which is the swelling of a kidney due to a buildup of urine.

In PUV, since the bladder outlet is obstructed, the hydronephrosis is bilateral, meaning it affects both kidneys.

Severe obstruction in utero can also lead to oligohydramnios, which is a low volume of amniotic fluid, since normally a significant proportion of amniotic fluid comes from fetal urine, and also, that amniotic fluid normally gets breathed into the developing lungs, which helps expand the airways and also provides the amino acid proline, and both of these things are critical to normal lung development.