PathologiesPathologies of of postchiasmaticpostchiasmatic visual visual pathwayspathways and visual and visual

cortexcortex

Optic radiation: anatomyOptic radiation: anatomy

••PathologiesPathologies of the of the postchiamsaticpostchiamsatic visual visual pathwayspathways and visual and visual cortexcortex

••CharacterizedCharacterized byby homonymoushomonymous hemianopsiahemianopsia. .

••ThisThis visual visual fieldfield defectdefect maymay bebe the the consequenceconsequence of of lesionslesions of the of the opticoptic tracttract, ,

opticoptic radiationradiation or visual or visual cortexcortex..

OpticOptic tracttractRare Rare byby itselfitself. .

••LesionsLesions involvinginvolving the the opticoptic tracttract are are generallygenerally largelarge, and include the , and include the opticoptic nervenerve and and chiasmchiasm. .

••EtiologyEtiology••pituitarypituitary tumorstumors, , ••aneurisms, aneurisms, ••craniopharyngioma craniopharyngioma ••demyelinating lesions.demyelinating lesions.

••The presence of The presence of neurologicalneurological symptomssymptoms isis notnot determinantdeterminant forfor the the diagnosisdiagnosis..

••ClinicalClinical characteristicscharacteristics suggestingsuggesting the the involvementinvolvement of the of the opticoptic tracttract include: include: ••homonymoushomonymous hemianopsiahemianopsia, complete or , complete or incongruousincongruous, , ••relative afferent pupillary defect (RAPD) in the relative afferent pupillary defect (RAPD) in the eyeeye withwith the the defectdefect involvinginvolving the the temporaltemporalhemifieldhemifield ((contralateralcontralateral toto the the lesionlesion) ) ••sectorialsectorial opticoptic atrophyatrophy. .

Optic tract field defectOptic tract field defect

••The The laterallateral geniculategeniculate nucleusnucleus rappresentsrappresents the the terminationtermination of of nervenerve fibersfibers whichwhich originate originate fromfrom the retina, and the retina, and isis generallygenerallyinvolvedinvolved in in lesionslesions of the of the temporaltemporal lobelobe. .

••-- homonymoushomonymous conecone--shapedshaped visual visual fieldfield defectsdefects

••The The fibersfibers constitutingconstituting the the opticoptic radiationradiation originate originate fromfrom the the superoposteriorsuperoposterior portionportion of the of the laterallateral geniculategeniculate nucleusnucleus. The first . The first portionportion of the of the opticoptic radiationradiation, , mademade up of up of inferiorinferior fibersfibers((correspondingcorresponding toto the the superiorsuperior visual visual fieldfield), ), runrun anteriorlyanteriorly infrontinfrontof the of the temporaltemporal hornhorn of the of the laterallateral ventricleventricle formingforming a a looploop knownknownasas ««MeyerMeyer’’s s looploop»».. In In thisthis site the visual site the visual fibersfibers are are adjacentadjacent toto the the internalinternal capsule and the capsule and the temporaltemporal isthmusisthmus. . LesionsLesions of the of the temporaltemporalopticoptic radiationsradiations determinedetermine the the onsetonset of a of a homonymoushomonymous defectdefectprevalentlyprevalently involvinginvolving the the superiorsuperior quadrantsquadrants. .

Optic radiation: anatomyOptic radiation: anatomy

••Visual Visual defectsdefects:: maymay involveinvolve a a sectorsector of the of the superiorsuperior quadrantquadrant and and havehave verticalverticalmarginsmargins whichwhich exactlyexactly correspondcorrespond toto the the verticalvertical meridianmeridian, , whilewhile horizontalhorizontalmarginsmargins are are ratherrather irregularirregular and and undefinedundefined. .

••The The defectdefect maymay obviouslyobviously extendextend towardstowards the the inferiorinferior quadrantquadrant, , causingcausing a a hemianopsiahemianopsia. .

••AssociatedAssociated neurologicalneurological symptomssymptoms::••hemiplegia hemiplegia ••hemianaestesia, hemianaestesia, ••aphasia (when the dominant hemisphere is involved), aphasia (when the dominant hemisphere is involved), ••agnosia, hallucinations (sight, agnosia, hallucinations (sight, hearinghearing, , smellsmell, taste), , taste), ••epilespsyepilespsy and and behaviouralbehavioural changeschanges. .

