visual pathway-localisation of lesion

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Optic nerve –anatomy and localization of lesion : 

Optic nerve –anatomy and localization of lesion P.Ranjith Kumar, Postgraduate in neurology, Gandhi Medical College

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Prosencephalon Optic vesicles Optic cup Optic stalk Outgrowths of brain specialized for sensitivity of light, encoding sensory data, transmission to cortex Neuronal pattern of retina resembles gray mater Optic nerve-white matter

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The rod cells contain rhodopsin, a conjugated protein inwhich the chromophore group is a carotenoid akin to vitamin A. The rods function in the perception of visual stimuli in subduedlight (twilight or scotopic vision), and the cones are responsible for color discrimination and the perception of stimuli in bright light (photopic vision). Most of the cones are concentrated in the macular region, particularly in its central part, the fovea, and are responsible for the highest level of visual acuity. There are no ganglion cells in the fovea.

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The axons of the retinal ganglion cells, as they stream across the inner surface of the retina, pursue an arcuate course. The axons of ganglion cells are collected in the optic discs and then pass uninterruptedly through the optic nerves, optic chiasm, and optic tracts to synapse in the lateral geniculate nuclei, the superior colliculi, and the midbrain pretectum

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Fibers derived from macular cells form a discrete bundle that first occupies the temporal side of the disc and optic nerve and then assumes a more central position within the nerve (papillomacular bundle). These fibers are of smaller caliber than the peripheral optic nerve fibers. The retinal ganglion cells and their axonic extensions are, properly speaking, an exteriorized part of the brain and that their pathologic reactions are the same as in other parts of the CNS.

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ophthalmic branch of the internal carotid artery supplies the retina, posterior coats of the eye, and opticnerve head. posteriorciliary arteries- form a rich circumferential plexus of vessels (arterial circle of Zinn-Haller)located deep to the lamina cribrosa. This arterial circle supplies the optic disc and adjacent part of the distal optic nerve, the choroid, and the ciliary body; it anastamoses with the pial arterial plexus that surroundsthe optic nerve. central retinal artery. It supplies the inner retinal layers and issues from the optic disc, where it divides into four branches, each of which supplies a quadrant of the retina;The inner layers of the retina, including the ganglion and bipolar cells, receive their bloodsupply from these arterioles and their capillaries the deeper photoreceptor elements and the fovea are nourished by theunderlying choroidal vascular bed, by diffusion through the retinal pigmented cells and the semipermeable Bruch’s membrane uponwhich they rest.

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In optic chiasma fibers derived from nasal half of each retina decussate and continue in the optic tract with uncrossed temporal fibers of the other eye . but the tract fibers are not evenly admixed. A variable bundle of fibers from the inferior nasal part of the optic nerve turns anteriorly into the opposite optic nerve as it crosses in the chiasm (Wilbrand’s knee). .

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80 percent of the the fibersof the optic tract terminate in the lateral geniculate body and synaps with the six laminae of its neurons. Three of these laminae (1, 4, 6),which constitute the large dorsal nucleus, receive crossed (nasal) fibers from the contralateral eye, Three(2, 3, 5)receive uncrossed (temporal)fibers from the ipsilateral eye. The geniculate cells project to the visual(striate)cortex of the occipital lobe,also called area 17 or V1 . Other optic tract fibers terminate in the pretectum and innervate bothEdinger-Westphal nuclei, which subserve pupillary constriction and accommodation . A small group of fibers terminate in the suprachiasmatic nuclei in animals and presumably also in humans. The vascular supply of the lateral geniculatebody is from both the posterior and anterior choroidal and thalamogeniculate arteries; it is therefore rarely infarcted.

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In their course through the temporal lobes, the fibers from the lower and upper quadrants of each retina diverge. The lower ones arch around the anterior pole of the temporal horn of the lateral ventricle before turning posteriorly; the upper ones follow a more direct path through the white matter of the uppermost part of the temporal lobe and possibly of the adjacent parietal lobe. Both groups of fibers merge posteriorly at the internal sagittal stratum. .

