Effect of multichannel transcranial direct current stimulation to reduce hypertonia in individuals with prolonged disorders of consciousness: A randomized controlled pilot study.

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1

2

Original

article

3

Effect

of

multichannel

transcranial

direct

current

stimulation

to

reduce

4

hypertonia

in

individuals

with

prolonged

disorders

of

consciousness:

5

A

randomized

controlled

pilot

study

§

6 Q1

Aurore

Thibaut

a,b,

*

,

Andrea

Piarulli

a,c

,

Ge´raldine

Martens

a

,

Camille

Chatelle

a,b,d,1

,

7

Steven

Laureys

a,b,1

8 a

ComaScienceGroup,UniversityHospitalofLie`ge,Lie`ge,Belgium

9 b

GIGA-Consciousness,UniversityofLie`ge,Lie`ge,Belgium

10 c

Departm

Q3 entofSurgical,MedicalandMolecularPathologyandCriticalCare,UniversityofPisa,Italy

11 d

LaboratoryforNeuroImagingofComaandConsciousness-DepartmentofNeurology,MassachusettsGeneralHospital,HarvardMedicalSchool,Boston,MA,

12 USA

13

14 1. Introduction

15 Many patients with severe brain injury and disorders of 16 consciousness(DOC)areaffectedbyspasticity,whosetreatment 17 isachallenge[1,2].Voluntarymovementsandcollaborationare 18 usuallylimitedifnotabsentinthispopulation[3,4].Treatments 19 areoftenlimitedtopassivephysical therapy(e.g.,conventional 20 stretchingortilttable[5])orpharmacologicalinterventionssuch

21 asanti-spasticdrugs(e.g.,baclofen,rivotril,sirdalud)orbotulinum

22 toxininjections, asprescribed forother neurologicalconditions

23 such as stroke (for review see [6]). In addition, the patients’

24 condition aggravates the symptoms because of inactivity and

25 positioning;hence,ahighproportionofpatientswithDOChave

26 severehypertonia:89%presentsignsofhypertoniaonaleastone

27 segment,and61.5%haveseverehypertonia(i.e.,scoreof3ormore

28 ontheModiﬁedAshworthScale[MAS])[1].

29 Transcranialdirect-currentstimulation(tDCS)involvesusinga

30 weak electrical current to modulate the threshold for action

31 potentialgeneration[7].Positive(anodal)ornegative(cathodal)

32 current facilitate the depolarization or hyperpolarization of

33 neurons, respectively [8]. In both cases, tDCSseems tohave a

34 long-termeffectintermsoflong-termpotentiation-orlong-term ARTICLE INFO

Articlehistory:

Received16January2019 Accepted8May2019 Keywords:

Uppermotorneuronsyndrome Spasticity

Hypertonia

Transcranialdirectcurrentstimulation Minimallyconsciousstate

Vegetativestate

ABSTRACT

Background:Spasticitymanagementinseverelybrain-injuredpatientswithdisordersofconsciousness (DOC)isamajorchallengebecauseitleadstocomplicationsandseverepainthatcanseriouslyaffect qualityoflife.

Objectives:Weaimedtodeterminethefeasibilityofusingtranscranialdirectcurrentstimulations(tDCS) toreducespasticityinchronicpatientswithDOC.

Methods:Weenrolled14patientsinthisdouble-blind,sham-controlledrandomizedcrossoverpilot study.Twocathodeswereplacedovertheleftandrightprimarymotorcortexand2anodesovertheleft andrightprefrontalcortex.Hypertoniaoftheupperlimbsandlevelofconsciousnesswereassessedby the Modiﬁed AshworthScale (MAS) and the Coma Recovery Scale-Revised (CRS-R). Resting state electroencephalographywasalsoperformed.

Results:Atthegrouplevel,spasticitywasreducedinonlyﬁngerﬂexors.Fourresponders(29%)showed reducedhypertonicityinatleast2jointsafteractivebutnotshamstimulation.Wefoundnobehavioural changesbytheCRS-Rtotalscore.Atthegrouplevel,connectivityvaluesinbeta2werehigherwithactive versusshamstimulation.Relativepowerinthethetabandandconnectivityinthebetabandwerehigher forrespondersthannon-respondersaftertheactivestimulation.

