MRI fistulogram

Magnetic Resonance Imaging in Anal fistulas

1. MR FISTULOGRAM

2. AIM
•Demonstrateaccuratelytheanatomyoftheperianalregion.
•Theanalsphinctermechanism,MRimagingclearlyshowsthe
relationshipoffistulastothepelvicdiaphragm(levatorplate)
andtheischiorectalfossae.
•Thisrelationshiphasimportantimplicationsforsurgical
managementandoutcomeandhasbeenclassifiedintofive
MRimaging–basedgrades.

3. 1.Iftheischio-analandischiorectalfossaeareunaffected,
diseaseislikelyconfinedtothesphinctercomplex(simple
intersphinctericfistulization,grade1or2),andoutcome
followingsimplesurgicalmanagementisfavorable.
2.Involvementoftheischio-analorischiorectalfossabya
fistuloustrackorabscessindicatescomplexdisease
relatedtotrans-sphinctericorsuprasphinctericdisease
(grade3or4).Correspondinglymorecomplexsurgery
mayberequiredthatmaythreatencontinenceormay
requirecolostomytoallowhealing.
3.Ifthetracktraversesthelevatorplate,atranslevator
fistula(grade5)ispresent,andasourceofpelvicsepsis
shouldbesought.

4. •Muchofourunderstandingofperianalfistulas
comesfromtheworkofsurgeonsatStMark’s
Hospital:DrSalmon,whooperatedonCharles
Dickens.
•Goodsall:-who described the course of
fistulous tracks from the skin to the anus.
•Parks:-whose classification of fistulas in
relation to anal anatomy is widely used in
surgical practice.

5. •Goodsalldescribedtherelationshipofthecutaneous
openingtotheexpectedsiteoftheentericopening.
•Therulestatesthatcutaneousopenings–
Anteriortothetransverseanallineareassociatedwith
Directradialfistuloustracksintotheanalcanal,
•whereasopeningsPosteriortothelinehavetracksthat
enterthecanalinthemidlineposteriorly.
•

6. •Thecutaneousopeningisevidenttothesurgeonand
thusoflittleimportancefortheradiologistto
demonstrate.
•Thesurgeonpassesbluntprobesalongthetrackfrom
thecutaneousopeningandmaydeterminetheenteric
openingofthefistulaatproctoscopy.
•Thechallengeinthemanagementoffistulasisto
definethecourseofthetrackbetweentheseopenings
sothattheappropriatesurgicaloptioncanbeused.

7. •MRimaging–basedgradingsystemfor
perianalfistulas(theStJames’sUniversity
Hospitalclassification)thathasbeenvalidated
bysurgicallyprovedcaseswithdocumented
long-termoutcome.

8. •AnatomyoftheAnalRegion:-
•Surgeonsdescribethesiteanddirectionoffistuloustracks
byreferringtothe“analclock”,thatis,theviewoftheanal
regionwiththepatientinthelithotomypositionusually
usedforfistulasurgery.
•At12o’clockistheanteriorperineumandat6o’clock,the
natalcleft;3o’clockreferstotheleftlateralaspect,and9
o’clock,totherightoftheanalcanal.
•**Fortunatelythesedescriptionscorrespondexactlywith
theviewoftheanalcanalonaxialMRimages,anditis
helpfulforsurgicalcolleaguesiftheradiologistsrelatethe
MRimagingfindingstotheanalclock.

9. •Tounderstandthesurgicaloptionsfortreatingfistulousdisease,
onemustfirstconsidertheanatomyandfunctionoftheanal
sphinctersandthecausesofperianalfistulas.
•Theinternalsphincterisinvoluntaryandiscomposedofsmooth
musclecontinuouswiththecircularsmoothmuscleoftherectum.
Itisresponsiblefor85%ofrestinganaltone.Inmostindividuals,it
canbedivided(OPERATED)withoutcausingalossofcontinence.
•Theexternalsphincteriscomposedofstriatedmuscleandis
continuoussuperiorlywiththepuborectalisandlevatorAni
muscles.
•Itcontributesonly15%ofrestinganaltone,butitsstrongvoluntary
contractionsresistdefecation.
•Adivisionoftheexternalsphinctercanleadtoincontinence.