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It is in Brodmann’s area 17, embedded in the medial lip of the occipital pole, that cortical processing of the retinogeniculate projections occurs. The receptive neurons are arranged in columns, some of which are activated by form and others by moving stimuli or by color. The neurons for each eye are grouped together and have concentric, center-surround receptive fields.

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The deep neurons of area 17 project to the secondary and tertiary visual areas of the occipito temporal cortex of the same and opposite cerebral hemispheres and also to other multisensory parietal and temporal cortices. Several of these extrastriate connections are just now being identified. Separate visualsystems are utilized in the perception of motion,color, stereopsis, contour, and depth perception. The classic studies of Hubel and Wiesel have elucidated much of this visual cortical physiology.

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For purposes of description of the visual fields, each retina and macula are divided into a temporal and nasal half by a vertical line passing through the fovea centralis. A horizontal line is represented roughly by the junction of the superior and inferior retinal vascular arcades also passes through the fovea and divides each half of the retina and macula into upper and lower quadrants. Visual field defects are always described in terms of the field defect from the patient’s view (nasal, temporal, superior, inferior)rather than the retinal defect or the examiner’s perspective. The retinal image of an object in the visual field is inverted and reversed from right to left, like the image on the film of a camera. Thus the left visual field of each eye is represented in the opposite half of each retina,with the upper part of the field represented in the lower part of the retina .

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From the retina there is a point-to-point projection to the lateralgeniculate ganglion and from the latter to the calcarine cortex of the occipital lobe. Thus the visual cortex receives a spatial pattern of stimulation that corresponds with the retinal image of the visual field. Visual impairments due to lesions of the central pathways usually involve only a part of the visual fields, and a plotting of the fields provides fairly specific information as to the site of the lesion.

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Prechiasmal Lesions Lesions of the macula, retina, or optic nerve cause a scotoma (an island of impaired vision surrounded by normal vision) rather than a defect that extends to the periphery of one visual field (“fieldcut”). Scotomas are named according to their position (central, cecocentral)or their shape (ring, arcuate). A small scotoma that is situated in the macular part of the visual field may seriously impairvisual acuity.

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Scotomas are the main features of optic neuropathy Demyelinative disease (opticneuritis), Leber hereditary optic atrophy,toxins (methyl alcohol,quinine, chloroquine, and certain phenothiazine drugs), nutritionaldeficiency (so-called tobacco-alcohol amblyopia), and vascular disease. Orbital or retro-orbital tumors andinfectious or granulomatous processes (e.g., sarcoid, retinal toxoplasmosisin AIDS)are other common causes. Certain toxic andmalnutritional states are characterized by more or less symmetricalbilateral central scotomas (involving the fixation point)or cecocentralones (involving both the fixation point and the blind spot).The cecocentral scotoma, which tends to have an arcuate border,represents a lesion that is predominantly in the distribution of thepapillomacular bundle.. Demyelinative disease is characterized by unilateralor asymmetrical bilateral scotomas. Vascular lesions that take the form of retinal hemorrhages or infarctions of the nerve-fiber layer(cotton-wool patches)give rise to unilateral scotomas;

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The visual field pattern created by a lesion in the optic nerve as it joins the chiasm typically includes a scotomatous defect onthe affected side coupled with a contralateral superior quadrantanopia (“junctional field defect”). this is caused by interruption of nasal retinal fibers which, after crossing in the chiasm, project into the base of the affected optic nerve(Wilbrand’s knee). A sharply defined pattern of this type is relatively uncommon. Variations in the pattern of visual loss are frequent, in part accounted for by the location of the chiasm in an individual patient—a prefixed chiasm making unilateral eye findings more common.