Conclusion:This pilotstudyhighlightsthepotentialbeneﬁt ofusingtDCSforreducing upper-limb hypertoniainpatientswithchronicDOC.Large-sampleclinicaltrialsareneedtooptimizeandvalidate thetechnique.

C ₂₀₁₉_Elsevier_Masson_SAS._All_rights_reserved.

§

Clinic

Q1 alTrials.govregistration:NCT03797573.

* Correspondingauthor.Coma ScienceGroup, GIGA-Consciousness,Alle´ede l’hoptial,1–B34,4000Lie`ge,Belgium.

E-mailaddress:athibaut@ulg.ac.be(A.Thibaut).

1 Co-lastauthors.

Available

online

at

ScienceDirect

www.sciencedirect.com

https://doi.org/10.1016/j.rehab.2019.05.009

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35 depression-like plasticity[9,10]. Several studies involving tDCS 36 haveassessedtheeffectofthistechniqueonreducinghypertonia 37 in stroke patients, showing improved strength or reduced 38 spasticity,amongothereffects[11–13].

39 Froma pathophysiological pointofview, brainlesionsaffect 40 tractsinbothpyramidalandextrapyramidalsystems.Increased 41 muscle tone results from neuroplastic changes (e.g., collateral 42 sprouting)and/orreleaseeffects(disinhibition)asaresultofthe 43 lesion[14].Ina1-yearlongitudinalfunctionalMRI(fMRI)study, 44 theauthors demonstrated an evolution in sensorimotor cortex 45 (S1M1)activationfromearly(20daysafterstroke)contralesional 46 hyperactivation to later (4 months after stroke) ipsilesional 47 hyperactivationconcomitantwithrecovery[15].Another electro-48 myography-fMRIstudyof10chronicstrokesurvivorswith upper-49 limbdysfunction demonstrated wide bilateralactivation in the 50 S1M1,supplementarymotorarea,andcerebellumwhilesubjects 51 movedtheparetichand[16].Thesedatasuggestthatipsilesional 52 S1M1 hyperactivation plays an important role in hypertonia 53 causedby uppermotor-neuron syndromes suchasstroke.This 54 findingcouldexplainwhya decrease,via cathodalstimulation, 55 couldreducethishyperactivationanddecreasethehypertonia. 56 Inthisstudy,weevaluatedtheeffectofmultifocaltDCSofthe 57 primarymotorcortex(M1)onhypertoniaofthearms,wrists,and 58 finger flexors in individuals with chronic DOC. Our secondary 59 outcomesweretheeffectoftDCSonthelevelofconsciousnessand 60 motorfunctionandoncorticalactivity.

61 2. Methods 62 2.1. Design

63 Thiswasadouble-blindsham-controlledrandomizedcrossover 64 pilotstudy.

65 2.2. Participants

66 AllparticipantswererecruitedfromtheUniversityHospitalof 67 Liege during a week of assessments involving behavioral 68 evaluations and neuroimaging acquisitions. Inclusion criteria 69 were age18 years; diagnosis of unresponsive wakefulness 70 syndrome, minimally conscious state (MCS), emergence from 71 MCSorlocked-insyndrome;signsofpyramidalsyndromewith 72 upper-extremityhypertonicityinﬂexionasdocumentedbythe 73 MAS;>3 months post-insult; stability of vital signs; and 74 obtaininginformedconsentfromtheparticipant’s legal repre-75 sentative.Exclusioncriteriawerepremorbidneurological condi-76 tionandcontraindicationtotDCS(e.g.,metalliccerebralimplant, 77 pacemaker,uncontrolledepilepsy).Weincludedindividualswho 78 were not taking sedative drugs or Na+ or Ca++ channel 79 blockers (e.g., carbamazepine) or NMDA receptor antagonists 80 (e.g., dextromethorphan). Medications, physical therapy and 81 rehabilitationremainedunchangedthroughouttheexperiment. 82 Thestudywasapprovedbytheethicscommitteeoftheuniversity 83 anduniversityhospitalofLie`ge,Belgium.

