Exoanal ultrasound of the anal sphincter: normal anatomy and sphincter defects

Objective To describe the sonographic appearance of normal anal sphincter anatomy and sphincter defects evaluated with a conventional 5 MHz convex transducer placed on the perineum.

Design Prospective, single-blind study.

Setting Department of Obstetrics and Gynecology, University of Michigan Medical Center, USA.

Population Twenty-five women with symptoms of faecal incontinence, 11 asymptomatic nulliparous women, and 32 asymptomatic parous women.

Methods A convex scanner was placed on the perineum with the woman in lithotomy position. Images were taken at three levels of the sphincter canal. Pictures were evaluated by two examiners who were blinded to the case history of the women and to the results of each other for the presence or absence of sphincter defects.

Main outcome measures Description of anal sphincter appearance on endoanal ultrasound. Reproducibilty of the evaluation of sphincter defects.

Results The internal anal sphincter is visible as a hypoechoic circle; the external anal sphincter shows a hyperechoic pattern. Proximally the sling of the puborectalis muscle is visible. Sphincter defects were detected in 20 women. In all five women who subsequently underwent surgery, the presence and location of the defect was confirmed at the time of surgery. Examiners were in agreement 100% of the time on the presence or absence of internal defects. They disagreed in one patient on the presence of an external defect.

Conclusion Exoanal ultrasound provides information on normal anatomy and on defects of the anal sphincter.