Purse-string morphology of external anal sphincter revealed by novel imaging techniques.
The external anal sphincter (EAS) may be injured in 25-35% of women during the first and subsequent vaginal childbirths and is likely the most common cause of anal incontinence. Since its first description almost 300 years ago, the EAS was believed to be a circular or a "donut-shaped" structure. Using three-dimensional transperineal ultrasound imaging, MRI, diffusion tensor imaging, and muscle fiber tracking, we delineated various components of the EAS and their muscle fiber directions. These novel imaging techniques suggest "purse-string" morphology, with "EAS muscles" crossing contralaterally in the perineal body to the contralateral transverse perineal (TP) and bulbospongiosus (BS) muscles, thus attaching the EAS to the pubic rami. Spin-tag MRI demonstrated purse-string action of the EAS muscle. Electromyography of TP/BS and EAS muscles revealed their simultaneous contraction and relaxation. Lidocaine injection into the TP/BS muscle significantly reduced anal canal pressure. These studies support purse-string morphology of the EAS to constrict/close the anal canal opening. Our findings have implications for the effect of episiotomy on anal closure function and the currently used surgical technique (overlapping sphincteroplasty) for EAS reconstructive surgery to treat anal incontinence.
Division of Gastroenterology, Department of Medicine, University of California San Diego, San Diego, California;
Ravinder K
RK
Mittal
Valmik
V
Bhargava
Geoff
G
Sheean
Melissa
M
Ledgerwood
Shantanu
S
Sinha
D000328:Adult; D001003:Anal Canal / Q000033:anatomy & histology* / Q000000981:diagnostic imaging / Q000187:drug effects; D004576:Electromyography / Q000379:methods; D005242:Fecal Incontinence / Q000209:etiology; D005260:Female; D006801:Humans; D008012:Lidocaine / Q000494:pharmacology; D008279:Magnetic Resonance Imaging / Q000379:methods; D009119:Muscle Contraction / Q000502:physiology; D018482:Muscle, Skeletal / Q000187:drug effects; D014463:Ultrasonography
D008012:Lidocaine
anal incontinence; childbirth-related injury; external anal sphincter muscle architecture; magnetic resonance diffusion tensor imaging