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

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(A)Thefourlobesofthecerebrum.(B)Thebrainstemandcerebellum.
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ACTofthehead,showingthecerebralcortex,subcorticalwhitematter,anddeepgraymatterstructures, includingthethalamusandbasalganglia(whichincludesthecaudateandlentiformnucleus).Aswewill discusslaterinthischapter,whitematter(suchastheinternalcapsule)actuallyappearsdarkonCT (owingtothehighcontentofmyelin,whichisafattysubstanceandthereforerelativelylowdensity comparedwiththecellulargraymatter),andgraymatter(suchasthecortex,thebasalgangliaand thalamus)appearsbright.(ModifiedfromFarrellTA.Radiology101.5thed.WoltersKluwer;2019.)
Thebrainisprotectedbytheskullandthethreemeninges.Thebloodsupplytothebraincanbedivided intotwosources:(1)theanterior(carotid)circulationand(2)theposterior(vertebrobasilar)circulation. TheyarelinkedbyanastomosesatthebaseofthebraininastructurecalledthecircleofWillis(see Chapter2,page50).Venoussinusesliewithinthedura,andthesedrainbothbloodandCSF.
Thebrainisalsohometofourventricles:twolateralventricles,thethirdventricle,andthefourth ventricle.TheventriclesarehollowspaceswithinthebrainandbrainstemthatcontaintheCSF.CSFis producedbymodifiedependymalcellsknownasthechoroidplexusthatarelocatedwithintheventricles. CSFflowsfromthelateralventriclesdownthroughthethirdventricle,fourthventricle,thecentralcanal ofthespinalcord,andintothesubarachnoidspace,whereitwillultimatelybepassivelyreabsorbedby arachnoidvilli(alsoknownasgranulations;thesearesmallprotrusionsofarachnoidintothedura)into theduralvenoussinuses.
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Thepathwayoftheventricularsystem.TheCSFflowsfromthelateralventriclesintothethirdventricle(via theforamenofMonro),intothefourthventricle(viathecerebralaqueduct),intothecentralcanalofthe spinalcord(viatheforamenofMagendieandLuschka),andfinallyintothesubarachnoidspacethat surroundsthespinalcordandthebrain.
ElectrophysiologyinTwoPages
Thebasicunitofthenervoussystemistheneuron,aspecializedcellthatcantransmitelectriccurrent. Neuronsarenotjustpassiveconduitsforelectricity;theyalsoreceive,integrate,transform,andsend signalstootherneurons.MostneuronsinthePNSareunipolar,connectingtojustoneothertargetneuron ormusclecell.NeuronsintheCNScanbemultipolar,oftenmakingconnectionswiththousandsofother neurons.
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(A)unipolarand(B)multipolarneurons.
Actionpotentialsarecarefullychoreographedelectricaleventsthatinvolvetheopeningandclosingof potassium,sodium,andcalciumchannelsintheneuronalmembraneandthatpropagatedowntheneuron, creatinganelectriccurrent.Theabilityofneuronstoconductactionpotentialsisenhancedbyanexternal insulatinglayercalledmyelin,producedbyspecifictypesofglialcells(thesearecalledSchwanncells intheperipheryandoligodendrocytesinthebrain),whicheffectivelypreventscurrentfromleakingoutof theneuronsandtherebysignificantlyspeedsupnerveconduction.
Neuronstalktoeachotheracrossaspacecalledthesynapse,andthisconversationiscarriedonby chemicalscalledneurotransmitters,whicharereleasedbythearrivalofanactionpotentialatthe synapse.Therearemanytypesofneurotransmitters;mostoftheonesyouknowalreadyaresmallpeptides suchasacetylcholine,GABA,glutamate,serotonin,andthecatecholamines(dopamine,epinephrine,and norepinephrine).
Box1.2Voltage-GatedSodiumChannelToxins
Voltage-gatedsodiumchannelsareacrucialcomponentofactionpotentialsandtherefore fundamentalforneuronalfunctioning.Asyoumightimagine,ingestionoftoxinsthatblock thesechannelscanhaveadevastatingeffect.Tetrodotoxin,presentinpufferfishandother
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animals,isapotentsodiumchannelblockerthat,wheningested,preventsneuronsfrom communicatingwitheachother.Symptomsdeveloprapidlyandincludeparesthesias, dizziness,nausea,vomiting,tremorand,ifsevere,seizures,paralysis,cardiacarrhythmias, andevendeath.Treatmentconsistsofactivatedcharcoal(tobindthetoxin)andgutlavage (togetitoutofthebody).Thereisnoknownspecificantidote.
Indisease,thingscangowrongatanyoftheselocations—thenervecell,myelinsheath,orsynapse.
Butkeepinmind,aswemovefromonedisordertothenext,thatourneurologicsystemisalsowhat makeslifeworthliving.It’snotallaboutdisease.Ourabilitytosensepleasureandbeautyareallrooted inourneurons.Withsomuchatstake,itisnowonderneurologicdiseasecanbesocatastrophic.
Thesynapseistheplacewhereneuronscommunicatewitheachother.
TheGoodNews!
