Objective Mapping of the Anal Sphincter Complex Using 3-d Endoanal Ultrasound

Introduction: Despite the subjectivity of the method, endoanal ultrasonography (EAUS) is the gold standard to evaluate the external (EAS) and Internal (IAS) anal sphincters. Differentiating a sphincter defect from normal anatomic variation is challenging. We developed a new standardized protocol to evaluate the anal sphincter complex and establish normal values of lengths and thickness of the EAS and IAS for men and women. Methods: 19 nulliparous women and 19 men were recruited to the study. Eligible subjects had no history of anal trauma or surgery. EAUS was done with a 3-dimensional endo-probe and 14 mHz transducer. To minimize recall bias, interpretation of the 3-D “image blocks” was done by one examiner (JN) at least 3 months after clinical examination and EAUS. We measured the length of the EAS and IAS in the anterior and posterior midlines and the thickness of the muscles at 10 mm from the distal end of the IAS as well as at mid EAS in 4 quadrants (20 measurements per subject). Means and standard deviations were calculated for the measurements and Wilcoxon signed-rank test was used to compare means of the two groups. To account for anatomic variation, a sphincter score was calculated by dividing the anterior lengths by posterior lengths for EAS and IAS. Results: Mean length (mm) of the anterior EAS was 17.2 (sd 3.13) for men and 12.6 (sd 3.08) for women (p=0.00043) versus posterior lengths of 21.1 (sd 3.81) for men and 18.4 (sd 3.60) for women (p=0.05036). Mean length (mm) of the anterior IAS was 15.3 (sd 4.01) for men and 11.6 (sd 2.52) for women (p=0.004545) versus posterior lengths of 16.8 (sd 3.38) for men and 13.4 (sd 3.52) for women (p=0.006598). Men’s mean anterior mid-EAS thickness was 4.1 mm (sd 1.81) compared to 2.8 mm (sd 0.81) for women (p=0.0089). The EAS sphincter score was .84 for men vs .70 for women (p=0.0576). Conclusions: Men’s EAS and IAS are longer than women’s and thicker anteriorly. Objective mapping of the anal sphincter with 3-D EAUS may improve the evaluation of patients with suspected anal sphincter defects.

Session: Poster

Program Number: P190

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