Rev. Bras. Anestesiol. vol.65 número1

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REVISTA

BRASILEIRA

DE

ANESTESIOLOGIA

OfficialPublicationoftheBrazilianSocietyofAnesthesiology

www.sba.com.br

SCIENTIFIC

ARTICLE

Changes

in

retinal

nerve

fiber

layer

thickness

after

spinal

surgery

in

the

prone

position:

a

prospective

study

夽

Baran

Gencer

a,∗

,

Murat

Cos

¸ar

b

,

Hasan

Ali

Tufan

a

,

Selcuk

Kara

a

,

Sedat

Arikan

a

,

Tarik

Akman

b

_,

_Hasan

_Ali

_Kiraz

c

_,

_Arzu

_Taskiran

_Comez

a

_,

_Volkan

_Hanci

d

a_Department_of_{Ophthalmology,}_Canakkale_Onsekiz_Mart_University,_Canakkale,_Turkey b_Department_of_{Neurosurgery,}_Canakkale_Onsekiz_Mart_University,_Canakkale,_Turkey

c_Department_of_{Anesthesiology}_and_Reanimation,_Canakkale_Onsekiz_Mart_University,_Canakkale,_Turkey d_Department_of_{Anesthesiology}_and_Reanimation,_Dokuz_Eylül_University,_Izmir,_Turkey

Received19August2013;accepted13March2014 Availableonline30April2014

KEYWORDS

Retinalnervefiber layerthickness; Proneposition; Spinalsurgery; Intraocularpressure

Abstract

Backgroundandobjectives: Changesinocularperfusionplayanimportantroleinthe pathogen-esisofischemicopticneuropathy.Ocularperfusionpressureisequaltomeanarterialpressure minusintraocularpressure.Theaimofthisstudywastoevaluatethechangesintheintraocular pressureandtheretinalnervefiberlayerthicknessinpatientsundergoingspinalsurgeryinthe proneposition.

Methods:Thisprospectivestudyincluded30patientsundergoingspinalsurgery.Retinalnerve fiber layerthickness were measured oneday before andafter thesurgery by usingoptical coherencetomography.Intraocularpressurewasmeasuredbytonopensixtimesatdifferent positionandtime-duration:supineposition(baseline);10minafterintubation(Supine1);10 (Prone1),60(Prone2),120(Prone3)minafter proneposition;andjustafterpostoperative supineposition(Supine2).

Results:Ourstudyinvolved10maleand20femalepatientswiththemedianageof57years. Whenpostoperativeretinalnervefiberlayerthicknessmeasurementswerecomparedwith pre-operativevalues,astatisticallysignificantthinningwasobservedininferiorandnasalquadrants (p=0.009andp=0.003,respectively).Weobservedastatisticallysignificantintraocular pres-suredecreaseinSupine1andanincreaseinbothProne2andProne3whencomparedtothe baseline.Meanarterialpressureandocularperfusionpressurewerefoundtobesignificantly lowerinProne1,Prone2andProne3,whencomparedwiththebaseline.

夽

Apartofthisstudywaspresentedatthe46thNationalCongressofTurkishOphthalmologySociety,October2012,Antalya,Turkey.

∗_{Corresponding}_author.

E-mail:barangencer@gmail.com(B.Gencer).

http://dx.doi.org/10.1016/j.bjane.2014.03.005

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Conclusions:Ourstudyhasshownincreaseinintraocularpressureduringspinalsurgeryinprone position.Astatisticallysignificantretinalnervefiberlayerthicknessthinningwasseenininferior andnasalquadrantsonedayafterthespinalsurgery.

PALAVRAS-CHAVE

Espessuradacamada defibrasnervosasda retina;

Pronac¸ão;

Cirurgiadacoluna vertebral;

PressãoIntraocular

Alterac¸õesdaespessuradacamadadefibrasnervosasdaretinaapóscirurgiada

colunavertebralempronac¸ão:estudoprospectivo

Resumo

Justificativaeobjetivos: Asalteraçõesdeperfusãooculardesempenhamumpapelimportante napatogênesedaneuropatiaópticaisquêmica.Apressãodeperfusãoocularéigualàpressão arterialmédiamenosapressãointraocular.Oobjetivodesteestudofoiavaliarasalterações dapressãointraocularedaespessuradacamadadefibras nervosasdaretinaempacientes submetidosàcirurgiadacolunavertebralempronação.