11. a = anal canal, IAF = ischioanal fossa, IRF = ischiorectal fossa, R = rectum

12. Line diagram shows the normal anatomy of the perianal region in the
axial plane.

13. •CausesofPerianalFistulas:-
•Idiopathicfistulasaregenerallybelievedtorepresent
thechronicphaseofintramuscularanalglandsepsis
(ie,thecryptoglandularhypothesis).
•However,perianalfistulasmayalsobecausedbyother
conditionsandevents,including
-Crohndisease,
-tuberculosis,
-traumaduringchildbirth,
-pelvicinfection,
-pelvicmalignancy,and
-radiationtherapy.

14. •Analglandslieatthelevelofthedentatelinein
themidanalcanalandcanpenetratetheinternal
sphinctertolieintheintersphinctericplane.
•Fromthisspace,theinfectionmaytrackdown
theintersphinctericplanetotheskin,andabout
70%offistulasbehaveinthisway.
•Alternatively,infectionmaypassthroughboth
layersoftheanalsphincter(trans-sphincteric
fistulization)toentertheischiorectalfossa;this
developmentpatternoccursinabout20%of
cases.

15. a = anal canal, IAF = ischioanal fossa, IRF = ischiorectal fossa, R = rectum

16. •Abscesscavitiesmaydevelopalongthecourseof
fistuloustracks.Characteristically,theabscesses
associatedwithintersphinctericfistulasareperianalor
indeedencystedwithintheintersphinctericspace.
•Trans-sphinctericfistulasaretypicallyassociatedwith
ischiorectalfossaabscesses.
•Inrarecases,infectiontracksupwardoverthe
sphinctercomplextoentertheischiorectalfossa
(suprasphinctericfistulization).
•Sepsisarisingwithinthepelvismaytrackdowntothe
skinthroughtheischiorectalfossa,resultinginfistulas
thatarereferredtoasextrasphinctericortranslevator.

17. •SurgicalApproaches/Surgicalmanagementof
perianalfistulasdependsonthe
-natureoftheprimaryfistulaand
-anysecondaryfistuloustracksor
-associatedabscesses.
•Forsimpleintersphinctericfistulas,thesurgeon
performsafistulotomyorfistulectomy,inwhichthe
internalsphincterisdividedtolayopenthetrack.
•Alternatively,inpatientswithperianalabscess,the
surgeonperformsasimpleincisionanddrainagefirst.

18. •Preservation of fecal continence is the paramount
consideration, and treatment strategies aim to
preserve the integrity of the external sphincter.
•Over the years, surgeons have been acutely
aware of the damage to their reputations that
has resulted from rendering patients incontinent.
Surgical papers abound with quotes, such as “if
this ring be cut, loss of control surely follows”

19. Preoperative Evaluation of Perianal
Fistulas
1.Todeterminetherelationshipofanyfistulous
tracktothesphinctercomplex.
-Isthesphincterinvolved?(YesorNO)
-Doesthetracktraversebothlayersofthe
sphincter(trans-sphincteric)oronlythe
internalsphincter(intersphincteric)?

20. 2.Toidentifyanysecondaryfistuloustracksandthe
sitesofanyabscesscavities.
Failuretodetectanderadicatethesemayleadto
relapseandthustherapeuticfailure.
•Secondarytracksorramificationsmaybefound
withintheintersphinctericplane,theischiorectal
fossa,orthesupralevatorspace.
•“Horseshoe”tracksmaypasscircumferentiallyin
theseplanesandmaycrossthemidline.