84 2.3. Procedures

85 Direct current was applied by a battery-driven constant 86 currentstimulatorvia2cathodesplacedovertheleftandright 87 M1(C3andC4accordingtothe10–20internationalsystem[17] 88 for electroencephalography [EEG] placement) and 2 anodes 89 positionedovertheleftandrightdorsolateralprefrontalcortex 90 (F3andF4accordingtothe10–20internationalsystemforEEG 91 placement;Fig.1).DuringactivetDCS,thecurrentwasincreased 92 to1mAfromtheonsetofstimulationandappliedfor20min.The

93 shamtDCSsessionwasprecededby15-secramp-upand

ramp-94 downperiodsatthebeginningandtheendofthe20-minsession

95 tomimicactivestimulation.Electrodeimpedancewas

maintai-96 nedat<10k

V

andvoltage<26V.tDCSandshamstimulation

97 weretestedinrandomorderin2separatesessionsseparatedbya

98 minimumof2days.

99 Hypertonia was assessed by the MAS in upper extremities

100 bilaterally (arms, wrists, and ﬁnger ﬂexors) and level of

101 consciousnesswasevaluatedbytheComaRecoveryScale-Revised

102 (CRS-R)[0(worst)and23(best)].Bothscaleswereadministered

103 directly before and after the tDCS and sham sessions by an

104 examinerwhowasblindedtotreatment.

105 ThetDCSdeviceallowsforrecordingEEGactivity,includingthe

106 sitesofstimulation.Therefore,wecollecteddatafrom6-minEEG

107 (resting state) before and after the 2 sessions at the sites of

108 stimulationinadditionto4otherelectrodesites(Fig.1).

109 2.4. Studyoutcomes

110 OurprimaryoutcomewastheeffectofactivetDCSascompared

111 with sham stimulation on decreasing hypertonia of the upper

112 limbs. Secondaryoutcomes weretheeffect oftDCS on level of

113 consciousness,asmeasuredbytheCRS-Rtotalscore,andonmotor

114 function,asmeasuredbythemotorsubscaleoftheCRS-R.

115 To assess the effect on hypertonia, we took the highest

116 difference(post-minuspre-intervention)ofbothjoints(leftand

117 right)andanalyzedthedataforthearmﬂexors,wristﬂexorsand

118 ﬁngerﬂexors,separately.

119 2.5. Randomizationandmasking

120 Eachpatientreceivedbothanodalandshamstimulationsina

121 randomized order. A computer-generated randomization

se-122 quence wasused toassigntheﬁrst session as anodalor sham

123 tDCSina 1:1ratio.For shamtDCS, thetDCSdevice(8channels

124 Startsim,Neuroelectrics,Barcelona)offersabuilt-inplacebomode.

125 Thus, both the operator who administered tDCS and the

126 participantscouldnotidentifytheshamtDCS.

Fig. 1. The placement of the 8 electrodes used for stimulation and electroencephalography (EEG) recording. Anodes: F3-F4; cathodes: C3-C4. Recordingelectrodes:Fp1,Fp2,F3,F4,C1,C2,C3,C4.

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127 2.6. Statisticalanalysis

128 BecausetheMASandtheCRS-Rarenon-normallydistributed 129 and our sample size was small, treatment effects (post-active 130 minuspre-activetDCScomparedtopost-shamminuspre-sham 131 tDCSscores)werecalculatedbyusingtheWilcoxonranksumtest 132 forMASassessments(arm,wristandﬁngerﬂexors)andtheCRS-R. 133 Effectsizeswerecalculatedasr=z/H2n,wherezisthestatisticof 134 theWilcoxonsignedranktest.WeusedSpearmancorrelationto 135 evaluatecorrelationsbetweenordinalvariables.Asanexploratory 136 analysis,weexamineddifferencesinproportionofresponders(i.e., 137 decreaseinhypertoniainatleast2jointsafteractivebutnotsham 138 tDCS)betweenthe2groupsbyaproportionaltest.

139 2.7. EEGanalysis

140 Allrecordingswereband-pass–ﬁlteredbetween0.7and45Hz, 141 with5 bandsofinterest chosen:delta(1–4Hz),theta(4–8Hz), 142 alpha(8–12Hz), beta1 (12–18Hz) and beta2 (18–30Hz). Both 143 sham and active stimulation pre- and post-recordings were 144 visuallyinspected,andnoisyepochswerediscarded.Independent 145 Component Analysis [18] was used to detect and discard 146 componentsrelatedtonearlystationaryartifacts(i.e.,eye-blinks, 147 electrocardiography effects). For each participant, the session

148 (active/sham)andelectrodeforeachperiod(pre/post)wasdivided

149 into4-secepochs,witha50%overlapbetweencontiguousepochs.

