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Ростовский Государственный Медицинский Университет

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Removethebrainandspinalcord,andyouarelookingattheperipheralnervoussystem.Well,notexactly. Thisstatementisabitofanoversimplification.Thereareafewimportantexceptions:thecranialnerves (exceptforCN1andCN2),aswellastheanteriorhorncellswithinthespinalcord(thecellbodiesoflower motorneurons),arepartoftheperipheralnervoussystemaswell.

WewillstartthischapterwithanillnessthatafflictsboththeCNSandPNS,asmoothtransitionforus, butadevastatingillnessforthosewhoareafflicted.

AmyotrophicLateralSclerosis(ALS)

Amyotrophiclateralsclerosis(ALS)isadiseaseofunknowncausethataffectsbothupperandlower motorneurons.Itisprogressiveanduniformlyfatal.Althoughtherearesomefamilialcaseswithan identifiableunderlyinggeneticmutation,mostcasesaresporadicandidiopathic.ALSisrare,withan incidenceoflessthan3per100,000personyears.However,thisnumberhasbeenslowlyincreasingover thepastfewdecades,likelyaresult,atleastinpart,ofourlongerlifeexpectancy.

Symptoms.ThekeytodiagnosingALSistorecognizetheinvolvementofbothupperandlowermotor neurons.AsareminderfromChapter1(thismaterialshouldlookfamiliar!):

Uppermotorneurons(UMNs)includeallneuronsthatruninthemotorpathwaysabovethelower motor neurons.These include the neuronsinthe corticospinal andcorticobulbar tracts. Weakness, increased tone andspasticity, hyperreflexia, clonus, and upgoing toes (akathe Babinski sign) are classicUMNfindings. Lowermotorneurons(LMNs)arethefinalnervesinthemotorpathwaysthatinnervatethemuscles. These include the anterior horn cells in the spinal cord and the cranial nerves that have motor components (i.e., all the cranial nerves except 1, 2, and 8). Like UMN disease, LMN disease presentswithweakness,butitcanalsocausemuscleatrophy,decreasedmuscletone,hyporeflexia, andfasciculations(ormuscletwitching).

PatientswithALSwillalleventuallyhavefindingsconsistentwithbothUMNandLMNdisease. Initially,however,theremaybeevidenceofonlyupperorlowermotorneurondisease,complicatingthe diagnosis.Themostcommonpresentationisasymmetriclimbweakness,typicallyinvolvingthehands and/orfeet,althoughasignificantminority(∼20%)ofpatientswillfirstpresentwithweaknessofthe bulbarmuscles.

Bulbarsymptoms(i.e.,symptomslocalizabletothemedulla),suchasdysarthriaanddysphagia,are alsocommoninALSandcanbecausedbyeitherUMNdisease(specificallythecorticobulbartracts, whichbegininthemotorcortexandsynapseonthemotornucleiofthecranialnervesinthebrainstem)or LMNdisease(thebrainstemcranialnervesthemselves:9,10,11,and12).Pseudobulbaraffectisa commonUMNbulbarsymptomthat’scharacterizedbyinappropriatelaughingorcrying,oftentriggeredby stimulithatundernormalconditionswouldnothaveelicitedsuchresponses.Spasticspeech,increased massetertone,andlaryngospasm(oftendescribedasabriefsqueezingsensationinthethroat)areother commonUMNbulbarsymptoms.TonguefasciculationsarethemostcommonLMNbulbarsymptom.

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(A)Theuppermotorneuronsofthecorticospinaltractbegininthemotorcortexandprojectdownward, throughthecoronaradiata,internalcapsule,andbrainstem(wheretheycrosstothecontralateralside) intothespinalcordwheretheysynapseonlowermotorneurons.(B)Theuppermotorneuronsofthe corticobulbartractalsobegininthemotorcortexbutsynapseinthebrainstem(again,aftercrossing)on thenucleiofCN9,CN10,CN11andCN12.

AbouthalfofpatientswithALSwillultimatelyexhibitsomedegreeofcognitiveimpairment.Some patientsmayhavesensorycomplaints,suchasparesthesias,butthesensoryexaminationisalmostalways normal.Ifnot,youneedtoconsiderotherdiagnoses.

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LouGehrig,thecelebratedHallofFamebaseballplayerwhoplayed17seasonsfortheNewYork Yankees,wasdiagnosedwithALSinhis30s.Thediseaseisnowcommonlyandcolloquiallyreferredtoas LouGehrigdisease.

