Imaging of the optic chiasm and retrochiasmal visual pathways

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CONTINUING

EDUCATION

PROGRAM:

FOCUS

.

Imaging

of

the

optic

chiasm

and

retrochiasmal

visual

pathways

N. Menjot

de

Champﬂeur

a,∗

,

S. Menjot

de

Champﬂeur

a

,

D. Galanaud

b

,

N. Leboucq

a

,

A. Bonafé

a

a_Service_de_{Neuroradiologie,}_Hôpital_{Gui-de-Chauliac,}_CHRU_de_Montpellier,_80,_avenue Augustin-Fliche,34295Montpelliercedex5,France

b_Service_de_{Neuroradiologie,}_Groupe_Hospitalier_{Pitié-Salpétrière,}_Paris,_France

KEYWORDS

Brain;

Cranialnerves; Orbits; MRI

Abstract Theexplorationofthechiasmalandretrochiasmalvisualpathwaysisbasedon

mag-neticresonanceimaging.Abitemporalhemianopsissuggestsalesionoftheopticchiasmwhile

homonymouslateralhemianopsisshouldleadtoasearchforalesionoftheretrochiasmalvisual

pathways.Thecausesofchiasmalimpairmentaremainlytumoral.Theexplorationprotocolis

basedonMRIwithT1-weightedsagittalsections,thenT2-andT1-weightedcoronalsections

withandwithoutinjection.Incaseofaretrochiasmalsyndrome,theMRIexplorationprotocol

isafunctionofthetypeofoccurrenceofthedeﬁciencyandthecontext.

Exploration

of

a

disorder

in

the

visual

ﬁeld

Methods

to

study

the

visual

ﬁeld

Thevisualfieldisstudiedseparatelyforeacheye.Theexaminationmayinvolveasimple clinicalexaminationwheretheexaminerpresentseitheroneofhisfingersorawhiteball, startingfromtheperipheryandmovingtowardsthecentreindifferentsectorsofthevisual field.Campimetryisadynamicmonocularexplorationofthevisualfieldonaflatscreen whileperimetryisadynamicmonocularexplorationofthevisualfield.

To analyse the results, it is necessary to take into account the fact that, for each eye,thenasalhemiretinareceiveslightraysfromthetemporalvisualhemiﬁeldandthat the temporal hemiretina receives light rays from the nasal visual hemiﬁeld. Both left

∗_{Corresponding}_author.

E-mailaddress:nicolasdechampﬂ[email protected](N.MenjotdeChampﬂeur).

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visualhemifields,correspondingtotheright temporaland leftnasalhemiretinas,projectthemselvesontheright fis-sure,whiletherightvisualhemifieldscorrespondingtothe lefttemporalandrightnasalhemiretinasprojectthemselves ontheleftcalcarinefissure.

Campimetry

Thecampimetrescreen: atest lightis movedalonga ﬂat screenanditspositionisnotedassoonasitisperceived.This simplemethodanalysesthecentralvisualﬁeldinadynamic manner.

Friedmann’sanalyzer(staticperimetry)isafastmethod toexplorethecentralvisualﬁeld:staticwhitetestlightsof thesamediameterarepresentedandtheirlightintensityis modiﬁed.

Amsler grids:thesubjectlooksat the smalldot inthe centreofa10cmgridandspeciﬁeswhetherheseeswavy lines,a deformationor if linesaremissingfrom thegrid. Thismethoddetectssmallcentralscotomas.

Perimetry

Perimetryusesabowl-shapedscreen,adaptedtothecurve oftheeye.Theprojectedtestsaretherebyatequaldistance fromtheeye.

Kineticperimetry

KineticperimetryiscarriedoutwithGoldman’sperimeter (Goldman bowl). The visual field registers on a diagram wherethecentre isthepoint offixation correspondingto themacula. TheMariotteblindspotcorrespondstoahole inthevisual field,duetotheopticpapilla(zone of abso-lutescotomaornegativetemporalparacentralphysiological scotoma).

Automatedperimetry

More speciﬁc, automated perimetry helps visualize the intensityofimpairment.Humphreyperimetryismostoften usedtostudythecentralvisualﬁeld.Thisperimetryisstatic, carriedoutbyacomputer.

What

imaging

protocol?

Theexplorationofthechiasmandretrochiasmalvisual path-waysrelies onmagnetic resonance imaging.If alesion of theopticchiasmissuspected,themillimetricacquisitions inT2andT1weightingwithandwithoutcontrastinfusion arecarriedoutinthethreeplanes.Ifthelevelofsuspected impairmentis thatof theoptictracts oropticradiations, the explorationwill cover the entire brain,in T2and T1 weighting, without and then after contrast infusion, and include diffusion imaging. The examination will be com-pleted, if necessary, by perfusion imaging, spectroscopic imaging, and in case of vascular lesion, by angiographic imaging.

Anatomical-physiological

review

The

optic

chiasm

Itreceivestheopticnervesbyitsanterioranglesandemits the optic tracts by its posterior angles. The nerve ﬁbres

from the two nasal hemiretina cross over there. In fact, thenerveﬁbresarisingineachofthetemporalhemiretina reachthehomolateraloptictract,whilethosecomingfrom each of the two nasalhemiretina reachthe contralateral optictract,themacularbundlecomprisingbothdirectand crossedﬁbres.Itislocatedinthechiasmalcistern,behind the tubercle of sellaturcica, behind the chiasmalgroove (located at the posterior part of the sphenoid planum), abovethesellaturcica.The normaldimensionsofthe chi-asmare8mm(4—13mm)inanterior-posteriordiameterand 3—5mmthick.

