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.
Incidence is estimated at ~1:10 000 1, with a recognised male predilection of 2-4:1.
The most commonly accepted pathophysiology is the cryptoglandular hypothesis, which posits that obstruction of the deep submucosal glands results in infection and abscess formation in the intersphincteric space, which consequently drains inferiorly between the sphincters, opening onto the skin surface or, less commonly, erodes through both the internal and external sphincters into the ischiorectal space, then onto 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' (i.e. 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 surgical classification for distinguishing four types of fistula. The course of the fistula and its relationship to the anal sphincters is described in the coronal plane 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 the 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 is 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 fistulous tract. 7
It has two major drawbacks:4
difficult to assess secondary extensions secondary to lack of proper filling with contrast material
inability to visualise the anal sphincters and to determine their relationship to the fistula
The benefits of ultrasonography over MRI are the former's ubiquity and lower operating costs 9-10.
There are three ultrasonography methods for the evaluation of perianal fistulae, whether cryptoglandular or Crohn disease-associated 8:
- endoanal ultrasound (EAUS)
- transvaginal ultrasound (TVUS)
- transperineal ultrasound (TPUS)
These can also be applied in combination. Infusion of hydrogen peroxide into the fistulous tract renders it hyperechoic, thus facilitating its delineation. Doppler interrogation can show hyperaemia in active disease (Crohn).
Endoanal ultrasonography is deemed less sensitive than endoanal MR for deep supralevator disease 10.
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
- T2: low signal compared to fat
- T2-FS: high signal compared to fat
- T1 C+: enhancing
Old, healed fistulae typically demonstrate low T1 and T2 signal without contrast enhancement, reflecting fibrosis.
- 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
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