Predicting success of vagus nerve stimulation (VNS) from EEG symmetry

Predicting

success

of

vagus

nerve

stimulation

(VNS)

from

EEG

symmetry

Janneke

Hilderink

*

,

Marleen

C.

Tjepkema-Cloostermans

,

Anita

Geertsema

,

Janita

Glastra-Zwiers

,

Cecile

C.

de

Vos

a_{DepartmentofNeurologyandClinicalNeurophysiologyandDepartmentofNeurosurgery,MedischSpectrumTwente,Enschede,TheNetherlands} b

SEINCentreofExcellenceforEpilepsyandSleepMedicine,Zwolle,TheNetherlands

c

MIRAInstituteforBiomedicalEngineeringandTechnicalMedicine,UniversityofTwente,Enschede,TheNetherlands

ARTICLE INFO Articlehistory: Received2January2017 Accepted31March2017 Keywords: Epilepsy Quantitativeelectroencephalography (qEEG)

Brainsymmetryindex(BSI) Vagusnervestimulation(VNS)

ABSTRACT

Purpose:Vagusnerve stimulation(VNS) hasshowntobeaneffectivetreatmentfordrugresistant epilepsy,withachievingmorethan50%seizurereductioninonethirdofthetreatedpatients.Inorderto predictwhichpatientswillprofitfromVNS,wepreviouslyfoundthatalowpairwisederivedBrain SymmetryIndex(pdBSI)couldpotentiallypredictgoodresponderstoVNStreatment.Thesefindings howeverhavetobevalidatedbeforetheycanbegeneralized.

Methods:39patients(age18–68years)withmedicallyintractableepilepsywhowerereferredforan implantedVNSsystemwereincluded.RoutineEEGregistrations,recordedbeforeimplantation,were analyzed.Artefact-freeepochswitheyesopenandeyesclosedwerequantitativelyanalyzed.ThepdBSI wastestedforrelationwithVNSoutcomeoneyearaftersurgery.

Results:Twenty-threepatients(59%)obtainedareductioninseizurefrequency,ofwhomten(26%)hada reductionofatleast50%(goodresponders)andthirteen(33%)areductionoflessthan50%(moderate responders).Sixteenpatientswithoutseizurereductionaredefinedasnon-responders.Nosignificant differenceswerefoundinthepdBSIofgoodresponders(mean0.27),moderateresponders(mean0.26) andnon-responders(mean0.25)(p>0.05).Besidesseizurereduction,manypatients(56%)reported additionalpositiveeffectsofVNSintermsofseizureduration,seizureintensityand/orpostictalrecovery. Conclusion:EEGfeaturesthatcorrelatewithVNStherapyoutcomemayenablebetterpatientselection andpreventunnecessaryVNSsurgery.Contrarytoearlierfindings,thisvalidationstudysuggeststhat pdBSImightnotbehelpfultopredictVNStherapyoutcome.

©2017BritishEpilepsyAssociation.PublishedbyElsevierLtd.Allrightsreserved.

1.Introduction

Vagusnerve stimulation(VNS) hasshown tobean effective treatment for drug resistant epilepsy in numerous patients. However,long-termstudiesshowedthatagoodresponse(>50% seizurereduction)isonlyachievedin20–55%ofthepatients[1–3], which meansthat a substantial number of patientsonly show moderateorevennoresponsetoVNStreatment.Determiningthe successofVNS is importanttocounselpatientsand givethem information about the expected seizure reduction. Potential respondersmight notneedtotryotherkindsof therapybefore theyreceiveaneffectiveVNSsystemandontheotherhand,alow

likelihood to respondcouldprevent someone fromundergoing surgeryandhavinganexpensiveVNSsystemimplantedwhileonly minimaleffectswillbeobtained.Despitethegrowingapplication ofVNS,itisstillnotpossibletopredictwhichpatientsrespondto whatextenttoVNStherapy.Moststudiesthatattempttopredict the success of VNS are based upon patient characteristics[4], epilepsysyndrome[5]orlocalizationoftheseizurefocus[2,6,7].A meta-analysisbyEnglotetal.onpredictorsof responsetoVNS therapy,showedthatyoungpatients(_<6years)respondslightly better in terms of seizure reduction compared to adults [2]. However,goodpredictorsofefficacyofVNStherapyforindividual patientsarestillelusive.