The

tractus

optici

or

optic

tract

Eachoptictractis formedbythetemporalbundlecoming fromthehomolateralretina,andbythenasalbundlecoming fromthecontralateralretinaaswellasthemacular ﬁbres originatinginbothretinas.

Thetractusopticioroptictractstartsatthe posterior-lateralangleofthechiasm.Itrunslaterallyandbehindthe anteriorperforatedsubstanceandthetubercinereum,and therebyformstheanterior-laterallimitofthe interpedun-cular cistern. It runs around the upper part of the brain peduncle, to which it adheres (Fig.1). In this portion of itspath,itisindivisiblefromtheuncusand parahippocam-palgyrus.The optictractispositioneddirectly abovethe posterior cerebralarteriesand endsinthelateral genicu-latenucleus,attheposterior-lateralsideofthethalamus. Eachoptictractsendstwocontingentsoffibres,thefirstand mostabundanttothelateralgeniculatenucleusandanother minoritycontingentoffibrestothesuperiorcolliculus.The optictractsdivideatthelateralgeniculatenucleusintotwo pathways:alateralpathwaythatentersthelateral genic-ulatenucleusandamedialpathwaythatentersthemedial geniculatenucleus.Theoptictractstherebyendinthe lat-eralgeniculatenucleus,wherenervefibresprovidearelay, butbefore,thebundleofpapillaryfibresseparatesfromit andreaches the pretectalregion,therebyentering in the formationofthepathwayofthepapillarylightreflex.

The

lateral

geniculate

nucleus

Thelateralgeniculatenucleusisanovoidformation associ-atinggreymatterandwhitematter,locatedattheposterior andlateralsideofthepulvinarthalamus.Theanteriorpole mingleswiththeoptictracts.Opticradiationsemergefrom thelateralgeniculatenucleus,whichruntowardsthevisual cortex.

Optic

radiations

or

geniculocalcarine

tract

Gratioletradiationsariseinthelateralgeniculatenucleus. They leave the lateral geniculate nucleus by forming the opticpeduncle.Theopticradiationsthendivideintothree contingents of ﬁbres that occupy the outer part of the sagittalstratum,directlyoutsidetheatriumofthelateral ventricle.Theopticradiationsthenruntowardsthestriate cortexof theoccipitallobe anddivideintotwogroups of ﬁbres:

• a ventral bundle goes around the temporal lobe of the lateral ventricle and reaches the lower lip of the calcarineﬁssure.Thisventralbundlemakesaloopwithin

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Figure1. Anatomyoftheretrochiasmalvisualpathways.Seriesofinversion-recoveryT1-weightedcoronalacquisitionsat3Tesla identi-fyingtheprechiasmalportionoftheopticnerves(a,tipofwhitearrow),theopticchiasm(bandc,whitearrows),theoptictracts(dtoi, tipofgreyarrows)andthelateralgeniculatebodies(jandk,tipofblackarrows).

the temporallobe, movingforwardandoutside,around the temporal lobe of the lateral ventricle, then runs towardsthereartojointhestriatecortex.Thisanterior deviationoftheloweropticradiationsismorecommonly calledMeyer’sloop.Itislocatedatthelevelofthe ante-rior end of the temporal lobe of the lateral ventricle, about1cmoutsideofit;

• adorsalbundlegoesaroundtheoccipitallobeofthe lat-eralventricleandendsintheupperlip ofthecalcarine ﬁssure.

Cortical

centre

of

the

visual

ﬁeld:

striate

cortex

The cortical centre of the visual ﬁeld (striate cortex or brodmann area 17) is located at the level of the upper

andlowerlipsof thecalcarine fissure,atthe medialside of the occipital lobe. Its anterior limit is the parieto-occipitalfissure,theposteriorlimitistheoccipitalpoleor, ifpresent,thelunatesulcus.Theupperlipofthecalcarine fissurebelongstothe cuneus,the lowerlip tothelingual gyrus.

Classiﬁcation

of

disorders

of

the

visual

ﬁeld

by

chiasmal

and

retrochiasmal

impairment:

anatomofunctional

correlation

Thesemiologyofdisordersofthevisual ﬁeldaccording to thesiteofthelesionissummedupinTable1.

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Table1 Summarytableofvisualdisordersaccordingtothesiteofthelesion.