Métodos: Esteestudoprospectivoincluiu 30pacientessubmetidos àcirurgia dacoluna ver-tebral. A espessurada camadade fibras nervosasdaretina foi medida um dia antese um depoisdacirurgia, como usodatomografia de coerência óptica.A pressão intraocularfoi medidaseisvezescomoTonopeneotempodeduraçãoemposiçõesdiferentes:emsupinação (basal),10minutosapósaintubação(Supinação1);apósapronaçãoaos10minutos(Pronação 1),60minutos(Pronação2) e120minutos (Pronação3) e logoapósasupinação noperíodo pós-operatório(Supinação2).

Resultados: Nossoestudoenvolveu10pacientesdosexomasculinoe20dofeminino,commédia de57anos.Nopós-operatório,quandoasmedidasdaespessuradacamadadefibrasnervosas daretinaforamcomparadascomosvaloresdopré-operatório,umafinamentoestatisticamente significativodaretinafoiobservadonosquadrantesnasaiseinferiores(p=0,009ep=0,003, respectivamente).Observamos uma diminuiçãoda pressãointraocularestatisticamente sig-nificanteemSupinação1eumaumentoem ambosmomentosPronação2ePronação3, em comparaçãocomosvaloresbasais.A pressãoarterialmédiae apressãodeperfusão ocular foramsignificativamentemaisbaixasemPronação1,Pronação2ePronação3,emcomparação comosvaloresbasais.

Conclusões:Nossoestudomostrouaumentodapressãointraocularduranteacirurgiadacoluna vertebralempronac¸ão.Umafinamentoestatisticamentesignificantedaespessuradacamada defibras nervosasdaretinafoiobservadonosquadrantes nasaiseinferioresum diaapósa cirurgiadacolunavertebral.

Introduction

Postoperativevisionloss(POVL)isararebutserious compli-cationthatmaydevelopafternon-ocularsurgeries.1---3_POVL

mostcommonlyoccursafterspinalsurgery.Theetiologyof POVLhasbeenidentifiedasposteriorischemicoptic neurop-athy(PION),anteriorischemicopticneuropathy(AION)and central arterial occlusion, respectively.4---6 _A _multi-center

studydemonstratedthatmalegender,obesity,useofWilson frame,long durationof anesthesia, excess blood lossand low ratio of colloids administration are the independent riskfactorsforperoperative IONdevelopmentafterspinal surgery.7_Changes_in_ocular_perfusion_play_an_important_role

inthe pathogenesisof ischemicopticneuropathy.8 _Ocular

perfusion pressure (OPP) is calculated by the subtraction of intraocular pressure (IOP) from meanarterial pressure

(MAP).8 _Several _studies _have _demonstrated _that _spinal

surgery in the pronepositioncauses an IOP elevation.9---13

Ithasbeen speculatedthatasaresultofIOP elevationin the prone position, OPP reduces (in patients where MAP remainsstableorreduces)andmayleadtoAION.9,10,12

Although IOP changes during spinal surgery in prone position have been well demonstrated, there is no study evaluatingtheeffectofthesechangesontheopticnerve. Retinalnervefiberlayerthickness(RNFLT)is animportant objectivemarkerforearlyperiodchangesduetoopticnerve damage,whichinturniscausedbyischemicoptic neurop-athyandsuddenincreaseinIOP.14---18

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Materials

and

methods

The study wasperformed with informedpatient consent, and conducted under a protocol approved by the local Ethics Committee of C¸anakkale Onsekiz Mart University (26.04.2012,050-99-79)andinaccordancewiththeethical standardsstatedinthe1964DeclarationofHelsinki. Partici-pantsforthestudywereenrolledfromconsecutivepatients whounderwentspinalsurgerybetweenMayandNovember 2012,andwhosephysicalstateswereI-IIIinaccordancewith AmericanSocietyofAnesthesiologists(ASA).

