(India)
INTRODUCTION
The visual pathway starts from the retina and ends in the cortical areas.1,2 There are basically seven levels through which the visual impulses pass. They are: (i) retina, (ii) optic nerve, (iii) optic chiasma, (iv) optic tract, (v) lateral geniculate body, (vi) optic radiation, and (vii) cortical areas.
RETINA
The end organ of the visual pathway is the neural epithelium of the rods and cones. The first conducting nerve cell or neuron of the first order is the bipolar cell. From the bipolar cells the impulses travel to the ganglion cells. From the ganglion cells to the lateral geniculate body (LGB) is the second-order neuron and from the LGB to the occipital cortex is the third-order neuron. This is done via the optic radiations. In the optic nerve, head the peripheral fibers from the retina insert in the periphery of the disk and those from the central retina insert in the center of the disk.
OPTIC NERVE
The optic nerve is the second cranial nerve and is about 5 cm in length. It has basically four portions (read Anatomy of the Optic Nerve). They are:
- Intraocular portion
- Intraorbital portion
- Intracanalicular portion
- Intracranial portion.
OPTIC CHIASMA
Definition
Optic chiasma is a commissure formed by the junction of the optic nerve. This provides for crossing of the nasal retinal fibers to the optic tract of the opposite side and for passage of temporal fibers into the optic tract of the ipsilateral side.
Dimensions
It is a flattened oblong band, some 12 mm in its transverse diameter and 8 mm from before backwards.
Types
Central chiasma This is present in about 80 percent of cases. It lies directly above the sella, so that expanding pituitary tumors will involve the chiasma first.5
Fig. 2: Arrangement of nerve fibers in the disk from the retina. The peripheral fibers insert in the periphery of the disk while the central fibers insert in the center of the disk
Prefixed chiasma This is seen in about 10 percent of cases. In these cases, the chiasma is present more anteriorly over the tuberculum sellae. In such a situation, the pituitary tumor may involve the optic tracts first.
Postfixed chiasma This is seen in about 10 percent of cases. In these cases, the chiasma is located more posteriorly over the dorsum sellae so that pituitary tumors are apt to damage the optic nerve first.
Anatomy
The optic chiasma lies over the diaphragma sellae and is ensheathed in pia mater surrounded by cerebrospinal fluid. As it lies over the diaphragma sellae, presence of a visual field defect in a patient with a pituitary tumor indicates suprasellar extension. Posteriorly, the chiasma is continuous with the optic tracts.
Relations
The relations of the optic chiasma are:
Anteriorly—are the anterior cerebral artery and their anterior communicating branch.
Laterally—is the internal carotid artery, as it passes upwards after having pierced the roof of the cavernous sinus. It lies on each side in contact with the chiasma in the angle between the optic nerve and tract. Laterally too is the anterior perforated substance. The medial root of the olfactory tract lies laterally.
Posteriorly—is the tuber cinereum-a hollow elevation of gray matter situated between the mamillaria bodies behind and the optic chiasma in front. Laterally, it is continuous with the gray matter of the anterior perforated substance. From its inferior aspect the infundibulum, which is a hollow conical process, passes downwards and forwards and through a hole in the posterior part of the diaphragma sellae attaches itself to the posterior lobe of the pituitary gland. The infundibulum is thus in close contact with the posteroinferior part of the chiasma, which it joins at an acute angle.
Above—is the third ventricle, in the floor of which the chiasma makes a prominence.
Inferior-is the hypophysis (pituitary gland) and under the lateral edge of the chiasma is the cavernous sinus.7
OPTIC TRACT
Definition
Each optic tract is a cylindrical band, which runs from the optic chiasma to the crus cerebri.
Course
It runs laterally and backwards from the posterolateral angle of the chiasma between the tuber cinereum and the anterior perforated substance. Becoming more flattened and strap-like it is united to the upper part of the anterior then lateral surface of the cerebral peduncle (crus cerebri).
Relations
The course can be divided into three parts:
Anterior part In the first section of its course, the optic tract lies superficial on the under aspect of the brain. It runs above the dorsum sellae and crosses the third nerve from medial to lateral. Above is the posterior part of the anterior perforated substance and the floor of the third ventricle while medially is the tuber cinereum.