150 Twosetsoffeatureswereestimatedforeachbandandperiod:1)

151 relativebandpower(RBP),deﬁnedforeachelectrodeastheratio

152 betweenthetotalpowerinthebandandtotalpowerinthe1-to

153 30-Hzrange (Supplementary Materialssection1 [SM1]),and2)

154 weightedphaselagindex(WPLI,Vincketal.,2011)betweeneach

155 pairofelectrodes(SM1).Foreachparticipant,session,bandand

156 electrode(RBP)orelectrodepairs(WPLI),thedifferencebetween

157 post-andpre-interventionwasthenestimated(

D

RBPand

D

WPLI).

158 Foreachbandandelectrode(orcoupleofelectrodes;

D

WPLI),

159 paired ttestwasusedtocompareactiveandshamstimulation,

160 extractingthet-statistics.Thet-statisticsigniﬁcancewasassessed

161 byanon-parametricpermutationtest(SupplementaryMaterials

162 section2[SM2]).

163 Differences between groups (respondersvs. non-responders)

164 foreachbandandelectrode(orcoupleofelectrodes;

D

WPLI)were

165 assessedbyunpairedttest,extractingrelatedtvalues.Analogously

166 tothebetween-conditioncomparisons,thesigniﬁcanceofeach

t-167 statistic was assessed by a non-parametric permutation test

168 (69randomizations,seeSM2).Between-groupcomparisonswere

169 assessedfortheactivesessionand,asacontrol,theshamsession.

170 For all tests, thesigniﬁcance threshold wasset at P=0.05. All

171 ofﬂine analyses were conducted using tailored Matlab codes Table1

Demographiccharacteristicsandstructuralbrainlesionsforeachindividualincludedinthestudy. ID Diagnosis Age(sex) Etiology Timesinceinjury Baseline

CRS-Rscore

MRIlesions

1 UWS 50F TBI 378days

(>1year)

3 Temporalandfrontallobes,temporo-occipitalareas(R>L), hippocampi,thalami,cerebellum

2 MCS 26F TBI 1397days

(>3years)

4 Hippocampi,temporallobes,sensorimotorcortices

(>2years)

5 Majorhydrocephalus,corpuscallosum,thalami,hippocampi(R>L) 4 MCS 39M Hemorrhagicstroke 253days

(>8months)

12 R:perirolandic,frontolateralandinsularregions L:pallidum,putamen

5 MCS 39M Cardiacarrest 2806days (>7years)

6 Diffuseaxonalinjurywithglobalatrophy 6 MCS 73M Hemorrhagicstroke 3065days

(>8years)

8 Rfrontalregion,basalnuclei,anteriormesialfrontaland temporo-parietalregionsbilaterally

7 UWS 40M TBI 315days

(>10months)

6 Temporallobes,hippocampi,thalami,leftpallidum, Rcaudatenucleus

8 UWS 25F TBI 233days

(>7months)

3 Rbasalganglia,frontallobes,mesiotemporalregions,anterior periventricularregions

9 LIS 35F Hemorrhagicstroke 1143days (>3years)

22 Protuberance 10 MCS 27F Hemorrhagicstroke 90days

(>3months)

4 Rparietallobe,rightthalamus,temporo-parietal andoccipitalregions

(>3years)

9 Rlenticularcapsule,Rinsula,Rcoronaradiata, corpuscallosum,Lthalamus

12 EMCS 61M Hemorrhagicstroke 409days (>1year)

22 Thalami,Lposteriorpons

13 UWS 62M Cardiacarrest 318days 6 Diffuseaxonalinjurywithglobalatrophy 14 UWS 46F Cardiacarrest 170days 5 Diffuseaxonalinjurywithglobalatrophy

L:left;R:right;TBI:traumaticbraininjury;CRS-R:ComaRecoveryScale-Revised[0(worst)and23(best)];UWS:unresponsivewakefulnesssyndrome;MCS:minimally consciousstate;EMCS:emergencefromMCS.

Table2

Demographiccharacteristicsandlevelofspasticityofthe4responders.

ChangeinMAS–activetreatment ChangeinMAS–shamtreatment

Age(sex) Etiology/timesinceonset(days) Armflexors Wristflexors Fingerflexors Armflexors Wristflexors Fingerflexors

73M Stroke/3065 -1 -1 -3 0 0 0

39F TBI/1292 1 -1 -1 2 1 1

61M Stroke/409 1 -2 -2 0 -1 1

62M Cardiacarrest/318 1 1 0 0 1 1

Median(IQR) 0( 1–1) 1( 1.75– 1) 1.5( 2.75– 0.25) 0(0–1.5) 0.5( 0.75–1) 1(0.25–1) M:male;F:female;MAS:ModiﬁedAshworthScale;TBI:traumaticbraininjury;IQR:interquartilerange.

