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Female Anal Sphincter: Age-related Differences in Asymptomatic Volunteers with High-Frequency Endoanal US¹

¹ From the Intestinal Imaging Centre (A.F., S.H., C.I.B.), Department of Cellular Pathology (A.B.P.), and Physiology Unit (A.F., M.A.K.), St Mark’s Hospital, Level 4V, Northwick Park, Watford Rd, Harrow, London HA1 3UJ, England; and Department of Obstetrics and Gynecology, University Graz, Austria (A.F., R.W.). Received May 31, 2001; revision requested June 18; revision received August 27; accepted October 10. Address correspondence to S.H. (e-mail: s.halligan@ic.ac.uk)

PURPOSE: To evaluate endoanal ultrasonographic (US) anatomy in a large group of nulliparous women by using a high-frequency 10-MHz transducer to define normal age-related differences in sphincter morphology.

MATERIALS AND METHODS: One hundred fifty asymptomatic nulliparous women (mean age, 31 years; range, 19–80 years) underwent endoanal US with a high-frequency 10-MHz transducer. Anal canal structures were measured at high, middle, and low levels and were correlated with age by using the Pearson simple linear correlation coefficient.

RESULTS: Internal sphincter thickness showed a highly significant positive correlation with age at both sites at which it was measured (high anal canal, r = 0.34, P < .001; middle anal canal, r = 0.33, P < .001). External sphincter thickness showed a highly significant negative correlation with age at all sites measured (high anal canal, r = -0.65, P < .001; middle anal canal, r = -0.49, P < .001; low anal canal, r = -0.21, P = .012). There was no significant correlation between age and thickness of subepithelial tissue, longitudinal muscle, or puborectalis muscle. Subjects whose internal sphincter showed mixed echogenicity were significantly older than those whose internal sphincter was uniformly hypoechoic (mean, 47.4 vs 34.6 years; P < .001). Subjects with mixed internal sphincter echogenicity also had a significantly thinner external sphincter at high (mean thickness, 3.8 vs 4.6 mm; P < .001) and middle (mean thickness, 3.7 vs 4.1 mm; P = .03) anal canal levels.

CONCLUSION: At older ages there are increased internal anal sphincter thickness and decreased external anal sphincter thickness. Diagnosis of external sphincter atrophy on the basis of sphincter thinning requires that one distinguish between abnormal thinning and age-related differences.

© RSNA, 2002

Index terms: Aging • Anus, abnormalities, 757.79 • Anus, US, 757.12981

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