HEMIANOPSIA --and neuroanatomy

Paul Pietsch, PhD, Professor Emeritus, School of Optometry Indiana University Web Contact: pietsch@indiana.edu

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INTRODUCTION

DEFINITIONS

FIELD DEFECTS AND NEUROANATOMY

QUIZZES

ENDNOTES

INTRODUCTION

Prerequisites (recommended)

• basic familiarity with the qualitative aspects of human visual fields
• an understanding of human neuroanatomy in general
• a working knowldege of the primary visual pathways in particular
• Instruction in the latter subject exists on the internet.

Caveats

DEFINITIONS

• Hemianposia (hemianopia) implies partial blindness in the visual fields of one or both eyes, and not necessarily half-blindness as the etymology of the term may suggest.
• Quadrantanopsia is a class of hemianopsia but confined to visual field quadrant(s).
• Scotoma is one or more islands of blindness within a visually patent or "seeing" zone. (The physiological blind spot is an example of a normal scotoma.)
  
• Homonymous implies correspondence and means (from ancient Greek) 'with the same name.' This adjective
  --can modify:
  • hemianopsia
  • quadrantanopsia
  • scotoma
  -- and refers to:
  • the corresponding visual fields of the two eyes (temporal and nasal' fields)
  • the corresponding parts of the visual pathways (e. g., temporal and nasal' hemiretinas -- T and N')
• Heteronymous is an occasionally used antonym of homonymous. Heteronymous implies a bilateral defect of the non-corresponding visual fields of an eye (i. e., T+T' or N'+N). In most contexts, the term is redundant, unnecessary and (mercifully) seldom used, outside of some textbooks. (Why? To say, for instance, 'a bitemporal hemianopsia' is to imply the involvement of the temporal fields of both eyes. Since the two temporal fields are non-corresponding, and ipso facto heteronymous, why engage in prolixity (and thus risk swallowing the tongue? The same is true of the rare ,but extant, binasal hemianopsia.)
• By convention, a field defect is named for the field, not the pathway nor the projection onto the retina. Recall that the fields rotate 180^o. The axis of rotation passes through the center of the field and the foveola of the retina. Thus in maps, the antomical pathways are the reverse of visual fields.
• When referring to a homonymous hemianopsia, "right" or "left" signify either the right or the left half of the binocular visual field (even though the mapping typically is done one eye at a time). In other words, a left homonymous hemianopsia involves the left eye's temporal field (which projects onto the left nasal hemiretina) and the right eye's nasal field (and right temporal hemiretina).
• Note: when discussing a unilateral defect, some authors use "right" and "left" to refer to the field of an affected eye (not to the binocular field); e. g., by a "right nasal hemianopsia," those authors mean a blindness in the right eye's nasal field. (Suggestion: one can avoid confusion by including 'eye' in the description; e. g., a 'nasal hemianopsia of the right eye' (which would involve the right eye's nasal field and, therefore, its T hemiretina ); or, a 'temporal hemianopsia of the left eye' would involve the left eye's temporal field and, therefore, those portions of its visual pathways that originate in its nasal hemiretina.
  
• Altitudinal hemianopsia refers to a field defect above or below the equator with the blinded zone extending across the prime meridian (midline), thus simultaneously involving both the crossed and uncrossed parts of the visual pathways.
• Altitudinal defects can be:
  • superior, inferior or mixed
  • unilateral or bilateral
  • quadrantic or hemispheric
  • scotomatous
• At the optic chiasm nasal fibers cross, temporals fibers do not.
• Therefore, posterior to the optic chiasm, homonymous members of the pathways are T+N' or T'+N.
• A hemianopsia may involve all or part of a field, or all or part of a quadrant. Therefore, hemianopsias (and quadrantanopsias) are referred to as
  • complete (total) or
  • incomplete (partial).
• Geometrically speaking, the corresponding blind zones in an incomplete hemianopsia can be either
  • congruous
  • or incongruous.
• Compound hemianopsias (complex, mixed or superimposed), although rare, do exist. Some are the result of more than one lesions. (Appreciate that hemorrhage in the posterior cerebral artery, for instance, would not necessarily preclude, say, a sclerotic internal carotid artery from abrading the lateral edge of the optic chiasm.)
  Other compound hemianopsias can be a consequence of a single lesion; (e. g., an aneurysm at the arterial circle of Willis could impinge upon the terminal portion of the optic nerve with involvement of both ipsilateral optic nerve fibers and contralateral optic nerve fibers forming the anterior genu; more on this later in the lesson).
• Physiological Blind Spots map in the temporal fields (and are thus heteronymous)
• Monocular sectors (temporal crescent) are the far peripheral portions of the visual fields -- what disappears on one side when an eyelid is shut. A temporal crescent registers on only the nasal hemiretina of the ipsilateral eye. Since nasal fibers cross (decussate) at the optic chiasm, the temporal crescents register exclusively on the opposite side of the brain.
• In the field diagrams used in the following discussion,
  • black is the blind zone, white the seeing (visually patent) zone
  • blind spots, essential in in clinical perimetry, have usually been omitted in the illustrations for simplicity.
  • also, for the sake of simplicity, no attempt is made to represent the monocular fields (click to see);
  • the left and right field exemplars correspond to the reader's left right.

FIELD DEFECTS AND NEUROANATOMY

Generalizations

A. Total Blindness, right eye B. Nasal hemianopsia of right eye C. Left homonymous hemianopsia a. with macular splitting b. with either macular splitting or macular sparing D. Bitemporal (heteronymous) hemianopsia Reference: Carpenter, M.B. and Sutin, J. Human Neuroanatomy, Williams and Wilkins, Baltimore and London, 1983, p. 544.

