MRI for assessment of anal fistula

Authors: Michael R Torkzad Urban Karlbom

Publish Date: 2010/05/27

Volume: 1, Issue: 2, Pages: 62-71

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Abstract

Magnetic resonance imaging MRI is the best imaging modality for preoperative assessment of patients with anal fistula MRI helps to accurately demonstrate disease extension and predict prognosis This in turn helps make therapy decisions and monitor therapy The pertinent anatomy fistula classification and MRI findings will be discussedA fistula is defined as a pathologic tract connecting two hollow organs or one hollow organ and the skin Sinuses are defined when only one hollow organ or skin in involved Anal fistula is a somewhat uncommon condition It affects approximately ten individuals in 100000 It usually affects men in their fourth decade 1 Men are affected two to fourtimes more commonly the reason is thought to be partially due to the higher abundance of anal glands 2 Infection of the anal glands and crypts is thought to be the cause of later fistula formation The disease usually begins as an abscess and in chronic stages develops into a fistula in 60 of cases 3 There are of course other etiologies as well such as trauma during childbirth Crohn’s disease see below and malignancies The cryptoglandular form of the fistula disease usually manifests itself in the form of chronic discharge and pain Traditionally treatment has been surgical with recurrence happening in up to a quarter of cases 4Cryptoglandular fistulas are usually distinguished from fistula due to Crohn’s disease This distinction is due to the fistula being more complex in the latter group This distinction is not always clear however since at least half of fistulas in patients with Cohn’s disease are simple Crohn’s fistulas result from transmural spread of chronic granulomatous inflammation More than onethird of Crohn’s patients have perirectal disease which could lead to fistula disease 5The role of imaging is therefore to outline all hidden tracts and define the relationship of the fistula to the anal sphincter Inadvertent damage to the anal sphincter can lead to anal incontinence hence the importance of knowledge of the relation between the fistula tract and the anal sphincter There are however other indications for imaging in anal fistula Occasionally general physicians or gastroenterologists wish to know if there are any fistulas present at all For these physicians knowledge of exact extension of fistula is not required and a more simple magnetic resonance imaging MRI protocol could be sufficient Also with the advent of new nonsurgical treatment modalities monitoring therapy response is becoming more frequently performedMRI performed adequately should be regarded as the “gold standard” for preoperative assessment replacing surgical examination under anesthetic EUA in this regard 6 7 However endoanal ultrasonography is used by many surgeons in the preoperative workup of anal fistulas Although there are some conflicting results hydrogen peroxideenhanced endoanal ultrasonography may be comparable with MRI 8 Endoanal ultrasound alone is sufficient in more simple cases however MRI is generally is superior to endoanal ultrasonograhy 9 10MRI helps not only to accurately demonstrate disease extension but also to predict prognosis make therapy decisions and monitor therapy 11 12 Missed extensions at surgery are usually the cause of recurrence and adequate surgery is warranted in more extensive disease 9 MRI has been shown to reduce recurrent disease and therefore reoperation In patients with Crohn’s disease the recurrence could be due to inadequate medical treatment MRI can be used for monitoring therapy and predicting prognosis even in patients with Crohn’s disease 13The anal canal begins at the anal verge which corresponds to the lowermost portion of the external sphincter The upper part of puborectalis muscle forms the radiologic upper boundary of the anal canal Thus the anus is the infralevator portion of the gastrointestinal tract surrounded by the ischioanal fossa to each side The ischioanal fossa is sometimes incorrectly termed the ischiorectal fossaThe external sphincter muscle is a voluntary striated muscle which continues 15–2 cm upward until it ends and the fibers of the puborectalis muscle continue as part of the pelvic floor 14 For radiologic purposes the levator ani muscle is the muscle that forms the pelvic floor

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