Diagnosis.ThediagnosisofALSismadebyhistoryandneurologicexamination,anditisconfirmedby electromyography(EMG)andnerveconductionstudies(NCS).MagneticresonanceimagingMRIofthe brainandspinalcordaredoneinordertoruleoutotherpossiblecauses;MRIisusuallynormalinALS, althoughyoumay(rarely)seeT2signalchangesbeginninginthemotorcortexandextendingdowninto thecorticospinaltracts.Cerebrospinalfluid(CSF)analysiscanalsobeusefultoruleoutothercausesof polyneuropathy,suchasinflammatorydisorders,HIVinfection,lymphoma,andLymedisease.

Prognosis.Theprognosisispoor.Mediansurvivalis2to5years,althoughtherearesomepatientswho livemuchlonger,albeitwithrelentlesslyprogressivedisability.Unlikemultiplesclerosis(seeChapter

9),progressionisnotoneofexacerbationsandremissionsbutratheroflineardecline.Themostcommon causeofdeathisrespiratoryfailureduetorespiratorymuscleinvolvement.

Treatment.Treatmentislargelysymptombasedandrequiresmultidisciplinarycare.Asthedisease progresses,patientsoftenrequireafeedingtubeandtracheostomywithmechanicalventilation.Thereare medicationsapprovedspecificallyforpatientswithALS;riluzole,aglutamateinhibitor,andedaravone,a freeradicalscavenger,canslowprogressiontoamodestdegreeandmayextendsurvivalbyseveral months.

Box11.1ALSMimicsandVariants

Severaldisorders,allevenlesscommonthanALS,needtobeconsideredinthedifferential diagnosisofpatientswhopresentwithamotorneuropathy.

ALSmimicsinclude:

 Multifocal motor neuropathy (MMN), an autoimmune demyelinating disease that can presentjustlikeALSbutwithexclusivelylowermotorneuroninvolvement.Patchy,often asymmetricweakness typically sparesthecranialnervesand bulbarmuscles. In many but not all patients MMN is associated with antibodies to ganglioside GM1. It is importanttodistinguishMMNfromALSbecause,unlikeALS,itrespondstointravenous immunoglobulin (IVIG). EMG (along with the clinical picture) is critical to help differentiatethesetwoconditions. Stenosisof thecervicalspine, theresultofprogressivedegenerationofthevertebrae and intervertebraldiscs of the cervical spine, usually from osteoarthritis. Patients may presentwith:

neck,shoulder,orarmpain lowermotorneuronfindingsintheupperextremities(whichcanoccuratthelevelofcordcompression,aresult ofdamagetotheanteriorhorncellsand/ornerveroots) uppermotorneuron findingsintheupper and/orlowerextremities(aresultofdamagetothecorticospinaltract withinthespinalcord) sensorylossinthearmsanddecreasedsensationbelowthelevelofthelesion(followingadermatomalpattern) gaitimpairment,whichisverycommonandisduetoacombinationofthesensoryandmotordeficitsdescribed above

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Thediagnosisofcervicalstenosisismadebyimagingandelectrodiagnostictesting.

ALSvariantsinclude:

Primarylateralsclerosis: This disorder presentswithsolelyupper motor neuronsigns and symptoms. Only late in the course do some patients exhibit lower motor neuron involvement. It progresses more slowly than ALS—patients need to be followed for several years before this diagnosis can be made—and their life expectancy is considerablybetter. Progressivemuscularatrophy: This conditionpresentswithsolelylower motor neuron signs and symptoms, although late in thecourse some patients will also exhibit upper motorinvolvement.SurvivalmaybeafewmonthslongerthanwithclassicALS.

Foreachofthesedisorders,history,neurologicexamination,andEMGarecriticaltohelp distinguishthemfromALS.

PeripheralNeuropathies:AnOverview

Let’sbeginwithaquickreviewofanatomy(justthisparagraph,wepromise!).ThePNSisdividedinto twocomponents:thesomaticdivision,whichincludesthespinalnerves(thesensoryafferentsandmotor efferents)andtheautonomicdivision,whichisfurtherdividedintotheparasympatheticandsympathetic divisions.Keepinmindthatthecranialnerves,withtheexceptionofCN1andCN2arepartofthePNS. Dependingonthespecificcranialnerve,theycanbemadeupofsensory,motor,and/orautonomic components.

Whenwetalkaboutperipheralneuropathies,wearetalkingaboutpathologyaffectinganyoftheabove: thesomaticsensoryandmotornerves,yes,andalsotheautonomicandcranialnerves.Thusgastroparesis, orthostatichypotension,anddiplopiacanallbeduetoperipheralneuropathy.

