MRI analysis shows three-part external anal sphincter

December 1, 2005

Montreal--Putting to rest a long-standing debate about the number of elements that the external anal sphincter (EAS) contains, magnetic resonance imaging and three-dimensional modeling here at the annual meeting of the International Continence Socity shows the EAS has three components, according to researchers from the University of Michigan, Ann Arbor.

Montreal-Putting to rest a long-standing debate about the number of elements that the external anal sphincter (EAS) contains, magnetic resonance imaging and three-dimensional modeling here at the annual meeting of the International Continence Socity shows the EAS has three components, according to researchers from the University of Michigan, Ann Arbor.

Previous data on the structure of the EAS have come from anatomists who have conducted cadaveric dissections.

Dr. Hsu, working with John O.L. DeLancey, MD, and Dee E. Fenner, MD, conducted detailed three-dimensional re-construction of the EAS from MRIs of three subjects with clear anal sphincter anatomy. The subjects were participating in an ongoing study on the pelvic anatomy of women who hadn't delivered children.

The images of the three subjects were imported into a 3-D modeling program. Outlines of each visible component were traced to create 3-D models in the three subjects.

The criteria that researchers used for establishing separate components were the presence of a clear separation visible between one element and adjacent structures, or differing muscle origin or insertion. They cataloged characteristic features of the EAS components from different scan planes, including axial, coronal, and sagittal.

Researchers morphologically reviewed the magnetic resonance scans of 50 women who hadn't delivered children to ensure consistency with the results of 3-D modeling in the three study participants. Two senior examiners independently reviewed tracings of the structures and the 3-D models.

"The MRI is a great method of doing this because we don't disrupt the anatomy, and we can do the modeling on living women," said Dr. Hsu, the study's principal investigator and a member of the University of Michigan's pelvic floor research group. "It's very difficult when dissecting a cadaver to separate the subdivisions."

Visibly different features

The winged portion (EAS-w) has fibers that are directed ventrally and laterally in a similar orientation to the puborectalis muscle. However, the winged portion's fibers form a distinct body of muscle visibly separate from the puborectalis, which substantiates their anatomic designation as part of the anal sphincters. Indeed, the discovery of a winged portion of the EAS was surprising, Dr. Hsu said.

"In 62% of patients, you can see a distinct separation between that and the puborectalis muscle," she said. "That led us to conclude that it was not an aberration, but was a distinct subdivision of the external anal sphincter."

The study's findings have no immediate clinical relevance, but knowledge of the anatomy of the EAS will be of benefit in the longer term, according to Dr. Hsu.

"While there isn't a clinical application of this finding now, the hope is that, if you can see a specific subdivision, then you can correlate the disruption in that specific subdivision with symptoms," she said. "These are preliminary data in the hope of doing that in the future."

Amanda Clark, MD, associate professor in the department of obstetrics and gynecology at the Oregon Health and Science University in Portland, commented on the utility of the research.

"My impression has always been that the external anal sphincter is an annular ring, and this research shows that it is not," Dr. Clark said. "This is important to know because it helps us to know how to approach it clinically."