anesthesia.

peroxideintheexternalopeningishelpfultobeusedasanadjunctin difficultcases,withanaccuracyof90%.Becausetheadventof3D-EUS, thisaccuracyhasfurtherimproved.Inarecentstudy,theagreementof 3D-EUSwithoperativefindingswasexcellentintermsofdelineatingthe primarytract(84%),findingtheinternalopening(84%),anddescribing anysecondarytracts(82%).Intheonlyprospectivestudythatcompared 3D-EUSandMRIforthedefinitionofanalfistulas,bothmodalitieswere showntobeequallyaccurate.TheadvantageofEUSisthatitisalow­cost,rapid,andsimpleprocedure.However,EUShassomelimitations: asitisahighlyoperator-dependentprocedure,itislimitedinitsabilityto detectsupralevatorabscessesandfistulas,maynotbetoleratedwithout anesthesia,anditcannotreliablydistinguishbetweeninfectionand fibrosis(Fig.7-10).

MRIhastheadvantageofexcellentintrinsicsoft-tissueresolution, whereitcandepictthefistulatractinthecontextofthesurrounding structures.Thesphinctermusclesandthelevatoranicanbeclearly visualized,andthetrajectoryofthefistulatractorabscesscanbestudied withaccuracyusingsagittal,axial,andcoronalreconstructions(Fig.7-

11).MRIismoresuitedfortheassessmentofcomplexbranchingtracts;

lateralextensionofatractintotheperianalspaces;andthecranial extensionabovethelevatorani.MRIissuperiortoEUSinthe identificationofsuprasphinctericandextrasphinctericfistulas.Another advantageofMRIisthatitcanreliablydifferentiateinfectionsfrom

beidentifiedandassessed.Locationoftheexternalopening,itsdistance fromtheanalmargin,andpalpationofthetractshouldgivethesurgeon hintsaboutthelocationoftheinternalopening.Oncethesesignsare assessed,probingisusedtocannulatethetractandlocalizetheinternal opening.Thisstepisperformedbydelicatelyinsertingablunt,slightly curvedfistulaprobewithintheexternalopeningandgentlyprobingto findtheinternalopening.Thismaneuvershouldbedonewithgreatcare topreventcreationoffalsepassages.

Iftheinternalopeningisnotidentifiedbygentleprobing,several maneuverscanbeusedtoassistwithitsidentification.Dilutehydrogen peroxidecanbeinjectedintotheexternalopeningusinganolive-tipped metalcatheter(Fig.7-13AandB).Thisistheauthors’preferencebecause itisnotmalleableandisnoteasilyobstructed.Otheragentsthatcanbeen usedincludeaweaksolutionofmethyleneblue(1:10)andmilk.The drawbackofamethylenebluesolutionisthatitstainsthesurrounding tissues.

Insomecases,theinternalopeningandtheexternalopening(s)are

easilyidentified,buttheprobecannotreachfromoneopeningtothe other.Thisisusuallyduetoahighextensionofthetract;inthissituation, gentleprobingthroughbothopeningsmaydelineatetheprimarytract.

Oncethecourseofthefistulaisfullyidentifiedandaprobeisgently insertedwithinthetractfromtheexternalopeningthroughtheinternal openingandintotheanus,therelationshipofthetractwiththesphincter musclesshouldbeconfirmed(Figs.7-14and7-15).Ifthefistulais deemedappropriateforfistulotomy,theroofoftheentiretractislaid open,usingascalpelbladeorelectrocauterytodividetheskin, subcutaneoustissue,andanyfibersoftheinvolvedsphinctermuscles. Theinfectedanalcryptisobliteratedintheprocess.Thegranulation tissuewithinthetractiscuretted.Ifthereisanyconcernforanetiology otherthanthecryptoglandulardiseasesuchasCrohn’sdiseaseor tuberculosis,thisgranulationtissuecanbesentforbiopsy.Thechronic woundedgesoftheexternalopeningareexciseduntilhealthytissueis reachedtoallowforproperhealing.Hemostasisisobtainedthrough electrocoagulation.