radiology anal fistula in ano

1. Imaging of Anal Fistula
Dr. Chetan Prakash Chol

2. Introduction
Pre-operative confirmation of fistula
complexity has been shown to facilitate
surgical planning of sphincter saving
techniques and to reduce the incidence of
unidentified sepsis, which is the leading
cause of fistula recurrence .

3. Imaging Objectives
• Determine relationship of fistula to
sphincter complex
• Identify any secondary fistulous tracks

4. Imaging Modalities
• Fistulography
• Endoanal ultrasound
• Magnetic resonance

5. Fistulography
• Acute tracks may not have a patent lumen
• Difficult to relate the track to the sphincter
and levator ani
• Shown to be accurate in only 16%
• Helpful for chronic fistulae with an external
opening distant from the anus

6. Endoanal ultrasound
• Operator dependent
• Highly accurate at identifying the internal
opening
• Depicts fewer secondary extensions than
MR
• Difficulty differentiating active track from
fibrosis

7. Magnetic Resonance
• Most accurate technique for evaluation of
the primary track and any extensions .
• More accurate predictor of patient outcome
than surgical findings at EUA.

8. MR Technique
• The pelvic MRI protocol for perianal fistula
evaluation consists of T1-weighted and high-
spatial-resolution T2-weighted imaging sequences
without fat saturation for delineation of the muscle
groups,fat planes, and the fistula tract.
• T2-weighted imaging with fat suppression (and
STIR) is used to assess edema and fluid-containing
tracts and cavities.

9. • whereas fat-suppressedT1-weighted unenhanced and
contrast-enhanced sequences are used to assess the
presence and degree of inflammation.
• Diffusion-weighted imaging has been shown
to increase the radiologist’s level of confidence
and to add value to fat-suppressed T2-
weighted and gadolinium-enhanced imaging
in the diagnosis of anal fistulae

10. Anatomy
• The anal canal extends from the levator ani muscle
cranially to the anal verge caudally and is
surrounded by the internal and external anal
sphincters.
• The internal sphincter is the inferior extension of
the inner circular smooth muscle of the rectum and
is primarily responsible for resting involuntary
anal continence

11. • The external sphincter is composed of striated
skeletal muscle, which is contiguous with both
the levator ani and puborectalis muscles
superiorly and is primarily responsible for
voluntary continence.
• As such, injury to the external sphincter during
surgery (e.g., episiotomy) can lead to fecal
incontinence.

12. • The internal and external sphincters are separated by
the intersphincteric space, which is composed of
loose fat containing areolar tissue.
• Surrounding the anal canal, superficial to the
sphincter complex and inferior to the puborectalis,
are two contiguous pyramidal fat-containing spaces
known as the ischiorectal space cranially and the
ischioanal space caudally

14. Normal perianal anatomy. Coronal T1-
weighted MR images show the normal
anatomy of the perianal region. a = anal canal,
es = external sphincter, iaf = ischioanal fossa,
irf = ischiorectal fossa, R = rectum, straight
arrow in a = the levator plate.

16. Normal perianal anatomy. axial T2-
weighted MR images show the normal
anatomy of the perianal region. a = anal
canal, es = external sphincter, iaf =
ischioanal fossa, irf = ischiorectal fossa, R
= rectum, straight arrow in a = the levator
plate.

17. Pathophysiology
• The mucosal surface of the proximal anal canal is
lined by a series of longitudinal mucosal columns,
known as the columns of Morgagni .
• The spaces between the columns are known as the
anal sinuses (or crypts of Morgagni), which receive
drainage from the anal glands. Distally, the columns
are connected to each other circumferentially by
small anal valves, which collectively form the
dentate line.

18. • Up to 90% of perianal fistulas are believed
to arise secondary to impaired drainage of
the anal glands, according to the “cryptoglandular
hypothesis” .
• Infection and anal gland drainage obstruction may
lead to an acute perianal abscess.
• Some abscesses may resolve spontaneously via
internal drainage into the anal canal, whereas others
may require surgical incision and drainage

19. • Abscesses that are inadequately or incompletely
drained will persist and may ultimately seek
additional drainage pathways through the
intersphincteric space or across the sphincter
complex and, in the process, create fistulous tracts.
• As such, perianal abscess and perianal fistula are
thought to be acute and chronic manifestations of
the same disease process, and as many as 87% of
patients with acute perianal abscess may
subsequently develop a fistula

