Perianal fistula is a presence of a fistulous tract across the anal sphincters. It is usually an inflammatory condition that affects the region around the anal canal 4.
There is a recognised male predilection of between 2-4:1 with an estimated incidence of ~1:10 000 1.
Most commonly accepted pathophysiology is cryptoglandular hypothesis which is due to obstruction of the deep submucosal glands resulting in infection and abscess formation in intersphincteric space which consequently drain between sphincters inferiority and tracts into the skin surface or less commonly erodes through the both intrinsic and extrinsic sphincters into the ischiorectal space then into the skin surface 4.
Trans-sphincteric fistulae are secondary to ischiorectal abscesses, with a resultant extension of the tract through the external sphincter. Intersphincteric fistulae are due to perianal abscesses. Suprasphincteric fistulae are due to supralevator abscesses.
- diverticulitis: perhaps the most common cause in developed countries
- inflammatory bowel disease: Crohn disease
- other bowel or pelvic infections
- iatrogenic- post-surgical complication:
- post ultralow anterior resection
- post pelvic radiotherapy
- pelvic malignancies
Goodsall's rule states that the internal opening of the fistula is dependent on where the fistula is located relative to the 'anal clock' (ie with the patient in the lithotomy position, anterior is 12 o'clock and posterior is 6 o'clock) and the transverse anal line (a line drawn from 9 o'clock to 3 o'clock):
- if the internal opening is anterior to the transverse anal line there will be a (usually simple) direct radial fistulous tract
- if the internal opening is posterior to the transverse anal line there will be a tortuous (and often more complex) fistulous tract that enters posteriorly in the midline (6 o'clock)
The Parks classification has become the most widely used method for distinguishing four types of fistula. It is a surgical clarification. The fistula course is described in the coronal plane and its relationship to the anal sphincters 4,6,7:
- inter-sphincteric (~70%): fistula crosses the intersphincteric space and does not cross the external sphincter
- trans-sphincteric (25%): fistula crosses from the intersphincteric space, through the external sphincter and into the ischiorectal fossa
- supra-sphincteric (5%): fistula passes superiorly into the intersphincteric space, and over the top of the puborectalis muscle then descending through the iliococcygeus muscle into the ischiorectal fossa and then skin
- extra-sphincteric (1%): fistula crosses from perineal skin through the ischiorectal fossa and levator ani muscle complex into the rectum (i.e. is outside the external anal sphincter)
Radiologists have developed another grading system for perianal fistulae, which is based on landmarks on axial plane and incorporates abscesses and secondary extensions to the grading system, is called St James’s university hospital classification 1:
- grade 1: simple linear intersphincteric
- grade 2: intersphincteric with abscess or secondary track
- grade 3: trans-sphincteric
- grade 4: trans-sphincteric with abscess or secondary track within the ischiorectal fossa
- grade 5: supralevator and translevator extension
Fistulography is a traditional radiologic technique used to define the anatomy of fistulas yet it is an unreliable technique and difficult to interpret.1
In fistulography, the external opening is catheterized with a fine cannula, and a water-soluble contrast agent is injected to define the fistula tract. 7
It has two major drawbacks:4
difficult to assess secondary extensions secondary to lack of proper filling with contrast material
inability to visualize the anal sphincters and to determine their relationship to the fistula
Computed tomography (CT)
MRI is the imaging modality of choice. See: pelvic MRI protocol for anal canal fistulae assessment.
Active fistulous tracts are typically:
- T1: isointense to muscle, enhancing with contrast
- T2: low signal compared to fat
- T2FS: high signal compared to fat
Old, healed fistulas typically demonstrate low T1 and T2 signal without contrast enhancement.
- detection of the primary fistulous tract and its activity:
- active tract has high T2 signal and demonstrates intense enhancement
- chronic tracks have low signal on both T1- and T2-WI and will not show contrast enhancement
- location (right/left) and course
- relationship to the sphincter complex
- Parks classification: trans-, inter-, supra-, or extrasphincteric
- distance of the internal mucosal defect to the perianal skin on coronal images
- position of the internal mucosal opening on axial images
- use the "anal clock": anterior = 12 o'clock
- identify secondary fistulous tracks and the sites of any abscess cavities in order to avoid therapeutic failure and recurrence.
- cranial extension above the levator ani muscle
- 1. Morris J, Spencer JA, Ambrose NS. MR imaging classification of perianal fistulas and its implications for patient management. Radiographics. 20 (3): 623-35. Radiographics (full text) - Pubmed citation
- 2. Spencer JA, Ward J, Beckingham IJ et-al. Dynamic contrast-enhanced MR imaging of perianal fistulas. AJR Am J Roentgenol. 1996;167 (3): 735-41. AJR Am J Roentgenol (abstract) - Pubmed citation
- 3. Hussain SM, Stoker J, Schouten WR et-al. Fistula in ano: endoanal sonography versus endoanal MR imaging in classification. Radiology. 1996;200 (2): 475-81. Radiology (abstract) - Pubmed citation
- 4. De miguel criado J, Del salto LG, Rivas PF et-al. MR imaging evaluation of perianal fistulas: spectrum of imaging features. Radiographics. 32 (1): 175-94. doi:10.1148/rg.321115040 - Pubmed citation
- 5. Engin G. Endosonographic imaging of anorectal diseases. J Ultrasound Med. 2006;25 (1): 57-73. J Ultrasound Med (full text) - Pubmed citation
- 6. Parks AG. Pathogenesis and treatment of fistuila-in-ano. Br Med J. 1998;1 (5224): 463-9. Free text at pubmed - Pubmed citation
- 7. Halligan S, Stoker J. Imaging of fistula in ano. Radiology. 2006;239 (1): 18-33. doi:10.1148/radiol.2391041043 - Pubmed citation