Middle part In the middle region of its course, the optic tract lies hidden between the uncus and the cerebral peduncle (crus cerebri). It is here also the flattening commences to conform to the upper aspect of the uncus. The optic tract here crosses the pyramidal tract, which occupies the middle segment of the basis pedunculi. Nearby, just dorsal to the substantia nigra, are the lemnisci carrying sensory fibers. It thus comes about that a single lesion here can affect vision and also the great motor and sensory roots.
Posterior part In the posterior part of its course, the optic tract lies in the depths of the hippocampal sulcus. Below and parallel to it runs the posterior cerebral artery.
Roots
In the posterior part of its course, the optic tract divides into two roots-the medial root and the lateral root.
Medial root The medial root passes to the medial geniculate body. These are not light fibers but commissural auditory fibers between the two medial geniculate bodies, whose course in the white matter is via the optic tracts and chiasma. It is called the Gudden's commissure.
Lateral root This spreads over the LGB and for the most part ends in it.
Terminations
The fibers of the optic tract coming from the ganglion cells of the retina reach three major destinations. They are:9
- Lateral geniculate body—for relay to the visual cortex
- Each pretectal nucleus—as part of the pupilloconstrictor pathway
- Superior colliculus—for general reflex responses to light.
LATERAL GENICULATE BODY
Definition
Lateral geniculate bodies (LGBs) are a pair of bodies, which are part of the thalamus and form an end station for all fibers subserving vision in the optic tracts.
Shape
Oval or cap-like structure.
Situation
On the posterior aspect of the thalamus.
Development
During the process of development from lower forms of life, there is a lateral rotation of the LGB as well as changes in its structure. As a result, in man, the 10original external surface has become ventral. This is of importance as regards the disposition of retinal fibers in the LGB.
Parts
The LGB is an asymmetrical cone, with a rounded apex to its main bulk or body and an incomplete rim inferiorly. The rim is drawn out laterally as a peak or spur, which is largely responsible for the surface elevation known as the LGB. The anterior part of the rim is observed by the entry of the optic tract. The medial part of the rim is superior to the medial root and is variably responsible for the surface elevation, which appears to lead dorsally into the medial geniculate body. Inferiorly, the nucleus is hollowed; producing a kind of hilum, which also extends onto the dorsal aspect of the nucleus which here, has no rim. The hilum may be represented by a superficial cleft or depression.
Relations
On coronal section It appears like a peaked cap, the peak projecting laterally.
On horizontal section It is shown to be related anteriorly with the optic tract which ends therein. Laterally, it is related to the retrolenticular portion of the internal capsule. Medially, it is related to the medial geniculate body, posteriorly with the hippocampal convolution and posterolaterally with the inferior horn of the lateral ventricle.
On sagittal section It is seen that the fibers of the optic tract divide into two layers: the inferior of these forms the white layer of the hilum; and the superior forms the dorsal portion of the saddle. Between these laminae which form the capsule of the LGB are alternating layers of myelinated fibers and cells which give the body its characteristic appearance. From the dorsal portion of the LGB pass a mass of fibers (which form its peduncle) into the area of Wernicke. This is a small region of myelinated fibers enclosed by the thalamus medially, the internal capsule laterally and the LGB posteriorly. The main constituents of the area of Wernicke are the fibers of the optic radiation. It also contains the vertical temporothalamic fibers of Arnold.
Superior Brachium
The LGB is connected to the superior colliculus by a slender band called the superior brachium.
OPTIC RADIATIONS
Definition
Optic radiation or optic radiation of Gratiolet is a fresh relay of fibers that carry the visual impulses from the LGB to the occipital lobe.11
Course
They pass forwards and then laterally through the area of Wernicke as the optic peduncle, anterior to the lateral ventricle and traversing the retrolenticular part of the internal capsule behind the sensory fibers and medial to the auditory tract. The fibers spread out fanwise to form the medullary optic lamina.
Meyer's Loop
The ventral portion of the optic radiation instead of sweeping back into the occipital lobe plunges forwards into the temporal pole before passing backwards as an inferior longitudinal fasciculus of Meyer. This is known as Meyer's loop. Interference with this loop causes a superior homonymous quadrantic hemianopia.