20. • The remaining 10% of cases result from other
causes, such as Crohn disease, tuberculosis,
diverticulitis, pelvic infection, trauma, anorectal
cancer, or radiation therapy.
• Perianal fistulas frequently complicate Crohn
disease, with a cumulative incidence of up to
26% after 20 years

21. Morris J, Spencer JA, Ambrose S. MR Imaging
Classification of Perianal Fistulas and Its implications
for Patient Management. Radiographics 2000;
20:623-635

22. Grade 1: Simple Linear
Intersphincteric Fistula
• grade 1 fistula arises from the anal canal, penetrates
the internal anal sphincter, and extends caudally
through the intersphinctericspace to its cutaneous
opening.
• There are no secondary tracts, cranial extension
above the levator ani, or associated abscess.
• The tracts are confined by the external sphincter
with no disruption of the external sphincter.

23. Grade 1 Simple Intersphincteric
Fistula
Grade 1 perianal fistula. (a) Line diagram of the
coronal view shows a right intersphincteric
fistula (yellow track) extending from the dentate
line down to the skin through the intersphincteric
plane.

24. Line diagram of the axial view
shows the posterior midline
intersphincteric fistulous track
(yellow spot) confined by the
external sphincter.

25. Grade 1 perianal fistula. Coronal
dynamic contrast-enhanced MR
image shows a right
intersphincteric fistula entering the
anal canal in the midline posteriorly
(arrow).

26. Grade 1 perianal fistula. Axial T2-
weighted MR image shows a
posterior midline intersphincteric
fistula (arrowhead).

27. • Grade 2 consists of an intersphincteric fistula
with associated abscess or secondary tract .
• Similar to grade 1, the primary tract, secondary tract,
or abscess cavity all lie within the intersphincteric
space, confined by the external sphincter.
• The “horseshoe”fistula subtype has a secondary tract
that crosses the midline and surrounds both
sides of the internal sphincter.
Grade 2 Intersphincteric track
with secondary track or abscess

28. Grade 2 Intersphincteric track with
secondary track or abscess
Grade 2 perianal fistula with an abscess.
Line diagram of the coronal view shows
a left intersphincteric abscess (a).

29. Grade 2 perianal fistula with an abscess.
Coronal dynamic contrast-enhanced MR
image shows a left intersphincteric abscess
cavity (arrowhead) above the primary
intersphincteric track (curved arrow). The
enteric entry point is suggested by a medial
track (straight arrow).

30. Grade 2 horseshoe perianal fistula.
Axial T2-weighted image shows an
intersphincteric horseshoe fistula
(arrow).

31. Grade 2 perianal fistula with an abscess. Coronal
(a), axial (b), MR images show an intersphincteric
abscess, which is peripherally enhanced (curved
arrow) and contains a central focus of nonenhancing
pus (straight arrow). As viewed in all planes, the
fistula is confined by the external sphincter and the
ischiorectal fossa is unaffected.

32. Grade 2 perianal fistula with an
abscess. Axial T2-weighted MR
image of a patient with Crohn
disease shows an intersphincteric
abscess (arrow) containing gas
bubbles (arrowheads).

33. Grade 3 Trans-sphincteric Fistula
• A grade 3 fistula arises from the anal canal,
penetrates both the internal and external anal
sphincters, and extends through the ischiorectal or
ischioanal fossae in its pathway to its cutaneous
opening .
• There is no associated secondary tract, abscess, or
extension cranial to the levator ani.

34. Grade 3 Trans-sphincteric Fistula
Grade 3 perianal fistula. Line diagram of the coronal
view shows a right trans-sphincteric fistula (yellow
track) crossing the ischiorectal fossa and piercing both
layers of the sphincter complex.

35. Grade 3 perianal fistula. Coronal dynamic
contrast-enhanced MR image shows a right
trans-sphincteric fistula (arrow) and
inflammatory change in the right ischiorectal
fossa. Note the entry site in the middle third of
the anal canal.

36. Grade 3 perianal fistula. Axial
dynamic contrast-enhanced MR
image shows a left trans-sphincteric
fistula within the ischiorectal fossa
and piercing the external sphincter
(arrow).