One day before the surgery, the patients underwent a full ophthalmic examination including visual acuity, slit-lampexam,IOPmeasurementusingGoldmannapplanation tonometryandfundusexamination.Patientsunder18year andpatientswithsphericalvalues±5dioptryand/or cylin-dricalvalues±3dioptryrefractoryerror,glaucoma,uveitis, historyofeyesurgery,opticnerveanomaly,historyofallergy totopicalanesthesia, ormeasurement qualitybelow5/10 onopticcoherencetomography(OCT)wereexcluded.

TheRNFLTweremeasuredonedaybeforeandafterthe surgeryby usingOTI SpectralOCT/SLO(Ophthalmic Tech-nologies Inc.(OTI), Toronto, Ontario,Canada). OCT scans were made after placing a 3.4mm diametercircular ring aroundtheopticnerveheadonimageoftheconfocal scan-ninglaserophthalmoscopy.Thepatients’RNFLTmapswere evaluatedtogetherwithnormativeRNFLTratios.The aver-ageRNFLTmeasurementsfortemporal,superior,nasaland inferiorquadrantswerecollected(Fig.1).

No premedication was given before anesthesia. Anes-thesia was induced with2␮g/kg fentanyl, and 2.5mg/kg propofol.Trachealintubationwasfacilitatedwith0.6mg/kg rocuronium, and the lungs were mechanically ventilated at 0.5 FiO2. Anesthesia wasmaintained withsevoflurane,

and remifentanyl 0.15---0.2␮g/kg/min. Additional rocuro-niumwasadministeredasrequired.Aftertopicalanesthesia (right eye0.5% proparacaine hydrochloride,0.5% Alcaine, Alcon) IOP measurements were taken for six times with Tono-PenAVIAapplanationtonometer(ReichertInc.,Depew, NY):beforepremedicationinthesupineposition(baseline); 10minafterintubation(Supine1);10min(Prone1),60min (Prone 2), 120min (Prone 3) after being turned to prone positionand10minafterthepatientwasturnedtosupine position(Supine 2).Prone position cushionswere used to stabilizethepatients’headsinproneposition.

Systolic blood pressure (SBP), diastolic blood pressure (DBP),MAP,heart rate(HR),end-tidalcarbondioxide (ET-CO2) and oxygen saturation (SatO2) were also recorded

simultaneouslywithIOPmeasurements.OPPwascalculated using the MAP-IOP formula. The anesthesia, prone posi-tionandsurgerydurationswerealsonoted.Theamountof bloodandliquidsgiventothepatientwasmeasuredagainst bloodlossandurineoutputduringtheoperationtocalculate liquidbalance[liquidbalance=(givenblood+fluids)−(lost blood+urine)].Preoperativeandpostoperativehemoglobin (Hb)andhemocrit(Htc)levelswererecorded.Inthe recov-eryroom, thepatients were asked whetherthey hadany visionlossordiscomfortintheireyes.

Date

Description

Eye

SNR: 6/10

RE

Jun 8, 2012 Jun 10, 2012

RE

SNR: 6/10

114 103134106

92

83

80 116 133 133 84 57

76 110

118

93

137 69 T

I S

N

136 109

101

89

89 133 149 131 77 53

78

S N 85 72 T

I

127

300

200

100

0

300

200

100

0

TEMP SUP NAS INF TEMP TEMP SUP NAS INF TEMP

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Poweranalysis

Themain focusof ourstudy wastodetermine postopera-tiveRNFLT. The samplingsizewasdetermined followinga studybyHongetal.,19_where_inferior_quadrant_RNFLT

aver-agewasreportedas139.9±11.8␮m.Thepatientnumber requiredtodetermineanaveragereductionininferior quad-rantRNFLT(with5%ofnormalvalues,0.05˛errorand80% power) wascalculatedas 30.The sample sizewas deter-minedbyapowercalculator.