22. •MRimagingexaminationsperformedwithabodycoil
requirenopatientpreparationandarewelltolerated.
•Useofendoanalcoilswasinitiallyhopedtofurtherimprove
theMRimagingevaluationofperianalfistulas(14).
•However,thistechniqueispoorlytoleratedinsymptomatic
patients,andalthoughitprovidesexcellentanatomicdetail
oftheanalsphincters,itfailstoprovidetheoverview
requiredforsurgicalmanagement(15).
•Itseemslikelythatuseofpelvicsurfacecoilsmayfurther
improveimagequality.

24. •MRImagingofPerianalFistulasNormalAnatomy
Theanatomyoftheperianalregioniswell
demonstratedoncoronalandaxialMRimages
(Fig4).
•Theinternalandexternalsphinctersarenot
separatelyresolvedinnormalsubjectsonMR
imagesobtainedwithabodycoil,
butthesphinctercomplex,ischiorectalfossae,
andlevatoranislingareclearlyseen.

26. •TechniquesforImagingPerianalFistulasUnenhancedT1-weighted
imagesprovideanexcellentanatomicoverviewofthesphincter
complex,levatorplate,andtheischiorectalfossae.
•Fistuloustracks,inflammation,andabscesses,however,appearas
areasoflowtointermediatesignalintensityandmaynotbe
distinguishedfromnormalstructuressuchasthesphinctersand
levatoranimuscles.
•OnT2-weightedandSTIRimages,pathologicprocessesincluding
fistulas,secondaryfistuloustracks,andfluidcollectionsareclearly
depicted(Table2).
•Theyappearasareasofhighsignalintensityincontrastwiththe
lowersignalintensityofthesphincters,muscles,andfat(especially
onSTIRimages).Theonlycomparativestudyofimagingsequences
suggestedforuseinthisconditionshowedthatSTIRimaginghas
certainlimitations(10).

27. •Insomecases,STIRimagingfailedto
demonstratesecondarytracks,andinothersit
didnotrevealsmallresidualabscesseswithin
edematousinflammatorychange.Furthermore,
spuriousincidencesofhighsignalintensityin
inactivetrackswerealsoobserved.
•FatsuppressiontechniquesusedwithT2-
weightedimaginghavealsobeenproposed(11)

29. •Weusegradient-echoT1-weighteddynamic
intravenouscontrastmaterial–enhancedMR
imagingcombinedwithT2-weightedimagingto
assessperianalfistulasandtheircomplications.
•Withuseofthistechnique,activefistuloustracks,
secondaryramifications,andabscessesare
clearlydemonstrated.
•Thetracksbrilliantlyenhanceasdothewallsof
abscesscavities.
•Retainedpusremainsunenhanced,withresulting
ringenhancement,anappearancethatistypical
ofabscessformationelsewhereinthebody.

30. •UnenhancedT1-weightedimagingmaybehelpfulin
postoperativeassessment.
•Firstly,ifMRimagingisperformedintheimmediate
postoperativeperiod,hemorrhagewillappear
hyperintenseandmaythusbedifferentiatedfroma
residualtrack.
•Secondly,fat-containing“grafts”maybeplacedtofill
cavitiesandresectionvoidsinrestorativesurgery.
Thesehyperintensestructuresarereadilyidentifiedon
unenhancedT1-weightedimagesandcanbe
distinguishedfromactivedisease,whichappears
hyperintenseonlyafterenhancementwithgadolinium.

31. St James’s University Hospital
Classification
•the demonstration of the primary fistulous
track but also with secondary ramifications
and associated abscesses.

32. Grade 1:
•SimpleLinearIntersphinctericFistula.—Inasimplelinear
intersphinctericfistula,thefistuloustrackextendsfromthe
skinoftheperineumornatalclefttotheanalcanal,and
theischiorectalandischioanalfossaeareclear(Figs5–7).
•Thereisnoramificationofthetrackwithinthesphincter
complex.
•Theenhancingtrackisseenintheplanebetweenthe
sphinctersandisentirelyconfinedbytheexternal
sphincter.
•Fistuloustracksarisingbehindthetransverseanalline,
whicharebyfarthemostcommontype,entertheanal
canalinthemidlineposteriorly(Figs6,7).