Siteoftheimpairment Visualdisorder Associatedsigns Opticchiasm Sellarorsuprasellarlesion:

heteronymousbitemporal hemianopsia,centralbitemporal scotoma

Supra-chiasmatallesion:

heteronymousbinasalhemianopsia

Optictract Homonymouslateralhemianopsia Absenceorpupilcontractiontolight withvisualstimulationoftheblind hemiﬁeld

Opticradiations(temporalcontingent) Contralateralhomonymoussuperior quadranopiawithoutrespectof macularvision

Persistenceofthepupilcontractionto lightwithstimulationoftheblind hemiﬁeld

Opticradiations(parietalcontingent) Contralateralhomonymousinferior quadranopia,fullhomonymouslateral hemianopsia

Persistenceofthepupilcontractionto lightwithstimulationoftheblind hemiﬁeld

Abolitionofcontralateraloptokinetic nystagmus

Visualcortex Deﬁciencyinareasofthevisualﬁeld: homonymouscentralhemianopsal scotoma

Unilaterallesion:homonymouslateral hemianopsiawithrespectofmacular vision

Bilaterallesions:doublehemianopsia Lesionslocatedintheareastriate(lips ofthecalcarineﬁssure):homonymous superiororinferiorquadranopia Bilateralandsymmetricallesionsof theupperorlowerlipofthecalcarine ﬁssure:inferiororsuperioraltitudinal hemianopsia

Bilateralimpairmentofthevisual cortex:corticalblindness

The

scotomas

Scotomasaregapsinthevisualfield.Theonesmost impor-tantto know arecentral scotomasrelatedtoimpairment ofthemacularbundles.The centralscotomaoccupiesthe macularvisualfieldaroundthefixation point.Itinducesa majorlossofvisualacuityresultingindiscomfortin every-daylife(reading).Theclinicalaspectvariesaccordingtothe seatofthelesion:

• asingleorbilateralcentralscotomaatteststoalesionin theopticnerve;

• abitemporalcentralscotomaatteststoachiasmallesion; • ajunctional scomota of Traquair attests to a compres-sionoftheanteriorangleofthechiasmandassociatesa centraltemporalhemiscotomaononesideandanupper temporalperipheralnotchontheotherside;

• ahomonymouscentralhemianopticscotomaatteststoa retrochiasmallesion. Hemianopticscotomasaregapsin thevisualﬁeldaffectingeachsideofthemacularvision orperipheralvision.

The

hemianopsias

This consistsofa lossinvisual acuityinhalf ofthe visual ﬁeld.When thelimit ofthe impairmentis horizontal,the hemianopsia is said tobealtitudinal, superior or inferior, dependingonwhetherthelossinvisualacuityinvolvesthe upperorlowerhalfofthevisualﬁeld.

The limit of the loss of the visual field may be verti-cal.The hemianopsiais saidtobehomonymouswhen the lossinvisualacuityinvolvesbothrightvisualhemifieldsor both leftvisual hemifields. It is saidto be heteronymous when it involves both nasal hemifields, or both temporal visualhemifields.Macularvisionisoftennotaffected dur-inghemianopsiaduetoitsbilateralprojectiononthevisual cortex.

Thealtitudinalhemianopsias

Thesehemianopsiasarerare,oftenduetobilateraloccipital lesionsaffectingtheopticradiationsorthecortexitself,in

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particularduringtraumas.Inferioraltitudinalhemianopsia isthemostcommonofthetwo.

Theheteronymoushemianopsias

Bitemporalheteronymoushemianopsia,relativelyfrequent andcharacteristicofthechiasmalsyndrome,provokesaloss ofvisualacuityinbothtemporalhemifieldsbyaffectingthe fibresleavingbothnasalhemiretinas.Itatteststoalesion oftheopticfibresthatcriss-crossintheopticchiasmand usuallyresultsfromanexternalcompression,inparticular duringtheevolutionofatumorinthesellarregion.Initially, itmayonlyaffectonequadrant,usuallytheupperquadrant. Binasalheteronymoushemianopsiaisrare.Itprovokesa lossofvisualacuityinbothnasalhemifieldsbyimpairment ofthefibresleavingboth temporalhemiretinas.Itfollows directimpairmentofthevisualfibrestotheopticnerveor chiasm. This typeof hemianopsiais especiallyfound with tumorsofthethirdventricle.

Therefore,heteronymoushemianopsiasaredueto chias-malimpairment.Thechiasmasyndromeisbasedonatriad ofsymptomsassociatingcampimetricdeﬁciency,areduction invisualacuityandopticatrophy.

Thehomonymouslateralhemianopsias

Deﬁnition

The homonymous lateralhemianopsiasaremost common, attestingtoaretrochiasmallesionofthevisualpathways. It mayaffect allof the right or leftvisualhemifields and arecontralateraltothelesion,thatis,aleftretrochiasmal lesion will induce right homonymous lateral hemianopsia affecting thenasal fieldofthe lefteye andthe temporal fieldoftherighteye.

The impairment may only involve the upper or lower halfofthevisualhemiﬁelds,therebycreatinghomonymous lateralquadranthemianopsia(orquadranopsia).The impair-ment is sometimes evenmore localized, occurring in the formofahemianopticscotoma.

Location of thelesions responsiblefor homonymous lateralhemianopsia

Thelesionsarefoundontheretrochiasmalvisualpathways andstudyofthepapillarylightreﬂexmayspecifythe loca-tionofthelesion.

Lesionoftheoptictract. Inunilaterallesionsofthe optictract,thehomonymouslateralhemianopsiais charac-terisedbythelackofpupilcontractioninresponsetovisual stimulation inthe blind hemiﬁeld (Wernicke’shemianopic pupilreaction),whilethepupilcontractionisnormalwhen thestimulationisonthehealthyvisualhemiﬁeld.

Lesion of the optic radiations. When the lesion affects theoptic radiations,in their parietal or temporal pathway,theresultinghemianopsiaischaracterisedbythe persistenceofthepupilcontractioninlightduringthe stim-ulationoftheblindvisualhemiﬁeld.

