Perianal fistulas and abscesses are among the most serious manifestations of Crohn's disease and non-Crohn's related anorectal disease (picture 1 and picture 2). Complications can lead to difficulties with recurrent or non-healing fistulas or abscesses. In addition, these patients are at risk of incontinence as a result of the destructive nature of the fistulizing process and/or inadvertent damage to the anal sphincters during surgical exploration.
The lifetime risk for developing a fistula in patients with Crohn's disease is 20 to 40 percent [1-4]. The frequency of perianal fistulas/abscesses in patients without Crohn's disease has not been well-established, but in a telephone survey of 102 randomly selected individuals, 20 percent of the individuals contacted had perianal symptoms (hemorrhoids, fistulas, etc) . Despite the significant prevalence of perianal disease, the evaluation of this problem was, in the past, largely limited to digital rectal examination.
The inability of the clinician to directly visualize the fistula or abscess makes it difficult to assess the lesions. The physician must essentially discern the perianal anatomy by touch. This task is made even more problematic by the induration and inflammation that is usually present in these patients. Even surgical evaluation is only 35 to 85 percent accurate when compared to the results of other diagnostic tests and clinical evaluation [6-9].
The importance of accurately characterizing the perianal process prior to embarking on therapy cannot be overemphasized. The risk of incomplete healing, a recurrent fistula, or even inadvertent sphincter injury is increased if fistula anatomy is incorrectly delineated or an occult abscess missed. An imaging modality should ideally provide a virtual road map that the physician can use to plan therapy. This is especially true with fistulas that involve a significant portion of the anal sphincter complex. Such patients are at the greatest risk of developing incontinence from the destructive fistulizing process or from overly aggressive surgical treatment.
Thus, patients with simple fistulas that only involve a small portion or none of the external anal sphincter generally do well with either medical or surgical treatment. Imaging of the fistula is helpful in determining the type of fistula to guide treatment but is not always needed. By contrast, for patients with a complex fistula (ie, one that involves a significant portion of the sphincter complex), preoperative imaging is mandatory.