33. Grade1perianalfistula.
(a)Linediagramofthecoronalviewshowsarightintersphinctericfistula(yellowtrack)
extendingfromthedentatelinedowntotheskinthroughtheintersphinctericplane.
(b)Linediagramoftheaxialviewshowstheposteriormidlineintersphinctericfistulous
track(yellowspot)confinedbytheexternalsphincter.

34. Figures6,7.Grade1perianalfistula.
(6)Coronaldynamiccontrast-enhancedMRimageshowsarightintersphinctericfistula
enteringtheanalcanalinthemidlineposteriorly(arrow).
(7)AxialT2-weightedMRimageshowsaposteriormidlineintersphinctericfistula
(arrowhead).

35. Grade 2:
•IntersphinctericFistulawithAbscessorSecondaryTrack.—
Intersphinctericfistulaswithanabscessorsecondarytrack
arealsoboundedbytheexternalsphincter.
•Secondaryfistuloustracksmaybeofthehorseshoetype,
crossingthemidline(Figs8,9),ortheymayramifyinthe
ipsilateralintersphinctericplane(Figs10,11).
•Evenwhenthereisabscessformation,thisprocessis
confinedwithinthesphinctercomplexregardlessof
imagingplaneorsequence(Fig12).

36. •OnT2-weightedimages,pushashighsignalintensityandthus
cannotbereliablydistinguishedfromedemaandinflammation,
•butgaswithinabscesseshasalowsignalintensitysimilartothatof
theanorectallumen(Fig13).
•Intersphinctericabscessesandsecondaryfistuloustracksarewell
shownbydynamiccontrast-enhancedMRimaging.
•Onthesecontrast-enhancedimages,thepusinthecentralcavity
haslowsignalintensityandissurroundedbyabrightlyenhancing
rim(Figs11,12).
•Ahorseshoefistula,inwhichtheprocessextendstotheopposite
side,isbestdemonstratedintheaxialplane(Figs10,12b).

37. Figures 8, 9. Grade 2 horseshoe perianal
fistula.
(8) Line diagram of the axial view shows an
intersphincteric horseshoe fistula (yellow track,
arrow) confined by the external sphincter.
(9) Axial T2-weighted image shows an
intersphincteric horseshoe fistula (arrow).

38. Figures 10, 11. Grade 2 perianal
fistula with an abscess.
(10) Line diagram of the coronal view
shows a left intersphincteric abscess
(a) .
(11) 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).

39. Grade 3: Trans-sphinctericFistula.
•Insteadoftrackingdowntheintersphinctericplanetotheskin,thetrans-
sphinctericfistulapiercesthroughbothlayersofthesphinctercomplex
andthenarcsdowntotheskinthroughtheischiorectalandischioanal
fossae(Fig14).
•Thus,atranssphinctericfistulamaydisruptthenormalfatofthe
ischiorectalandischioanalfossaewithsecondaryedemaandhyperemia
(Figs15,16).
•Thesefistulasaredistinguishedbythesiteoftheentericentrypointinthe
middlethirdoftheanalcanal(ie,correspondingtothepositionofthe
dentateline),asseenoncoronalimages.
•Becausethesefistulasdisrupttheintegrityofthesphinctermechanism,
theirtracksmustbeexcisedbydividingbothlayersofthesphincter,thus
riskingfecalincontinence.

40. Figures15,16.GRADE3PERIANALFISTULA.
(15)Coronaldynamiccontrast-enhancedMRimageshowsarighttrans-sphinctericfistula(arrow)and
inflammatorychangeintherightischiorectalfossa.Notetheentrysiteinthemiddlethirdoftheanal
canal.
(16)Axialdynamiccontrast-enhancedMRimageshowsalefttrans-sphinctericfistulawithinthe
ischiorectalfossaandpiercingtheexternalsphincter(arrow).