Lesion of thetemporal lobe. Inthe temporal syn-drome, the deficiency in the visual field is due to the impairment of the fibres that reach the lower lip of the calcarine fissure,resulting in a contralateralsuperior homonymousquadranopsia.Itdoesnotrespectthemacular vision.

Thesemiologyofimpairmenttothetemporallobeis com-plexandheterogeneous.Othersensoryandgnosticdisorders

maybeinvolved(corticaldeafnessbybilateralimpairment ofthetransversetemporalgyrus,auditiveagnosia,auditory illusionsandhallucinations,olfactorydisordersandbalance disorders),aphasia,orﬁnallyseizures.

When present, these seizures may be simple partial seizures.Complex visual hallucinationsandillusions of an aestheticnaturearethenpossible.

Lesion of theparietal lobe. The parietallobe syn-drome involves a deficiency in the visual field related to impairment of the fibres that reach the upper lip of the calcarine fissure. It consists of contralateral inferior homonymousquadranopsia,eveniftheoccurrenceof com-pletehomonymouslateralhemianopsiaisnotexceptional.

Theblinkreﬂexinresponsetoathreatmaybeabolished withparietallesionsevenwithouthemianopsia.Aconjugate eyedeviationispossiblebut,aboveall,theabolitionof con-tralateraloptokineticnystagmus(rapidmovementtowards theoppositesideofthelesion)shouldbesearchedfor.

The otherelements oftheparietal lobesyndromemay befound:objectivesensorydisordersand tactileagnosia, body image disorders (hemiasomatognosia, anosognosia, anosodiaphoriain caseof lesionoftheminorhemisphere; autotopoagnosia,digital agnosiaand right-left disorienta-tion,incaseofalesionofthedominanthemisphere),spatial agnosia, praxis disorders, language disorders and trophic disorders(amyotrophy).

Cortical lesionsof theoccipital lobe. Impairment ofthevisualcortex(Brodmannarea17)mayinduce homony-mouslateralhemianopsiawithrespectofthemacularvision incaseofaunilaterallesion,oradoublehemianopsiaincase ofbilateral lesions.Respect of macular vision is certainly duetoitsbilateralprojectiononthevisualcortex.

Homonymousquadranthemianopsia,whethersuperioror inferior,isfoundwithlocalizedlesionsofthestriatearea, onlydamaging oneofthelipsofthecalcarinefissure.The loss of vision affects the upperor lower half of the con-tralateralvisualhemifielddependingonwhetherthelesion involvesthelowerorupperlipofthecalcarinefissure. Bilat-eraland symmetricallesions of theupper or lower lip of thecalcarinefissurearemanifestedbyinferiororsuperior altitudinalhemianopsia.

Centralhomonymoushemianopicscotomashavealready beenmentionedandcorrespondtodeﬁcienciesinareasof thevisualﬁeld.

Finally,thedoublehemianopsisis possibleandis mani-festedbythe lossof peripheral visionin theentire visual ﬁeldwhilethemacularvisionmayberespected.

Bilateral impairmentofthevisual cortexisresponsible for corticalblindless. It creates a bilateral destructionof the Brodmann area 17. Here, the loss of vision is total andaffectsbothmacularvisionandperipheralvision. How-ever,thepupilreflexesaremaintained.Theblinkreflexin responsetoathreat,andoptokineticnystagmusare abol-ished,voluntaryandreflexeyemovementsaremaintained. Thedurationofsuchcorticalblindnessisvariable(transitory ordefinitive)andmaybeaccompaniedbyanosognosiawith recognitiondisordersandvisualhallucinations.

Theotherelementsoftheoccipitalsyndromeshouldbe searched for: dyschromatopsia and achromatopsia, visual illusionsorhallucinationsincase ofperi-andparastriated lesion, seizures with visual manifestations, visual agnosia (trouble recognizingobjects, people or graphic symbols),

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spatialagnosia,oculomotorandvisualmotordisordersand ﬁnallypsychicdisorders(transitoryorlastingﬁxation amne-sia).

The

causes

of

chiasmal

impairment

Chiasmallesionsmainlyresultfromsellartumors(pituitary adenomas) as well as suprasellar tumors (craniopharyn-gioma,meningiomaofthetubercleofsellaturcica,chiasm glioma).

Tumoral

causes

Infrachiasmaltumors

Pituitaryadenoma

The macroadenomas (diameter greater than 10mm) and the invasive adenomas may be responsible for chiasm compression in case of extrapituitary extension towards the suprasellar regions. Most cases are non-secretory as secreting adenomas are most often discovered at the microadenomastage.

Solid macroadenoma. The (non-necrotic) solid macroadenomas most often appear isointense to the cerebral cortex in T1, isointense or slightly hyperintense andheterogeneousinT2.Contrastinfusion determinesan intense and homogenous enhancement. The dura mater may be thickened and enhanced. GH adenomas often appearhypointenseinT2.