WepreviouslyshowedthatquantifyingEEGasymmetryusing the pairwise derived Brain Symmetry Index (pdBSI) could potentiallypredictwhichpatientswillbenefitfromVNStreatment. Itwasobservedthatnon-respondersshowsignificantlyhigherEEG

* Correspondingauthorat:MedischSpectrumTwente,P.O.Box50.000,7500KA Enschede,TheNetherlands.

E-mailaddress:j.hilderink@mst.nl(J.Hilderink).

http://dx.doi.org/10.1016/j.seizure.2017.03.020

1059-1311/©2017BritishEpilepsyAssociation.PublishedbyElsevierLtd.Allrightsreserved.

ContentslistsavailableatScienceDirect

Seizure

asymmetry,reflectedinhigher pdBSIvalues, ascompared with responders[9].Thesefindingshoweverrequirevalidationinnew patientgroupsbeforetheycanbegeneralized.Wehavetherefore conducted this prospective study in adult epilepsy patients withoutcognitiveimpairment;expectingtoconfirmthepredictive valueofEEGsymmetry,asdefinedbypdBSI.

2.Materials/methods 2.1.Patients

Wehaveincludedotherwisehealthyepilepsypatients(age>18 years)whowerereferredbySEINCentreofExcellenceforEpilepsy andSleepMedicine(EpilepsyCentre)toMedischSpectrumTwente hospitalforimplantation ofa VNSsystem.Allpatientssuffered frommedicallyintractable(generalizedorlocalized)epilepsywith varyingfocuslocations.Patientshadtokeepseizurediariesforat leastsixmonthspriortoimplantation andduringtheoneyear followupperiodofthestudy.

2.2.VNSoutcome

After implantation, patientshave regular follow upvisits at theirEpilepsyCentre.Duringthesevisits,stimulationparameters areoptimized for the individualpatient and the effect of VNS therapyismonitored.ThesuccessofVNSwasdeterminedbythe amountof seizure reduction due tothetherapy. Patients were groupedasgood(>50%reduction),moderate(<50%reduction)and non-responders (no reduction) based on seizure diary data provided by the treating nurse practitioner from the Epilepsy Centre.

Inaddition,patientsareaskedtofillinaquestionnaireabout the effects of VNS and their satisfaction with the device approximately one year after implantation. The questionnaire addressesvariousparameterssuchasseizurefrequency,duration and intensity, postictal recovery, patient satisfaction, and side-effects. Patients indicate how much various parameters have improved or worsened upon VNS therapy on a 7-point scale (ClinicalGlobalImpression–Improvementscale,CGI-I),where1 meansverymuchimprovedand7meansverymuchworsened. Data from questionnaires was used to compare patient's own perceptionwiththedatafromtheEpilepsyCentreandisusedto make a second classification, where CGI-I scores 1–2=good responder, score 3=moderate responder, scores 4–7= non-re-sponder.

2.3.EEGanalysis

Thirty-minute routine EEG registrations were made several weeksbeforeVNSimplantation.Electrodeswereplacedconform theinternational10_–20system,usinganelectrocapandsignalwas recorded using a BrainLab EEG recording system (OSG BVBA, Belgium) with a sampling frequency of 250Hz. During the registration,thepatientswerecomfortablylyingdowninaquiet, shielded room. Artefact-free epochs with eyes open and eyes closed were selected for quantitative analysis. Selected epochs werefilteredwithabandpassfilterbetween0.5and30Hz.Epochs of500sampleswith50%overlapwereFouriertransformedwith pwelchinMATLAB(TheMathworks,Inc.,USA)usingaHamming window.

Brain symmetry was quantifiedusing the pair-wise derived BrainSymmetry Index(pdBSI),which was describedpreviously

[9–11].Briefly,thepdBSIevaluatesasymmetrybycalculatingthe powerperfrequencycoefficientalong homologousEEGchannel pairs.LowpdBSIvaluesrepresentsymmetricEEGactivity,whereas higherpdBSIvalues indicatehigher asymmetryof theEEG.For

each patient, pdBSI values were determined for four different frequencybands:delta(0.5–4Hz),theta(4–8Hz),alpha(8–12Hz) andbeta(12–30Hz).EEGsymmetry,definedbypdBSI,wastested forrelationwitheffectofVNStherapyafteroneyear.

2.4.Statistics

Statisticalanalyseswereperformedusingt-testswhen(normal distribution) and Mann–Whitney Utests (non-normal distribu-tion)withaconfidenceintervalof95%.

