Initiation of Voiding1

SUMMARY The urethral smooth musculature is the primary sphincteric mechanism. The striated external sphincter contributes about 50 per cent. of urethral closure pressure. The entire pelvic floor is a part of the sphincteric mechanism, not only by its direct contribution‐the striated external sphincter‐but also as a unit, as it indirectly influences the urethral closure mechanism. A prevoiding drop in urethral pressure, followed 5 to 15 seconds later by detrusor contraction, constitutes the normal pattern of initiation of voiding. In patients with low urethral closure pressure, voiding may be initiated and completed with straining and increased intra‐abdominal pressure with or without minimal detrusor contraction. In other instances, the prevoiding urethral pressures may drop below the level of the resting intravesical pressure and voiding proceeds without detrusor activity. These other modalities, although occasionally encountered in “normal” people, are seen only when closure pressure is low and they are more commonly seen in pathological states. The prevoiding drop in urethral pressure is due to relaxation of the pelvic floor and the striated external sphincter.