Structure-function relationship of the human external anal sphincter

Abstract

Introduction and hypothesis: Obstetrical external anal sphincter (EAS) injury and subsequent dysfunction are leading risk factors for female fecal incontinence (FI). Limited knowledge of the EAS structure-function relationship hinders treatment optimization. We directly measured functionally relevant intrinsic parameters of human EAS and tested whether vaginal delivery alters the EAS structure-function relationship.

Methods: Major predictors of in vivo EAS function were compared between specimens procured from vaginally nulliparous (VN, n = 5) and vaginally parous (VP, n = 7) cadaveric donors: operational sarcomere length (L_s), which dictates force-length relationship; physiological cross-sectional area (PCSA), which determines isometric force-generating capacity; fiber length (L_fn), responsible for muscle excursion and contractile velocity; and muscle stiffness. Data were analyzed using unpaired and paired t tests, α < 0.05. Results are presented as mean ± SEM.

Results: The VN and VP (median parity 3) groups were similar in age and BMI. No gross anatomical defects were identified. EAS L_s (2.36 ± 0.05 μm) was shorter than the optimal L_so (2.7 μm), at which contractile force is maximal, P = 0.0001. Stiffness was lower at L_s than L_so (5.4 ± 14 kPa/μm vs 35.3 ± 12 kPa/μm, P < 0.0001). This structural design allows active and passive tension to increase with EAS stretching. EAS relatively long L_fn (106 ± 24.8 mm) permits rapid contraction without decreased force, whereas intermediate PCSA (1.3 ± 0.3 cm²) is conducive to maintaining resting tone. All parameters were similar between groups.

Conclusions: This first direct examination of human EAS underscores how EAS intrinsic design matches its intended function. Knowledge of the EAS structure-function relationship is important for understanding the pathogenesis of FI and the optimization of treatments for EAS dysfunction.

Keywords: External anal sphincter; Fecal incontinence; Muscle architecture.

Fig. 1

Human cadaveric en bloc external anal sphincter with arrows indicating divisions used for thorough sampling

Fig. 2

Human skeletal muscle active length–tension curve and external anal sphincterpassive tension curve, with sarcomere length in micrometers (μm) on the x-axis, percentage of maximum active muscle tension on the left y-axis, and stress in kilopascals (kPa) on the right y-axis. Maximum contractile force is produced at the optimal sarcomere length, 2.7 μm, corresponding to the plateau region of the active length–tension curve

Fig. 3

Scatterplot of external anal sphincter normalized fiber length in millimeters (mm), which determines excursion and contractile velocity, vs physiological cross-sectional area (PCSA), a predictor of isometric force-generating capacity, comparing vaginally nulliparous (VN) and vaginally parous (VP) groups. Individual pelvic floor muscles, coccygeus (C), iliococcygeus (IC), and pubovisceralis (PV) are shown for comparison [21]