Therelationshipbetweentheupperpoleoftheadenoma ontheonehandandthechiasmandtheopticnervesonthe otherhandisbestassessedoncoronalsectionsinT2andT1 withoutinjection.Thechiasmalcompressionmaybe associ-atedwithahypersignaloftheoptictracts.Thishyperintense signaloftheoptictractsinT2atteststoanedemainduced by theblocked communication of the perivascular spaces of the nervous system with the subarachnoid space, due tothetumoral compression[1]. The macroadenomasmay beenclosed,therebypresentingregularlimits,orinvasive, wheretheir contoursareirregularandtheyoften present an extension to the cavernous sinus space. The intracav-ernousextensionmaybeassessedusingtheKnospmethod [2].

CT imaging, carried out in bone window, reveals an enlargedsellaturcica,anerosionofthedorsumsellae, thin-ning and depression of the sellar ﬂoor. Giant adenomas, ofteninvasive,developtowardsthesuprasellarregions,the cavernoussinusspaces,thesphenoidsinusandthe basisphe-noid.

Necroticmacroadenoma. Thenecrotic macroadeno-masoftenpresentahypointensecentralpartinT1,highly hyperintense in T2, with peripheral enhancement. These macroadenomaswithnecrosisarehemorrhagicin30%ofthe cases.InCTimaging,theyappearhypo-,iso-,orhyperdense withpossibilityofliquid-liquidlevel.

InMRI,thehemorrhagicnecrosisappears iso-or hyper-intense in T1. A liquid-liquid level may be identiﬁed on the sagittal and transverse sections, where the anterior part appears hyperintense in T2, and the posterior part hypointenseinT2.The enhancementis usuallyperipheral andannular.

Thesuddenhemorrhagicnecrosisofthemacroadenoma may be associated with a clinical picture of pituitary apoplexy. In this case, the T1 hyperintensity is generally absent and the T2 gradient echo sequence alone reveals intratumoralhyperintenseareas.Thehemorrhagicnecrosis is identiﬁedinthe acutephasebyCTscaninthe formof intratumoralareasofhyperdensities.Thispituitaryapoplexy is often associated with the thickening of the sphenoid sinus.

Thedifferentialdiagnosisofmacroadenomawith hemor-rhagicnecrosisshouldbecarriedoutwith:

• Rathke’scleftcyst:mediantopography,absenceof con-trastenhancement,absenceofliquid-liquidlevel; • acysticcraniopharyngioma;

• agiantthromboticaneurysm.

Other intrasellar lesions responsible for chiasmal compression

Fleshy lesions enhanced by the injection of con-trastproduct:hypophysitis. Thegroupofhypophysitis includesthreeentities:

• lymphocytic adenohypophysitis (touches the anterior pituitary)correspondstoalymphocytic,plasmocyticand eosinophilicinﬁltrationofthepituitaryandthepituitary stalk, with progressive appearance of a ﬁbrosis. It is especiallyfoundin thepost-partumwoman.The height of the gland seems to be higher, of iso- or hypoitense signal in T1 and hyperintense in T2, with intense enhancement extending to the thickened pituitary stalk;

• lympocyticinfudibulo-neurohypophysitisreachesthe pos-teriorpituitary,pituitarystalkandthehypothalamus. It isclinicallymanifestedbydiabetesinsipidus;

• giantcellgranulomateoushypophysitis.

Otherﬂeshyandenhanced pituitarylesionsmayﬁnally lead to a compression of the intracranial portion of the opticchiasmortheopticnerve:pituitarymetastases, ger-minomes,choristomas,chordomas,giantarterialaneurysms withthrombosis.

Liquid lesions. Certain liquid pituitary lesions may causecompressionofthechiasmalvisualpathways: intrasel-lar subarachnoid cysts, Rathke’s cleft cyst, intrasellar craniopharyngiomasandmorerarelyabscesses,epidermoid cystsorcolloidcysts.

Suprachiasmalandprechiasmaltumors

Fifteento20%ofallbraintumorsarelocatedintheregion ofthechiasm,including50%pituitaryadenomas,25% cran-iopharyngiomas,10%meningiomas,and5%gliomas.

Meningiomas

They account for 20% of all intracranial tumors. When they are responsible for clinical signs, the meningiomas may cause visual signs, inducing unilateral central sco-tomaormonocularblindness,bycompressionofoneofthe anterioranglesofthechiasm.Onlymeningiomaswith pos-teriordevelopmentinduceachiasmalsyndrome.Theyare sometimesresponsibleforaFoster-Kennedysyndrome, asso-ciatingunilateralopticatrophyandcontralateralpapillary edema, sometimes with anosmiaon the side ofthe optic atrophy.Itisaboveallprovokedbymeningiomaofthesmall

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wing of the sphenoid, olfactory meningiomas and frontal tumors.

Craniopharyngiomas

The craniopharyngiomas are embryonal tumors usually foundin thechild.Thesetumorspresentafirstfrequency peakinthechildandasecondpeakintheadultafterthe ageof50.Inmostcases,theyarelocatedinthesuprasellar regionandmorerarelywithinthethird ventricle,or even intrasellar.Adamantinecraniopharyngiomasaremost com-moninthechild,associatingfleshyandcysticcomponents, andareoftencalcified.InMRI,thecysticportionmostoften appears hyperintense in T2 and FLAIR, of variable signal inT1.The wallsofthe cystmaybeenhancedafter injec-tion.Afterinjection,thefleshycomponentpresentsintense enhancement.Papillarycraniopharyngiomasaremore com-monintheadult.Theyaresolidorsolidandcysticandmost oftenwithoutcalcification(Fig.2).

