The evaluation of the patient with anal fistula includes a detailed history of past anal or colorectal operations; past obstetrical history; comorbidities such as diabetes, human immunodeficiency virus (HIV), neurologic or neuromuscular disorders, prior radiation therapy to the pelvis, baseline level of continence, and use of tobacco. Smoking has been associated with a higher failure rate of the endorectal flap and therefore patients who smoke are advised to quit prior to operative intervention (10,11). Physical examination includes visual inspection of the external fistulous opening (location and number of openings if more than one), digital examination to assess sphincter tone, probing of the external opening with the index finger of the examiner inside the anus to evaluate the depth of the fistula and muscle involvement, and anoscopy to visualize the quality of the anorectal mucosa. Occasionally the examination can be limited due to the patient’s discomfort. Under such circumstances an examination under anesthesia is required to ensure patient comfort and cooperation and may reveal an abscess that requires drainage as initial step of treatment. Proper incision and drainage of an underlying abscess yields a higher success rate of the endorectal flap with one study reporting an overall success rate of 73% versus 49% (prior drainage vs. none) (12). The use of noncutting seton prior to definitive endorectal flap improves overall success rate (12). Usually 2–3 months of seton drainage is advised before performing the endorectal flap except for Crohn’s disease where the response of inflammation to the medical therapy is the major determinant of when to proceed with definitive operation.

Endoscopic evaluation of the colorectum is indicated in the setting of inflammatory bowel disease or if there is suspicion of it and in patients who meet screening criteria or who warrant diagnostic colonoscopy for history of polyps, cancer, or for symptoms such as bleeding or abdominal pain. Occasionally a high internal fistulous opening can be visualized during the endoscopic retroflexion inside the rectum. Routine imaging of the anorectal area or pelvis to determine the anatomy of a fistula is not warranted. Imaging is used selectively for recurrent or persistent disease, deep tracts, multiple external openings, or when assessing the integrity of the anal sphincter muscles in conditions such as rectovaginal fistula. If an associated sphincter defect is present and a concomitant sphincter repair is performed, success rate of the endorectal advancement flap for rectovaginal fistula is higher. Computed tomography plays little role in the management of anal fistulas but can be helpful in the setting of acute sepsis especially when a deep abscess is suspected. Magnetic resonance imaging and endoanal ultrasound (EUS) are useful diagnostic studies in patients with anal fistulas with similar accuracy. However, EUS is more widely available and more commonly used; EUS is less costly than magnetic resonance imaging and can be performed by the treating surgeon. Fistulous tracts can be confirmed by hydrogen peroxide injection through the external fistulous opening (Fig. 32.1A,B). Fistulogram, while commonly used in the past, is less frequently performed because of the availability of EUS but can be helpful when delineating complex fistulas such as horseshoe (Fig. 32.2).

Physiologic testing of the anorectum is not routinely performed but can be helpful in a subgroup of patients. Sainio et al. (13) advocate preoperative anorectal manometry in women with previous obstetrical trauma, elderly patients, patients with Crohn’s disease or HIV, or patients with recurrent disease and prior surgery. Preoperative manometric evaluation can elicit sphincter dysfunction already present and may alter the treatment plan.