Peripheralneuropathiesareverycommon.Youwillseepatientswithperipheralneuropathiesnomatter whatbranchofmedicineyouchoose.

Theterm“peripheralneuropathy”actuallyencompassesanumberofdifferentdisorders:

Polyneuropathy—Thisiswhatmostpeoplethinkofwhentheyusetheterm peripheralneuropathy. Polyneuropathy refers to damage of multiple nerves by a single disease process. Involvement is usuallysymmetricallybilateralandsynchronous;thatis,whensymptomsprogress,theydosoonthe rightsideandleftsideatmoreorlessthesametime.Wewillspendmostofthischapterlookingat thesedisorders,astheyarebyfarthemostcommon. Mononeuritismultiplex—This term referstodamageofatleasttwoseparateperipheral nerves; unlikepolyneuropathy,itneednotbesymmetricandthedamagetothevariousnervesdoesnotneed tooccuratthesametime. Mononeuropathy—Thistermmeansdamagetoasinglenerve.Thesedisordersareusuallycaused by trauma, entrapment, or compression. A common example is carpal tunnel syndrome (median neuropathy at the wrist). However, not all mononeuropathies are mechanical in origin; the big exceptionis cranial mononeuropathy, which is more often the resultofaninfectious/inflammatory process(e.g.,aCN7,orBellpalsy)orischemicevent(CN3palsy).

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 Plexopathy—Plexopathies affect either of two discrete networks of nerves, the brachial plexus, whichinnervates the muscles and skin oftheshoulder andarm,or the lumbosacral plexus, which innervatesthemusclesandskinofthelowerextremities.

Box11.2PeripheralNeuropathyVersusRadiculopathy

YouwillrecallfromChapter10thatthetermradiculopathyreferstocompressionofanerve root,oftenbyaherniateddiscorosteophyte.Ingeneral,radiculopathiesarepainful, whereasperipheralneuropathiesbyandlargearenot,oratleastnotpredominantlyso(one importantexceptiontothisruleissmallfiberneuropathy;seepage288).Radiculopathies tendtocauseincompletesymptoms(i.e.,mildweaknessasopposedtototalparalysis, becausetheinvolvedmusclesarealsogettingneurologicinputfromothernerveroots), whereasperipheralneuropathiesaremorelikelytocausecompletesymptoms.Knowing yourperipheralneuroanatomyhereiscrucial.Luckily,however,whenyourbestneurologic examinationleavesyouuncertain,electrodiagnostictestingcanhelplocalizethelesion.

Polyneuropathies

Onewaytothinkofthepolyneuropathiesistoclassifythemintoprimaryaxonalandprimary demyelinatingdisorders.Althoughthisclassificationcanbehelpfulfromapathophysiologicviewpoint, andwewon’tignorethisapproachentirely,fromapracticalstandpointitispreferabletothinkintermsof whatyouarelikelytoseeintheclinic.Andthisissurprisinglystraightforward,easilybrokendowninto onlythreecategories:

Lengthdependentsensorimotorpolyneuropathies Inflammatorydemyelinatingpolyneuropathies Smallfiberneuropathies

LengthDependentSensorimotorPolyneuropathies

Thesepolyneuropathiesarevery,verycommon.Theypresentwithbilateral,symmetricdeficits.Because thelongestnervesinthebodyarepreferentiallyaffected,symptomstendtostartinthefeet(and,toa lesserdegree,thehands;whenbothhandsandfeetareaffected,thepatient’sneuropathyissaidtohavea “stockingglove”distribution)andthenprogressupward.Sensorydeficitsarethemostcommon presentation,resultinginnumbnessandparesthesias,butmotorabnormalitiescanbepresentandcaneven bethepredominantfeature.Absentordecreaseddeeptendonreflexesoftenaccompanythesensory findings(remember,theperipheralmotornervesareLMNs!).Autonomicdysfunctionmayalsodevelop. Inmostcasestheneurologicdeficitsevolveslowly,overyears.

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Theclassicstockingglovedistributionofapolyneuropathy.