Statisticalanalyses

Statistical analyses were performed using the Statisti-cal Package for Social Sciences (SPSS), version 15.0, for Windows (SPSS, Chicago, IL). The Shapiro---Wilk normal-ity test was used to examine the compatibility between the measured variables and the normal distribution. The Wilcoxon test was used to compare pre- and postop-erative RNFLT measurements. During the operation the changes in IOP and hemodynamic parameters measured at six different times were later evaluated using the Friedmantest.Pairwisecomparisonswereperformedusing the Wilcoxon and Mann---Whitney U test. The relationship between RNFLT changes and parameters showing signifi-cant changes during the operation were examined using the Spearman correlation test. The data were shown as median(minimum---maximum),andthelevelofsignificance wasacceptedasp<0.05.

Results

Atotalof32patientsunderwentspinalsurgeriesbetween May and November 2012. Among them, 30 patients were included in our study, while two patients were excluded due to tilted disk. There were 10 male and 20 female patients, with an average age of 54.9±13.4 years. The patientcharacteristicsandpropertiesrelatingtothe oper-ationaresummarizedinTable1.

Preoperative and postoperative median values of superior,temporal,inferiorandnasalquadrantRNFLT mea-surements are given in Table 2. There was no significant differencebetween postoperative and preoperative mea-surementsfor superior and temporalquadrants(p>0.05). However,astatisticallysignificantthinningwasobservedin inferior(p=0.009)andnasalquadrants(p=0.003)(Table2). The patients’IOPand hemodynamicmeasurementsare summarized in Table 3. When compared to the base-line, a reductionin IOP wasseen at Supine1 (p<0.001).

Table1 Patientvariables.

Variable Data

Age(years) 57(28---80)

Gender(male/female) 10/20

BMI(kg/m2₎ _28.3_(18.7---45)

Anesthesiatime(min) 140(82---260) Pronetime(min) 130(70---255) Operationtime(min) 120(60---242) Bloodloss(mL) 350(100---1200) Colloidinfusion(mL) 400(0---1200)

Variablesareshownasthemedian(range). BMI,Bodymassindex;min,minutes;mL,mililitres

AstatisticallysignificantIOPelevationwasobservedinProne 2(p<0.001)andProne3(p=0.01)whencomparedto base-linevalues.HemodynamicchangesshowedthatSBP,DBPand MAPweresignificantlyloweratProne1(p=0.02,p=0.03, p=0.01),Prone 2(p<0.001 forall) andProne3 (p=0.001 for all) when compared to baseline. Average OPP values weresignificantlylowerthanbaselineatProne1(p=0.01), Prone 2 (p<0.001) and Prone 3 (p=0.004). Heart rates were significantly lowerat Prone 2 (p=0.01)and Prone 3 (p=0.01) when compared tobaseline. At all times, SpO2 values were significantly higher when compared to base-line (Supine 1and Prone 2;p<0.001, Prone 1and Supine 2;p=0.001, Prone3;p=0.005).Calculated liquidbalance forallpatientswaspositive,andaveragevalueswere1325 (550---3000)mL.PreoperativeandpostoperativeaverageHb valueswere13.2(11---16.3)g/dLand11.1(8.1---14.1)g/dL, respectively; while Htcvalueswere 39.3(33.8---48.7)g/dL and 34.2 (25---43.1)g/dL. The reductions in postoperative HbandHtcvalueswerestatisticallysignificant(p<0.001for both).

Nosignificantcorrelationwasfoundbetweentheamount of RNFLTthinningandpatients age,BMI,anesthesia dura-tion,bloodloss,andadministeredcolloidamount,changes ofIOPandOPPduringtheoperation(p>0.05)(Table4).

Discussion

ThisprospectivestudyaimedtoexaminechangesinRNFLT andfactorsaffectingthosechangesinpatientswho under-went spinal surgery in the prone position. When we evaluatedthe changes in RNFLTmeasured bySD-OCT, we observed a thinning in the inferior and nasal quadrants on the first postoperative day compared to preoperative

Table2 Preoperativeandpostoperativemeasurementsofretinalnervefiberlayerthicknessatfourquadrants.

