Endosonography of the anal sphincters: normal anatomy and comparison with manometry

To determine the normal and anatomy in vivo, and endosonography and manometry were performed in 93 nulliparous females, and endosonography alone in 21 healthy males. Endosonography did not reveal any plane of cleavage between the components of the external and sphincter, though a changing pattern at different levels conforming to a trilaminar arrangement was apparent. The deep (proximal) aspect of the external sphincter was annular in 72% of females and 76% of males. The superficial external sphincter was elliptical in 76% and 86%, the subcutaneous part conical in 56% and 57%, respectively. The external sphincter was shorter anteriorly in females. Aberrant insertions from the external sphincter anteriorly were identified in 14%. The longitudinal muscle layer could be distinguished sonographically in all males, as the external sphincter was relatively hypoechoic, but in 60% of the females the longitudinal muscle and external sphincter were of similar echogenicity and sonographically indistinguishable. The subepithelial tissues and internal sphincter were identified in each subject. The external sphincter was thicker bilaterally (P = 0.001) in males (8.6 +/- 1 mm, mean +/- S.D.) compared to females (7.7 +/- 1.1), which related to the higher weight of the males (73 +/- 7 vs 65 +/- 11 kg, P < 0.0001). The mean maximum lateral thickness of the internal sphincter (1.8 +/- 0.5 vs 1.9 +/- 0.6) and the longitudinal muscle (2.5 +/- 0.6 vs 2.9) in females and males were not significantly different. There was no relationship between the manometric resting or squeeze pressures in the anal canal, and the internal or external sphincter thickness.