Further Course
The optic radiations as they pass back into the white matter of the cerebral hemisphere lie deep to the middle temporal gyrus, so that tumors of this portion of the temporal lobe may give rise to visual defects. The optic radiations end in the occipital lobe in an extensive area of thin cortex in which is the white striae of Gennari.
Other Fiber
The other fibers in the optic radiations are:
- Fibers that pass from the cortex to the LGB and the superior colliculus
- Descending nerve fibers passing to the ocular motor nuclei.
VISUAL CORTEX
Calcarine Sulcus
In man, the visual projection cortex is situated along the superior and inferior lips of the calcarine sulcus. This area is usually called the striate cortex because of the prominent band of geniculocalcarine fibers termed as striae of Gennari, after its discoverer who discovered it in 1776. The striate cortex is also referred to as area 17 of Brodman. The anterior part of the sulcus is called the calcarine fissure and the posterior part is called the postcalcarine fissure. The striate cortex is situated on the inferior and superior lips of the postcalcarine fissure and on the inferior lips of the calcarine fissure.
Course of Calcarine Sulcus
The calcarine sulcus is a deep sulcus extending from near the occipital pole. The parieto-occipital sulcus joins the calcarine sulcus at an acute angle a little in front of its middle, dividing it into anterior and posterior portions and forming an Y-shaped figure.13
Cuneus
If the lips of the parietooccipital sulcus and calcarine sulcus are widely separated, it will be seen that although on the surface they appear to be continuous, they are separated from each other by a small buried vertical cuneate gyruscalled cuneus.
Histology of the Visual Cortex
There are six layers of the visual cortex histologically. From external to internal they are:
- Layer I-plexiform lamina
- Layer II—xternal granular lamina
- Layer III—pyramidal lamina
- Layer IV-internal granular lamina: this in turn is divided into layer IV A, layer IV B and layer IV C alpha and layer IV C beta
- Layer V-ganglionic lamina
- Layer VI—multiform lamina.
Parastriate Cortex
The visual picture from both the eyes unites in the parastriate cortex called area 18. The lips of the lunate sulcus separate area 17 from area 19. Area 18 is buried within the walls of the sulcus and is in between area 17 and area 19.
Peristriate Cortex
This is area 19. Most of area 19 lies in the posterior parietal lobe but inferiorly it forms part of the temporal lobe. In area 19 the object seen is recognized.
LOCALIZATION IN THE VISUAL PATHWAY
RETINA
The nerve fibers converge towards the disk on the temporal side in the important papillomacular bundle. There is no overlap between the upper and lower halves of the fibers of the peripheral parts of the retina. In the retina the line dividing nasal from temporal fibers, in the sense of those that will cross the chiasma and those that will not, passes through the center of the fovea. Hence, the temporal macular fibers remain on the same side while the nasal ones cross. The upper temporal retinal fibers are separated from the lower by the macular fibers—an arrangement, which holds throughout the central visual pathway.15
OPTIC NERVE
In the optic nerve head In the optic nerve head the arrangement of the nerve fibers is exactly as in the retina.
In the optic nerve just behind the eyeball Here the nerve fibers are distributed like in the retina. The upper temporal and lower temporal fibers which are situated on the temporal half of the optic nerve and are separated from each other by a wedge-shaped area occupied by the papillomacular bundle. The upper and lower nasal fibers are situated on the nasal side.
In the optic nerve near the chiasma Here the macular fibers are centrally placed.
OPTIC CHIASMA
The Nasal Fibers
The nasal peripheral fibers constitute about three-quarter of all the fibers and cross over to enter the medial part of the opposite optic tract in the following manner.
The lower nasal fibers in the optic nerve traverse the chiasma low and anteriorly So they are first affected in the tumors of the pituitary body producing upper temporal quadratic field defects. These fibers form convex loops called Wilbrand's knee in the terminal part of the opposite optic nerve therefore ipsilateral blindness due to lesions of the proximal most part of the optic nerve is associated with contralateral field defects. They then cross to the opposite tract and occupy its lower quadrant.
The upper nasal fibers of the optic nerve traverse the chiasma high and posteriorly Therefore, they are involved first by lesions coming from above the chiasma, e.g. craniopharyngioma. After crossing they occupy the upper nasal quadrant of the opposite optic tract. Some of these fibers make a loop in the ipsilateral optic tract before crossing.
