M m

Fig. 20. Intersphincteric fistula with internal opening at 6 o'clock position and horseshoe extension in the left lateral intersphincteric space line (in relation to the presumed location of the dentate line at the middle third of the anal canal) or in the rectal ampulla. In addition, the site of the internal opening can also be characterised by the clock position, being classified from 1 o'clock to 12 o'clock. The internal opening can be identified as hypoechoic (when acute inflammation is present) or hyperechoic (when chronically inflamed).

After standard EAUS examination, in patients in whom the external fistula opening is patent,

Fig. 19. Intersphincteric fistula at 3 o'clock position
Fig. 21. Posterior transsphincteric tract extending through the external sphincter

1.0-2.0 ml of 3% hydrogen peroxide (HP) can be injected very slowly using an 18-gauge plastic cannu-la via this opening while ultrasonic scanning of the anal canal is performed [16]. When no obvious external opening is present, a focal, elevated erythematous region immediately adjacent to the anal orifice is frequently identified. The soft catheter tip should be firmly pressed onto or probed into the center of this region, where the skin is easily broken, and the external opening located. Gas is a strong ultrasound

Fig. 22. Anterior transsphincteric tract at the level of transverse perineii

Fig. 23. Three-dimensional reconstruction in the sagittal plane showing a transsphincteric fistula that traverses the middle part of the external sphincter

Fig. 22. Anterior transsphincteric tract at the level of transverse perineii

Fig. 23. Three-dimensional reconstruction in the sagittal plane showing a transsphincteric fistula that traverses the middle part of the external sphincter reflector, and after injection, fistula tracks become hyperechoic, and the internal opening is identified as an echogenic breach at the submucosa (Figs. 31, 32). This method can be particularly useful when an active fistulous tract needs to be distinguished from postsurgical or posttraumatic scar tissue that can cause tissue alterations that are difficult to analyse. During this technique, however, the operator must be careful because the injected HP often results in bubbling into the anal canal, which then acts as a bar rier to the ultrasound wave. Another disadvantage inherent to HP injection is the very strong reflection that occurs at a gas/tissue interface, which blanks out any detail deep into this interface. The bubbles produced by HP induce acoustic shadowing deep into the tract, so all information deep into the inner surface of the tract is lost. To reduce this potential pitfall of imaging, a volume-rendered 3-D data set can facilitate the following of a tortuous fistula tract due to the transparency and depth information (Fig. 33) [16].

Fig. 24. Three-dimensional reconstruction in the sagittal Fig. 25. Suprasphincteric tract extending through the pubplane showing a high transsphincteric tract traversing both orectalis muscle sphincters in the higher part of the anal canal
Fig. 26. Extrasphincteric fistula with direct communication between the perineum and rectum and no anal canal involvement
Fig. 27. Anovaginal fistula (arrow)

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