3.Results

3.1.Patientcharacteristics

Between March 2011 and January 2015, 39 patients had a routineEEGrecordedafewweekspriortoimplantationofaVNS systemandgaveinformedconsenttoanalyzeEEGcharacteristics andlookforrelationwithVNSeffects.Patientcharacteristicsare summarized in Table 1. Patients were not considered surgical candidatesandtheirintellectualabilityvaried,howevernoneof thepatientswasseverelycognitivelyimpaired.Forall39patients, dataprovidedbythenursepractitioner(follow-uptimeonaverage 14months,range8–24months) andpdBSIvaluesareavailable. Fiveoutof39patientsdidnotsendbackthequestionnaireabout theirperceptionofeffectsandsatisfactionwiththeVNStherapyso thisinformationisonlyavailablefor34patients(follow-uptimeon average14months,range6–36months).

3.2.EffectsofVNS 3.2.1.Seizurereduction

Basedondataprovidedbythenursepractitioner,twenty-three patientsobtainedareductioninseizurefrequency,ofwhomten hadareductionofatleast50%(goodresponders)andthirteena reduction of less than 50% (moderate responders). The other sixteenpatientsdidnotshowanyreductioninseizurefrequency andweredefinedasnon-responderstoVNStherapy.Neitherthe patient'sagenorthetypeofepilepsycorrelatedwiththeseizure reductionobtainedwithVNSandthereforetheseparameterscould notpredicttheeffectofVNS(Table1).

Besides seizure reduction, other positive effects of VNS treatmentwerereportedbythenursepractitioner.Outofsixteen patientswhoweredefinedasnon-responders,sevenpatientsstill experiencedotherpositiveeffectsofVNS.Also,themajorityofthe goodresponders(7outof10)andmoderateresponders(8outof 13)showedadditionalpositiveeffectsintermsofseizureduration, seizureintensityand/orpostictalrecovery.

3.2.2.Patientperception

Inadditiontothedataprovidedbythenursepractitioner,VNS outcome was determined using patient questionnaires. Ten patients indicated that their seizure frequency has improved very much or much (CGI-I score 1 or 2). Fourteen patients mentionedasmall improvementin seizurefrequency(score3) andten patients indicated thatthe seizurefrequency hadnot changed (score 4). No patient indicated worsening of seizure frequency. The patient's general impression regarding seizure frequency only partly corresponded with the percentages of seizurereductionthatwereprovidedbythenurse practitioner (Table1).

All patients who indicated that their seizure frequency has (very)muchimprovedalsoexperienced(some)improvementin seizureintensityand/orpostictalrecovery.Majorityofthefourteen patientswhoreportedminimalimprovementinseizurefrequency also report little or no improvement in seizure intensity and

recovery after seizures. However, three of these patients mentioned much improvement in intensity, recovery, or both. Thenon-responders,reportedlittleornoimprovementintermsof seizureintensityandpostictalrecovery.

To assess their overall satisfaction with the VNS therapy, patientswereaskedhowsatisfiedtheywereonascaleof1(very satisfied)to7 (verydissatisfied).Thistimethewholescalewas usedbythepatientsandonaveragetheyscored3.4.Asexpected, theextentofsatisfactionisstronglycorrelatedwiththeeffectsof thestimulator.Inthegoodrespondergroup(accordingtothenurse

practitioner)theaveragesatisfactionis2.1,whereasthemoderate andnon-respondersscore3.2and4.2,respectively.

3.2.3.Sideeffects

Mostpatients(33outof39)experienceside-effectsfromVNS treatment, independently of theeffectivenessof VNS(Table 2). Somepatientsexperiencemorethanonetypeofside-effect. Side-effects that werementioned occur only during stimulation. In general, comparable occurrence and type of side effects are experiencedbyallthreegroups.

Table2

Reportedsideeffects.Numbersindicatenumberofpatientspergroup(good,moderate,non-responder)reportingtherespectiveside-effect.Somepatientsexperience multipletypesofside-effects.

Sideeffect Goodresp.(#8/10) Moderateresp.(#12/13) Non-resp.(#13/16) Total(#33/39)

Hoarseness 7 8 8 23 70%

Throatdiscomfort 3 2 6 11 33%

Voicealterations 2 5 1 8 24%

Shortnessofbreath(duringexercise) 1 2 3 6 18%

Tinglingsensationinneck/shoulder/arm 2 0 1 3 9%

Painfuljaw/teeth 0 0 2 2 6%

Coughing 0 1 0 1 3%

Snoring 1 0 0 1 3%

Table1

PatientcharacteristicsandeffectofVNS.EffectofVNSreductionisbasedonthedatafromseizurediariesprovidedbythetreatingnursepractitioner.Patient'sperceptionof seizurereductionandpatient'soverallsatisfactionwiththeVNSdevicearebasedontheClinicalGlobalImpression–Improvement(CGI-I)scores.(NA:informationisnot available).

