| Discussion: |
Posterior urethral valve
Anomalous developmental membranes, formed by mucosal folds, which occur in the distal prostatic urethra of males. Posterior urethral valves are the commonest cause of urethral obstruction in children, with an incidence of 1 in 5,000 - 8,000 male births. Two types of posterior urethral valve have been described. In type I, the membrane slopes anteroinferiorly from the verumontanum to the membranous urethra. In type III, the membrane is a horizontal disk at the level of the prostatamembranous junction. Type III valves are rare. Type II valves were described in the original classification, and consisted of vertical folds running upwards from the verumontanum. Type II valves are now believed to be an effect rather than a cause of bladder outlet obstruction, and therefore no longer belong in the classification. However, the terminology of type I and type III valves is retained, because this has been ingrained by historical usage. The clinical presentation of posterior urethral valves is highly variable, and related to the degree of obstruction. Severe cases may cause intrauterine or postnatal death.
Antenatal ultrasound may suggest the diagnosis by demonstrating bilateral hydroureteronephrosis, an enlarged bladder and oligohydramnios. After birth, children may present with mild to severe symptoms of outflow obstruction. The urinary stream is often poor and dribbling.
Hydronephrotic kidneys may be palpable, as may the enlarged bladder. Occasionally, the only clinical feature may be failure to thrive. Urinary tract infections may occur, as may renal impairment or failure. Voiding cystourethrography (VCUG) is the definitive radiographic study. The bladder is typically trabeculated. The posterior urethra is elongated and dilated, with a prominent bladder neck. The valves may be seen as linear radiolucent defects. Typically, retrograde urethrography is normal, because the retrograde flow compresses the valves against the urethral wall.
In posterior urethral valves the dilated prostatic urethra has a cylindrical outline with a rounded inferior margin, whereas in bladder neck dysfunction the dilated urethra has a triangular configuration with a sharp inferior beak. However, endoscopy may be required for definitive distinction between these two conditions.