41. Grade 4:
•Grade 4: Trans-sphinctericFistula with Abscess or Secondary Track
within the Ischiorectal Fossa.—
•A trans-sphinctericfistula can be complicated by sepsis in the
ischiorectal or ischioanalfossa(Figs 17, 18).
•Such an abscess may manifest as an expansion along the primary
track or as a structure distorting or filling the ischiorectal fossa.
•Axial and coronal dynamic contrast-enhanced MR imaging clearly
depicts a trans-sphinctericabscess, which characteristically has a
central focus of low-signal-intensity pus (Figs 18–20).
•As with grade 3 lesions, the key anatomic discriminator of a grade 4
fistula is the track crossing the external sphincter (Figs 21, 22).

42. Grade 5:
•Grade 5: Supralevatorand TranslevatorDisease.—
•In rare cases, perianal fistulous disease extends above the insertion
of the levator animuscle.
•Suprasphinctericfistulas extend upward in the intersphincteric
plane and over the top of the levator anito pierce downward
through the ischiorectal fossa.
•Extrasphinctericfistulas reflect extension of primary pelvic disease
down through the levator plate (Fig 23). These fistulas pose
problems for management because further assessment is needed
to detect pelvic sepsis. Coronal dynamic contrast-enhanced MR
imaging elegantly demonstrates breaches of the levator plate,
which is clearly shown in this plane. In some translevatorfistulas,
horseshoe ramifications to the contralateralside may occur (Fig 24).

44. Figures17,18.Grade4perianalfistulawithanischiorectalfossaabscess.
(17)Linediagramofthecoronalviewshowsalefttrans-sphinctericfistulawithaleftischiorectal
fossaabscess(a).
(18)Coronaldynamiccontrast-enhancedMRimageshowsalefttrans-sphinctericfistula(arrow)
withaleftischiorectalfossaabscess(arrowheads)containingnonenhancingpus.Notethe
contractionoftheischiorectalfossa.

45. Figures19,20.Grade4perianalfistulawithanabscess.
(19)Linediagramoftheaxialviewshowsalefttrans-sphinctericfistulaandleftischioanal
fossaabscess(a).
(20)AxialT2-weighted(a)anddynamiccontrastenhanced(b)MRimagesshowaleft
trans-sphinctericfistula(straightarrow)withaleftischioanalfossaabscess(curvedarrow)
andnonenhancingpusinthecavity(arrowhead).

46. Figure23.Grade5perianalfistulawith
anabscess.Linediagramofthecoronal
viewshowsapelvicabscess(a)witha
translevatorfistulatraversingthe
ischiorectalfossa
Figure 24. Grade 5 perianal fistula.
Coronal dynamic contrast-enhanced
MR image shows a right translevator
fistula (straight arrow) with extensive
supralevatorhorseshoe ramification
(curved arrows).

47. Advantages of MRI
•AparticularadvantageofMRimagingisitsabilityto
demonstrateoccultintersphinctericspacesepsis(ie,
pusistrappedwithintheintersphinctericspacewithno
cutaneousexitandthuscannotbefoundbyprobing).
•Incasesof“high”fistulas(trans-sphinctericand
extrasphincteric,grades3–5),probingorexploration
maybeabandonedwhenanatomiclandmarksare
uncertainandtheoperatorisunsurewhetherheorshe
isaboveorbelowthelevatorplate.

48. Teaching points
•1.IftheischioanalandischiorectalfossaeareunaffectedatMR
imaging,diseaseislikelyconfinedtothesphinctercomplex
(intersphinctericfistulization,grade1or2).Outcomefollowing
simplesurgicalmanagementisfavorable.
•2.Ifthereisanytrackorabscesswithintheischiorectalfossa,itis
usuallyrelatedtoacomplexperianalfistula(typicallytrans-
sphinctericfistulization,grade3or4).Correspondinglymore
complexsurgerymayberequiredthatmaythreatencontinenceor
mayrequirefecaldiversion(colostomy)toallowhealing.
•3.Whenthetracktraversesthelevatorplate,atranslevatorfistula
(grade5)ispresent,andasourceofpelvicsepsisshouldbesought.

49. •The End…