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Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_186_библиотеки_им_акад_М_И_Перельмана

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denotebrightwhite.Thissequenceisderivedfrommeasuringtherandommotionofwatermolecules withinagivenvolumeoftissue.Whencellsdie,theyswell,makingitdifficultforwatertomovearound: thus,infarctedtissue“restricts”waterdiffusion.
Box1.13TheADCSequence
Yetanothersequence,theapparentdiffusioncoefficient(ADC)sequence,iscalculatedfrom theDWIscanandcanbeusedtoconfirmthatwhatappearsbrightonDWIisinfacttrue restricteddiffusionandnotwhatisreferredtoas“T2shine-through,”thatis,whenbrightT2 signal“shinesthrough”toDWI.
AcuteinfarctionwillbebrightonDWIanddarkonADC(werefertothisasan“ADC correlate”).IfyouseeabrightlesiononDWIbutnocorrespondingdarklesiononADC(i.e., noADCcorrelate),theDWIlesionismostlikelyT2shine-throughandnotacuteinfarction. Shine-throughismostoftenattributedtosubacute(olderthan1week)infarctions,butcan alsobecausedbyotherlesionssuchascysts.
NotethatstrokestaysbrightonDWIforabout1monthafteritoccurs,butstaysdarkon ADCforonlyabout1week.
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Anacuteinfarctoftheleftcoronaradiata(thesubcorticalbundleofwhitematterfibersthatcontinue inferiorlyastheinternalcapsuleandcarrythedescendingmotorfibers;seeMotorSystemAnatomy
101),brightonDWI(left)withdarkADCcorrelate(right).(ReprintedfromKleinJ,VinsonEN,Brant WE,HelmsCA.BrantandHelms’FundamentalsofDiagnosticRadiology.5thed.WoltersKluwer;
2018.)
Box1.14Diffusion-RestrictingLesions
Thisgoesabitbeyondthescopeofthisbook,butjustsoyouknow,lesionsotherthan infarctioncanrestrictdiffusion.WhenyouseebrightwhiteonDWIwithcorrespondingdark onADC,alwaysthink“stroke”first,butkeepinmindthathypercellulartumors(suchas lymphomaandmeningiomas)andbacterialabscesses,amongotherlesions,canrestrict diffusionaswell.ClinicalcontextandotherMRIsequencesareoftensufficienttohelpyou figureoutwhat’sgoingon.
VesselImaging.Noneedtogointothedetails,butyoushouldknowthattherearebothCT-basedand MR-basedangiographystudies.Thesescansenableustolookspecificallyatthebloodvesselswithinthe neckandhead.Theyarefrequentlyutilizedinacutestroketodetectvesselocclusions.Theycanalsohelp diagnosedissection,vasculitis,andotherangiopathies.CTangiography(CTA)alwaysrequires intravenous(IV)contrast;MRangiography(MRA)doesnot.
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ThecircleofWillis,asvisualizedonaCTAofthehead.(CourtesyofJonathanHoward.)
Box1.15AQuickNoteonContrast
IVcontrastcanbeusedwithbothCTandMRItohelpvisualizeandenhancespecific abnormalities.Neurologistsoftenordercontrast-enhancedstudieswhenthey’reconcerned aboutmalignancyorabscess,asbothoftheselesionstypicallyenhanceonimaging.CT usesiodinatedcontrast;MRIusesgadolinium.Bothcancauseallergicreactions,and
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patientsshouldalwaysbeaskedpriortocontrastadministrationiftheyhaveanyhistoryof reactionstocontrast,includingrashandshortnessofbreath.Iodinatedcontrastcancause contrast-inducednephropathy(CIN),whichpresentsasanacutekidneyinjury(AKI)within24 to48hoursofcontrastadministration,andisgenerallyreversiblewithsupportivetreatment. Gadoliniumcancausenephrogenicsystemicsclerosis(NSF),characterizedbythickening andhardeningoftheskinthatcancausejointcontracturesanddiffusefibrosisaffectingvital organs.NSFonlyoccursinpatientswithadvancedkidneydisease;likewiseCIN,mostofthe time.Thus,especiallyinpatientswithkidneydisease,thebenefitsandrisksofadministering contrastmustalwaysbetakenintoaccountbeforeorderingtesting.
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Aleftfrontalring-enhancingabscessseenonCTwithcontrast.ModifiedfromDaffnerRH.Clinical Radiology.3rded.WoltersKluwer;2007.
LumbarPuncture
AnLPprovidesaccesstotheCSF.IndicationsforanLPinclude:
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1. CSFanalysis:todetectbleeding,infection,inflammation,malignantcells,andsoforth
2.  CSFpressure measurement:anormal adult“openingpressure” is10to18cmH2O;this can onlybe accuratelyobtainedwith the patientlyinginthelateraldecubitusposition
3. CSFremoval:thiscanbeindicatedforbothdiagnosisandtherapyinconditionssuchasnormalpressurehydrocephalusandidiopathic intracranialhypertension
4. Medicationinjection:suchasanesthesiaorchemotherapy
AnLPcanbeperformedatthepatient’sbedside.Wetypicallypositionpatientsontheirside(the “lateraldecubitusposition”),withtheirkneescurledintotheirchesttohelpopenupthespacesbetween thevertebrae.ThespinalcordendssomewherearoundtheL1-L2vertebrae,soweaimtoinsertour needlebetweentheL3-L4orL4-L5vertebraetoavoidpotentialinjurytothecord.
