Anatomy and Manifestations of Visual Pathway Rae L d e Be sion hbehan s i , MD, FRCSC
Visual pathways Prechiasmal: optic nerve-chism. Retrochiasmal: optic tract, the optic radiations, and the occipital cortex.
Optic Neuropathy Unilateral. RAPD, dyschromatopsia. Central, cecocentral. Arcuate (superior, rior inferior). Altitudinal. Generalized decrease in sensitivity. y
Optic Nerve Axoplasmic transport : clearance of expired organelles, structural maintainance, and energy requirements. Interruption of axoplasmic transport : ischemia, compression, inflammation. Orthograde axonal transport : away from the cell body LGN. N Retrograde axonal transport : toward cell body. y
Intra-orbital Optic Nerve Myelination (oligodendrocytes). 20-30 mm Long. Axons: mylein and glial cell (metabolic support at the nodes of Ranvier).
Intracranalicular Optic Nerve Within the two bases of the LWS L . Medial wall of canal forms lateral wall of sphenoid sinus (can be absent !). Within canal : meninges, ophthalmic artery and sympathetic plexus. 10 mm length. Tight space ! Internal carotid artery. y
Intracranial Optic Nerve Leaves the cranial end of the optic canal (medially, dially backwards, upwards). 4-15 m (depending on the position of chiasm). Upward 45 degree-angle. Anterior cerebral and anterior comunicating artery lie superior. r
Arcuate Early Late
Chiasm Floor of the third ventricle. 5-10 mm above the diphragma sella and the hypophysis cerebri. 12mm wide, 8mm A-P , 4 mm thick. Important relations: 3rd ventricle, hypothalmus, pituitary stalk, sella, dorsum sellam anterior and posterior clinoid processes, cavernous sinus. Nasal fibers cross ; temporal fibers do not (53:47). Wilband’s knee.
Chiasmal syndrome Unilateral or Bilateral. Junctional scotoma. Bitemporal defect. Homonymous defects. Diplopia (III, IV, V VI cranial nerves or hemi-field slide phenomenon).
Traq T uair scotoma A monocular hemianopic visual field loss is referred to as junctional scotoma of Traquair. uair
Posterior Chiasmal Syndrome 90% of chiasmal fibers have macular origin (superior and posterior portions of chiasm).
Band atrophy From (Practical viewing of the optic disk)
Retrochiasmal Visual Pathway Lesions Bilateral. Homonymous. Complete or incomplete. Congrous or incongrous.
Optic Tr T act Lesions Contralateral RAPD (may be an ipsilateral afferent pupillary defect if a concomitant optic neuropathy exists) A specific form of optic atrophy (band atrophy) due to the involvement of nasal fibers (temporal field) in the contralateral eye An incongruous homonymous hemianopsia.
Optic Tra ic T ct Travel around the cerebral peduncles at dorsal midbrain. Divides into lateral root LGN , and a smaller medial root pretectal area (pupillary light reflex)
Optic Tra ic T ct
Optic tract lesions Band Atrophy due to compression Hoyt Wf, of the left tract. Kommerell G. Der fundus oculi bei homonyermeinaopia. Klin Monatsblat Augenheilkd 1973; 162: 456-464)
Lateral Geniculate Bodies Lesions Part of the thalamus. Hilum, medial and lateral horn. Six laminae (layers 1-6), crossed fibers 1,4,6 , uncrossed fibers 2,3,5. medial lateral
LGB Upper quadrant medial aspect of LGN, N Lower quadrant lateral aspect of LGN. N Macular fibers central wedge of LGN. N
LGB 1- Optic nerve 2- Optic chiasma 3- Optic tract 4- Lateral geniculate body 5- Optic radiation 6- Visual cortex 7-Superior colliculus of the midbrain 8- Putamen 9- Long association bundle - inferior occipitofrontal fasciculus 10- Pulvinar of the thalamus 11-Calcarine fissure 12- Posteroinferior horn of the lateral ventricle
Lateral Geniculate Nucleus Upper quadrant medial aspect of LGN, N Lower quadrant lateral aspect of LGN. N Macular fibers central wedge of LGN. N Layers 1,2: magnocellular. lar (motion) Layers 3-6: Parvocellular. llular (color)
An incongruous wedge defect tending to point toward fixation (spears to fixation)
Usually complete or nearly complete field homonymous defect.
Optic radiations Nerve fibers bundles with cell bodies in the LGN. N Loop of Meyers (around temporal and inferior horn of LV L ). Inferior fascicle. Superior fascicle.
Optic radiations Inferior fascicle anterior pole of temporal lobe lower calcarine cortex. Superior fascicle parietal lobe upper calacrine cortex.
“Pie on the floor” homonynous defect.
Associated neurologic signs and symptoms (e.g., hemiplegia, hemisensory loss, visual, or neglect) may be present .
Anterior temporal lobe “Pie on the sky” homonymous. Often incongrous. Seizures, hemiparesis, hemianesthesia. Contralateral neglect (Non- dominant). Aphasia (Dominant).
Optic radiation lesions
Occipital lobe lesions
Primary Visual Cortex Optic radiations terminate in layer 4 (lamina granularis) . Layer 4 is divided into 3 layers (Line of Gennari). P- P cells 4C bets. M-cells 4C alpha. Macular fibers – terminate posterioly. y Lateral fibes – termriate anteriorly. y
Primary Visual Cortex ( V1) Upper bank and lower bank (Calcarine fissure). Inferior visual filed (upper bank) , Superior visual field (lower bank). Macular projections represented by 50%-60% of the area of the calcarine cortex. Occipital tip is for foveal vision.
Occipital cortex lesions Isolated (i.e ., without other neurologic deficit)ز Congruous. Paracentral or peripheral. Complete or incomplete Macular involvement or macular sparing of the central 5 degrees may occur (occipital pole involvement).
Occipital cortex lesions
Visual cortex -Anterior striate cortex (8%-10%) is monocularly innervated (temporal crecsent of contralateral eye).
Visual association areas
Visual Association Areas V2: input from V1. V3: sends info to basal ganglia and midbrain. V3a: perceive motion and direction. V4 : (lingual and fusiform gyrus) color. or V5 : (medial temporal visual region) speed and direction, origin of pursuit movemen. V6 : (parietal) represent “extra personal space”.
“What” Pathway Ve V ntral stream (occipitotemporal) : object recognition , color, r shape, and pattern. Continuation of the parvocellular pathway. y V1 V2 V4 inferotemporal cortex angular gyrus limbic structures. Alexeia, anomia, agnosia, amenesia.
“Where” Pathway Dorsal stream (occipitoparietal): Spatial orientation ,visual guidance of movement. V1 V3 V5 Parietal and superotemporal cortex. Continuation of magnocellular pathway. y Simultagnosia, optic ataxia, acquired oculomotor apraxia, and hemispatial neglect.
Cortical blindness Due to bilateral occipital lobe lesions. Often misdiagnosed as functional vision loss. Stroke, severe blood loss, Eclampsia, hypertension, angiography, aphy CO poisoning, cyclosporine.
Dyschromatopsia Bilateral occipital lobe lesions in the lingual or fusiform gyri of the medial occipital lobe (medial occipito- temporal lobe). Rarely no field defect. Unilateral involvement may cause hemidyschromatopsia.
Alexia without Agraphia Loss of ability to read but can write. Left occipital lobe and splenium of corpus callosum.
Palinopsia Persistant or recurrence of visual stimulus after it has been removed.