Posterior urethral valves: Viva

Branch 1: Classification and pathophysiology

The candidate should define posterior urethral valves as congenital membrane-like folds in the male posterior urethra that obstruct the urinary outflow, and should state that they are the most common cause of lower urinary tract obstruction in male infants, occurring only in boys at approximately 1 in 4000 to 8000 live male births. A strong candidate classifies the valves by the Young system: the Type I valve, two leaflets fusing in the midline from the verumontanum, is by far the most common, the Type II valve is now regarded as non-obstructing and rare, and the Type III valve is a distal diaphragm with a central pinhole. ^[1]

If the examiner presses on the pathophysiology, the candidate should trace the urine. The valve blocks the outflow, the bladder thickens into a hard muscular wall, and the back-pressure travels up the ureters to cause bilateral hydronephrosis and renal dysplasia. The reduced fetal urine output causes oligohydramnios, which in the most severe cases produces the Potter sequence with pulmonary hypoplasia. The candidate should name pulmonary hypoplasia as reason worst-affected boys die of respiratory failure at birth, and should explain that the valve bladder persists after ablation. ^[1]