MRI studies identify three components of external anal sphincter

September 1, 2005

With the help of magnetic resonance imaging, University of Michigan researchers have determined that the external anal sphincter (EAS) has three different components: a main component (EAS-M), a lateral wing portion (EAS-W), and a subcutaneous component (SQ-EAS).

With the help of magnetic resonance imaging, University of Michigan researchers have determined that the external anal sphincter (EAS) has three different components: a main component (EAS-M), a lateral wing portion (EAS-W), and a subcutaneous component (SQ-EAS).

Investigators studied MRIs of 50 women who hadn't had children and conducted detailed 3-dimensional reconstruction of the EAS of three other subjects with clear anal sphincter anatomy. Outlines of each visible component were traced to create 3-D models in the three subjects. The criteria 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.

"The previous data has been gathered from anatomists who have conducted cadaveric dissections," said Yvonne Hsu, MD, a third-year urogynecology fellow at the University of Michigan, Ann Arbor. "It's very difficult when dissecting a cadaver to separate the subdivisions."

The precise number of subdivisions of the EAS has been a considerable source of controversy, with speculations including one, two, three or four components, Dr. Hsu said. She and colleagues morphologically reviewed the MRI scans of 50 subjects to ensure consistency with the results of 3-D modeling in three subjects.

"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. "This is preliminary data in the hopes of doing that in the future."

In particular, the discovery of a winged portion of the EAS was surprising, said Dr. Hsu."In quite a few patients, you can see a distinct separation between that and the puborectalis muscle. That led us to conclude it's part of the external anal sphincter," she said.