Three-dimensional endoanal sonography in assessing anal canal injury

Instrument design limits endosonography of the anal canal to the axial plane, with no capability for longitudinal imaging or measurement. Using three-dimensional reconstructions, the relationship between the radial and linear extent of an anal sphincter tear has been explored, and sex differences in anal canal and sphincter length have been established.

Three-dimensional reconstructions were performed in 20 controls and 24 patients with faecal incontinence found to have 25 external and five internal sphincter defects. The radial and linear extent of any sphincter tear was measured. In controls the length of the sphincters was compared with the total anal canal length, and the maximum and mean internal sphincter thickness was compared.

The radial angle of an internal or external sphincter defect was significantly related to its length (R2 = 96.8 per cent and R2 = 84.4 per cent respectively; both P < 0.001). The anal canal was longer in men than in women (mean(s.d.) 32.6(5.3) versus 25.1(3.4) mm; P < 0.001). The internal anal sphincter was also longer in men (25.6(6.3) versus 19.8(4.0) mm; P < 0.02), but the mean internal sphincter length as a percentage of total anal canal length did not differ (78.3 versus 78.7 per cent; P not significant). The anterior external anal sphincter was longer in men than in women (32.6(5.3) versus 15.3(2.8) mm; P < 0.001), and formed a greater percentage of total anal canal length (100 versus 62.9 per cent; P < 0.001).

Multiplanar imaging has revealed a direct relationship between the length of a sphincter tear and its radial extent as shown on axial scanning. Marked sex differences in sphincter configuration have been demonstrated. In women the shorter anterior sphincter length highlights the risk of complete sphincter disruption with extensive tears.