الفهرس 
Anatomy of the anal regionOnly 14 pages are availabe for public view
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Abstract
Perianal fistula is a track, lined by granulation tissue that connects deeply in the anal canal or rectum and superficially on the skin around the anus. The cryptoglandular hypothesis is the most accepted theory in its pathogenesis; also there are some other causes e. g Crohn’s disease, ulcerative colitis, actinomycosis, foreign body, lymphogranuloma venerium and trauma.
The estimated incidence among population is about 1:10.000 and male to female ratio is of between 2:1 and 4:1. The most widely used classification is that of Parks’ in which the anal fistulae are classified into; intersphincteric fistulae in which there are simple and complicated, trans-sphincteric fistulae in which there are simple and complicated, suprasphincteric fistulae and extrasphincteric fistulae. Perianal fistula often recurs despite seemingly adequate surgery, usually because of infection that was missed at surgery.
MRI is now generally available and high quality examination does not require special equipment. The added advantage of multiplanar imaging enables fistula anatomy to be imaged in a way that is directly related to surgical planes and exploration.
The standard coil for imaging of perianal fistula is the external coil; an endo-anal coil is only employed if the internal opening is inapparent.
Axial scans best relate the primary track to the sphincter complex and are the basis from which to start fistula classification. But, coronal scans best visualise the levator plate and facilitate diagnosis of both supralevator sepsis and the level of the internal opening.
There are a variety of approaches for imaging perianal fistula including STIR imaging, T2-weighted scanning, or via gadolinium enhancement. Whatever the technique used, it must be able to highlight both pus and adjacent structures.
The relationship between the fistula and the anal sphincter and the extensions from the primary tract that need to be treated to prevent recurrence are the two surgical points need to be assessed preoperatively.
There are a variety of imaging modalities available to try to define fistula anatomy. MRI suffers from none of the drawbacks associated with other imaging modalities. As contrast material–enhanced fistulography fails to image the sphincter muscles and the levator plate. Also, extensions from the primary tract may fail to fill with contrast material.
CT cannot be used for this purpose with sufficient accuracy. This is because the CT attenuation of the anal sphincter and pelvic floor is similar to that of the fistula itself. The limited field of view of anal endosonography and inability to image in the surgically important coronal plane render it inferior to MRI in accurate imaging of perianal fistula. It also should displace examination under anesthesia (EUA).
The ability of MRI to contrast pus and granulation tissue against pelvic floor anatomy renders it so suitable for fistula classification.
MRI accurately delineates the presence and course of a primary fistulous tract and also demonstrates the site and presence of any secondary extensions. It also provides the most accurate imaging technique of localizing the site of the internal opening because its location can be inferred from the proximity of the tract in the intersphincteric space.
MR imaging clearly shows the relationship of fistulas to the pelvic diaphragm (levator plate) and the ischiorectal fossae. This relationship has important implications for surgical management and outcome and has been classified into five MR imaging–based grades. If the ischioanal and ischiorectal fossae are unaffected, disease is likely confined to the sphincter complex (simple intersphincteric fistulization, grade 1 or 2), and outcome following simple surgical management is favorable. Involvement of the ischioanal or ischiorectal fossa by a fistulous track or abscess indicates complex disease related to trans-sphincteric or suprasphincteric disease (grade 3 or 4). Correspondingly more complex surgery may be required that may threaten continence or may require colostomy to allow healing. If the track traverses the levator plate, a translevator fistula (grade 5) is present, and a source of pelvic sepsis should be sought.
In particular, magnetic resonance (MR) imaging findings influence surgery and markedly diminish the chance of recurrence; thus, preoperative imaging will become increasingly routine in the future.