Fistula-in-Ano Workup

Updated: Jul 29, 2022
  • Author: Juan L Poggio, MD, MS, FACS, FASCRS; Chief Editor: John Geibel, MD, MSc, DSc, AGAF  more...
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Laboratory Studies

No specific laboratory studies are required in the diagnosis of fistula-in-ano (though the normal preoperative studies are performed, based on age and comorbidities). Instead, physical examination findings remain the mainstay of diagnosis.


Imaging Studies

Radiologic studies are not performed for routine fistula evaluation, because in most cases, the anatomy of a fistula-in-ano can be determined in the operating room. However, such studies can be helpful when the primary opening is difficult to identify or when recurrent or persistent disease is present. In the case of recurrent or multiple fistulas, such studies can be used to identify secondary tracts or missed primary openings. [14]  Several imaging diagnostic modalities are available to evaluate fistula-in-ano. The efficacy of each modality is reviewed.


This technique involves injection of contrast via the internal opening, which is followed by anteroposterior, lateral, and oblique radiographic images to outline the course of the fistula tract.

Fistulography is relatively well tolerated but it can be painful when injecting the contrast material into the fistulous tract. It requires the ability to visualize the internal opening. Questions have been raised about its accuracy, which has been reported to range from 16% to 48%. [15] .

Because of these limitations, fistulography is generally reserved for cases in which there is a concern about a fistulous connection between the rectum and adjacent organs such as the bladder, where it may be slightly more useful than a careful examination under anesthesia.

Endoanal or endorectal ultrasonography

Endoanal or endorectal ultrasonography involves the passage of a 7- or 10-MHz ultrasound transducer into the anal canal to help define the muscular anatomy and thereby help differentiate intersphincteric from transsphincteric lesions. A standard water-filled balloon transducer can facilitate evaluation of the rectal wall for any suprasphincteric extension.

Investigations have shown that the addition of hydrogen peroxide via the external opening can aid in outlining the course of the fistula tract. This may be useful for helping to identify missed internal openings.

Endoanal/endorectal ultrasonography has been reported to be 50% better than physical examination alone in helping to detect an internal opening that is difficult to localize. This modality has not been used widely for routine clinical fistula evaluation. [16]

Magnetic resonance imaging

Findings on magnetic resonance imaging (MRI) show 80-90% concordance with operative findings when a primary tract course and secondary extensions are observed. MRI is becoming the study of choice for the evaluation of complex fistulas and recurrent fistulas. It has been shown to reduce recurrence rates by providing information on otherwise unknown extensions. [17, 18]

Computed tomography

Computed tomography (CT) is more helpful in the setting of perirectal inflammatory disease than in the setting of small fistulas because it is better for delineating fluid pockets that require drainage than for delineating small fistulas. CT requires administration of oral and rectal contrast. Muscular anatomy is not well delineated.

Barium enema/small bowel series

These studies may be useful for patients with multiple fistulas or recurrent disease to help rule out inflammatory bowel disease.


Anal Manometry

Anal manometry is rarely used in the evaluation of patients with fistula-in-ano. However, pressure evaluation of the sphincter mechanism is helpful in certain patients for operative planning, including the following:

  • Patients in whom decreased tone is observed during preoperative evaluation
  • Patients with a history of previous fistulotomy
  • Patients with a history of obstetrical trauma
  • Patients with a high transsphincteric or suprasphincteric fistula (if known)
  • Very elderly patients

If a decrease in pressure is found, surgical division of any portion of the sphincter mechanism should be avoided.



Examination under anesthesia

Examination of the perineum, digital rectal examination (DRE), and anoscopy are performed after the anesthesia of choice is administered. This must be done before surgical intervention is initiated, especially if outpatient evaluation causes discomfort or has not helped to delineate the course of the fistulous process.

Several techniques have been described to help locate the course of the fistula and, more important, identify the internal opening. They include the following:

  • Inject hydrogen peroxide, milk, or dilute methylene blue into the external opening and watch for egress at the dentate line; in the authors' experience, methylene blue often obscures the field more than it helps identify the opening
  • Traction (pulling or pushing) on the external opening may also cause a dimpling or protrusion of the involved crypt
  • Insertion of a blunt-tip crypt probe via the external opening may help to outline the direction of the tract; if it approaches the dentate line within a few millimeters, a direct extension likely existed (care should be taken to not use excessive force and create false passages)


Rigid sigmoidoscopy can be performed at the initial evaluation to help rule out any associated disease process in the rectum. Further colonic evaluation is performed only as indicated.