••PathologiesPathologies causingcausing lesionslesions of the of the temporaltemporal lobelobe maymay havehave neoplastic (neoplastic (gliomaglioma, ,

meningiomameningioma etc.), etc.), vascularvascular and and rarelyrarely inflammatoryinflammatory nature.nature.

AsAs the the fibersfibers procede procede towardstowards the the calcarinecalcarine sulcussulcus theythey becomebecomemore and more more and more segregatedsegregated. .

ThusThus lesionslesions of the of the parietalparietal lobelobe cause a more cause a more congruouscongruoushomonymoushomonymous defectdefect thanthan temporaltemporal lobelobe lesionslesions. .

AssociatedAssociated neurologicalneurological symptomssymptoms: : ••severe severe hemianaesthesiahemianaesthesia••hemiparesishemiparesis••alexiaalexia••asomatognosiaasomatognosiaTheseThese are are generallygenerally so severe so severe thatthat in some in some casescases do do notnot allowallow the the carryingcarrying out of the visual out of the visual fieldfield examinationexamination. .

••In In additionaddition itit isis possiblepossible toto observeobserve::••tonictonic deviationdeviation of the of the eyeseyes toto the the oppositeopposite side of the side of the parietalparietallesionlesion, , tryingtrying toto reproducereproduce BellBell’’s s phenomenonphenomenon. . ••reductionreduction or or absenceabsence of the of the opticooptico--kinetickinetic nystagmusnystagmus whenwhenrotatingrotating the the stimulusstimulus toto the side of the the side of the lesionlesion. .

The The mostmost common common causescauses of a of a parietalparietal lobelobe syndromesyndrome are:are:••tumors tumors ••vascular lesionsvascular lesions••infectious pathologies (infectious pathologies (coccidiomycosiscoccidiomycosis, , candidiasiscandidiasis etc.). etc.).

In In subjectssubjects withwith a a parietalparietal lobelobe syndromesyndrome, visual , visual acuityacuity, , pupillarypupillaryreactionsreactions and and opticoptic disc disc aspectsaspects are are normalnormal..

Temporal lobe defectTemporal lobe defect

CT scan of CT scan of parietal lobe parietal lobe tumour with tumour with

loss of loss of optokineticoptokineticnystagmusnystagmus

••OccipitalOccipital lobelobe••mostmost of the visual of the visual cortexcortex lieslies deeplydeeply in the in the

medialmedial portionportion of the of the occipitaloccipital lobelobe;;

••macular macular projectionsprojections havehave a a posterioposterio--laterallateralposition in the position in the occipitaloccipital lobelobe;;

••the the peripheralperipheral visual visual fieldfield hashas the the mostmostanterioranterior projectionprojection alongalong the the

interhemisphericinterhemispheric fissurefissure..

••ClinicalClinical characteristicscharacteristics of of occipitaloccipital lesionslesions include:include:••CongruityCongruity of the of the homonymoushomonymous visual visual fieldfield defectdefect. . ••DefectsDefects of the visual of the visual fieldfield withwith occipitaloccipital originorigin maymaypresentpresent asas••a) a) congruouscongruous scotoma, scotoma, ••b) b) quadrantanopsiaquadrantanopsia, complete , complete hemianopsiahemianopsia or or homonymoushomonymous bilateralbilateral defectsdefects. . ••c) Of c) Of paricularparicular interest, and interest, and typicaltypical of of occipitaloccipital lesionslesions, , isis the the phenomenonphenomenon of macular of macular sparingsparing rappresentedrappresented bybythe the conservationconservation of the of the centralcentral 55-- 10 10 degreesdegrees of the of the visual visual fieldfield on the side of the on the side of the hemianopsiahemianopsia. .

•This is explained by the double vascularizationof the occipital pole by the middle and posterior

cerebral arteries and by the possibility of a bilateral macular representation. Some patientsaffected by cortical lesions show a static-kinetic

dissociation (Riddoch’s phenomenon), and preceive movements of a stimulus without

perceiving its shape. This phenomenon is notconsidered pathognomonic of occipital lesions asit occurs in the presence of other pathologies of

the visual pathways.

