visual pathway & its lesions.pptx

Visual pathway

1. Visual  Pathway & Its Lesions   JATIN YADAV  MBBS 2021-2022

2. Visual pathway starting from retina consists of— Components: Optic nerve Optic chiasma. Optic tract Lateral geniculate body (of the thalamus). Optic radiations Visual cortex

3. Optic Nerve 2 nd cranial nerve. 47-50 mm in length. Starts from optic disc & extends upto optic chiasma where the two nerves meet. Backward continuation of nerve fibre layer of retina which consist of axons originating from ganglion cells. Contains the afferent fibres of light reflex Has 4 parts : 1)intraocular (1mm) 2)intraorbital (30mm) 3)intra canalicular (6-9mm) 4)intracranial (10mm)

4. Optic chiasma Flattened structure,12 mm horizontally & 8mm anteroposteriorly. Ensheathed by pia & surrounded by CSF. Lies over diaphragma sellae so visual field defects seen in patient with pituitary tumor having suprasellar extension. Posteriorly chiasma continuous with the optic tracts & form the anterior wall of 3 rd ventricle. Nerve fibres arising from nasal half of two retina decussate at the chiasma .

5. Optic tract Cylindrical bundle of nerve fibres. Run outwards & backwards from posterolateral aspect of optic chiasma. Fibres from temporal half of retina of same eye & nasal half of opposite eye. Posteriorly each ends in Lateral Geniculate Body.

6. Lateral Geniculate body -A nucleus in the thalamus, which projects t o the 1 ° visual c or t e x and serves visual perception. - This body is the site of termination of all optic nerve fibres except few which reach and relay in the pretectal region and superior colliculus of Midbrain. Consists of 6 lamina. Contralateral retina – 1,4,6 Ipsilateral retina – 2,3,5

7. 0ptic radiation From LGB to the occipital cortex. Pass forwards then laterally through the area of wernicke as optic peduncles. Anterior to lateral ventricle ,traversing the retrolenticular part of internal capsule, medial to auditory tract. Its fibres then spread out fanwise to form medullary optic lamina. Inferior fibres subserve upper visual fields & sweep anteroinferiorly in meyer’s loop & temporal lobe to visual cortex. Superior fibres subserve inferior visual field proceed posteriorly through parietal lobe to visual cortex.

8. Visual cortex It is located on the medial aspect of occipital lobe, above and below the calcarine fissure. Visual cortex V i suo p s y ch i c area P er i s tr i a t e area 18 P a r a s tr i a t e area 19 V i suosen s o r y area Striate area 17

9. feature Visual sensation Somatic sensation Sensory end organ Rods and Cones Nerve endings in the skin Neurons of 1 st order Lie in bipolar layer of retina Lies in posterior root ganglion Neurons of 2 nd order Lies in ganglion cells of the retina Lies in nucleus gracilis and nucleus cuneatus Neurons of 3 rd order Lie in geniculate body Lie in geniculte body

10. Neural pathway for vision Ist order sensory neurons – Arise from the bipolar cells of the retina. II nd order neurons are the Multipolar neurons whose axons run along the optic nerve to the optic chiasma Nasal fibres – Cross to opposite side and terminate in LGB of opposite side. Temporal fibres – Doesn’t cross and terminates in ipsilateral LGB.

11. The cell bodies of III order sensory neurons are located in LGB. Their axons form optic radiation which project into the visual cortex.

12. Visual reflexes Light reflex or pupillary reflex: When light is shown to one eye, normally the pupils of both eyes constrict. Direct light reflex: The constriction of pupils upon which light is shown is called direct light reflex. Indirect or consensual: The constriction of pupil on the other eye even though no light is shown

13. ACCCOMODATION REFLEX When the eyes are focussed from a distant to near object, three reactions take place 1. Constriction of pupils 2. thickening of lens due to contraction of ciliary muscles 3. Convergence of both eye balls These three reactions together constitute Accommodation or near reflex

14. Clinical correlation Loss of vision in one half of the visual field (Rt or Lt) is termed as hemianopia. Homonymous hemianopia: Loss of vision in the same halves of the visual field. Heteronymous Hemianopia: Loss of vision in the different halves of the visual field.

15. Lesions of the visual pathway Lesions of the optic nerve: Characterised by marked loss of vision or complete blindness on the affected side associated with abolition of direct light reflex on the ipsilateral side and consensual on contralateral side. Causes : optic atrophy indirect optic neuropathy acute optic neuritis traumatic avulsion of optic nerve Eg : right optic nerve involvement

16. 2)Lesions through proximal part of optic nerve : ipsilateral blindness. contralateral hemianopia abolition of direct light reflex on affected side & concensual light reflex on contralateral side. near reflex intact. Rt optic nerve Involvement in Proximal part

17. Central lesions of chiasma (sagittal): Characterised by: Bitemporal hemianopia Bitemporal hemianopic paralysis of pupillary reflex. (usually lead to partial descending optic atrophy) causes: suprasellar aneurysm tumors of pituitary gland craniopharyngioma suprasellar meningioma & glioma of 3 rd ventricle. obstructive third ventricular dilatation due to hydrocephalus. chronic chiasmal arachnoiditis

18. Lateral chiasmal lesions : Characterised by Binasal hemianopia Binasal hemianopic parallysis of pupillary reflex (usually lead to partial descending optic atrophy) causes: Distension of 3 rd ventricle causing pressure on each side of optic chiasma Atheroma of carotids & posterior communicating artery.

19. Lesions of optic tract : Characterised by : Incongruous homonymous hemianopia with C/L hemianopic pupillary reaction( wernicke’s reaction) These lesions usually lead to partial descending optic atrophy & may be associated with C/L 3 rd nerve paralysis & ipsilateral hemiplegia. Causes: Syphilitic meningitis/ gumma. Tuberculosis Tumors of optic thalamus Aneurysm of superior cerebellar or posterior cerebral arteries.

20. Lesions of lateral geniculate body : leads to homonymous hemianopia with sparing of pupillary reflexes & may end in partial optic atrophy

21. Lesions of optic radiations : Causes: Vascular occlusion Primary & secondary tumors Trauma Characterised by : TOTAL OPTIC RADIATION INVOLVEMENT COMPLETE HO M O N YM OUS HEMIANOPIA( sometimes sparing macula)

22. LESIONS OF PARIETAL LOBE (involving superior fibres of optic radiations) INFERIOR QUADRANTIC HEMIANOPIA( PIE ON THE FLOOR) LESIONS OF TEMPORAL LOBE (i n v o lving in f e r i o r fibres of optic radiations) SUPERIOR QUADRANTIC HEMIANOPIA( PIE ON THE ROOF)

23. Pupillary reactions are normal as fibres of light reflex leave the optic tracts to synapse in the superior colliculi. Lesions of optic radiations do not produce optic atrophy as the 1 st order neurons (optic nerve fibres) synapse in LGB.

24. Lesions of visual cortex: pupillary light reflex is normal & optic atrophy does not occur following visual cortex lesions. Congruous homonymous hemianopia(sparing macula) Occlusion of posterior cerebral artery supplyin anterior part of occipiatl cortex Congruous homonymous macular defect Head injury/gun shot injury leading to lesions of tip of occipital cortex+