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Fig.2.Electrodepairswithhigherdifferenceinweightedphaselagindex(DWPLI)betweentheactiveversusshamstimulationforthebeta2bandatthescalplevel(upper panel,redlines).Dataaremean(SD)intervalforeachsigniﬁcantpairforactiveandshamstimulation.

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172 (MathWorks,Natick,MA,USA),andwhendealingwith Indepen-173 dentComponent Analysis, takingadvantage of EEGLABtoolbox 174 functions[20].

175 3. Results

176 Between January 2014 and December 2017, we screened 177 17patients,and14(mean[SD]age47 [19],range25–73years; 178 7women)wereenrolledinthestudy(mean[SD]timesinceinjury 179 30[32],range3–102months,6withtraumaticbraininjury).No 180 patients droppedout. Individualdemographic informationis in 181 Table1.

182 Atthegrouplevel,wedidnotobserveanytreatmenteffectby 183 theMASforthearmflexors(z=1.500;P=0.134;r=0.28)orwrist 184 flexors(z=-1.341;P=0.180;r=0.25).Weidentifiedatreatment 185 effect for the finger flexors (z=-2.344; P=0.019; r=0.44); 186 however,post-hocanalysesdidnotdemonstratea differencein 187 MASscoresafter theactivetreatment (decrease of 0.25points, 188 z=1.102; P=0.270; r=0.21) or sham treatment (increase of 189 0.75points,z=-1.781;P=0.075;r=0.34).

190 WedidnotobserveanytreatmenteffectintermsofCRS-Rtotal 191 scores(z=1.223;P=0.221;r=0.23)orthemotorsubscaleofthe 192 CRS-R (z=0.169; P=0.865; r=0.03) or any effect of etiology 193 (R=0.166;P=0.616),timesinceinsult(R=-0.397;P=0.200)or 194 diagnosis(R=-0.031;P=0.924).

195 At the individual level, 4 participants showed a decrease 196 inhypertoniainatleast2jointsafteractivebutnotshamtDCS

197 (i.e.,tDCS-responders),butnoneshowedadecreaseinMASscore

198 onmorethanonejointwithshamtDCS(Table2).Theproportion

199 of responders was higher with active than sham treatment

200 (z=-2179;P=0.029).

201 ForEEGresults,becausethiswasapilotstudy,nocorrectionfor

202 multiple comparisons was applied. Nevertheless, to ensure a

203 certain robustness of the ﬁndings, when dealing with

D

RBP

204 comparisons,weconsideredonlybandsshowingsigniﬁcanceforat

205 least 2 electrodes and when dealing with

D

WPLI, only bands

206 showingmorethan3signiﬁcantcomparisons.

207 Eightparticipants(4responders)wereretainedfortheanalyses

208 (6/14wererejectedbecauseofnoisyrecordings intheshamor

209 active stimulation session). We found no between-condition

210 differenceforanybandwhenconsidering

D

RBP(Supplementary

211 Materialssection 3[SM3], TableA).

D

WPLIvalues werehigher

212 withactivethanshamstimulationfor4electrodepairsinbeta2(all

213 P<0.05; Fig. 2; SM3, Table B). When considering the active

214 session,mean

D

WPLIwaspositiveforall4pairs,whichindicates

215 highersynchronizationinpost-stimulationthanpre-stimulation

216 (Fig. 2). In the active session,

D

RBP values were higher for

217 3electrodepairsinthetawhencomparingrespondersand

non-218 responders(allP<0.05;Fig.3;SupplementaryMaterialssection

219 4 [SM4], Table C). No signiﬁcant difference was found when

220 consideringtheshamsessionfor

D

RBPor

D

WPLI(Supplementary

221 Materialssection5[SM5],TablesE–F).

D

WPLIvalueswerehigher

222 forrespondersthannon-respondersfor4electrodepairsinbeta1

223 duringtheactivesession(allP<0.05;Fig.4;SM4,TableD).

