ANATOMIC AND PHYSIOLOGICAL MEASUREMENTS OF THE INTERNAL AND EXTERNAL ANAL SPHINCTERS IN NORMAL FEMALES

Objective: To assess the correlation between anal sphincter magnetic r esonance imaging (MRI) measurements and manometric anorectal vectograp hy pressures. Methods: Ten healthy, nulliparous women underwent anal s phincter MRI with examination of sagittal, axial oblique, and coronal planes. Anal manometry was performed with a radial eight-channel cathe ter. Customary functional measurements were recorded, including anteri or 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 sq ueeze 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 poster ior sphincter length by MRI (P = .042). The mean +/- standard deviatio n (SD) posterior sphincter length was 27.3 +/- 6.0 mm, Anatomically, t he cylindric shape of the anal sphincter is characterized by a gradual increase in muscle thickness cephalad. The external striated sphincte r 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 smo oth muscle sphincter was nearly equal in thickness anteriorly and post eriorly (9.0 +/- 1.4 mm and 9.6 +/- 1.7 min, respectively). Although v ariation in the thickness of both the smooth and striated muscle was f ound, manometric pressures did not correlate with the muscle thickness along the sphincter. Conclusion: The length of the anal sphincter cor related positively with the functional information, as determined by m anometry. An anal sphincter length of 3 cm is consistent, from an anat omic and functional view, in these ten normal women. (C) 1998 by The A merican College of Obstetricians and Gynecologists.