Structural aspects of the extrinsic continence mechanism.

The changes in urethral pressure that occur during a cough, and microtransducer measurements of urethral pressures at rest, suggest that structures extrinsic to the urethra and vesical neck influence sphincteric function. The present study examined the structure of the extrinsic continence mechanism as it relates to these physiologic observations. Serial histologic sections of the pelvic viscera made from eight female cadavers and dissections of 34 cadavers were examined. In the proximal urethra, the anterior vaginal wall is attached to the muscles of the pelvic diaphragm and to the arcus tendineus fasciae pelvis. Contraction of the pelvic diaphragm would pull the vagina against the posterior surface of the urethra, causing an increase in posteriorly measured "pressures." Support of the urethra at rest comes from both its attachment to the arcus tendineus fasciae pelvis and the resting tone of the pelvic diaphragm muscles. Two arches of striated muscle (compressor urethrae and urethrovaginal sphincter) run over the distal urethra in the region of the perineal membrane (ie, urogenital diaphragm). Activity of these muscles could compress the urethra distally, causing the urethral pressure rise that precedes and exceeds the rise in abdominal pressure during a cough and explaining asymmetry of pressure measurements in this area. Recognition of these structural relationships can help us understand urethral pressures during a cough and directional "pressure" variations seen in patients with stress incontinence. They may also contribute to the understanding of surgical failures that occur despite successful urethral suspension.