Longitudinal and radial variations of pressure in the human anal sphincter.

The pressure profile of the human anal canal integrates the effects of several separate muscles including internal and external and sphincters and the puborectalis. To define the topography of pressure along the anal canal, we developed a new probe that features four perfused side-hole channels spaced at 90 degrees circumferentially. In 18 healthy volunteers (10 women, 8 men), simultaneous four-quadrant pressures were measured at orad, mid-, and distal stations in the anal canal, at rest and during a maximum voluntary "squeeze." Anterior quadrant pressures were reduced significantly (p less than 0.001) at the orad station at rest and during maximum voluntary effort. In the midcanal, radial pressures were equal in all quadrants. Distally, pressures in the posterior quadrant were significantly lower than those in the other three quadrants. Men had longer sphincters than women, but men had higher pressures at the orad station only. Orad pressures were reduced in women who had had vaginal deliveries. We conclude that the human anal canal has marked, but consistent, radial and longitudinal variations of pressure. Based on these results, a structural and physiological explanation for their genesis is suggested. The findings may also be relevant to the development of cystoceles, rectoceles, and chronic anal fissures.