Functional morphology of anal sphincter complex unveiled by high definition anal manometery and three dimensional ultrasound imaging

Background Anal sphincter complex consists of anatomically overlapping internal anal sphincter (IAS), external anal sphincter (EAS) and puborectalis muscle (PRM). We determined the functional morphology of anal sphincter muscles using high definition anal manometery (HDAM), three dimensional (3D)-ultrasound (US) and Magnetic resonance (MR) imaging. Methods We studied 15 nulliparous women. High definition anal manometery probe equipped with 256 pressure transducers was used to measure the anal canal pressures at rest and squeeze. Lengths of IAS, PRM, and EAS were determined from the 3D-US images and superimposed on the HDAM plots. Movements of anorectal angle with squeeze were determined from the dynamic MR images. Key Results High definition anal manometery plots reveal that anal canal pressures are highly asymmetric in the axial and circumferential direction. Anal canal length determined by the 3D-US images is slightly smaller than that measured by HDAM. The EAS (1.9 +/- 0.5 cm long) and PRM (1.7 +/- 0.4 cm long) surround distal and proximal parts of the anal canal, respectively. With voluntary contraction, anal canal pressures increase in the proximal (PRM) and distal (EAS zone) parts of anal canal. Posterior peak pressure in the anal canal moves cranially in relation to the anterior peak pressure, with squeeze. Similar to the movement of peak posterior pressure, MR images show cranial movement of anorectal angle with squeeze. Conclusions & Inferences Our study proves that the PRM is responsible for the closure of the cranial part of anal canal. HDAM, in addition to measuring constrictor function can also record the elevator function of levator ani/pelvic floor muscles.