The Temporal Fibers
The temporal fibers from the retina which occupy the temporal half of the optic nerves, remain uncrossed and run backwards in the lateral part of the optic chiasma to reach the dorsolateral part of the optic tract.
The Macular Fibers
The macular fibers, which occupy the central part at the proximal end of the optic nerve, keep this position in the anterior part of the chiasma. Then the crossing (nasal) macular fibers get separated from the uncrossed fibers and pass as a bundle obliquely backwards and upwards to decussate in the posterior most part of the chiasma, which is related to the supraoptic recess. Lesions here may produce central temporal hemianopic scotoma.17
Fig. 12: Arrangement of visual fibers in the optic chiasma UN — Upper nasal fibers; LN — Lower nasal fibers; UT — Upper temporal fibers; LT — Lower temporal fibers; M — Macular fibers
OPTIC TRACT
In the chiasma, crossed and uncrossed fibers are intermingled and when they reach the optic tract they are rearranged to correspond with their position in the LGB. The macular fibers occupy area of the cross-section dorsolaterally. The fibers from the lower retinal quadrants are lateral and those from the upper are medial.
LATERAL GENICULATE BODY
The fibers from the upper part of the retina go to the medial part of the LGB and those from below to the lateral part. The macular area is somewhat cuneiform and is confined to the posterior two/third of the nucleus, broadening towards the caudal pole.
The neurons of the LGB go to the visual cortex. The axons of the ganglion cells synapse with the dendrites of the neurons of the LGB. There is a regular point-to-point localization of the retina in the LGB nucleus, which is also carried from the latter to the visual cortex. The LGB has 6 lamina (1–6). The crossed fibers end in the laminae 1, 4 and 6 while the uncrossed fibers end in the laminae 2, 3 and 5, in such a way that those from the corresponding parts of the two retinae end in neighboring part of the adjacent laminae. Therefore, the smallest lesion of the retina results in degeneration of three laminae of the LGB in which the retinal fibers end. Hence, the conducting unit in optic nerve fibers is a 3-laminae unit. Since the optic radiations commence from all the six laminae (6-laminae unit), so a lesion in the visual cortex results in degeneration of all the six laminae of the LGB.
OPTIC RADIATIONS
In the optic radiations, there occurs a temporal rotation of the fibers. So, the upper retinal fibers occupy the upper part of the optic radiations and the lower retinal fibers occupy the lower part of the optic radiations. The macular fibers lie in the central part of the optic radiations separating the upper retinal fibers from the lower retinal fibers.
VISUAL CORTEX
The visual cortex is also called the cortical retina, since a true copy of the retinal image is formed here. It is only in the visual cortex that the impulses originating from the two retinae meet. There is a point-to-point representation of the retina in the visual cortex. The right visual cortex is concerned with perception of objects situated to the left of the vertical median line in the visual fields and left visual cortex with the objects situated to the right half. In other words, the right visual cortex receives impulses arising from the temporal half of the right retina and nasal half of the left retina.19
The left visual cortex receives impulses arising from the temporal half of the left retina and nasal half of the right retina.
The visual fibers contained in the optic radiations are relayed in the visual cortex in the following manner. Fibers from the macular area relay in an extensive area placed posteriorly in the visual cortex. Fibers from the peripheral retina end anterior to the macular fibers. Those from the upper retina go above the calcarine sulcus.
BLOOD SUPPLY OF THE VISUAL PATHWAY
INTRODUCTION
The visual pathway receives its blood supply from the two arterial systems, the carotid and the vertebral connected to each other at the base of the brain by the circle of Willis. The branches of the carotid system which contribute to the blood supply of the visual pathway are ophthalmic artery, small branches of the internal carotic artery, posterior communicating artery, anterior cerebral artery and middle cerebral artery. The arteries of the vertebral systems are cortical, central and choroidal branches from the posterior cerebral artery. Similar to the brain, the visual pathway is mainly supplied by the pial network of vessels except the orbital part of the optic nerve, which is also supplied by an axial system derived from the central retinal artery.
