Visual Pathway and its Defects

By Mutahir ShahResident M Phil VS2nd SemesterPakistan Institute of Community Ophthalmology

Visual Pathway and its Defects

Brief Anatomy of Visual PathwayAfferent Visual Pathways:-It is important to recognize that any disturbance in afferent function may result in the same symptoms of vision loss as observed with pathology affecting the retina, optic nerve,and visual pathway.RetinaThe afferent visual pathway begins within the retina.Optic NerveThe optic nerve begins anatomically at the optic disc but physiologically and functionally within the ganglion cell layer that covers the entire retina. The first portion of the optic nerve, representing the confluence of approximately 1.0- 1.2 million ganglion cell axons.

The combination of small channels and a unique blood supply (largely from branches of the posterior ciliary arteries) probably plays a role in several optic neuropathies.Retinal fibers enter optic discs in a specific manner.

Nerve fiber bundle (NFB) defects are of the following:Papillomacular bundle.Sup. & Inf. Arcuate bundle.Nasal bundle.

PAPILLOMACULAR BUNDLE-DEFECTS:

Optic ChiasmThe optic chiasm measures approximately 12 mm wide, 8 mm long in the anteroposterior direction, and 4 mm thick.Within the chiasm, the fibers coming from the nasal retina (approximately 53% of total fibers) cross to the opposite side to join the corresponding contralateral fibers.Extramacular superonasal fibers cross directly to the opposite tract. Extramacular temporal fibers remain uncrossed in the chiasm and optic tract. The macular projections are located centrally in the optic nerve and constitute 80%-90% of the total volume of the optic nerve and the chiasma! Fibers.

The extramacular fibers from the inferonasal retina cross anteriorly in the chiasm at the "Wilbrand knee" before passing into the optic tract.

Optic Tract Starting from the posterior part of the chiasma upto the LGB.Fibers (both crossed and uncrossed) from the upper retinal projections travel medially in the optic tract; lower projections move laterally. The macular fibers adopt a dorsolateral orientation as they course toward the lateral geniculate bodyAll retrochiasmatic lesions result in a contralateral homonymous hemianopia.Optic tract lesions tend to produce markedly incongruous field defect.

Lateral Ganiculate Body or NucleusThe LGN is a peaked, mushroom-shaped structure that is divided into 6 levels.The 4 superior levels are the termini of P-cell axons, which are the ganglion cells with smaller receptive fields and are responsible for mediating maximal spatial resolution and color perception. The 2 inferior layers receive input from the M-cell fibers, which are the ganglion cells with larger receptive fields and are more sensitive to detecting motion. Axons originating in the contralateral eye terminate in layers 1, 4, and 6; the ipsilateral fibers innervate 2, 3, and 5.

As the fibers approach the LGN, the superior fibers move superomedially and the inferior fibers swing inferolaterally.Overall, the retinal representation rotates almost 90, with the superior fibers moving medially and the inferior fibers laterally. The macular fibers tend to move super laterallyMacular vision is subserved by the hilum and peripheral field by the medial and lateral horns.

LGN field defects:Incongruous homonymous hemianopia.Unique sector & sector-sparing defects due to dual blood supply of LGN from anterior & posterior choroidal arteries.

Optic RadiationFollowing a synapse in the LGN, the axons travel posteriorly as the optic radiations toterminate in the primary visual (calcarine) cortex in the occipital lobe.The most inferior of the fibers first travel anteriorly, then laterally and posteriorly to loop around the temporal horn of the lateral ventricles (Meyer loop) .More superiorly, the fibers travel posteriorly through the deep white matter of the parietal lobe. The macular (central) fibers course laterally, with the peripheral fibers concentrated more at the superior and inferior aspects of the radiations.

Right superior quadrantanopia >> temporal lobe lesion

Left inferior quadrantanopia >> parietal lobe lesion

Visual CortexThe visual cortex, the thinnest area of the human cerebral cortex, has 6 cellular layers It occupies the superior and inferior lips of the calcarine fissure on the posterior and medial surfaces of the occipital lobes. Macular function is extremely well represented in the visual cortex and occupies the most posterior position at the tip of the occipital lobe. The most anterior portion of the calcarine fissure is occupied by contralateral nasal retinal fibers only. The visual cortex usually have dual blood supply.The macular visual cortex is supplied by terminal branches of posterior & middle cerebral arteries.

A lesion affecting the tip of the occipital lobe tends to produce a central homonymous hemianopia

Left homonymous hemianopia with macular sparing

Related terms used in Visual Field defectsScotoma It is a defect in Visual Field surrounded by normal visual field.Relative scotoma - an area where objects of low luminance cannot be seen but larger or brighter ones can. Absolute scotoma - nothing can be seen at all within that area.Hemianopia - binocular visual defect in each eye's hemifield. Bitemporal hemianopia the two halves lost are on the outside of each eye's peripheral vision, effectively creating a central visual tunnel.

Homonymous hemianopia - the two halves lost are on the corresponding area of visual field in both eyes, i.e. either the left or the right half of the visual field.

Altitudinal hemianopia - refers to the dividing line between loss and sight being horizontal rather than vertical, with visual loss either above or below the line.

Quadrantanopia - is an incomplete hemianopia referring to a quarter of the schematic 'pie' of visual field loss.

Sectoral defect - is also an incomplete hemianopia Congruous: when the defect is not complete (does not occupy the entire half of the field) & the defect extends to the same angular meridian in both eyes

Congruity describes incomplete homonymous hemianopic defects that are identical in all attributes: location, shape, size, depth, slope of margins.Remember: the more posterior toward the occipital cortex the lesion in the postchiasmal visual pathways, the more likely the defects will be congruous.

b. Anterior parietal radiation les ion a.Temporal radiation lesion c. Main radiation lesion d. Anterior visual cortex lesion e. Macular cortex lesion Temporal horn of lateral ventricle Lateral geniculate body Calcarine fissure a b c d e

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