RNFLT Superior(␮m) Temporal(␮m) Inferior(␮m) Nasal(␮m)

Preoperative 115(98---149) 66.5(46---96) 129(91---158) 78.5(55---119) Postoperative 115.5(81---156) 69.5(47---105) 126.5(87---164)a ₇₂_(59---101)a

p NS NS 0.009 0.003

Variablesareshownasthemedian(range).

RNFLT,retinalnervefiberlayerthickness;␮m,micrometer;NS,nonsignificant.

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Table3 Intraocularpressure,bloodpressure(systolic,diastolic,mean),ocularperfusionpressureandheartratemeasurements duringoperation.

IOP(mmHg) SBP(mmHg) DBP(mmHg) MAP(mmHg) OPP(mmHg) HR(beats/min)

Baseline(n=30) 15(7---20) 130(91---202) 77(60---111) 95(69---149) 79.5(53---134) 75(55---100) Supine1(n=30) 10(5---24)a ₁₂₀_(86---190) ₇₅_(54---112) ₈₆_(63---133) _75.5_(47---119) _78.5_(55---100)

Prone1(n=30) 14.5(6---31) 117(78---156)a ₇₆_(44---99)a ₉₂_(54---122)a ₇₁_(38---109)a ₇₆_(57---109)

Prone2(n=30) 19(10---37)a _107.5_(81---148)a ₇₁_(53---94)a ₈₂_(68---104)a _62.5_(35---92)a _69.5_(54---93)a

Prone3(n=18) 15(11---35)a ₁₀₉_(90---132)a ₇₀_(58---85)a _81.5_(72---105)a ₆₄_(46---90)a ₆₉_(62---98)a

Supine2(n=30) 15.5(9---32) 128(81---170) 77.5(62---99) 98(70---127) 80(46---115) 78.5(55---99)

Variablesareshownasthemedian(range).

IOP,intraocularpressure;SBP,systolicbloodpressure;DBP,diastolicbloodpressure;MAP,meanarterialpressure;OPP,ocularperfusion pressure;HR,heartrate.

a _p_<_0.05,_compared_with_baseline_value,_Wilcoxon_signed_ranks_test.

measurements.WeobservedasignificantincreaseinIOPin thepronepositionwhiletherewasasignificantdecreasein hemodynamicfactors.However,wedidnotfindany corre-lationbetween the changingof all parametersduring the operationandtheRNFLTthinning.

POVL,occurringafterspinalsurgeryintheproneposition is a serious complicationwhich has been linked,in order ofdecreasingfrequency,toposteriorION,anteriorIONand central artery occlusion, but its etiology is still not well understood.1---3,6_It_has_been_proposed_that_IOP_elevation_in

the pronepositionreduces ocular perfusion, especiallyin caseswhoseMAP remainsstable,therebycausing anterior IONorcentralretinalarterialocclusion.12

ThefirststudyinvestigatingIOPchangesinspinalsurgery in the prone position was carried out by Cheng et al.10

Theyobservedasignificant IOPelevationat thebeginning (27±2mmHg) andat the end (40±2mmHg) of the oper-ation in theprone positionwhen compared withbaseline values(19±1mmHg)measuredinthesupineposition. How-ever, theydidnot investigateintraoperative IOP changes. Inanotherstudyevaluatingpatientswhounderwentspinal surgery, it was found that the average IOP values in the supinepositionafteranesthesiainductionwere11.5mmHg, while in the prone position at the beginning and at the endofsurgeryIOPvalueswere23.5mmHgand30.5mmHg, respectively.10 _They_determined_that_in_the_prone_position,

these values were significantly higher than in the supine

position. In various studies using healthy volunteers, IOP valuesinthepronepositionwerefoundtobesignificantly higherthaninthesittingposition.20,21

Tothebestofourknowledge,thisstudyisthemost com-prehensivestudytilldate investigatingIOPchangesinthe proneposition.IOPwasmeasured inthepronepositionat the10th,60thand 120thminutesof operation.We found that IOP was highest at the 60th minute, and it reduced slightly at the 120th minute, which wasstill significantly higherthanbaselinevalues.WhileourstudyonIOPchanges duringtheoperationhasavariationcurvesimilartoother studies, any differences may be linked toerroneous high measurementsduetoexcessiveeyelidopeningorpressure ontheglobe.