Circle of WillisCircle of Willis

Blood supply of the optic radiationsBlood supply of the optic radiations

Diagram of the visual cortexDiagram of the visual cortex

Occipital lobe defectOccipital lobe defect

CT scan of occipital infarctCT scan of occipital infarct

PathologiesPathologies involvinginvolving the the occipitaloccipital lobelobe are are generallygenerally of of vascularvascular((adultadult ageage) or neoplastic () or neoplastic (gliomaglioma and and meningiomameningioma) ) originorigin

••AssociatedAssociated symptomssymptoms::••headache, nausea and headache, nausea and vomitingvomiting, , papilloedemapapilloedema and and signssigns of of cerebellarcerebellar involvementinvolvement

••SinceSince the the occipitaloccipital lobelobe isis connectedconnected toto secondarysecondary associative associative areasareas and the and the twotwo hemisphereshemispheres are are connectedconnected through the through the spleniumspleniumof the corpus of the corpus callosumcallosum, the , the involvementinvolvement of of thesethese structuresstructuresdeterminesdetermines the the onsetonset of of characteristiccharacteristic clinicalclinical manifestationsmanifestations. . ForForexampleexample the the extensonextenson of the of the lesionlesion towardstowards the the occipitaloccipital lobelobe and and spleniumsplenium causescauses alexiaalexia withoutwithout agraphiaagraphia, , associatedassociated toto rightrighthomonymoushomonymous hemianopsiahemianopsia..

••CorticalCortical blindnessblindness isis the the consequenceconsequence of of bilateralbilateral lesionslesions of of the the retrogeniculateretrogeniculate visual visual pathwayspathways

or visual or visual cortexcortex. . ••LesionsLesions generallygenerally developdevelopsimultaneouslysimultaneously and and mustmust bebeanatomicallyanatomically symmetricalsymmetrical..

••EtiologyEtiology::••vascularvascular nature.nature.••infectious nature, infectious nature, ••toxic (eg. toxic (eg. CarbonCarbon monoxidemonoxide) ) ••severe severe hypoxichypoxic or or embolicembolic phenomenaphenomena duringduring cardiaccardiac surgerysurgery. .

••ClinicalClinical characteristicscharacteristics of of corticalcortical blindnessblindness include complete include complete losslossof visual of visual functionfunction, , normalnormal pupillarypupillary reflexesreflexes, , normalnormal ocularocular fundusfundusand and normalnormal ocularocular motilitymotility. .

••SubjectsSubjects withwith bilateralbilateral blindnessblindness maymay referrefer visual visual hallucinationshallucinations. .

••Visual Visual hallucinationshallucinations are are defineddefined asas the visual the visual perceptionperception of of somethingsomething thatthat the the subjectsubject isis convincedconvinced hehe//sheshe seessees, , butbut whichwhich isis notnot

visiblevisible toto otherother people people presentpresent in the in the samesame room. room.

••TheyThey are are presentpresent in in subjectssubjects withwith neurologicalneurological and visual and visual defectsdefects. . Visual Visual hallucinationshallucinations maymay alsoalso bebe causedcaused byby::••drugsdrugs suchsuch asas amphetaminesamphetamines, , ••antianti--Parkinson drugs, Parkinson drugs, ••antidepressives, antidepressives, ••cardiovascular drugs and numerous antibiotics cardiovascular drugs and numerous antibiotics ••Some forms of Some forms of migrainemigraine may may bebe responsibleresponsible forfor visual visual hallucinationshallucinations. The . The differentialdifferential diagnosisdiagnosis isis howeverhowever facilitatedfacilitated byby the the presencepresence of of characteristiccharacteristic scintillatingscintillating or or fortifyingfortifying scotomasscotomas in in migrainemigraine. .

••Visual agnosia Visual agnosia isis a rare a rare disorderdisorder of the of the superiorsuperior corticalcorticalfunctionsfunctions in in whichwhich patientspatients are are notnot ableable toto recognizerecognizefamiliarfamiliar objectsobjects, , notwithstandingnotwithstanding normalnormal visual visual acuityacuity. . OnlyOnly utilizingutilizing hearinghearing and and smellingsmelling are are patientspatients ableable totoidentifyidentify the the objectobject. .

••The The originorigin of of thisthis disorderdisorder isis the the presencepresence of of bilateralbilateraloccipitaloccipital or or parietoparieto--occipitaloccipital lesionslesions. Some . Some formsforms of of agnosia are agnosia are characterizedcharacterized byby the the inabilityinability toto recognizerecognizefamiliarfamiliar facesfaces ((prosopagnosiaprosopagnosia)) untiluntil the the knownknown personpersonspeaksspeaks. The . The latterlatter seemsseems toto bebe the the consequenceconsequence of of bilateralbilateral lesionslesions of the of the inferiorinferior occipitaloccipital--temporaltemporaljunctionjunction. .