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224 4. Discussion

225 Herewereportpilotdatafromarandomizedsham-controlled 226 double-blind study assessing the effect of a single session of 227 bilateralcathodal tDCSover theM1 on reducing hypertoniain 228 individualswithDOC.WedidnotﬁndaneffectoftDCSatthegroup 229 level,butattheindividuallevel,4participantsshowedaclinically 230 relevantdecreaseinspasticityaftertheactivesession(i.e., tDCS-231 responders:decrease in spasticity in at least 2 joints after the 232 active but not sham session). We foundno effects on signs of 233 consciousness.

234 RegardingEEGanalysis,anincreaseinbeta2band connec-235 tivity between motor areas and frontal areas has been 236 identiﬁed after active tDCS. Beta connectivity in the central 237 (or M1) and frontal regions are widely considered linked to 238 movementanddecision making.Forinstance,degreeof beta-239 frequency resting-state functional connectivity between M1 240 and the anterior prefrontal cortex were found to predict 241 subsequentdegreeofmotoradaptationinhealthyvolunteers, 242 whichsuggeststhattheresting-statesynchronization dynam-243 ics can predict the degree of motor adaptation in a healthy 244 population[21]. Instroke, betacoherence inthe somatosen-245 sory areas is increased during movement planning and 246 associatedwithvelocityofmovement[22].Inaddition,central 247 inter-hemisphericbeta coherencewas foundlinked tomotor 248 function recovery, patients with higher interhemispheric 249 coherence presenting higher motor function recovery after 250 stroke [23]. Our preliminary results highlight the possible 251 effectsof tDCS onmotorfunction inpatientswith DOC. 252 For the 4 patients with clinical response (i.e., reduced 253 hypertonia after active tDCS), we found a similar pattern of 254 connectivityinthebetaband(herebeta1)betweenthemotorand 255 frontalareas, asidentiﬁedatthegrouplevel.In addition,these 256 patientsdemonstratedanincreaseinconnectivityaftertheactive 257 stimulationinbeta1betweenthefrontal, prefrontaland fronto-258 polarareas.Previousstudiesfoundsimilarincreasedbetapower 259 afterasinglestimulationsessionovertheprefrontalcortex[24]or 260 primarymotorcortex[25].TheauthorsconcludedthattDCScould 261 primebrainactivitytoa‘‘readystate’’toperformcognitivetasks 262 (prefrontaltDCS)ormotor-relatedtasks(M1tDCS).Onthebasisof 263 thishypothesis,wecouldhaveexpectedbehaviouralchangesmore 264 than reduced muscle overactivity. In this scenario, repeated 265 sessionsoftDCSmaybeneededtoinducemotor-relatedclinical 266 improvement.Besidesmodulationofbetapower,ourresponders 267 showedincreased power in thetheta bandfor the frontaland 268 fronto-centralelectrodesafteractivestimulation.Increaseintheta 269 power is mainly linkedtomemory functionsand hippocampal 270 activity [26,27]. However, theta oscillations have also been 271 associated with sensorimotor integration arising from the 272 hippocampalformation[28]andcanbemodulatedafteramotor 273 task[29].Inthiscontext,thereducedmusclehypertoniaobserved 274 inresponderstogetherwithincreasedthetaactivityinthe fronto-275 centralregionscouldresultfromanormalizationofbrainactivity 276 orreducedcorticalmaladaptiveplasticity,leadingtospasticity. 277 Althoughwithasmallsamplesize,thispilotstudycouldhelpin 278 thedevelopmentofnewtrialsaimedatmanaginghypertoniain 279 patients with DOC by using non-invasive brain stimulation. 280 Repeated tDCS sessions are considered required to induce 281 clinicallyrelevantandlong-lastingeffectsindifferentneurological 282 conditions [30–34]. Although we had a few responders, they 283 represented 30% ofour smallsample. In this context, repeated 284 stimulationsessionswouldincreasethenumberofrespondersand 285 couldinducelastingclinicaleffectsduetomechanismsthoughtto 286 berelated tolong-term potentiationand long-termdepression 287 [35,36].

288 Regardingevaluationofconsciousness(i.e.,CRS-R),wedidnot

289 observeanysigniﬁcanteffectoftDCSonpatients’responsiveness.