Intrachiasmaltumor:opticchiasmglioma

Theopticchiasmgliomaarisesfromtheproliferationof neu-rologicaltissuefoundinnerveﬁbres.Itisfoundinthechild andissometimesassociatedwithtypeIneuroﬁbromatosis. The association of a chiasmalsyndrome, bilateral optical atrophyandsignsofintracranialhypertensionisindicative ofthediagnosis(Fig.3).

Vascular

causes

Aneurysms of the inner carotid artery and the circle of Willis(supraclinoidaneurysms,oftheanterior communicat-ingarteryor anteriorcerebralartery)maycausechiasmal compression.

Post-traumatic

chiasmal

syndrome

Itusually followsserious cranial traumaswithfracture of thelowerlevelofthebaseoftheskull.

Figure2. Craniopharyngiomaina51-year-oldpatient.Thisnodularlesioniscentredontheinfundibulumofthethirdventricle,inisosignal T1(a),heterogeneoushypersignalT2(bande),hypersignalFLAIR(c),enhancedinahomogenousandmassivewayaftertheinjectionof gadoliniumsalts(dandf).ItcomesintocontactwiththeposteriorsideoftheopticchiasmthatpresentsaT2hypersignal(g,tipofblack arrow).NarrowingoftheretrochiasmalvisualpathwaysisnotedintheformofaT2hypersignaloftheoptictracts(h,tipofwhitearrows). TheCT-scandoesnotrevealanycalciﬁcations(i).

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Figure3. Mesencephalo-diencephalicgliomaina 5-year-old.Mesencephalicgliomatousinﬁltration inhypersignalT2,hyposignalT1, presentingseveralenhancementsafterinjection,extendingtothediencephalicregion,theoptictracts,theopticchiasmandtheprechiasmal portionofbothopticnerves.

The

causes

of

retrochiasmal

impairment

Theoptictractmaybedamagedbyatumor,athrombosisof theanteriorchoroidalartery,orananeurysmoftheinternal carotidartery.

Lesions of the optic radiations are actually included in those of the parietal and temporal lobes, strokes and tumors being most common. Transitory deﬁciencies may followischemiainthevertebrobasilarterritoryorabasilar migraine.

Tumoral

The symptoms most often seen with brain tumors are headache,neurologicaldeﬁciency,seizures,orvisual symp-tomssuchasocularmotordisorders,colourvisiondisorders, papillaryedema orvisualﬁelddisorders.Thetumorsmost oftenseen areof neuroepithelialorigin(pilocytic astrocy-tomas,low-gradegliomas,glioblastomas),andofsecondary origin(intracranialmetastases).

Low-gradegliomas

Low-grade gliomas affect the subject between the age of 20 and 50. Beyond this age, high-grade gliomas are most often seen. They mainly involve the fronto-temporo-insular regions, and preferentially diffuse along the ﬁbres of white matter [3]. This accounts for the impairment of the optic radiations. Low-grade gliomas aretherebyresponsibleforhomonymouslateral hemianop-sias where the installation is slow, over several months (Fig.4).

These lesions appear in hyperintensity on T2 and FLAIR sequences, in T1hypointensity, notenhanced after injection, the increase in the cerebral blood volume (rCBVmax) if present, is moderate and the spectroscopy ﬁnds a moderate increase in choline, a drop in N-acetyl-aspartate (NAA), without a lipid or lactate peak. The borders of the lesions are poorly deﬁned and the mass effect is moderate. The growth of the tumor is slow, with an average diameter of under 4mm per year [4].

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Figure4. Homonymouslateralhemianopsiarightina31-year-oldpatient,revealingapilocyticastrocytomaoftheleftoptictract.The intraparenchymatouslesioninisosignalT1,hypersignalFLAIRpresentsanenhancementaftertheinjectionofgadoliniumsalts(c,tipof blackarrow).ItinducesahypersignaloftheleftoptictractontheFLAIRsequences(b,tipofwhitearrow)andT2(e,f,g,blackarrows).

High-gradegliomas

Theanaplasticgliomasandglioblastomasarethemost com-monhigh-gradegliomas.Especiallyfoundintheelderly,they allhaveanunfavorableprognosis.

The kinetics of tumor growth is superior than that oflow-grade gliomas,provoking fast-evolving neurological deﬁciencies.Apapillaryedemaofthefundusoculimaybe found,attestingtointracranialhypertension.Theselesions are heterogeneous, comprising hemorrhagic or necrotic rearrangements,enhancedbytheintravenousinjectionina nodularorannularfashion,andaresurroundedbyanedema. Inﬁrstpassageperfusion,therCBVmaxisincreased.In pro-tonspectroscopy,acholinepeakofgreatamplitude,adrop intheNAA, thepresence oflipidsand/or lactateindicate malignancy.

Vascular

Anysudden deﬁciency in thevisual ﬁeld,whether regres-sive or not, should raisethe possibility of an ischemic or hemorrhagic vascular origin. Once the vascular origin is eliminated,the main cause of transient visual disorder is migraine.