No. Sex Age(years) Focuslocation EffectVNS(%reduction) Patientperceptionofreduction Patientoverallsatisfaction

1 M 68 Multifocal >50 1 1 2 F 37 Multifocal >50 2 1 3 F 27 Rtemporal >50 1 1 4 M 45 Multifocal >50 4 4 5 F 46 Multifocal >50 3 2 6 F 40 Rtemporal >50 3 3 7 M 19 Multifocal >50 2 3 8 F 23 Multifocal >50 3 4 9 F 27 Ltemporal >50 2 2 10 M 62 Rtemporal >50 NA NA 11 F 31 Frontal <50 2 4 12 M 21 Rtemporal <50 2 3 13 F 54 Multifocal <50 4 6 14 F 25 Multifocal <50 3 3 15 F 49 Rtemporal <50 4 3 16 M 60 Ltemporal <50 3 3 17 F 35 Rtemporal <50 3 3 18 M 65 Multifocal <50 4 4 19 F 59 Multifocal <50 2 2 20 F 54 Rtemporal <50 3 2 21 F 22 Ltemporal <50 3 2 22 F 26 Multifocal <50 NA NA 23 F 20 Multifocal <50 NA NA 24 F 51 Ltemporal 0 3 3 25 F 48 Rtemporal 0 4 7 26 F 52 Multifocal 0 2 3 27 M 51 Multifocal 0 2 3 28 M 47 Multifocal 0 3 4 29 F 42 Frontal 0 4 6 30 F 18 Multifocal 0 3 3 31 M 31 Multifocal 0 4 6 32 M 20 Frontal 0 4 NA 33 F 19 Multifocal 0 3 5 34 M 62 Multifocal 0 3 3 35 F 24 Multifocal 0 3 4 36 M 32 Frontal 0 4 5 37 M 35 Multifocal 0 4 3 38 F 28 Multifocal 0 NA NA 39 M 46 Multifocal 0 NA NA

3.3.PairwisederivedBrainSymmetryIndex(pdBSI) 3.3.1.EEG

Artefact-free EEG signal of interictal episodes was used to determinethepdBSI.Epochsof10switheyesopenandeyesclosed wereselected.Foreachpatient–foreyesopenonaverage54sand foreyesclosedonaverage57sof–EEGsignalwasavailablefor quantitativeanalysis.Nostatisticallysignificantdifferenceswere observedbetweenpdBSIvaluesobtainedfromEEGperiodswith eyesopenandeyesclosed(Fig.1).

3.3.2.RelationpdBSIandseizurereduction

Patientsweregroupedasgood,moderateandnon-responders basedonseizurereductionasindicatedbythenursepractitioner. NosignificantdifferenceswereobservedbetweenpdBSIvaluesin delta,theta,alphaandbetabands,forgoodresponderscompared with moderate responders and non-responders (Fig. 1A).

Additionally,patientswereclassifiedbasedontheirownreported seizure reduction. Even though this is a partially different grouping,itresultsinonlyslightlydifferentaveragepdBSIvalues forallfrequencybandsandagainnosignificantdifferenceswere observed(Fig.1B).Similarly,patientsclassifiedbasedontheirown overallsatisfactionwithVNS,doesalsonotshowanysignificant differences in pdBSI values between good, moderate and non-responders(Fig.1C).

4.Discussion

To prevent patientswho willnot respond toVNStreatment from undergoing unnecessary surgery, it is necessary to find predictors of the expected success of VNS. Low pdBSI values showedpromisingresultsinpredictinggoodresponderstoVNS treatmentinapreviousstudy[9].However,wehavenotbeenable toreproducethepreviousfindingsinthiscurrentstudy.Although

Fig.1.Differencesbetweengoodresponders,moderateresponders,andnon-respondersinpdBSIforthedelta,theta,alphaandbetabandandaveragedoverallfrequencies, forEEGperiodswitheyesopenandeyesclosed.Classificationgood,moderateandnon-responderswasbasedon:(A)dataprovidedbythetreatingnursepractitioner,(B) patient'sperceptionofseizurereduction,and(C)patient'soverallsatisfactionwiththetherapy.Errorbarsrepresentthestandarddeviation.

thestudypopulationofthiscurrentstudywaslargerthaninour previousstudy(n=39versusn=19),thenumberofpatientsisstill relatively small, and again the study population consists of a heterogeneousgroupofpatientsthatarenotsuitableorwillingto undergoepilepsysurgery.Previouslywebasedouranalysesonly ontheinformationprovidedbynursepractitioners,whereasinthis study we also included two measures of the patients’ own perceptionofVNSoutcome, andevaluatedpdBSIvaluesof EEG recordingwitheyesopenaswellaswitheyesclosed.Nevertheless, nosignificantdifferencesinpdBSIvalues wereobservedwithin thesevariousconditionsoroutcomemeasures.

