Anal endosonography in asymptomatic subjects.

The anal sphincter thickness was measured ultrasonographically in 30 asymptomatic subjects (13 male and 17 female: 23 young and 7 old). The internal anal sphincter (IAS) was measured at its thickest portion in the anal canal, its borders lying between the submucosa and the external and sphincter (EAS). The EAS was measured at the lowermost point in the anal canal, its borders being the IAS and the coccyx. The EAS was thicker in men than in women (p < 0.02). The IAS showed no difference with regard to gender. The EAS became thinner with age (p < 0.005), whereas the IAS appeared thicker with age (p < 0.0002). The EAS thickness correlated inversely with the IAS thickness (p < 0.001). Anorectal manometry showed a correlation between the anal canal resting pressure and the sum of the thickness of the two sphincters (p < 0.001). Thus, although the visualized thickness of the two sphincters vary, they do so proportionally, and their sum reflects their tonic activity.