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Hemianopia (hemianopsia)means blindness in half of the visualfield. Bitemporal hemianopia indicates a lesion of the decussating fibers of the optic chiasm and is caused most often by the suprasellar extension of a tumor of the pituitary gland , craniopharyngioma, a saccular aneurysm of the circle of Willis, and a meningioma of the tuberculum sellae; sarcoidosis, metastatic carcinoma, ectopic pinealoma or dysgerminoma, Hand-Schu¨ller-Christian disease or hydrocephalus with dilation and downward herniation of the posterior part of the third ventricle. . Heteronymous field defects, i.e., scotomas or field defects that differ in the two eyes, are also a sign of involvement of the optic chiasm or the adjoining optic nerves or tracts; they are caused by craniopharyngiomas or other suprasellar tumors and rarely by mucoceles, angiomas, giant carotid aneurysms, and opticochiasmicarachnoiditis.

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The optic chiasm lies just above the pituitary body and also forms part of the anterior wall of the third ventricle; hence the crossing fibers may be compressed from below by a pituitarytumor, a meningioma of the tuberculumsellae, or an aneurysm and from above by a dilated third ventricle or craniopharyngioma. The resulting field defect is bitemporal .. Optictract lesions, in comparison with chiasmatic and nerve lesions, are relatively rare. .

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Homonymous hemianopia (a loss of vision in corresponding halves of the visual fields)signifies a lesion of the visual pathwaybehind the chiasm and, if complete, gives no more information than that. Incomplete homonymous hemianopia has more localizing value. As a general rule, if the field defects in the two eyes are identical (congruous), the lesion is likely to be in the calcarine cortex and subcortical white matter of the occipital lobe; if they are incongruous, the visual fibers in the optic tract or in the parietalor temporal lobe are more likely to be implicated. Actually, absolute congruity of field defects is rare, even with occipital lesions.

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The lower fibers of the geniculocalcarine pathway (from the inferior retinas)swing in a wide arc over the temporal horn of the lateral ventricle and then proceed posteriorly to join the upper fibersof the pathway on their way to the calcarine cortex This arc of fibers is known variously as the Flechsig, Meyer, orArchambault loop, and a lesion that interrupts these fibers will produce a superior homonymous quadrantanopia (contralateral upper temporal and ipsilateral upper nasal quadrants; . This clinical effect was first described by Harvey Cushing. Parietal lobe lesions are said to affect the inferior quadrants of thevisual fields more than the superior ones, but this is difficult todocument; with a lesion of the right parietal lobe, the patient ignores the left half of space; with a left parietal lesion, the patient is often aphasic. As to the localizing value of quadrantic defects,the report of Jacobson is of interest; he found, in reviewing the imaging studies of 41 patients with inferior quadrantanopia and 30with superior quadrantanopia, that in 76 percent of the former and83 percent of the latter the lesions were confined to the occipital lobe.

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If the entire optic tract or calcarine cortex on one side is destroyed,the homonymous hemianopia is complete. But often that part of the field subserved by the macula is spared, i.e., there is a 5- to 10-degree island of vision around the fixation point on the side of the hemianopia (sparing of fixation, or macular sparing). With infarction of the occipital lobe due to occlusion of the posteriorcerebral artery, the macular region, represented in the most posterior part of the striate cortex, may be spared by virtue of collateralcirculation from branches of the middle cerebral artery. Incomplete lesions of the optic tract and radiation usually spare central(macular)vision. Lesions of both occipital poles (as in embolization of the posterior cerebral arteries)result in bilateral central scotomas; if all the calcarine cortex or all the subcortical geniculocalcarine fibers on both sides are completely destroyed, there is cerebral or “cortical” blindness

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An altitudinal defect is one that is confined to the upper or lower half of the visual field but crosses the vertical meridian. Homonymous altitudinal hemianopia is usually due to lesions of both occipital lobes below or above the calcarine sulcus and rarely to a lesion of the optic chiasm or nerves. The most common cause is occlusion of both posterior cerebral arteries at their origin at the termination of the basilar artery. By contrast, the cause of a monocular altitudinal hemianopia is almost invariably an ischemic optic neuropathy that arises from occlusion of the posterior ciliary vessels.

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Thank you