Ultrasonographic Assessment of Perianal Crohns Disease

Accurate identification of all loculate purulent areas and definition of the anatomy of the primary fistulous tract, secondary extensions and internal opening is crucial for the treatment plan. EAUS has been demonstrated to be a very helpful diagnostic tool in accurately assessing all fistula or abscess characteristics [8-11, 16, 17]. Ultrasound examination with the 2050 transducer is generally started using 13 MHz, changing to 12 MHz or 9 MHz to optimise visualisation of the deeper structures external to the anal sphincters. The puborectalis muscle and EAS, LM and IAS should always be identified and used as referents for the spatial orientation of the fistula or abscess.

An anal abscess appears as a hypoechoic, disho-mogeneous area, sometimes with hyperechoic spots within it, possibly in connection with a fistulous tract directed through the anal canal lumen. Infection can spread in a number of directions, usually along the path of least resistance. Abscesses are classified as superficial, intersphincteric (Fig. 14), ischiorectal

Fig. 13. Three-dimensional endosonographic reconstruction demonstrating that the anterior anal sphincter is shorter in the female: male (a), female (b). Sagittal images: male (c), female (d)

Anus Abscess

Fig. 13. Three-dimensional endosonographic reconstruction demonstrating that the anterior anal sphincter is shorter in the female: male (a), female (b). Sagittal images: male (c), female (d)

(Fig. 15), supralevator (Figs. 16,17), pelvirectal and horseshoe (Fig. 18) [16]. An anal fistula appears as a hypoechoic tract, which is followed along its crossing of the subepithelium, internal or external sphincters and through the perianal spaces [16]. With regard to the anal sphincters, according to the classification by Parks et al. [4], the fistulous primary tract can be classified into four types:

1. Intersphincteric tract, which is presented as a band of poor reflectivity within the longitudinal layer, causing widening and distortion of an otherwise narrow intersphincteric plane (Fig. 19). The tract goes through the intersphincteric space without traversing the EAS fibres (Fig. 20).

2. Transsphincteric tract, in which the extension through the EAS is clearly shown by a poorly reflective tract running out through the EAS and disrupting its normal architecture (Figs. 21, 22).

The point at which the main tract of the fistula traverses the sphincters defines the fistula level (high, medium or low). The low transsphincteric tract traverses only the distal third of the EAS at the lower portion of the medium anal canal. The medium transsphincteric tract traverses both sphincters, external and internal, in the middle part of the medium anal canal (Fig. 23). The high transsphincteric tract traverses both sphincters in the higher part of the medium anal canal in the space below the puborectalis (Fig. 24).

3. Suprasphincteric tract, which goes above or through the puborectalis level (Fig. 25).

4. Extrasphincteric tract, which may be seen close to but more laterally placed around the EAS (Fig. 26).

Rectovaginal fistula appears as a hypoechoic tract connecting the rectal wall and the vagina. It can be detected on endorectal ultrasonography with a stan-

Eas Defect Ultrasound
Fig. 15. Acute abscess in the right ischiorectal space

Fig. 16. Acute supralevator abscess presenting as an area of low reflectivity deep to the puborectalis muscle

Crohn Disease Fistula

dard water balloon, performed immediately after EAUS. Anovaginal fistulae are frequently small and collapsed, and their assessment may be problematic for both noncontrast and contrast-enhanced EAUS (Fig. 27). On the other hand, EAUS is very useful in the recognition of coexisting occult sphincter defects.

The major problems while investigating primary tracts with EAUS occur because of the structure alterations of the anal canal and perianal muscles and tissues, which can overstage the fistula, or poor definition of the tract when filled with inflammatory tissue, which can downstage the fistula. Differentiation between granulated tracts and scars is sometimes difficult. Straight tracts are easily identified, but smaller and oblique tracts are more difficult to image. Secondary tracts, when present, are related to the main one and are classified as intersphincteric, transsphincteric, suprasphincteric or extrasphinc-

Puborectalis Muscle

Fig. 17. Acute supralevator abscess presenting as an area of low reflectivity in the right side of the anal canal deep to the puborectalis muscle (a). Three-dimensional reconstruction in the coronal plane (b). Volume render mode (c)

teric. Similarly, horseshoe tracts, when identified, are categorised as intersphincteric, suprasphincteric or extrasphincteric (Fig. 28-30). The use of 3-D EAUS, which enables reconstruction of transversal images of the anal canal in the coronal and sagittal planes, is very helpful in tracing the pathway of a tract.

The dentate line is not visible as an anatomical structure but is assumed to be just below the midpoint of the IAS. For this reason, the internal opening of an anal fistula is seldom clearly defined. Endosonographic criteria for the site of an internal opening, according to Cho [17], are the following: (1) an appearance of a root-like budding formed by the intersphincteric tract, which contacts the IAS; (2) an appearance of a root-like budding with an IAS defect; (3) a subepithelial breach connecting to the inter-sphincteric tract through an IAS defect. The site is categorised as being above, at or below the dentate

Photos Perianal Crohns Disease
Fig. 18. Horseshoe collection in the intersphincteric space

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