290 In previous studies targeting the left prefrontal cortex, clinical

291 improvements(i.e.,responsivenessassessedbytheCRS-R)were

292 notedinpatientswithMCS,evenafterasinglestimulationsession

293 of20minoftDCSat2mA[37].Severalfactorscouldexplainwhy

294 wedidnotreproducesuchbehaviouraleffects.First,thesample

295 sizewasrelativelysmall,withonly14individualsincluded,7with

296 MCS, a subgroup for which tDCS seems to be more efﬁcient.

297 Second,westimulatedtheleftprefrontalcortexat1mA,which

298 maynotbesufﬁcienttoinducerelevantclinicalimprovementafter

299 asinglesessionoftDCS.Third,byplacingcathodesovertheM1,we

300 mayhavealsoreducedtheabilityofparticipantstoinitiatethe

301 motor-mediateresponsesasassessedbytheCRS-R.However,at

302 thegrouplevel,patients’behaviouralresponsesdidnotworsenas

303 comparedtobaseline.

304 tDCSrepresentsaninterestingtool,especiallyforpatientswith

305 DOC,becauseitdoesnotrequiretheirparticipation.Inaddition,it

306 issafe,withfewsideeffects,whichisalsoanessentialfactorfor

307 this population of individuals unable to communicate their

308 feelings. Finally, the device is relatively inexpensive, portable,

309 and user-friendly, so it is a good technique to be used in

310 rehabilitationcentersandinnursingfacilitiesorevenathome.

311 Inconclusion,wereport4individualswithDOCshowinga

312 signiﬁcantreductioninmuscletoneaftertDCS,whichhighlights

313 thepotentialclinicaleffectofcathodaltDCSappliedovertheM1

314 for managing spastic symptoms in DOC. This ﬁnding is also

315 supportedbytheincreaseinEEGconnectivitywithinthemotor

316 andfrontalregionsinbetaaftertDCS.Muscleoveractivityaffects

317 many individualswith severe braininjury, whohave limited

318 treatmentoptions;therefore,tDCSrepresentsavaluabletoolto

319 help manage hypertonia in this critical population. Future

320 clinical trials including repeated sessions might conﬁrm the

321 effectsoftDCSasatherapeuticoptionfortreatinghypertoniain

322 individuals with chronic DOC. In addition, although

low-323 intensitystimulationprotocolswerepreviouslyrecommended

324 [38],arecentstudyofstrokepatientsdemonstratedthesafety

325 andtolerabilityofapplyingacurrentashighas4mAovertheM1

326 [39];therefore,increasingthecurrentintensityofourprotocol,

327 fora totalof 4mAof injectedcurrent,couldlead tostronger

328 clinicaleffects.Studiesshouldalsoassessparticipants’levelof

329 pain,knowntobelinkedtohypertoniaandmayberelatedto

330 quality of life (e.g., by meansof the NociceptionComa Scale

331 Revised)[40].ExcitatorytDCS(anodesappliedoverM1)could

332 alsobetestedtoreducehypertonia,aswaspreviouslyobserved

333 instrokepatientsreceivingintermittentexcitatorythetaburst

334 stimulation[41].

335 Funding

336 The study was supported by the University and UniversityQ4Q5

337 HospitalofLiege,theBelgianNationalFundsforScientiﬁcResearch

338 (FRS-FNRS), the Human Brain Project

(EU-H2020-fetﬂags-339 hiphbpsga1-ga720270), the Luminous project

(EU-H2020-feto-340 penga686764),theCenter-TBIproject(FP7-HEALTH-602150),the

341 Public Utility Foundation ‘‘Universite´ Europe´enne du Travail’’,

342 ‘‘FondazioneEuropeadiRicercaBiomedica’’,theBialFoundation,

343 theEuropeanSpaceAgency,theMindScienceFoundationandthe

344 European Commission, and the European Union’s Horizon

345 2020 research and innovation programme under the Marie

346 Skłodowska-Curie grant agreement No 778234. AT is a FNRS

347 post-doctoralresearchfellow.SLisaFNRSdirectorofresearch.CC

348 isaMarieSklodowska-Curiefellow

(H2020-MSCA-IF-2016-ADOC-349 752686).

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350 Disclosureofinterest

351 Theauthorsdeclarethattheyhavenocompetinginterest. 352Q6Uncitedreference

353 [19].

354 AppendixA. Supplementarydata

355 Supplementarydataassociatedwiththisarticlecanbefound,in 356 theonlineversion,athttp://dx.doi.org/10.1016/j.rehab.2019.05.009. 357 References

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