MDCT Fistulography in Evaluation of Fistula-In-Ano

Introduction:

Fistula or perianal fistula is a condition in which an abnormal perianal track is formed that connects the two epithelialized surfaces from the inner side of the anus and the outer skin around the anus. It is an infected cavity in the anus that connects the abscess to an opening on the skin near the anus. The anus is the outer opening through which the waste (feces) is excreted from the body. The treatment of fistula is done through surgeries, and it also has a 25 percent of recurrence rate even after surgical treatment.

Fistula-in-ano is a challenging condition for both diagnosis and treatment. Various imaging modalities, such as fistulography and endosonography, magnetic resonance imaging (MRI), computed tomography (CT), and perineal sonography, are available for its evaluation. Among all these, magnetic resonance imaging (MRI) is considered the modality of choice to identify the location and length of track and for the detection of associated complications. But it also has some limitations, such as its availability and affordability and its inability to show the fistula in relation to normal anatomical structures in a single image. Therefore, the multi-detector computed tomography fistulography (MDCTF) technique can be used.

What Is Multidetector Computed Tomography Fistulography (MDCTF)?

Multi-detector computed tomography fistulography (MDCTF) is an underutilized technique for detecting perianal fistulas. In this technique, the contrast medium is used and injected into the fistula, which delineates the tract and its components. After that, the post-processing of the images shows detailed images in multiple planes and formats. This technique helps demonstrate the type of fistula and its extent, whether it is simple or complex, the site of its opening, and other related complications.

What Is an Anal Fistula?

An anorectal fistula is described as a connection between the anorectal canal and the perianal area. It is a dreadful condition that usually occurs following the perianal abscess. The treatment includes the drainage of the abscess and requires a thorough follow-up to diagnose and treat. This commonly occurs when the anal glands in the intersphincteric plane become occluded and infected, resulting in a cryptoglandular abscess.

What Is the Clinical Anatomy of the Anorectal Region?

The anal area is described by the surgeons as an anal clock according to the position in which this area is viewed. The different positions of hours are specified as different locations, such as

• 12 o'clock is the anterior perineum.

• 6 o'clock is the natal cleft.

• 3 o'clock refers to the left lateral aspect.

• 9 o'clock to the right side of the anal site.

According to this, both the internal (mucosal in the bowel) and external (cutaneous or skin) openings of the fistulous tract are identified. The anal region consists of two sphincters, which include an internal and an external sphincter. The internal sphincter is composed of involuntary smooth and circular muscles, which are responsible for 85 percent of the resting anal tone. The external sphincter is composed of striated muscles, which are responsible for stool movement.

What Is the Classification of Anal Fistula?

The anal fistula can be classified into two broad categories, which include -

Surgical Classification -

Park’s classification is one of the most commonly used surgical classifications to describe four types of fistula according to the tract location relative to the internal (present at the inner surface of the canal) and external sphincters (layer of striated muscle surrounding the outside wall of the anal canal) which include -

• Intersphincteric (70 Percent - Most Common) - In this, the fistula crosses the intersphincteric space (space between the two sphincters) and does not cross the external sphincter.

• Transsphincteric (25 Percent) - In this, the fistula crosses the intersphincteric space via the external sphincter and into the ischiorectal fossa.

• Suprasphincteric (Five Percent) - In this fistula, it passes into the intersphincteric space and over the top of the puborectalis muscle (present around the lower rectum, has the shape of a sling), then descending through the other muscles to the external opening (skin).

• Extrasphincteric (One Percent) - In this fistula crosses from the perineal skin through the ischiorectal fossa into the rectum, which means it is present outside the external anal sphincter.

Radiological Classification -

Another grading system has been used by radiologists based on the landmarks on the axial plane and including abscess is called St James’s University Hospital classification -

• Grade 1 - Simple linear intersphincteric.
• Grade 2 - Intersphincteric fistula with an abscess.
• Grade 3 - Transsphincteric fistula
• Grade 4 - Transsphincteric with an abscess or secondary tract within the ischiorectal fossa.
• Grade 5 - Supralevator and translevator extension.

What Is the Pathology of Anal Fistula?

The idiopathic perianal fistula describes the most common type of fistula that may occur due to conditions such as Crohn's disease, tuberculosis, trauma, pelvic infection or malignancy, and radiation therapy. Anal glands present in the intramuscular plane in the anal canal may become infected, and persistent infection of the glands may result in different types of fistulas.

What Is the Technique of MDCT Fistulography?

Multi-detector computed tomography fistulography is done the same as conventional computed tomography, along with some modifications.

• In this technique, initially, the patient is asked to lie prone on the gantry table.

• Before the scan, cannulation (placing of the cannula) of the external perianal opening is done using an intravenous catheter, and five percent iodinated contrast is injected.

• After that, axial scanning is performed from the mid-sacrum (a bony structure present at the base of the lumbar vertebrae, which is connected to the pelvis) to the gluteal fold (horizontal crease formed by the buttocks in the inferior region).

• This scan helps in determining the location, course, and extent of the primary tract, whether the tract is simple or complex, and the position of the internal opening.

• Moreover, the presence of secondary tracts and abscesses may also be found at the site of the internal opening, and according to the finding, the fistula is classified.

• After that, the post-processing with thin sections allows the development of good-quality images for presentation in different planes and formats. Therefore, MDCTF can be utilized when MRI is contraindicated or not feasible.

Conclusion:

The diagnostic imaging tests for perianal fistula are a challenging task as any misdiagnosis may lead to its recurrence and repeated surgical treatment. Therefore, multi-detector computed tomography fistulography is considered a promising technique through which all the components of the perianal fistula can be viewed, and all the types of fistula can be distinguished with high-definition images.