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
 3rd 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

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

9.  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.

10.  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

12. 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.

13. 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

14. 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

15. 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.

17. 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

18.  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

19. 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 –
i. Surface layer – capillaries from retinal arterioles.
ii. Prelaminar region – centripetal branches of
peripapillary choroid + vessels of lamina cribrosa.
iii. Lamina cribrosa – posterior ciliary A & arterial circle
of Zinn.
iv. Retrolaminar part – centrifugal branches from CRA
& centripetal br from pial plexus.

20. Lesions Of Visual
Pathway

21. 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.

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

23. 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, 3rd ventricular dilatation
◦ Chronic chiasmal arachnoiditis.
◦ Ectopic pinealomas, dermoid tumours.

24.  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.

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

26. 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 3rd nerve palsy & hemiplegia.

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

28. 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.

29. Left homonymous hemianopia

30. Pie in the floor Pie in the sky

31. 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.

32.  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