OcularUltrasound:

AQuickReferenceGuidefortheOn-CallPhysician

AaronFairbanksBS,LorraineMyersMD,WilliamFlanaryMD,LauraWarner,H.CulverBoldtMD

Ocularultrasound,alsoknownasocularechography,“echo,”oraB-scan,isaquick,non-invasivetestroutinelyusedinclinicalpracticetoassessthestructuralintegrityandpathologyoftheeye.Itcanprovideadditionalinformationnotreadilyobtainedbydirectvisualizationofoculartissues,anditisparticularlyusefulinpatientswithpathologythatpreventsorobscuresophthalmoscopy,e.g.largecornealopacities,densecataracts,orvitreoushemorrhage(1).

Someacademic centersemployahighly trainedocularultrasonographer toperformocularultrasoundduringregular business hours. Consequently, ophthalmology residentsmay lack technical and practical experience in ocularultrasound.Thesedeficienciesarehighlightedwhenseeingpatientsafterhours,whileon-call.Proficiencyinperformingocularultrasoundisaninvaluabletooltotheon-callphysicianwhoseekstoquickly,safely,andinexpensivelyexaminetheglobeandproperlytriageapatient.Pleasenote,inthesettingofasuspectedopenglobeinjury,echographyshouldonlybeperformedbyanexperiencedechographer,aspressureontheeyecancausefurtherdamage.Here,wepresentasimple,introductory“on-callsurvivalguide”forophthalmologyresidentsusingocularultrasound.

OcularUltrasoundTechnique

Onecanexaminetheentireglobeinjustfivemaneuvers,i.e.fourdynamicquadrantviewsandonemorestaticslicethroughthemaculaandopticdisc,alsoknownaslongitudinalmacula(LMAC).ThequadrantsviewsaredesignatedT12,T3,T6,andT9.Thesenumberedquadrantscorrespondtoaclockfacesuperimposedontheeye.Forexample,T12isaviewthroughthesuperiorquadrantoftheeye,T3thenasalquadrantoftherighteye(temporalquadrantofthelefteye),andsoon(Figure1)(2).

Figure1:Schematicofultrasoundquadrants Figure2:Limbus-to-fornixrotationalmotion

Ultrasoundimagescanbeobtainedthroughthepatient’seyelids(asdepictedinthistutorial)orwiththeprobedirectlyonthesurfaceoftheeyewithappropriatetopicalanesthesia.Beginwiththegainonhigh.Thepatientshouldlookinthedirectionofthequadranttobeevaluated.Themarkerontheprobeisalwaysorientedsuperiorlyornasallybyconvention. Usea limbus-to-fornixrocking,rotationalmotionsothatthetipoftheprobemovesasmalldistance,whitethebaseoftheprobemovesalargerdistance(Figure2)(3).Theproberotatesaroundtheglobesothatthesoundwavesalwayspassthroughthecenteroftheeye.Thisrotationalmotionwillmaximizetheamountofretinavisualizedduringthescan.Seethe“AdditionalInformation”sectionformoredetail.

AStep-WiseApproach

TransverseView1:T12(quadrantcenteredat12o’clock)

Figure 3: Ask the patient to look up. Place your probe on the inferior aspect of the globewith themarker orientednasally.Beginatthelimbus(L),andlocatetheopticnerveshadow,bothtoorientyourselfandassureyouareimagingthe posterior segment. Slowly sweep your probe toward the fornix (F) until visualization of the T12 quadrant iscomplete.Repeatifnecessary.Remembertocenteranypathologyalongtheequatorialplaneoftheimageforthebestresolution.

TransverseView2:T6(quadrantcenteredat6o’clock)

Figure4:Ask thepatient to lookdown.Placeyourprobeon the superioraspectof theglobewith themarkeraimednasally.Again,beginatthelimbus(L)andensureyouhaveanimageoftheretinaandopticnervebeforesweepingtheprobetowardthefornix(F).Repeatifnecessary,centeringanypathology.

L

F

F L

TransverseView3:T3(quadrantcenteredat3o’clock)

Figure5:Remember,toscanthemedialandlateralquadrantsoftheeye,theprobemarkershouldpointsuperiorly.FortheT3quadrantofthepatient’srighteye,instructthepatienttolookleft.Placetheprobeonthetemporallimbus(L).Afterobtaininganimageoftheretinaandopticnerve,gentlysweeptheprobetothefornix(F)tocompleteevaluationofthisquadrant.ToviewtheT3quadrantofthelefteye,thepatientshouldstillgazetotheleft,buttheprobewillbeplacedatthemediallimbus,withthemarkerorientedsuperiorly.

