Nonobstructive posterior urethral widening (spinning top urethra) in boys with bladder instability.

Seven boys between the ages of 5 and 10 years with symptoms of urinary frequency and urgency and daytime wetting were studied with urodynamics and were shown to have bladder instability and a dilated posterior urethra. In two the dilatation occurred predominantly during bladder filling. Unstable contractions caused filling of the posterior urethra, and leakage was prevented by voluntary contraction of the distal urethral sphincter; with voiding, the urethra showed a more normal appearance. In the remaining five, there were similar changes during filling, but dilatation persisted during voiding. In six the measured urine flow rate was normal, and none showed any evidence of anatomic obstruction. The mechanism of urethral distention appears to be similar to that previously shown in girls with spinning top urethra: Unstable contractions resisted by voluntary sphincter contraction cause posterior urethral dilatation. Boys with dilated posterior urethras who have urinary frequency and urgency and daytime wetting and normal urine flow rates should be assumed to have bladder instability.