Figure 1: Soft-tissue stranding in the pelvis represents site of colectomy with ileoanal anastomosis. The fistula tract is again seen as a high T2 signal collection extending from the anus to the left buttock subcutaneous tissue.

Figure 2: Image demonstrates peripheral ring enhancement of the subcutaneous collection in the left buttock. There is also a linear soft tissue enhancement extending from this collection to the anus. There is transverse linear enhancement (arrow) from the anterior wall of the anus to the posterior wall of the urethra.

Figure 3: Axial image demonstrates a linear high T2-signal extending from the anterior wall of the anus to the posterior wall of the urethra. This linear enhancement corresponds to the transverse enhancement seen in Figure 2.

Movie 1: Images follow a high T2-signal collection extending from the anus, crossing both the internal and external sphincters and traversing the left ischioanal fossa to the left buttock skin surface.

Trans-sphincteric perianal fistula with a fistulous connection from the anus to the urethra.

Anorectal fistulas (fistula in ano) are granulation tissue-lined tracts between the anal canal or rectum to one or more openings in the perianal skin. Anorectal fistulas are associated with congenital anomalies, Crohn’s disease, ulcerative colitis, diverticulitis, appendicitis, malignancy, radiation treatment, penetrating trauma, birth trauma, TB, lymphogranuloma venereum, and human immunodeficiency viral infection. The pathogenesis of anorectal fistulas begins with infection in the anal crypts at the pectinate line. Infection leads to gland obstruction and secondary abscess formation with subsequent rupture or external drainage through a fistulous tract. Drainage of pus, blood, mucus, and occasionally stool are often the main presenting symptoms. Other symptoms include perianal pruritis or dermatitis and varying degrees of pain with sitting, walking, and defecation, involuntary passing of gas, and recurrent perianal swelling.

Depending on the location and relation to the sphincteric muscles one can classify a fistula as:

• Intersphincteric: courses between the internal and external sphincters
• Transsphincteric: crosses the sphincter complex to exit the skin around the anus
• Suprasphincteric: traverses the internal anal sphincter, extends cranially in the intermuscular space, passes the puborectalis muscle, and reaches the ischiorectal fossa
• Extrasphincteric: usually arises from a focus of inflammation originating in the pelvis above the levator ani muscle and its track crosses the ischiorectal fossa to exit in the perianal area

Godsall’s rule describes the relationship of the cutaneous opening of the fistula to the expected site of enteric opening. The rule states that cutaneous openings anterior to the transverse anal line are associated with direct radial fistulous tracts into the anal canal, whereas openings posterior to the line have tracks that enter the canal in the midline posteriorly.

MR imaging is useful for pre-operative imaging of anorectal fistulas. MRI has been shown to accurately demonstrate the anatomy of the perianal region and imaging of the fistulous tracks. MR imaging performed with an endorectal coil provides excellent delineation of the sphincteric components. It can also depict the internal origin of the fistulous track. Unenhanced T1-weighted images provide an excellent anatomic overview of the sphincter complexes, levator plate, and the ischiorectal fossa. Fistulous tracts, inflammation, and abscesses appear as areas of low to intermediate signal intensity and may not be distinguished from normal structures such as the sphincters and levator ani muscles. On T2-weighted and STIR images, fistulas and fluid collections can be clearly demonstrated as areas of high signal intensity in contrast with the lower signal intensity of the sphincters, muscles, and fat. Gradient-echo T1-weighted dynamic intravenous contrast-enhanced MR imaging combined with T2-weighted imaging are the best sequences to assess anorectal fistulas and their complications. Fistulous tracks and the walls of abscess cavities enhance while retained pus does not, resulting in ring enhancement. T1-weighted sequences can also be helpful in post-operative assessment of patients. If MR imaging is performed immediately post-operatively, hemorrhage will appear hyperintense and can thus be differentiated from a residual fistulous track. In addition, fat containing “grafts” which are used to fill cavities and resection voids in restorative procedures can be identified as hyperintense structures on unenhanced T1-weighted images and do not enhance substantially.

References:

1. Stein, E. Anorectal and Colon Diseases: Textbook and Color Atlas of Proctology. Berlin: Springer, c2003. pp. 101-106.
2. Eisenberg, RL. Gastrointestinal Radiology: A Pattern Approach, 4th ed. Philadelphia: Lippincott Williams & Wilkins, c2003. p.945.
3. Feldman M, Friedman S, Sleisenger M. Sleisenger & Fordtran's Gastrointestinal and Liver disease : pathophysiology, diagnosis, management, 6th ed. Philadelphia : Saunders, c2002. pp. 1969-1970.
4. Morris J, Spencer JA, Simon A. MR Imaging Classification of Perianal Fistulas and Its Implications for Patient Management. Radiographics. 200; 20: 623-635.