Perianal Fistula and Abscess: A Beginner’s Guide to Imaging

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The full digital presentation is available online.

TEACHING POINTS

  • ■ When identifying a perianal fistula, remember that it is an abnormal communication between the anal canal and the perianal or perineal skin from a sinus tract, which is a primary conduit that blind ends in the subcutaneous tissues.

  • ■ Correct anatomical assessment is crucial. Look for the levator ani muscle to define the supralevator space or pelvic cavity, as it is considered a surgical emergency when involved.

  • ■ An adequate radiological report of perianal fistulas must be clear and concise and must include the following three points: internal opening, route and external opening.

Perianal fistulas are a fairly rare entity and are associated with significant morbidity. Diagnostic imaging of the perianal region is key to the successful detection and treatment of perianal fistulas. For accurate interpretation and analysis of imaging studies, it is essential to have a thorough knowledge of the complex anatomy of the anal canal and anal sphincter as well as the imaging manifestations of fistulizing disease (Fig 1). This online presentation reviews normal CT and MRI anatomy, explains relevant imaging findings, and describes the pathophysiology of perianal fistulas and the most commonly used classification systems.

Figure 1. Normal anal anatomy. Coronal oblique T2-weighted MRI image shows how the external anal sphincter (EAS) contains in depth (D)superficial (S)and subcutaneous (CS) components, and how the levator ani (THE) muscles are composed of iliococcygeus (HE) and puborectal (RP) muscles. CLM = circular longitudinal muscle, IAF = ischio-anal fossa, IAS = internal anal sphincter, IS = intersphincter space, computer = pelvic cavity.

Perianal fistula is defined as an abnormal connection between the anal canal and the skin of the perineum. The cause is often idiopathic, but conditions that predispose to the disease include Crohn’s disease, pelvic infection, tuberculosis, diverticulitis, trauma during childbirth, pelvic malignancies, and radiation therapy. The main theory of pathogenesis is the cryptoglandular hypothesis, in which the primary event is infection of an intersphincteric gland with formation of an intersphincteric abscess. This can then lead to the formation of a fistulous tract.

Imaging of perianal disease is best performed using cross-sectional techniques, with MRI being the method of choice. MRI allows optimal evaluation of perianal fistulas by providing precise definition of the fistulous tract and possible associated complications in the surrounding pelvic structures. T2-weighted MRI sequences without fat saturation are most useful for depicting the anatomy of the anal canal complex and anal sphincter because they provide excellent soft tissue contrast. In addition to using the appropriate imaging sequences, it is important for radiologists to assess acquisitions in the true minor axial axis and the true coronal plane relative to the anal canal to avoid superposition of normal structures that can simulate abnormalities.

The treatment of perianal fistulas is essentially surgical. However, there is a significant incidence of postoperative recurrence and additional morbidity. Successful surgical management of perianal fistulas requires a detailed preoperative assessment of the progress of the fistula tract and the presence of accessory tracts, abscesses, or other complications. Classification systems have been developed to aid in the treatment of perianal fistulas, the two main systems being the Parks classification and the St James University Hospital classification (Fig 2). The Parks system describes the course of the primary fistulous tract and its relationship to the anal sphincter complex, while the St James system incorporates secondary findings that can be seen on MRI.

The axial fat-suppressed T2-weighted MRI image shows a simple linear intersphincteric fistula course (arrow) without complications at the 4 o'clock position.

Figure 2. The axial fat-suppressed T2-weighted MRI image shows a simple linear intersphincteric fistula course (arrow) without complications at the 4 o’clock position.

A practical guide is offered to facilitate the understanding of the imaging results of fistulous perianal disease. The online presentation reviews imaging modalities used in the evaluation of perianal fistulas, with an emphasis on MRI. The relevant imaging findings of each type of fistula and the most used classification systems are discussed. This is accomplished with didactic figures that detail the normal anatomy of the anal canal and perianal region, with case examples of the main types of fistulas and their associated complications.

Perianal fistulas can be associated with significant morbidity, and knowledge of their appearance, imaging, description and classification on radiological images is essential to ensure appropriate management.

Disclosure of conflicts of interest.—kt Member of the Board of the American Board of Radiology.

1 Current address: Centro Médico de Especialidades, Ciudad Juárez, Chihuahua, Mexico.

Recipient of a Certificate of Achievement for Educational Exhibit at the 2020 RSNA Annual Meeting.

KT provided information (see end of article); all other authors revealed no relevant relationship.

Suggested readings

  • by Miguel Criado J., Salto LG, Rivas PF, et al. MRI Evaluation of Perianal Fistulas: Spectrum of Imaging Features. RadioGraphics 2012;32(1):175–194. Link, Google Scholar
  • Erden A. MRI of the anal canal: normal anatomy, imaging protocol and perianal fistulas: part 1. Abdom Radiol (NY) 2018;43(6):1334–1352. Cross-reference, Medline, Google Scholar
  • Khati NJ, Sondel Lewis N., Frazier AA, Obias V, Zeman RK, MC Hill. CT of acute perianal abscesses and infected fistulas: an illustrated trial. emergency radiol 2015;22(3):329–335. Crossref, Medline, Google Scholar
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