Anatomic and physiologic measurements of the internal and external anal sphincters in normal females

Objective: To assess the correlation between anal sphincter magnetic resonance imaging (MRI) measurements and manometric 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.