Positionpatientsontheirsideinthelateraldecubitusposition.Placeyourfingersonthetopsoftheiliac crests,whichlieatapproximatelytheL4level,andstretchyourthumbsmidlinetothespine.Youcanthen palpatethevertebraeandthespacesinbetween,todeterminethebestplace—betweenthebonesofthe vertebrae,soyou’llhaveaccesstothesubarachnoidspace—toinsertyourneedle.
ComplicationsfromanLParerare.Post-LPheadacheisthemostcommonadverseeffectandisdueto rapidCSFremovalandthesubsequent“lowpressure”situation.Wetellpatientstolieflatfor1to2 hoursaftertheprocedure,whichismorethanenoughtimeforthebodytoreplacetheCSFthathasbeen removed.Thismakeslogicalsense,but,justsoyouknow,thereisnoclearevidencethatdoingsoactually preventstheheadache.Othercomplicationsincludelowerbackdiscomfort,bleedingattheneedle insertionsiteor,rarely,intotheepiduralspace,and—incrediblyrarely,nowthatweusesteriletechnique —infection.
Box1.16
Adultshaveabout150mLofCSFatanygiventime,butturnitoverrapidly.CSFisproduced bythechoroidplexusatabout20mL/hour,orapproximately500mL/day.Toputthese numbersinperspective,wetypicallyremoveanywherefrom5to20mLofCSFwhen performinganLP.
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Box1.17LPContraindications
1. Patientcannotlieflat(e.g.,inapatientwithdecompensatedheartfailure)
2. Concernforelevatedintracranialpressureandriskofherniation(CSFremovalinthepresenceofelevatedintracranial pressurecancausedownwarddisplacementofthebrainandcompressionofthebrainstem)
3. Highriskforbleeding(e.g.,lowplatelets,currentuseofanticoagulation)
4. Presenceofanepiduralabscess(riskofseedingtheCSF)
5. Significantpriorlumbarsurgery(withdistortedanatomy)
Electroencephalography
EEGdetectselectricalactivityinthecerebralcortexbyusingsmall,stickyelectrodesattachedtothe scalp.WhatexactlyanEEGisrecording(whethertheelectricalactivityisderivedfromactionpotentials, chronicdepolarizations,postsynapticpotentials,orothersources)iscomplicatedandnotentirely understood,althoughitisbelievedattheveryleasttooriginatefromneurons.Regardless,the electrocorticalactivityisverysmallandmustbeamplifiedbyafactorofonemilliontobedetectedona computerscreen.
EEGismostoftenusedtodiagnoseepilepsy.Seizuresareburstsofabnormalelectricalactivity,and
differentseizuredisordershavedifferentcharacteristicpatternsonEEG(seeChapter6).
TherearefourmainfrequenciesofEEGwaves,eachofwhichisassociatedwithdifferentstatesof normalbrainfunctioning.Eachfrequencycanalso,underspecificcircumstances,betheresultof underlyingdisease(excessivedeltaactivity,forinstance,canbeindicativeofencephalopathy).Thetable belowisbynomeanscomprehensive,butitwillbeusefulforyoutohaveseenthesewords—delta,theta, alpha,andbeta—andhaveabasicunderstandingofwhattheymean.
Freque ncy Hz BrainState Notes
Delta <4 Deepsleep Canoccurfocally,inthegeneraldistributionofunderlyingbrainlesions(i.e.,youcansee“deltaslowing”
overanareaofanoldstroke);orexcessivelyanddiffusely,indicativeofencephalopathyofnonspecific etiology
Theta 4–8 Drowsiness,
sleep
Diffusethetaisnormalinawakechildren;itcanbeseeninawakeadults,butitmayalsobecompletely absentduringwakefulness
Alpha 8–12Relaxed,
witheyes closed
You’llheartheterm“posteriordominantrhythm”:thisisthenormalalpharhythmthat’sseenoverthe posteriorregionofthebrainwhenpatientsarerelaxedwiththeireyesclosed(whentheyopentheireyes, the“PDR”attenuatesorevendisappears)
Beta 12–30Awakeand
active
Variousdrugs(includingbenzodiazepinesandbarbiturates)canincreasebetaactivity
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EEGfrequencies
NerveConductionStudiesandElectromyography
Thesearepredominantlyoutpatientteststhatcanhelpdiagnosevariousneuromusculardisorders.These includediseasesinvolvingtheperipheralnerves(forexample,diabeticneuropathy),neuromuscular junction(myastheniagravis),ormuscles(dermatomyositis).EMGmeasuresmuscleresponsetonerve stimulationvianeedlesinsertedintothemuscles.NCS,oftendoneatthesametime,measureshowwell andhowfastperipheralnervessendsignals.
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ObtaininganNCSandEMG.WewillshowyouwhatthetracingslooklikeinChapter12.
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Hailey’sMRI(FLAIRsequence)showsrightfrontalvasogenicedema.Thepostcontrastimages(not pictured)confirmanunderlyingmass.(ReprintedfromFisherRG,BoyceTG,CorreaAG.Moffet’s PediatricInfectiousDiseases.5thed.WoltersKluwer;2017.)
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