Thisprospectivestudyenableda long-termfollow-upof the samepatient population, eliminating theinfluenceof potential earlydropoutsthatmaybemissedinretrospectivestudies.Inour study, most patientsexperience moderate(33%) togood (26%) seizurereductionuponVNStherapy.Studiesinyoungerpatients haveshownabetterresponsetoVNStherapy[2,7,8].However,our studyincludedonlyadults.Eventhoughonly26%ofthepatients hadstrongseizurereductionuponVNStreatment(dataprovided by nurse practitioner), mostpatients (62%) indicated that they wereingeneral(somewhattovery)satisfiedwiththedevice.This mayberelatedtootherpositiveeffectsmanypatientsexperience besides seizure reduction, suchas reduced seizure intensityor durationand/orshorterpostictalrecoverytimes,alsoof impor-tancetothepatient.Inaddition,potentialreductionofnocturnal seizures, which may not be noticed and thus not reflected in reduced seizure frequency, may result in an improved sleep quality,increased daytimealertnessand a betteroverallhealth condition[12]. On the other hand, patientswho showseizure reduction upon VNS treatment may experience negative side-effectsthatcouldinfluencetheirperceptionofseizurereduction and decreasetheir overallsatisfaction withthetherapy. In our study,nearlyall patients(85%)experienced some,mostlymild, side-effects.Thispercentageis higherthandescribedbyothers, however,thetypeofreportedside-effectsissimilar[13,14].

Patients were defined as either good, moderate or non-responders based onthe amount of their seizure reduction as providedbynursepractitioners,basedonseizurediarieskeptby the patients themselves, which is still the standard, but has neverthelesslimitedaccuracy[15].Hence,thediscrepancy with thepatients’ownperceptionofseizurereductionaswellastheir satisfaction with the therapy, when asked one year after implantation. We found that seizure reduction as experienced bypatients(CGI-Iscale) differedsubstantially fromtheseizure reduction provided by the nurse practitioner. Patients’ own perceptioncouldbe bothbetteror worse thanwhat thenurse practitioners’ data suggested and 18 patients were classified differently. Possibly, this classification is influenced by other factorslikeside-effectsorgeneral satisfactionwiththetherapy, eventhoughwehavetriedtopreventthisbyaskingaboutthese aspectsinseparatequestions.Nevertheless,patients_’perceptionof seizurereductionisaskedonlyonceafteroneyearasascoreona 7-pointscale,whichisadifferentmeasurethantheinformation based on daily seizure diaries. Moreover, interpretation of the 7-pointscalemayvaryamongstpatients.Onepatientmayscorea 30%seizurereductionas‘verymuchimproved’whereasanother patientmayscore thesame percentageof seizure reductionas ‘minimally improved’, probably coloured by the impact that (varioustypesof)seizureshaveonthepatient'sdailylife.

Incontrasttoourearlierfindings,thecurrentdataindicatethat EEGsymmetryquantifiedaspdBSIvaluesmightnotcorrelatewith patientsatisfactionorwithseizurereductionduetoVNStherapy andcanthereforenotbeusedasapredictoroftheeffectofVNS. Newstudiesneedandwillbeconductedtocontinuethesearchfor biomarkers to reliably predict the effects of VNS therapy and enablebetterpatientselectionandpreventunnecessarysurgical procedures.

Acknowledgments

WewouldliketoacknowledgetheDepartmentofNeurosurgery andespeciallyM.J.DriesseMDandK.KhoMDwhoimplantedthe VNSdevices,fortheircontinuoussupportandtime.Inaddition,we wouldliketothanktheDepartmentofClinicalNeurophysiology fortheregistrationoftheEEGs.

Thisresearchdidnotreceiveanyspecificgrantfromfunding agenciesinthepublic,commercial,ornot-for-profitsectors.

WeconfirmthatwehavereadtheJournal'spositiononissues involved in ethical publication and affirm that this report is consistentwiththoseguidelines.

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