seminar on anatomy and physiology of visual system and localisation of lesions

2. • 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

9. • 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.

17. • 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

19. • 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.

24. • 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.

28. • 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).
.

30. • 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.

38. • 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.
.

46. • 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.

51. • 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.

55. • 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 .

58. • 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.

65. • 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.

66. • 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;

67. • 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.

69. • 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.

70. • 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. .

71. • 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.

74. • 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.

78. • 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

81. • 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.

90. Thank you