Ischemicvasculardisease

6Anischemicstrokeintheposteriorcerebralarteriesmaybe thecauseofasuddendeﬁciencyinthevisualﬁeld(Fig.5). Cerebralvenousthrombosis

Hemianopsis is found in 4% of all cerebral venous throm-boses,mostoftenattestingtodistressofthevisualcortex

[5].Inthesub-acutephase,thethrombusappearsin hyper-signalT1andT2.Venousinfarctionsarenotsystematizedto anarterialvascularterritory,ofcortical-subcortical topog-raphyandarevisibleintheformofsometimeshemorrhagic T2andFLAIRhyperintensities.Thepresenceofan endolu-minaldefect iscorroboratedby avenousangiography-MRI sequence.

Migrainewithvisualaura

Ascintillatingscotomalastingfor 5minutesto1hourmay sometimes precede a migraine. During the episode, the MRIismost oftennormal.However,theperfusionimaging

[6,7] and the T1 imaging after the injection of con-trast agent may be perturbed. The anomalies found are dilations of the pial vessels [8] on the one hand and, in ﬁrst passage perfusion, an elongation of the mean time of transit and the time until the crest value on the other hand, while the cerebral blood volume and the blood ﬂow remain normal. These perfusion disorders may be found in migraine with [9] or without aura [10]. Migrainesaresometimescomplicatedbyischemicaccidents [11].

Inﬂammatory

Theotherinﬂammatorycausesgroupdifferententities.Only twoofthemwillbementionedhere.

Multiplesclerosis

Multiplesclerosisisacommondisease,affectingtheyoung subject.Thediseaseevolvesinstages,theoriginofamajor

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Figure5. Ischemicstrokeintheareaoftheposteriorcerebralartery.Homonymousrightlateralhemianopsia,righthypoesthesiaand righthemibodyweaknessinan82-year-oldpatientrevealinganischemicstrokeintheleftposteriorcerebralartery.Thestrokeisvisiblein thediffusionimaginginhypersignalb=1000s/mm2_(a),_in_restriction_on_the_ADC_map_(b),_in_slight_hypersignal_FLAIR_(c).

disability.The lesionsaretypicallymultifocal,the seatof inﬂammatoryphenomena,demyelinationandthenagliosis. Themostcommonseatsinvolvetheperiventricularregions, thecorpus callosum,theopticpathwaysand inparticular theoptic chiasmor opticnerves, the brain stem andthe cerebellumandﬁnallythespinalcord.Thelesions suggest-ingthediagnosisareofovoidmorphology,callosofugal,with thegreataxisperpendiculartotheventricles.Thelocationis preferentiallyperiventricularpredominantaroundthe tem-poralhornsofthelateralventricles,orsub-tentorial(where theymayinvolvethespinalcord).MRIprovidesinformation abouttheactivityofthedisease.Theenhancementofthe lesionsofactivemultiplesclerosisisnodular,annularorin anopenring.

Sarcoidosis

Sarcoidosisisa multisystemicgranulomatosis. The impair-ment of the central nervous system results from an inﬁltration of the meningeal spaces that then diffuse to the cranial and spinal nerves, to the vessels, to the hypothalamus-pituitary axis or to the brain parenchyma. Theleptomeningeal impairmentis manifestedinthe form of a contrastenhancementaccording to the reliefof the brain.Theimpairmentof theduramater isrevealed bya diffuseorfocalthickeningofthemeninges. Hypothalamic-pituitaryandchiasmalimpairmentisclassic,byextensionof themeningealgranulomasofthesupracellarcisterns.The parenchymatousimpairmentmaytaketheappearanceofa pseudo-tumorbycoalescenceofthegranulomathatappears inhyposignalT2,enhancedafterinjection.Thecoexistence ofleptomeningealandparenchymateousenhancement sug-gests the diagnosis (Fig. 6). The edema and mass effect areingeneral discrete.Hydrocephalyis oftenassociated. Anomaliesofthewhitematter,intheformofT2 hypersig-nals,are common. Non-speciﬁc, theypredominate in the regions and ventricles, and may affect the deep regions (basal ganglia, brain stem), and are not enhanced after injection.

Degenerative:

posterior

cortical

atrophy

(Benson’s

syndrome)

Posterior cortical atrophy (PCA) is a neurodegenerative disease in the young subject, before 60years of age, associating visual disorders of progressive evolution with atrophyoftheposteriorcorticalregions.Itishistologically related to Alzheimer’s disease, with an atypical distribu-tion sparing the hippocampus and predominating in the parieto-occipitalregions.Thestructuralimagingclassically discovers focal, parieto-occipital corticalatrophy and the functional imaging (scintigraphy) demonstrates hypoper-fusion and hypometabolism of the same areas (Fig. 7) [12—15].

Toxic

and

metabolic

Hypoglycemia

The brainlesionsinduced byhypoglycemiamayaffectthe braincortex,thehippocampus,thespleniumofthecorpus callosum,theinnercapsules,andthewhitebrainmatter.In theMRI,theyappearintheformofhyperintensitiesin diffu-sionimaging,inrestrictiononthemappingoftheapparent diffusioncoefﬁcient(Fig.8),andaresometimesreversible. Statusepilepticus

A prolonged state of epilepsy (statusepilepticus) may be thecauseofcortical(Fig.9),orhippocampusimpairment. The atypical forms involvethe centralgrey nuclei (thala-mus,striatum,cerebellum).The diffusionimagingis most oftensuggestive,revealinganon-systematizedhypersignal atanarterialterritory,wheretheevolutionoccurstowards restitutionadintegrum.