RETINA
Choriocapillaries supply the outer four layers of the retina—the pigment epithelium layer, the layers of rods and cones, external limiting membrane, and the outer nuclear layer.
Central retinal artery supplies the inner six layers—outer plexiform layer, inner nuclear layer, inner plexiform layer, ganglion cell layer, nerve fiber layer, and internal limiting membrane.
The outer plexiform layer gets dual blood supply from chorio capillaries and central retinal artery. The rational vessels are end arteries.
OPTIC NERVE
See Anatomy of the Optic Nerve.
OPTIC CHIASMA
The vessels may enter the chiasma directly or through the pial plexus. The main blood supply of the chiasma is through the anterior cerebral artery and the internal carotid artery.
- The inferior aspect of the chiasma is supplied by branches form the internal carotid artery and the posterior communicating artery. A branch from the ophthalmic artery supplies the anteroinferior margin of the chiasm.The venous drainage of the chiasma is as follows:
- The superior aspect of the chiasma is drained by the superior chiasmal vein which ends in the anterior cerebral vein
- The inferior aspect of the chiasma is drained by the preinfundibular vein which drains into the basal vein.
OPTIC TRACT
The pial plexus supplying the optic tract receives contributions from:
- Posterior communicating artery
- Anterior choroidal artery
- Branches from the middle cerebral artery.
Though there is no anastomosis, there is considerable overlapping in the blood supply of the optic tract by the anterior choroidal artery and the branches of the middle cerebral artery. Therefore, occlusion of the anterior choroidal artery does not result in hemianopia.
The venous drainage from the superior aspect of the optic tract is through the anterior cerebral vein and from the inferior aspect of the optic tract through the basal vein.22
LATERAL GENICULATE BODY
The blood supply of the LGB is as follows:
- Posterior cerebral artery supplies the posteromedial aspect of the LGB and thus nourishes the fibers coming from the superior homonymous quadrants of the retina
- Anterior choroidal artery almost solely nourishes the anterolateral aspect of the LGB and thus supplies the fibers coming from the inferior homonymous quadrants of the retina
- The region of the hilum, which contains the macular fibers, is supplied by a rich anastomosis from both the posterior cerebral and the anterior choroidal arteries.Venous drainage of the LGB is through the basal vein.
OPTIC RADIATIONS
The blood supply of the optic radiations is as follows:
- Anterior choroidal artery supplies the optic radiations anteriorly
- Deep optic artery (lateral striate artery) which is a branch of the middle cerebral artery supplies the middle part of the optic radiations
- Calcarine branches of the posterior cerebral artery and perforating branches from the middle cerebral artery supply the posterior part of the optic radiations as the fibers spread out to reach the visual cortex.
Venous drainage from the optic radiations is mainly by the basal vein and in some parts by the middle cerebral vein
VISUAL CORTEX
Blood supply of the visual cortex is by:
- Posterior cerebral artery supplies the visual cortex mainly through the calcarine artery
- The terminal branches of the middle cerebral artery supply the anterior end of the calcarine sulcus and the lateral aspect of the occipital pole. At the posterior pole, there exists a rich anastomosis between the posterior and middle cerebral artery.
Venous drainage from the medial aspect of the occipital cortex is by the internal occipital vein, which ends into the great cerebral vein of Galen and straight sinus. The superolateral aspect of the cortex drains into the inferior cerebral vein, which ends in the cavernous sinus.
LESIONS OF THE VISUAL PATHWAY AND FIELD DEFECTS
OPTIC NERVE TYPE FIELD DEFECTS
Retinal nerve fibers enter the optic disk in a specific manner. So, nerve fiber bundle defects are of three basic types:23
Papillomacular Bundle
Macular fibers enter the temporal aspect of the disk. A defect in this bundle of nerve fibers results in one of the following:
- Central scotoma—a defect covering central fixation
- Centrocecal scotoma—a central scotoma connected to the blind spot (the cecum)
- Paracentral scotoma—a defect of some of the papillomacular fibers lying next to but not involving central fixation.
