Normal female anal sphincter: difficulties in interpretation explained

Our aims were to quantify the nature, characteristics, and frequency of variations in female anal sphincter anatomy.

Nulliparous patients from the antenatal clinic and healthy volunteers of both genders were studied. Sphincter length was determined by the position of the puborectalis sling. Defects in the external anal sphincter were defined at each level and recorded in degrees. Cylindric longitudinal images of the endoanal scans were created by a three-dimensional-representation software package. Manometry was performed by a pull-through technique.

Fifty-seven nulliparous patients and 18 healthy volunteers were included in the study. The mean age was 39 years for males and 28.35 years for females. There was no significant difference in overall sphincter length or in the internal anal sphincter length as a percentage of overall sphincter length between genders. All nine males had a complete ring of external anal sphincter along the full sphincter length. In the external anal sphincter below the level of the puborectalis sling, a natural gap occurred in 43 nulliparous (75 percent) and all 9 female volunteers. The greater the size of the defect, the greater its extent (mean 1.33 cm for >90 degrees and 1.16 cm for <90 degrees; chi-squared P = 0.008, eight degrees of freedom). Manometry provided confirmatory evidence of the gaps seen. Anal manometry was analyzed by Mann-Whitney U test for continuous nonparametric data and t-test for comparison between genders.

The female sphincter has a variable natural defect occurring along its anterior length. This makes interpretation of the isolated endoanal ultrasound difficult and explains previous overreporting of obstetric sphincter defects.