Anatomic and Physiologic Measurements of the Internal and External Anal Sphincters in Normal Females

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Abstract

Objective

To assess the correlation between anal sphincter magnetic resonance imaging (MRI) measurements and manometriic anorectal vectography pressures.

Methods

Ten healthy, nulliparous women underwent anal sphincter MRI with examination of sagittal, axial oblique, and coronal planes. Anal manometry was performed with a radial eight-channel catheter. Customary functional measurements were recorded, including anterior and posterior sphincter length, squeeze length, length of the high-pressure zone, and maximal resting and squeeze pressures. The Spearman rank correlation coefficient was used to assess correlation.

Results

The manometric squeeze length and the manometric length to maximum squeeze pressure were correlated negatively with the posterior sphincter length by MRI (P = .049 and .044, respectively). The manometric high-pressure zone squeeze length was correlated positively with the posterior sphincter length by MRI (P = .042). The mean ± standard deviation (SD) posterior sphincter length was 27.3 ± 6.0 mm. Anatomically, the cylindric shape of the anal sphincter is characterized by a gradual increase in muscle thickness cephalad. The external striated sphincter was much thicker posteriorly (24.7 ± 4.6 mm) than anteriorly (6.6 ± 1.7 mm) in the proximal or caudal third. The proximal internal smooth muscle sphincter was nearly equal in thickness anteriorly and posteriorly (9.0 ± 1.4 mm and 9.6 ± 1.7 mm, respectively). Although variation in the thickness of both the smooth and striated muscle was found, manometric pressures did not correlate with the muscle thickness along the sphincter.

Conclusion

The length of the anal sphincter correlated positively with the functional information, as determined by manometry. An anal sphincter length of 3 cm is consistent, from an anatomic and functional view, in these ten normal women.

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