Purse-string morphology of external anal sphincter revealed by novel imaging techniques

Abstract

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.

Keywords: anal incontinence; childbirth-related injury; external anal sphincter muscle architecture; magnetic resonance diffusion tensor imaging.

Fig. 1.

Schematic of our hypothesis (A) and sagittal and transverse ultrasound (US) images of the anal canal (B and C). A: schematic of conventional external anal sphincter (EAS) muscle architecture compared with our proposed model. B: sagittal US images of the anal canal. Note elliptical shape of the perineal body (PB), which has a well-defined structure. C: 16 serial axial images of the anal canal, spaced 15 mm apart. Note circular configuration of the internal anal sphincter (IAS), while the EAS merges with the transverse perineal (TP)/bulbospongiosus (BS) muscle at the ventral end. Cranial-caudal extent of the EAS and TP/BS is similar, and the EAS and TP/BS are continuous at the ventral end in the PB.

Fig. 2.

Proton density (PD) MRI of the anal canal (A–H) and 3-dimensional (3-D) reconstruction of the EAS muscle complex (I–N). In A–D, axial images of the anal canal (6 mm apart, from cranial to caudal direction) show various components of the anal sphincter complex. In E–H, muscle margins were marked manually to construct the 3-D anatomy (viewed from caudal to cranial direction, in 4 different subjects, I–N). I–K: images from 1 subject shown in 3 different projections. L–N: anatomy of the EAS muscle in the caudal-cranial projection from 3 additional subjects.

Fig. 3.

PD MRIs and diffusion tensor (DT) images of the EAS complex. A and B: PD images (3-mm thick, 0.4 mm apart) and corresponding DT images of 4 axial slices of the anal canal from cranial to caudal direction in 1 subject. C: DT images superimposed on PD images. DT images were calculated from diffusion-weighted images and placed closest to the location of PD images (since DT image slices were 5-mm thick and separated by 0.4 mm). PB identified in A (arrow) corresponds to that in Fig. 2B. Tensor images show direction of the principal eigenvector, which corresponds to direction of the muscle fiber in each voxel. Colors represent direction of the eigenvector: blue, cranial-caudal; red, left-right (LR); green, ventral-dorsal [anterior-posterior (AP)]. In 2nd and 3rd panels in B and C, mainly red color of the PB indicates left-to-right-oriented muscle fibers. In 1st and 3rd panels of B, mixed colors in PB suggest crossing of muscle fibers. All DT images show ventral-dorsal orientation of the EAS muscle fibers in lateral positions and no crossing of muscle fibers in the dorsal region. AP, anterior-posterior; LR, left-right; SI, superior-inferior or craniocaudal.

Fig. 4.

A: fiber tracking of the EAS muscle complex. Fiber tracking, utilizing the tensor images in Fig. 3, illustrates results obtained in 4 different subjects (a–d). Colors of the fibers at different points are indicative of their directions, as shown in d. As shown in a–d, EAS muscle fibers cross over from one side to the other in the PB to continue as the TP/BS muscles. Dorsally and caudally, muscle fibers continue as the anococcygeal raphe. Regions of the IAS and anal canal are depicted as a signal void in a–d. B: schematic of the EAS muscle complex. On the basis of US images, PD MRIs, DT images, and magnetic resonance fiber tracking, we propose “purse-string” morphology of the EAS complex.

Fig. 5.

Method of dynamic MRI and results. A: schematic showing signal processing to provide an MRI trigger used for gating the magnetic resonance imager. Audio-video prompt (a) is generated by the computer. In response, the subject generates an anal pressure trace (b, in red). This pressure signal is band-pass-filtered and differentiated (c, in green). Finally, a trigger is generated on the basis of the threshold amplitude of the 1st derivative (black spikes). This signal is used to trigger the magnetic resonance imager. Bottom trace: average anal pressure plot, along with timing (vertical lines) of the 3 magnetic resonance-tagged images in B, C, and D. B–D: 3 different phases (10, 690, and 1,020 ms) of the pressure (contraction/relaxation) cycle, after the start of the spin tag (top). Regions of the EAS, IAS, and anococcygeal raphe with the tag lines are highlighted to show distortions of the tag lines, depicting motion in different regions (bottom). With EAS relaxation, 1) the entire anal complex moves dorsally, 2) tag lines at the inner edge of the EAS, adjacent to the IAS, move dorsally significantly less than tag lines at the outer edge (away from the IAS) of the EAS (25% vs. 6%), and 3) tag lines do not distort within the IAS region. If the muscle fibers of the EAS were circularly oriented and contracted/relaxed isometrically, one would not observe differential movement across its transverse thickness, unlike movement in these images of EAS region. Finally, the direction of distortion of the EAS tag line on either side, when viewed in combination, indicates that the directions of the fibers cross in the PB. E: superimposed tag lines from B and D, bottom. Green region with black lines represents data from B; pink region with red and orange lines represents data from D. Note displacement of the IAS region with minimal distortion of tag lines and distortion of tag lines in the EAS region.

Fig. 6.

A: electromyographic (EMG) recording of the TP/BS muscle. Note tonic EMG activity (spikes) in EAS and TP/BS muscles at rest. Before lidocaine injection, push (defecatory) maneuver resulted in a decrease and voluntary contraction (squeeze) resulted in an increase in EMG activity in both muscles. Lidocaine injection into the TP/BS muscle abolishes EMG activity in the TP/BS muscle without affecting the EAS muscle, at rest and during the squeeze maneuver. B: anal canal pressures recorded by high-definition anal canal manometry before and after lidocaine injection into the TP/BS muscle. Anal canal pressures were recorded during voluntary squeeze before and 15 min after lidocaine injection into the TP/BS muscle. Subjects were instructed to make a similar voluntary effort (confirmed by amplitude of the EMG signal) before and after injection. Reduction of anal canal pressure after lidocaine injection was observed in the lower half of the anal canal. In subject 4, the cranial-caudal extent of the pressure profile decreased significantly because of marked reduction of EAS contraction-related pressure. Bar graphs show effect of lidocaine on anal canal pressure.