Posteriorreversibleencephalopathy

Posteriorreversibleencephalopathycreates aclinical pic-ture associating seizures, cortical blindness, headache,

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Figure6. Neurosarcoidosisina47-year-oldpatient.Multiplenodular,confluent,cortico-pialenhancementsoftheSylvianfissuresaswell astheedgesofthecalcarinefissure(tipofwhitearrows).

nauseaandvomiting, awarenessdisorders,arterial hyper-tension and kidney failure. Most often a favouring circumstance is found (eclampsia, transfusion, drug tox-icity). The signal modiﬁcations most often involve the posterior regions, in a bilateral and symmetrical manner withavasogenicedemaonthediffusionimaging(Fig.10). Theyclassicallyregressintentoﬁfteendayswithout seque-lae. The atypicalforms involve thecentral greynuclei or posteriorfossa,andmaybehemorrhagic.

Infectious:

prion

infections

Prion infections (sub-acute, transmissible spongiform encephalopathies or Creutzfedlt-Jakob’s disease) may be

responsible for posterior cortical impairment resulting in visualsymptomatologysuch ascortical blindnessor visual agnosia.Certaintypesofcorticalimpairmentare predom-inantly found in the sporadic form of the disease, often associatedwithbilateralstriatalimpairment.The topogra-phyof these corticalhypersignalsis variable,fully visible in FLAIR [16], and ondiffusion imaging (the ADC is most often reduced) [17,18], without mass effect or enhance-ment (Fig. 11). The Heidenhan form, also sporadic [19], involves an elective impairment of the parieto-occipital cortex.

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Figure7. Visualandcognitivedisordersina62-year-oldpatientrevealingposteriorcorticalatrophy(Benson’ssyndrome).TheMRIﬁnds focal,parieto-occipitalatrophycreatingabilateralsulcalenlargement(atoe).TheperfusionCT-scanrevealsparieto-occipitalhypoperfusion (fandg).

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Figure8. Sixty-eight-year-old,diabeticpatient.Thedaybeforehospitalization,spontaneouslyregressivephasicdisorderfollowedby theinstallationofacomathefollowingday.Uponcarebytheemergencymedicalaid,capillaryglycaemiaat0.14g/L.Signalanomaliesof theoccipitalforcepsandthebilateraloccipitalsubcorticalwhitematterrelatedtoseverehypoglycemia,inhypersignalonthediffusion imagingb=1000s/mm2_(a,_tip_of_arrows),_in_restriction_on_the_ADC_mapping_(b),_without_translation_on_the_FLAIR_sequence.

Figure9. Seventy-six-year-oldpatientwithapasthistoryofleftposteriorischemicstroke,hospitalizedforpartialstatusepilepticus. TheinitialMRIfoundtheoldischemicsequence(tipofarrow,d)andidentiﬁedthesignalanomaliesoftheleftoccipitalribbon(white arrows),inhypersignalonthediffusionimagingb=1000s/mm2(a),increaseintheapparentdiffusioncoefﬁcient(b),FLAIRhypersignal(c). Thefollow-upMRIcarriedout10daysaftertheepisoderevealedaregressionofthepreviouslydescribedlesions,inrelationtothestatus epilepticus.

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Figure10. Sixty-ﬁve-year-oldpatienttreatedbychemotherapyforapulmonaryadenocarcinoma.Emergencyhospitalizationfor general-izedseizuresandcorticalblindness.AnemergencyMRIrevealedposterior,bilateralandasymmetricalcortico-subcorticalsignalanomalies inT2(a)andFLAIR(b),inhyposignalT1(c)presentingseveralrupturesofthehemato-encephalicbarrieraftertheinjectionofgadolinium salts(d),inhypersignalonthediffusionimaging(e),attestingtoavasogenicedemaontheADCmap(f).ThecontrolMRIafter1month revealstheadintegrumrestorationontheT2(g)andFLAIR(h)sequences.

Figure11. Seventy-two-year-oldpatientpresentingvisual disordersfor thelast3months,thenprogressive,rapidlyinstalledataxia, followedbyabehaviourdisorderandextrapyramidalimpairment.Signal anomalyofthecorticalribboninhypersignalonthediffusion imaging(b=1000s/mm2)(a),inrestrictionontheADCmapping(b),inFLAIRhypersignal(c)andT2(d).

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Conclusion

The exploration of the chiasmal and retrochiasmal visual pathwaysisbasedonMRI.Abitemporalhemianopsisshould calltominda lesionofthe opticchiasmwhile a homony-mouslateralhemianopsisshouldleadtoasearchforalesion oftheretrochiasmalvisualpathways.Thecausesof chias-malimpairmentaremainlytumoral.Inviewofaseemingly retrochiasmal impairment, the tumoral or vascular origin shouldbesuspectedinitially.

TAKE-HOMEMESSAGES

• Theexploration of thechiasmaland retrochiasmal visual pathways is based on magnetic resonance imaging.

• A bitemporal hemianopsis should suggest a lesion of the optic chiasm while a homonymous lateral hemianopsisshouldleadtoa searchforalesionof theretrochiasmalvisualpathways.

• The causes of chiasmal impairment are mainly tumoral.

Disclosure

of

interest

Theauthorsdeclarethattheyhavenoconﬂictsofinterest concerningthisarticle.

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