TransverseView4:T9View(quadrantcenteredat9o’clock)

Figure6:ScanningtheT9quadrantoftherighteyeissimplythereversescanoftheT3quadrant.Withtheprobemarkerorientedsuperiorly,instructthepatienttodirecttheirgazetotheright.Placetheprobeontheglobeatthenasallimbus(L).FortheT9quadrantofthelefteye,placetheprobeatthetemporallimbus.Proceed,againwithalimbus-to-fornix(F)rotationalsweepingmovement.

FL

LF

LongitudinalMacula(LMAC)View

Figure7:TheLMACviewallowsforpropervisualizationofthemaculaandopticnerve.Gentlyplacetheprobeonthemedialaspectoftheeyewiththepatient’sgazedirectedtemporally.Note:Forthisposition,themarkeroftheprobeshouldbedirectedtowardthepupil,insteadofsuperiorly.Alongitudinalscanistheonlyscanwherethisoccurs!Inthisview,theopticnervewillbebelowthemacula.Maneuvertheprobetobringthemaculaintothecenteroftheimagetoobtainthebestresolution.

Summary

The on-call ophthalmologist must be proficient at ocular ultrasound, as it is an indispensible tool for thediagnosis and triage of ophthalmic emergencies. One can systematically examine the entire globe with just fivemaneuvers, i.e. fourdynamicquadrantviewsandone longitudinalcutthroughthemaculaanddisc.Onemustalwaysremember that this is simplyastartingpoint,andamoredetailed,comprehensiveultrasoundexaminationshouldbeguidedbyadditionalclinicaldataandpreliminaryultrasonographicfindings.

Appendix:Supplementalinformationonocularultrasonography(2)

1. High frequencies (approximately 10Mhz) areused inocular echographybecause theyproducean imagewithgreater resolution than low frequencies.While this comes at the expense of poorer tissue penetration, highfrequenciesretainenoughpenetrationtoproperlyexaminethedelicateocularstructures.

2. The B-scan creates a two dimensional image from a very thin slice of tissue oriented perpendicular to thecylinderoftheprobe.

3. Theareaofbestresolution isalongthecenteraxisoftheprobe,paralleltotheprobeitself.Thus,theareaofinterestshouldbeplacedalongtheequatoriallineoftheimage.Inocularultrasound,theretinawillappearontherighthandsideoftheimage;thisiswhereanypathologyshouldbefocused.

4. Thedenserthetissue,thebrighter(hyperechoic) itwillappearandviceversa. Ifthetissueisdenseenough, itwillcasta“shadow”directlybehindit,preventingthattissuefrombeingevaluated.

5. Asthegain isadjustedhigher,weakersignalsaremoreeasilyvisualized (vitreousopacities,posteriorvitreousdetachment,smallforeignbodies,etc.).Asthegainisadjustedlower,strongersignalsaremoreeasilyvisualized(masses,tumors,etc.)andtheweakersignalsmaybeabsent.

6. For transverse images, themarker on the probe is always oriented superiorly or nasally by convention. Thisallowsanyreadertointerpretyourimagesgiventhestatedcut(e.g.T12).

7. Themosteffectivemethod toexamine theextentof the retinaduringaB-scan is touse the limbus-to-fornixtechnique.Toperformthistechnique,theultrasonographershouldgentlyglidetheprobefromthelimbusoftheeyetothefornixinasweepingmotiontomaximizetheamountofretinavisualizedduringthescan.

8. Byconvention,aclockfaceissuperimposedoneacheyetoidentifythequadrantstobescanned,similartothemethodusedtodescribefunduslesions.WhiletheT12andT6remainsuperiorlyandinferiorly(respectively)oneach eye, the T3 quadrant on the patient’s right eye is located nasally,while on their left it is the temporalquadrant.ThesameistruefortheT9quadrant,whichislocatedtemporallyontherighteyeandnasallyonthelefteye.

References:

1. ByrneSF,GreenRL.Ultrasoundoftheeyeandorbit.2nded.St.Louis:MosbyYearBook;2002.2. BoldtHC,OssoinigKC,WarnerLL,FuhrmesiterL.Echography.http://www.medicine.uiowa.edu/

eye/echography.AccessedNovember25th,2015.3. WaldronRG.ContactB-ScanUltrasonography.EmoryEyeCenter.2003:1-9.