Visual pathway

Visual pathway

an Ophthalmology topic for MBBS

1. Visual Pathway

2. Anatomy Of Visual Pathway Visual pathway conveys visual (light) impulses from the retina to the visual cortex in the occipital lobe. Visual impulses are generated in rods & cones, acts as transducers converting light signals to electrical signals . Rods and cones of retina acts as photoreceptors

3. Rods concerned with scotopic vision (dim light) numerous at ora serrata (120 million) Cones concerned with photopic vision ( colour vision) numerous at fovea centralis (7 million) Both rods and cones absent at optic disc

4. First order neuron – bipolar cells of retina Second order neuron - ganglion cells of retina 3 rd order neuron – neurons in the 6 layers of L G B .

5. VISUAL PATH Optic nerve ↓ optic chiasma ↓ optic tract ↓ Lateral geniculate body ↓ optic radiation ↓ visual cortex

6. Optic Nerve 4 parts – Intra ocular portion ( 1mm ) Surface nerve fiber layer Prelaminar region Lamina cribrosa Retrolaminar region Intra orbital portion ( 25-30mm ) Closely surrounded by origin of rectus muscles . Intra canalicular part ( 5-9mm ) Sphenoid & ethmoid sinuses medial to it. Intra cranial part ( 10-16mm )

7. Consists of approx. 1.2 million axons. 80 % of fibres originate from macular region. No neurilemma – no regeneration. Acquire myelin sheaths proximal to lamina cribrosa . Covered by meningeal sheaths.

8. Fibres from the peripheral parts of retina → periphery of optic nerve. From parts of retina near optic disc → central area of nerve. Fibres from macula ( Papillomacular bundle ) - Enter nerve on outer aspect , apex towards centre of nerve. Become centrally – posterior part. Fibers from the 4 quadrants of retina run in the same relative positions of optic nerve

9. Optic Chiasma Flattened structure , 12mm horizontally & 8mm anteroposteriorly . Lies over tuberculum & diaphragma sella . Nasal fibres decussate. Von Willebrand knee – fibres from inferonasal retina of other side which loop forward slightly into opposite optic nerve after crossing over.

10. Optic tracts Starts at the posterolateral end of optic chiasma Optic tract winds round the upper part of cerebral peduncle to reach the LGB. Pupillary reflex fibres pass onto pretectal nucleus . Some fibers terminate in superior colliculus . Mode of termination of fibers of optic tract supr quadr of retina – lateral part of L G B infr quad of retina – medial part of L G B Macular fibres – central & posterior part of LGB

11. Lateral Geniculate Body Seen as small ovoid projection at posterolateral part of thalamus. It is the thalamic relay station for visual impulses. Grey matter split to form 6 lamina . 1& 2 lamina larger cells – magnocellular ( motion) 3 to 6 -smaller neurons – parvocellular ( colour , fine details,shape ) Crossed fibers of optic tract make synapse with cells in 1,4,and 6 layers. Uncrossed/ fibres of same side with 2,3,and 5. Fibers from fovea centralis has precise point to point projection is found through L G nucleus to the visual cortex

12. Optic radiations Extend from LGB to visual cortex ( Geniculo-calcarine tract ). Pass thru the retrolentiform part of internal capsule. Ventral fibres ( lower quadrant of retina) run forwards into temporal lobe – Meyers Loop . Dorsal fibres ( upper retinal quadrant ) run directly thru parietal lobe into visual cortex. Pass close to posterior cornu of lateral ventricle.

13. Visual cortex Medial aspect of occipital lobe, in & about the calcarine fissure. Line of Gennari – interpolated in grey matter. Visuosensory area - striate area 17, Visuopsychic area – peristriate area 18 & parastriate area 19. Rt ½ of field of vision is represented in the visual cortex of Lt hemisphere & vice versa

14. Spatial arrangement maintained- Parts above & below calcarine fissure represent upper & lower quadrants of both retinae . Peripheral retinal fibers end in antr part of visual area Macular fibers postrly

15. Blood supply of visual pathway Mainly supplied by pial network of vessels. Orbital part of optic nerve also supplied by an axial system – central artery of retina. Blood supply of optic nerve head – Surface layer – capillaries from retinal arterioles. Prelaminar region – centripetal branches of peripapillary choroid + vessels of lamina cribrosa . Lamina cribrosa – posterior ciliary A & arterial circle of Zinn . Retrolaminar part – centrifugal branches from CRA & centripetal br from pial plexus.

16. Lesions Of Visual Pathway

17. Lesions of optic nerve One optic nerve damaged → loss of vision on affected side, loss of ipsilateral direct & contralateral consensual pupillary reflexes. Lesion in proximal part of optic nerve near chiasma → ipsilateral blindness & contralateral superotemporal field defect – Traquair Junctional scotoma . Due to involvement of crossed fibres of opp side as they loop forward into the nerve.

18. Causes of optic nerve lesions – Optic atrophy Traumatic avulsion of optic nerve Indirect optic neuropathy Acute optic neuritis ischemia

19. Lesions of optic chiasma Sagittal lesions of chiasma :- Bitemporal heteronymous hemianopia & bitemporal hemianopic paralysis of pupillary reflexes. Lead to partial descending optic atrophy. pallor of optic disc Common causes – Tumours of pituitary gland Suprasellar tumours – craniopharyngiomata , suprasellar meningiomata Gliomas of third ventricle, 3 rd ventricular dilatation Chronic chiasmal arachnoiditis . Ectopic pinealomas , dermoid tumours.

20. Pattern of visual defect varies. As one side is compressed before the other, earliest defect – unilateral central scotoma . Involvement of von Willebrand knee. Altitudinal hemianopia Usually loss of upper halves of field – intra/ extra sellar tumours. Rarely loss of lower halves of field – suprasellar tumour .

21. Lateral lesions of chiasma :- Binasal hemianopia , with binasal paralysis of pupillary reflexes. Partial descending optic atrophy. Common causes – Distension of 3 rd ventricle Atheroma of carotids or posterior communicating arteries

22. Lesions of Optic tract Incongruous Homonymous hemianopia As arrangement of nerve fibres in the tract is not regular. Contralateral hemianopic pupillary reaction ( Wernicke’s reaction ). Partial descending optic atrophy. Atrophy of crossed retinal fibres – bow tie optic atrophy. Association with 3 rd nerve palsy & hemiplegia .

23. Causes – Syphilitic meningitis or gumma & tuberculous. Tumours of optic thalamus Aneurysms of superior cerebellar or posterior cerebral arteries. Tentorial meningiomas

24. Lesions of Optic radiations Involvement of total optic radiation → congruous homonymous hemianopia . Lesions of Meyers loop ( inferior fibres ) → superior homonymous quadrantic defects – Pie in the sky on the opp side. Lesions of temporal lobe → complete superior homonymous quadrantanopia . Lesions of parietal lobe (superior fibres ) → inferior quadrantic hemianopia – Pie on the floor . No optic atrophy.

25. Left homonymous hemianopia

26. Pie in the floor Pie in the sky

27. Lesions of Visual Cortex Congruous Homonymous hemianopia , usually sparing macular area. Pupillary reflexes normal, no optic atrophy. Causes – Injury by fall on the back of head Gunshot wounds Cerebral softening When angular gyrus involved → word blindness.

28. Riddoch Phenomenon – Appreciation of a dim kinetic target is retained within the defective visual field, with loss of appreciation of a static bright target. Typical of an occipital lesion. If lesion is more anteriorly in the occipital cortex, Contralateral temporal crescentic field defect

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