Perianal Fistulas

Perianal fistulas primarily occur as the result of fistulous disease originating in the anal glands near the anal crypts (cryptoglandular hypothesis), or in patients with Crohn's disease [13]. Infection of the anal glands may result in abscess formation. It is a relatively common condition with a prevalence of approximately 0.01%, predominantly affecting young adults.

The tracks may have a simple superficial course, or may have a complicated course. The complicated tracks may be intersphincteric (through the internal anal sphincter and then downward through the intersphinc-teric space), transsphincteric (transversing not only the internal sphincter but also the external sphincter or pu-borectis muscle), supralevator, or extrasphincteric (extension to the rectum without involvement of the anal sphincter). These complicated tracks require detailed imaging for proper therapy, as inadequate treatment may lead to recurrent disease. The surgeon must be aware of the presence and number of tracks and their extent, the location of internal opening, and presence of abscesses.

Preoperative evaluation of these fistulas may include physical examination, examination under anesthesia (EUA), EUS, or MRI. Physical examination has significant shortcomings, especially in patients with recurrent disease. EUA can be used to determinine the extent of the disease, immediately followed by treatment. However, EUA has limitations and disadvantages, mostly related to probing. Firstly, not all fistulas have an external opening that can be probed, and probing may miss secondary tracks. It is well-recognized that missed extensions are the commonest cause of recurrence, which reaches 25% in some series. Forceful probing may lead to perforation of the levator plate, worsening the extent of the disease. Patients with recurrent disease are likely to harbor missed disease, but are also most difficult to assess. Digital palpation frequently cannot distinguish between scarring due to repeated surgery and induration due to an underlying extension.

EUS, which can be enhanced by hydrogen peroxide installation in the track, may be used to determine disease extent. Initial reports were encouraging, but later studies have been less sanguine, especially when comparison is made to MRI. This discrepancy probably relates to operator expertise, since EUS is highly operator dependent. Insufficient penetration beyond the external sphincter, especially with high-frequency transducers, limits the ability to resolve ischioanal and supralevator sepsis, with the result that EUS may miss extensions from the primary tract. EUS is useful for demonstrating the internal opening. However, it is often difficult to differentiate infection from postoperative fibrosis by EUS.

MRI has been proven to provide the most comprehensive assessment of patients with perianal fistulas, facilitating accurate identification of tracks and extensions as well as abscesses. An MRI examination for perianal fistulas should include T2-weighted sequences in multiple planes, a fat-saturation sequence, and preferably a contrast-enhanced (fat-saturation) T1-weighted sequence [14].

Tracks (Fig. 6) are identified at T2 as hyperintense longitudinal structures, often with a hypointense, fibrous wall. Some collateral inflammation is often appreciated at fat saturation sequences. After intravenous contrast medium administration the lining of the wall enhances. Non-enhancing fluid can be identified in the center of the track or the track can be completely obliterated by granulation tissue. In the latter case, there is complete en-

Fig. 6. A male patient with Crohn's disease and perianal fistulas. a Axial T2-weight-ed turbo spin-echo demonstrates multiple tracks (arrowheads), both intersphincteric as well as outside the anal sphincter. At the right an abscess (A) is seen in the ischioanal space. b Coronal oblique T2-weight-ed TSE demonstrates the abscess with supralevator extension (arrow)

hancement of the part of the track that is hyperintense at T2 (Fig. 7). Abscesses are readily appreciated on fat saturation sequences, although some small fluid collections can be more difficult to identify. External phased array coils are used, which may have limitations for the detection of superficial extensions and difficulty in locating the precise level on the internal opening. Endoluminal MRI may provide more information on these findings.

Results of a prospective triple-blinded comparison of the accuracy of AES, pelvic MRI, and surgical EUA in perianal Crohn's disease showed that AES correctly classified fistulas in 91% of cases, compared with 87% for pelvic MRI, and 91% for surgical evaluation [15]. Combination of any two of the three modalities increased accuracy to 100%. Another study, in which MRI and AES were compared with surgical findings, showed MRI to be superior to AES for classification of fistulas, with sensitivities of 84% versus 60% for the two modalities, and specificities of 68% and 21%, respectively.

Several studies have indicated the positive effect of preoperative MRI on patient outcome. In one study, the therapeutic effect of MRI before EUA was 21.1% [16]. Additionally, it was shown that recurrence of disease after surgery could be reduced by about 75% if surgery was guided by MRI.

The differential diagnoses of perianal fistulas primarily concern fistulas originating from skin appendages (acne conglobata, suppurative hidradenitis and pilonidal sinus). The first of these are commonly easy to recognize

Fig. 7. a Coronal T2-weighted turbo spin-echo with en-doanal coil shows a fibrous track (arrowhead) extending through the left external sphincter. Normal sphincter anatomy at the right side: external sphincter (E), internal sphincter (I), puborectal muscle (PR) as well as the levator ani plate (LA). b Axial T2-weighted turbo spin-echo demonstrates the hyperin-tense track (arrow) surrounded by fibrous tissue (arrowhead), along the left external sphincter (E)

clinically, but diagnosis of pilonidal sinus is more difficult. Imaging can be used to differentiate between perianal fistula and pilonidal sinus. A study in seven patients with pilonidal sinus, and in 14 sex- and age-matched individuals with perianal fistulas has demonstrated that these conditions can be discriminated, due to the fact that the former condition has an absence of intersphincteric sepsis or enteric opening. Osteomyelitis of the pelvis or femur may give rise to abscesses and tracks that extend to the anorec-tal region, while osteomyelitis is a rare finding in perianal fistulas due to Crohn's disease. Differentiating these two conditions is usually not difficult, as the predominant disease localization (either extensive bone marrow edema or extensive tracks with intersphincteric extension and internal opening) establishes the diagnosis.

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