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Withourneurologicsystemresponsibleforsomuch,itwouldbeeasytothrowupourhandsandproclaim thatitmustbeimpossibletofigureoutwhatisgoingonwhenthingsgohaywire.Butourneurologic toolbox—theinstrumentswehaveatourdisposaltodiagnoseneurologicdisorders—iscompactand, withthehelpofthisbook,somethingthatyouwillsoongrowcomfortableapplyingtoanyandall neurologicissues.Learnhowtousethesetoolsandtheworldofneurologywillunfoldbeforeyou.The mostessentialelementsofourneurologictoolboxare:
History Neurologicexamination Lumbarpuncture(forCSFanalysis) Electroencephalography(EEG) Imaging(CTandMRI,amongothers) Nerveconductionstudies(NCS)andelectromyography(EMG)
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Solet’sgetfamiliarwitheachiteminourtoolbox,andthenwecanmoveontodiscussingthe neurologicdisordersyouwillneedtobefamiliarwith.Wearegoingtotakeeachitemonebyone, startingwiththeneurologichistory.
TheNeurologicHistory
BeforepickingupaspinalneedleororderinganMRI,neurologists,arguablymorethaninanyotherfield ofmedicine,relyonagoodhistorytoguidetheirdifferentialdiagnosisandsubsequentmanagement.A commonmisconceptionisthattakingagoodneurologichistoryneedstotakeWarandPeace-length amountsoftime.Itdoesnot.Wewouldinfactarguethatthemoreskilledatthisyoubecome,themore conciseanddirectedyourhistorycanbe.You’vealmostcertainlyalreadylearnedthegistofitinyour previoustraining.
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Step1:YourBasicOPQRSTDonotskimponthisstep!
Box1.3
Thankgoodnessformnemonics(theonlyword,besidesmnemophobia[yes,thisisaword], intheEnglishlanguagethatbeginswithan“m”followedbyan“n”;thetermisderivedfrom Mnemosyne,theGreekgoddessofmemory).OPQRSTisthefirstbutfarfromthelast mnemonicyouwillencounterinthisbook.
OPQRSTisaguidetothekeyitemsinyourhistorythatwillhelpyouteaseoutthedetailsofapatient’s illnessandhelpwiththediagnosisandsubsequentmanagement:
O: Onset—Did the symptom come onsuddenlyor gradually, andwhat was happeningwhen the symptombegan? P:Provocation—Whatmakesthesymptombetter,whatmakesitworse?Q: Quality—Ask the patient to describe the symptom. This can be difficult for even the most articulatepatient,sobepatient. R:Region orRadiation—Where is the symptomlocated? Isitlocalizedorgeneralized?Does it movetootherareasofthebody?  S: Severity—How bad is the symptom? If the symptom is weakness, for instance, is it subtle, completeparalysis, or something inbetween?If the symptom is pain,many cliniciansusea pain scaleof0to10,butagoodverbaldescriptionisoftenmorehelpful.  T: Time—How long has this been going on? Has it happened before? Has the pain/sensation changedovertime?
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Whenneurologicdiseaseaffectsthebrain,itmaybeimpossibleforyourpatienttogiveacoherent history.Inthatcase,enlistfamilymembers,friends,homehealthaides—anyoneyoucanfind!—tohelp out.
Step2:StartLocalizingThisisthebreadandbutterofneurology.Whileyou’reOPQRST-ing,start thinkingaboutwhereinthenervoussystemyoucouldsituateyourpatient’schiefcomplaint.Startatthe topandworkyourwaydown,tracingyourwayfromthebrainallthewaydownintothemuscles.Whynot theotherway—frombottomtotop?Anatomically,“top-down”isthesimple,tried-and-truewayof makingsureyoudon’tmissanything.Butperhapsthebestreasontostartthinkinginthiswayisthatmost ofthemoreserious“donotmiss”diagnoseslocalizetohigherupintheneuraxis.Forinstance,aparetic (i.e.,weak)handcanbecausedbyboth“lowerdown”(e.g.,peripheralnerveinjury)and“higherup” (e.g.,strokeorbraintumor)lesions.Allareimportantdiagnoses,butthehigherupetiologiesarethemore dangerousandthemorepressingtoruleout.
Thistop-downapproachworksbestformotorcomplaints(youareessentiallytracingthemotor pathwayfromstarttofinish;seepage18),butitcanbeusedforjustaboutanyneurologiccomplaint.For example,handnumbness,asensoryissue,canbecausedbyexactlythesamelesionsasmentionedabove (peripheralnerveinjury,strokeorbraintumor);justbeawarethatyouaretracingtheanatomicpathway backward(fromfinishtostart,sotospeak;thiswillmakesenseinafewpages,seepage23).
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Thebasiclocalizationpathway:Brain→SpinalCord→Nerveroots→Plexus→PeripheralNerves→ NeuromuscularJunction→Muscles.
Let’stakeanexampleofhowyoumightlocalizeaneurologiclesion.Don’tbediscouragedbyallthe neuroanatomyinthisparagraph.Wearejustmakingapointhereandwillgetintoallthemessydetails later.Let’ssaythatyourpatientispresentingwithacuteleft-sidedface,arm,andlegweakness.Youstart ashighaspossible,intheright-sidedmotorstripofthecerebralcortex.Younextfollowthosemotor fibersdownthroughthesubcorticalwhitematterandintothebrainstem,butonceyouhitthecervical spinalcord,you’redone.Why?Becauseyourpatienthasfacialinvolvement,thesymptomscannotbedue toalesionlowerdowninthecord.Nowstopandthink.Ifyourpatienthasalesioninthebrain,where
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