Arcuate Nerve Fiber Bundle
Fibers from the retina temporal to the disk enter the superior and inferior poles of the disk. A defect in these bundles may cause any of the following:
- Seidel scotoma—a defect in the proximal portion of the nerve fiber bundle causes a comma-shaped extension of the blind spot called a Seidel's scotoma
- Bjerrum, arcuate or scimitar scotoma—this arcuate portion of the field at 15 degrees from fixation is known as Bjerrum's area
- Isolated scotoma within Bjerrum's area—this is due to a defect of the intermediate portion of the arcuate nerve fiber bundle
- Nasal step of Ronne—a defect in the distal portion of the arcuate nerve fiber bundles produces a nasal step of Ronne. Since the superior and inferior arcuate bundles do not cross the horizontal raphe of the temporal retina, a nasal step defect respects the horizontal (180 degrees) meridian.
Nasal Nerve Fiber Bundle Defects
Fibers that enter the nasal aspect of the disk course in a straight (nonarcuate) fashion. The defect in this bundle results in a wedge-shaped temporal scotoma arising from the blind spot and does not necessarily respect the temporal horizontal meridian.
Remember, nerve fiber bundle defects arise from the blind spot and not from the fixation point. They do not respect the vertical meridian but respect the nasal horizontal meridian. If a person has a quadrantic field defect, then check if the field defect originates from the fixation point or from the blind spot. If it originates from the fixation point it is a retrochiasmal lesion and if it originates from the blind spot it is an optic nerve lesion. Other findings to check for an optic nerve lesion is decreased visual acuity, which generally will not occur in retrochiasmal lesions.
OPTIC CHIASMA LESIONS
Fig. 20: Quadrantic field defects – differentiation between retrochiasmal lesion and an optic nerve lesion
Fig. 21: Lesions in the optic chiasma UN — Upper nasal fibers; LN — Lower nasal fibers; UT — Upper temporal fibers; LT — Lower temporal fibers; M — Macular fibers
Bitemporal Hemianopia
The nasal retinal fibers including the nasal half of the macula of each eye cross in the chiasma, to the contralateral optic tract. The temporal fibers remain uncrossed. Thus, a chiasmal lesion will cause a bitemporal hemianopia due to interruption of the decussating nasal fibers.
Central Bitemporal Hemianopia
Macular crossing fibers pass in the posterior part of the chiasma and are related to the supraoptic recess. Lesions here can produce a central bitemporal hemianopia.
Junctional Scotoma
A central scotoma in one eye with a superotemporal quadrantic defect in the other eye indicates a lesion at the junction of the optic nerve (RE in this case) and the chiasma. The lower nasal fibers cross in the chiasma and course anteriorly approximately 4 mm in the contralateral optic nerve. This is Wilbrand's knee. Then they turn back to join uncrossed lower temporal fibers in the optic tract. A lesion involving the Wilbrand's knee creates the junctional scotoma. An important gem to remember this is the J Lawton Smith super gem. If a patient comes with poor vision in the right eye, the important eye for visual field examination is the left eye. There may be an upper temporal defect with respect for the vertical meridian, due to involvement of the Wilbrand's knee. The lesion is now intracranial at the junction of the right optic nerve and chiasma. The field defects constitute a junctional scotoma.
Upper Temporal Quadrantic Defects
The lower nasal fibers travel low and anteriorly in the optic chiasma. Thus, pituitary tumors can affect them. Thus, they produce upper temporal quadrantic defects.
Lower Temporal Quadrantic Defects
The upper nasal fibers travel high and posteriorly. Thus, a lesion from above the chiasma like a craniopharyngioma can produce a lesion here. These produce a lower temporal quadrantic defect.
OPTIC TRACT LESIONS
All retrochiasmal lesions result in a contralateral homonymous hemianopia. In the optic tracts and LGB, nerve fibers of corresponding points do not yet lie adjacent to one another. This leads to incongruous visual field defects. When we use the term congruous it means homonymous hemianopic defects that are identical in all attributes like location, size, shape, depth and slope of margins.27
Thus in optic tract lesions, there is an incongruous homonymous hemianopia.
LATERAL GENICULATE BODY LESIONS
A lesion in the lateral geniculate body is extremely rare. Two types of defects can occur. They are:
- Incongruous homonymous hemianopia
- Relatively congruous homonymous horizontal sectoranopia associated with sectorial optic atrophy. This is due to vascular infarction of the LGB.