Spectral domain OCT provides reliable and repeatable measuresofthenervefiberlayeroftheopticnerveatthe levelof7---15␮m.22Thesepropertieshaveledittobecoming themostimportantdiagnostictoolforearly diagnosisand follow-upfordisordersinvolvingthefrontpartofthevisual pathway.23_In_a_previous_study,_{interoperator}_(intraclass

cor-relationcoefficient(ICC),0.87;CV,2.89%)andintraoperator (ICC,0.94and0.95;CV,1.28%and1.26%,respectively,for operatorAandoperatorB)agreementhavebeenfoundfor averageRNFLTmeasurementsusingOTISpectralOCT/SLO.24

ThisstudyisthefirstintheliteraturetoevaluateRNFLT changes following spinal surgery in the prone position. On the first postoperative day, a significant thinning was

Table4 The correlations between inferiorand nasal quadrants’ retinal nerve fiber layer thickness changes and patients variables.

Variables(n=30) Inferiorquadrantchange Nasalquadrantchange

ra _pa _ra _pa

Age −0.13 0.50 0.05 0.80

BMI −0.02 0.90 0.05 0.81

Anesthesiaduration 0.22 0.24 0.33 0.07

Bloodloss 0.07 0.70 0.29 0.11

Colloidinfusion −0.09 0.64 0.29 0.12

IOPchange(Prone2−baseline) 0.25 0.18 0.16 0.41

OPPchange(Prone2−baseline) −0.14 0.45 −0.35 0.85

a _Spearman’s_rank_correlation.

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observedin the inferiorand nasalquadrants.While there wasa significant rise in IOP in the prone position, there wasa significant decrease in hemodynamic factors. How-ever,nosignificantcorrelationwasfoundbetweenpatients’ age,BMI,anesthesiaduration,bloodlossorgivencolloids, IOPorOPPchangesandthinninginRNFL.Similarly,Fortune etal.16_showed_that_acute_IOP_increase_caused_RNFL_thinning

in rat eyes, and this thinning returned to normal values afteramonth.Pietteetal.14 _found_that_acute_IOP_increase

causedbysuctioninLASIKpatients,whichresultedina sta-tisticallysignificant thinning ofRNFLT, andeven afterIOP valuesreturnedtonormalthisthinningremained.Inanother study,allGDxparametersexceptforsymmetryshowed sta-tisticallysignificantreductiononthefirstpostoperativeday in patients who underwent coronary artery bypass oper-ation. By the 5th postoperative day, superior/nasal ratio parameters returned to normal values, however, by the firstpostoperativemonththeinferiorratiovalueswerestill undernormalvalues.25_This_study_supports_our_results

show-ingthattheinferiorquadrantretinalnervefibersaremore sensitive.

Althoughthenumberofpatientsinourstudyis compa-rable to similar studies, low patient number is the most important limitation. Another limitation was the lack of long-term follow-ups.The progress of RNFLTchanges was notevaluatedinthisstudy.

Inconclusion,spinalsurgicalinterventionsintheprone positionleadasignificantinferiorandnasalquadrants thin-ninginRNFLattheearlypostoperativeperiodeventhough patientsdonot sufferfromvision loss. Buttherewasnot found any significant correlation between the amount of RNFLTthinningandchangesofIOP duringtheoperationin thisstudy.Largercontrolled studieswithlongerfollow-up willberequiredtofullydeterminetheroleofproneposition onRNFLTchanges.

Conflicts

of

interest

Theauthorsdeclarenoconflictsofinterest.

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