Diabetesisbyfarandawaythemostcommonlyidentifiedunderlyingetiology,butthereareseveral othersyoushouldbefamiliarwith.Themostfrequentalternativediagnosesare:

Infectiousdiseases(suchasHIV,hepatitisC,andLymedisease) Vitamindeficiencies(includingB1,B6,B12,D,andE)  Dysproteinemias (such as multiple myeloma, Waldenstrom macroglobulinemia, and monoclonal gammopathyofundeterminedsignificance) Hereditarydisorders(mostcommonlyCharcotMarieToothdisease)  Drug or toxin related (chronic alcohol use, various chemotherapeutic agents and heavy metal exposures)  Less common causes—but important ones not to miss!—include vasculitis, amyloidosis, paraneoplastic syndromes, andother systemic diseasessuch as hypothyroidism and endstage renal disease. Idiopathic—bydefinition,nounderlyingcausecanbefound

Mostofthesepolyneuropathiesareconsideredpredominantlyaxonal;however,somedegreeof demyelinationisoftenpresentonelectrophysiologictestingaswell.

Atypicalneuropathyscreen(i.e.,bloodworksentfromtheofficeforpatientswhopresentwithclassic length-dependentsensorimotorpolyneuropathy)includesvitaminlevels,serumproteinelectrophoresis andimmunoelectrophoresis(toruleoutdysproteinemias),andthyroidfunctiontests.Screeningfor diseaseslikeLymeandHIVshouldbeconsideredonacase-by-casebasis,basedonrisk.

Let’stakeaquicklookatafewofthemorecommonetiologiesoflength-dependentsensorimotor polyneuropathies.

Diabetes.Itisconventionallytaughtthatittakesdiabetesmanyyearstocauseneurologicdamage,andit istruethattheprevalenceandseverityofneuropathycorrelatewiththedurationandseverityofthe patient’sdiabetes.However,morethan10%ofpatientswillhaveevidenceofneuropathyatthetimetheir diabetesisfirstdiagnosed,andasmanyas25%ofpatientswithoutdiabetesbutwithimpairedglucose tolerancewillshowelectrodiagnosticchangesconsistentwithdiabeticneuropathy.Themessagehereis simple:youshouldnotdismissthepossibilityofhyperglycemiaasthecauseofperipheralneuropathyin patientsnotpreviouslydiagnosedwithdiabetesorwithoutlaboratorycriteriaforfrankdiabetes.A glucosetolerancetestmaybeworthdoing,becauseitwillbeabnormalinsomeofthesepatients. Neurologistsnotuncommonlyarethefirsttodiagnosediabetesinpatientsforwhomperipheral neuropathyisthepresentingsymptom.

Inapatientwithfrankdiabetesorimpairedglucoseintoleranceandadistal,symmetricpolyneuropathy, yourevaluationisdone.Patientstypicallycomplainofpainandparesthesiasintheirfeetand/orhands, andyourexaminationwillshowdiminisheddistalsensationtovibrationanddecreasedanklereflexes. Motorweakness,ifitdevelopsatall,isamuchlaterfinding.

Tightglucosecontrol,exercise,andmanagementofeachofthecomponentsofmetabolicsyndrome (obesity,hypertension,hyperlipidemia,etc.)candelaytheprogressionofdiabeticpolyneuropathyand mayimprovesymptoms.Ifneeded,anticonvulsants(oftengabapentinandpregabalin),tricyclic

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antidepressantsandselectiveserotonin-norepinephrinereuptakeinhibitorsmayhelpmitigateneuropathic painindiabeticpatients.

VitaminB12(Cobalamin)Deficiency.IttakesseveralyearstodepletehepaticstoresofvitaminB12. Themostcommoncausesincludepancreaticinsufficiency,ilealdamage(ascanoccurinpatientswith Crohndiseaseorfollowingbariatricsurgery),perniciousanemia,andvariousmedicationsthatinterfere withB12absorption(includingmetforminandprotonpumpinhibitorssuchasomeprazole).Adherenceto astrictvegandietcanalsoresultinB12deficiency.

ThemajorconsequencesofB12deficiencyincludemegaloblasticanemia,neuropsychiatric disturbances(includingdepressionandcognitiveslowing)andmyelopathyduetodorsolateralspinal columndisease(knownassubacutecombineddegeneration;seepage269fordetails).Peripheral neuropathyisalsocommon,usuallyaccompaniedbysignsofmyelopathy.Ittendstobedistaland symmetricandcancomeonacutely.Anelevatedserummethylmalonicacidisamorereliablediagnostic testthanadecreasedserumB12level(B12functionsasacofactorformethylmalonylCoAmutase,which catalyzestheconversionofmethylmalonicacidtosuccinylCoA;therefore,B12deficiencyresultsin elevatedlevelsofmethylmalonicacid).SupplementationwithvitaminB12willdelayprogressionand improvethepatient’ssymptomswithinweeks.

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