37. Grade4TranssphinctericFistula
With Abscess or Secondary Track
• A grade 4 fistula consists of a transsphincteric
fistula complicated by secondary tracts
or abscess .
• The abscess may occur anywhere along the primary
tract, secondary tracts, or within the ischioanal or
ischiorectal fossae, but not above the levator ani.

38. Grade 4 Trans-sphincteric Fistula
With Abscess or Secondary
Track

39. Grade 4 perianal fistula with an ischiorectal
fossa abscess. Coronal dynamic contrast-
enhanced MR image shows a left trans-
sphincteric fistula (arrow) with a left ischiorectal
fossa abscess (arrowheads) containing
nonenhancing pus. Note the contraction of the
ischiorectal fossa.

40. Grade 4 perianal fistula with an abscess. Axial
T2-weighted (a) and dynamic contrast-enhanced
(b) MR images show a left trans-sphincteric
fistula (straight arrow) with a left ischioanal
fossa abscess (curved arrow) and nonenhancing
pus in the cavity (arrowhead).

41. Grade 4 perianal fistula with an abscess.
Axial T2-weighted MR image shows a left
trans-sphincteric fistula (straight arrow) with
intersphincteric and left ischioanal fossa
components of the abscess (curved arrows).

42. Grade 5 Supralevator and
Translevator Disease
• Grade 5 fistulas encompass a variety of complex
tracts characterized by the above the levator ani that
can be extrasphincteric and suprasphincteric .
• Suprasphincteric fistulas arise from the anal canal,
penetrate the internal sphincter, and then ascend
within the intersphincteric space into the
supralevator space . The fistula then pierces the
levator ani as it descends through the ischiorectal
fossae en route to its cutaneous opening

43. • Extrasphincteric fistulas are a separate entity
caused by primary pelvic disease (e.g., Crohn
disease, diverticulitis, or carcinoma) that extends
caudally through the levator plate, traverses the
ischiorectal fossa, and terminates at the cutaneous
opening.
• Most importantly, an extrasphincteric fistula does
not involve the internal or external anal sphincters

44. Coronal illustration of perianal region depicts
grade 5 supralevator fistula that arises from anal
canal, penetrates both internal (1) and external (2)
anal sphincters, and then ascends into supralevator
space where it forms abscess before descending
through levator ani and coursing to skin

45. Grade 5 perianal fistula with an abscess.
Line diagram of the coronal view shows a
pelvic abscess (a) with a translevator fistula
traversing the ischiorectal fossa.

46. Grade 5 perianal fistula. Coronal dynamic
contrast-enhanced MR image shows a right
translevator fistula (straight arrow) with
extensive supralevator horseshoe ramification
(curved arrows).

47. MRI Interpretation
• Fistula Detection and Tract Activity
• Active fistula tract appears as a hypointense linear
structure on T1-weighted imaging and hyperintense
on T2-weighted imaging (best visualized with fat
saturation) relative to muscle and enhances with IV
contrast agent.
• Granulation tissue with increased vascularity is
thought to account for the T2-weighted imaging
hyperintensity and contrast enhancement

48. • Inactive tracts are also hypointense on T1-weighted
imaging but lack the associated T2-weighted
imaging hyperintensity and contrast enhancement.
• Tissues surrounding the tract may also show
hyperintensity on T2-weighted imaging if there is
edema or inflammation

49. • Tract Course
• Once a fistula tract is detected, its relationship
to the sphincter complex and location of
the internal and external openings should
be described by the radiologist interpreting
the MRI examination.
• The internal opening can be described according to
anterior-posterior and right-left locations or
according to the “anal clock” with the patient in the
supine position

50. • Detection of Secondary Tracts or Abscesses
• Secondary tracts will have features similar
to those of the primary tract, and their course
should be defined relative to the sphincters,
levator ani, and overlying skin.
• Perianal abscesses may occur anywhere along a
fistula tract and typically have central hyperintense
signal on T2-weighted imaging corresponding
to pus with peripheral rim enhancement
secondary to the fibrous wall and surrounding
inflammation.

51. Conclusion
• Adequate understanding of relevant pelvic
anatomy and fistula classification on MRI
examinations is essential in providing proper
assessment of perianal fistulas.
• Evaluation of clinically undetectable disease has a
significant bearing on guiding medical and surgical
therapy and can help minimize recurrence and better
predict outcome compared with surgical exploration

52. THANKS