[Perianal Fistula: An Overview]

Abstract

Perianal fistula is a common inflammatory condition in the general population and affects the area around the anal canal. Although most cases are benign, they cause serious morbidity and require surgical treatment due to a high risk of recurrence. MR imaging is a gold standard technique for the evaluation of perianal fistulas and provides accurate information on the anatomy of the anal canal, its relationship to the anal sphincter complex, accurate identification of secondary tracts or abscesses, and reporting associated complications. MR imaging can help monitor treatment effects and determine treatment methods. Crohn's disease-related fistulas often require medical rather than surgical treatment. The radiologist is required to know the anatomy and MR imaging findings of the perianal fistula to present an accurate diagnosis to the clinician.

Fig. 1. Normal anatomy of the rectum and anal canal in the coronal plane.

Fig. 2. Normal anatomy of the anal canal in the coronal plane.

A-C. A shows diagram of coronal scan of normal anal canal. Oblique coronal T2WI (B) and oblique axial T2WI (C) show the anatomy of anal canal.

Fig. 3. The relationship between the crypts of Morgagni and anal glands. If an intersphincteric anal gland infection penetrates the internal sphincter to enter the intersphincteric space or both internal, as well as external space, a fistula tract or an abscess is formed; this is known as the cryptoglandular hypothesis.

Fig. 4. MRI plane adjustment. It shows adjustment of (A) oblique axial and oblique coronal imaging planes according to the anal canal axis. (B) and (C) show the corresponding oblique axial view and represent the upper anal canal and midanal canal (white line, white arrows at each plane).

Fig. 5. Intersphincteric fistula (Courtesy of Kim M in Seoul Song Do Colorectal Hospital). It presents oblique axial T2WI without FS (A) and oblique axial FST2WI (B) shows an intersphincteric fistula (arrows) in a 25-year-old male. Oblique coronal FST2WI (C) shows a high signal intensity tract representing intersphincteric fistula (arrows). Postcontrast axial T1WI with FS (D) shows that the fistula wall (arrows) is significantly enhanced due to internal fluid (arrowhead).

FS = fat suppression

Fig. 6. Healed perianal fistula tract (Courtesy of Kim M in Seoul Song Do Colorectal Hospital).

A. Sagittal T2WI shows a low signal intensity tract (arrows). B, C. It presents a healed fistula tract after surgical treatment. Oblique coronal T2WI (B) and oblique axial fat-saturated T2WI (C) show low signal intensity fibrous tracts (arrows).

Fig. 7. Parks classification. The figure represents intersphincteric (arrow), transsphincteric (*), suprasphincteric (arrowhead), and extrasphincteric types (◁).

Fig. 8. Intersphincteric fistula (Courtesy of Kim M in Seoul Song Do Colorectal Hospital).

A. Figure represents an intersphincteric fistula. B, C. Oblique coronal (B) and axial (C) T2WI show a high signal intensity fistula tract (arrows) within the intersphincteric space. D. Fat-suppressed coronal T2WI shows the high signal intensity fistula (arrows). E. Postcontrast coronal T1WI shows an enhanced fistula tract (arrows) in the intersphincteric space.

Fig. 9. Transsphincteric fistula (Courtesy of Kim M in Seoul Song Do Colorectal Hospital).

A. This diagram represents a transsphincteric fistula. B, C. Oblique coronal T2WI (B) and oblique axial FS T2WI (C) represent a high signal fistula tract (arrows) that crosses the EAS. EAS = external anal sphincter

Fig. 10. Suprasphincteric fistula (Courtesy of Kim M in Seoul Song Do Colorectal Hospital).

A-C. The diagram (A), oblique coronal T2WI (B), and oblique axial FS T2WI (C) represent a wide fistula tract (arrows) that courses through the level of LA to the ischioanal fossa (arrowhead). D. Postcontrast coronal T1WI shows a well enhanced fistular tract (arrows). FS = fat suppression, LA = levator ani

Fig. 11. Extrasphincteric fistula (Courtesy of Kim M in Seoul Song Do Colorectal Hospital).

A, B. Diagram (A) and oblique postcontrast T1WI (B) show that the fistula tract (arrows) is completely outside the sphincters without the involvement of the anal canal.

Fig. 12. Patient is oriented in the supine position (lithotomy position) with axial slices used for localization (A, anal clock; B, quadrant location). (C) shows the anal clock of the surgeon’s view for the perianal region.

Fig. 13. A complex transsphincteric tract with multiple branches and abscesses in Crohn’s disease (Courtesy of Park SH in Asan Medical Center).

A, B. Oblique axial T2WI (A) and FS T2WI (B) show multiple fistulas with internal setons (arrows). C, D. Postcontrast coronal (C) and axial (D) show multiple branches (arrows) and abscesses (arrow).