OPTIC RADIATIONS AND VISUAL CORTEX LESIONS
Various lesions can occur in the optic radiations and visual cortex. Depending on the site of lesion, various field defects can occur.
Temporal Lobe Lesions
Inferior fibers course anteriorly from the LGB into the temporal lobe, forming Meyer's loop, approximately 2.5 cm from the anterior tip of the temporal lobe. They are separated from the superior retinal fibers, which course directly back in the optic radiations of the parietal lobe. Anterior temporal lobe lesions tend to produce midperipheral and peripheral contralateral homonymous superior quadrantanopia. This is called a pie in the sky field defect.
Parietal Lobe Lesions
The superior fibers cross directly through the parietal lobe to lie superiorly in the optic radiations. The inferior fibers course through the temporal lobe (Meyer's loop) and lie inferiorly in the optic radiations. Thus, there is a correction of the 90 degree rotation of the visual fibers that occurred through the chiasma into the tracts. Parietal lobe lesions tend to produce contralateral inferior homonymous quadrantanopia as they affect the superior fibers first.
Occipital Lobe Lesions
Central homonymous hemianopia In the visual cortex, the macular representation is located on the tips of the occipital lobes. A lesion affecting the tip of the occipital lobe tends to produce a central homonymous hemianopia.
Macular sparing The macular area of the visual cortex is a watershed area with respect to the blood supply.
Terminal branches of the posterior cerebral and middle cerebral arteries supply the macular visual cortex. Only the posterior cerebral artery supplies the visual cortex subserving midperipheral and peripheral fields. A more proximal (not terminal) vessel supplies the area.29
Fig. 25: Field defect due to temporal lobe lesions. Anterior temporal lobe lesions of Meyer's loop produce incongruous midperipheral and peripheral contralateral homonymous superior quadrantanopia. This is a pie in the sky field defect. This case is an example of a right temporal lobe lesion
Therefore, when there isobstruction of flow through the posterior cerebral artery, ipsilateral macular visual cortex may be spared, because of blood supply provided by the terminal branches of the middle cerebral artery. This may be an explanation of macular sparing. However, when there is a generalized hypoperfusion state (e.g. intraoperative hypotension), the first area of the visual cortex to be affected is that supplied by terminal branches, the macular visual cortex, resulting in a central homonymous hemianopia. To say the patient has macular sparing at least 5 degrees of the macular field must be spared in both eyes, on the side of the hemianopia.
Temporal crescents When we fixate with both eyes and achieve fusion of the visual information gained by both eyes, there is superimposition of the corresponding portions of the visual fields—the central 60 degrees radius of field in each eye. There remains in each eye, a temporal crescent of field for which there are no corresponding visual points in the other eye. This temporal crescent of field, perceived by a nasal crescent of retina, is represented in the contralateral visual cortex, in the most anterior portion of the mesial surface of the occipital lobe along the calcarine fissure. If a patient has a homonymous hemianopia with sparing of the temporal crescent, the patient has an occipital lobe lesion, since this is the only site where the temporal crescent of fibers are separated from the other nasal fibers of the contralateral eye.
Riddoch phenomenon This is a rare visual field sign. Riddoch believed that patients with severe field loss from occipital lobe involvement perceive from and movement separately. He postulated that perception of movement recovers before perception of form and that this phenomenon was of some prognostic value for recovery of field. This phenomenon is illustrated in the patient with extensive dense homonymous hemianopia as a result of an occipital lobe lesion. The patient cannot see a large stationary object in the blind field but can see a smaller object, if it is moving.
Altitudinal defect Injury to both occipital poles may result in altitudinal field defects. When the upper portions of the visual cortex or posterior radiation are damaged, the resultant field defects are altitudinal with loss of the entire lower field of vision of both eyes. If the lower portion of the lobes are damaged, death usually occurs after intracranial bleeding as a result of laceration of dural sinuses.
REFERENCES
- Sunita Agarwal, Athiya Agarwal, et al. Textbook of Ophthalmology 4th vol; Jaypee, India 2003.
Fig. 26: Field defect due to parietal lobe lesions. Parietal lobe lesions affect the superior fibers first and so produce a contralateral inferior homonymous quadrantanopia. This is a case of a right parietal lobe lesion
