Functional correlates of anal canal anatomy: Puborectalis muscle and anal canal pressure

BACKGROUND: Resting and squeeze pressures in the anal canal are thought to reflect the contributions of the internal anal sphincter (IAS) and the external anal sphincter (EAS) respectively. Role of the puborectalis muscle (PRM) in the genesis of anal canal pressure is not known. OBJECTIVES: To determine the functional correlates of anal canal anatomy. METHODS: Seventeen asymptomatic nulliparous women were studied using simultaneous 3D ultrasound images and manometry of the anal canal. Ultrasound images were recorded using a transducer placed at the vaginal introitus and pressures were recorded with a side-hole manometry catheter using a station (every 5 mm) pull-through technique. Pressures were recorded at rest and during voluntary squeeze. RESULTS: Anal canal high pressure zone was 39 +/- 1 mm in length. The IAS, EAS, and PRM were clearly visualized in the ultrasound images. EAS was located in the distal (length 19 +/- 1 mm) and PRM in the proximal part (length 18 +/- 1 mm) of the anal canal. The station pull-through technique revealed increases in pressure with voluntary squeeze in the proximal as well as distal parts of the anal canal. Proximal anal canal pressure, located in the PRM zone, showed greater circumferential asymmetry than the distal anal canal pressure, located in the EAS zone. CONCLUSIONS: (1) PRM contributes to the squeeze pressure in the proximal part of the anal canal and EAS to the distal anal canal. (2) PRM squeeze-related increase in anal canal pressure might be important in the anal continence mechanism.