Points about the generalizations

• A. Complete destruction of an optic nerve obviously blinds the eye it serves. But unilateral defects involving both the nasal and temporal halves of an eye's field tend to be pre-chiasmal. Why? Recall that at the optic chiasm: temporal and nasal fibers from an eye diverge, the nasal fibers crossing to the opposite side of the brain, the temporals remaining on the same side. (The right and left pathways do converge towards the occipital lobes; but recall that the tough, fibrous falx cerebri of the dura mater separates the right and left lobes. A lesion capable of biaterally damaging the visual cortex would doubtless be massive and would have long since manifest itself in other ways.)
  • Many forms of injury and disease can directly or indirectly cause minute foci of damage in the retina, thus creating a scotomas. But because of the complex arrangement of fibers either at or en route to the optic disc (e.g., the papillo-macular bundle) such lesions can produce oddly shaped visual field defects and deficits out of proportion to their size.
  • Points in visual space closely correspond to foci on the retina. Remembered, however, that division of the retina into quadrants refers to rods and cones, not the retinal ganglion cell fibers in the eye or at the optic disc.
• B. Nasal field defects (from temporal portions of the pathways) are not mere theoretical possibilities but have been reported in the scientific literature; the anatomical basis will be considered below.
• C a and C b. Homonymous hemianopsia implies both a post-chiasmal lesion and a lesion on the side of the brain opposite the compromised half of the binocular field. An important question about the latter: is the homonymous hemianopsia associated with macular splitting or macular sparing?
  
  Macular splitting can occur in lesions both before or after the LGB (lateral geniculate body or nucleus*). However, macular sparing tends to occur with lesions beyond the LGB. That both signs are a genuine phenomena, and not indicative of sloppy technique, was treated many years ago in a classic paper of Halstead and his coworkers.)
• D. Bitemporal hemianopsia, sometimes called 'tunnel vision' in lay terminology, is indicative of direct or indirect insult of the sagittal portion of the optic chiasm -- where the nasal fibers of the two eyes, as they cross, lie in close proximity. Before and after the chiasm, the two sets of fibers in question are physically separated.

SOME SPECIFIC HEMIANOPSIAS -- HEURISTIC EXAMPLES (7)

1. HOMONYMOUS HEMIANOPSIA

A. left, complete, congruous, with macular splitting

B. right, complete, congruous, with macular sparing

C. left, incomplete incongruous, with macular sparing

Figures A, B and C idealize field defects that would be consistent with lesions posterior to the optic chiasm.

In Figure A, the visual pathways on the right side of the brain would be involved (T+N' fibers); the left binocular field would lose its temporal crescent; but the nasal half of the left field projects to the right eye's temporal hemiretina and, therefore remains visible. The hypothetical patient, able to see the central and paracentral (near peripheral) fields, may not be conscious of the loss (consider general unawareness of the blind spot); and he or she may be surprised to learn of the perimetry data.
However, as Harrington observes, a complete and congruous homonymous hemianopsia with macular splitting is not a very informative field defect from a diagnostic standpoint. Why? Consider that the defect would be consistent with destruction of an optic tract; an LGB; an occipital lobe (as, for example, in the removal of the entire lobe [total right occipital lobotomy] to extirpate a tumor); i. e., the possibilities include virtually half the cranial contents.

Figure B shows macular sparing, a sign that the lesion probably -- but is not guaranteed -- to lie posterior to the LGB. But aside from that plus the fact that the afflicted pathways are on the opposite side of the brain, the field data are insufficent, in and of themselves, to locate the damaged site.

Figure C illustrates what is meant by incomplete, with reference to homonymous hemianopsia. Harrington also notes that visual field defects in this general category (incomplete) are the most common from lesions in the post-chiasmal portions of the visual pathways. Both the frequency and the incomplete character are a function of the anatomy of the optic radiations. Recall, the optic radiations fan out of the LGB, different fibers diverging in different directions and into different lobes of the cerebrum before converging toward the calcarine fissure. A lesion in one area may totally spare another:
Consider a diagram of a left LGB and left Optic Radiations:

In a coronal (frontal) section through the anterior occipital lobe, where the optic radiations are converging towards the calcarine fissure a map of field sectors looks like this:

(Note: the labels refer to fields, not fibers, per se.)
(Back to quadrantanopsias)

In the region idealized in Figure C, the affected radiations would most likely be those in the temporal and temporo-occipital cerebrum (down low) while those escaping would probably be in the parieto-occipital area (up high).

Concomitant neurological deficits may provide clues as to the location of lesions accompanying homonymous hemianopsias. Here are a few:

• Cortical blindness is a puzzling phenomenon, without adequate explanation, but of potential use in narrowing the site of the lesion to the visual cortex vis-à-vis the optic radiations or optic tracts. In cortical blindness, a patient with a frank and demonstrable hemianposia is subjectively unaware of the blindness, despite the objective evidence. ["My eyesight's just great, Doc..." even though it clearly is not.] If the denial sounds preposterous, recall that we are generally unaware , for example, of the physiological blind spot. (Note though, that while cortical blindness is an indicator of cortical involvement, absence of the sign does not necessarily rule out the cortex.)
• Lesions in structures neighboring on the optic tract may produce important comitants of partial blindness. Consider the intimate relationship of the optic tract to the uncus (of the temporal lobe) and to the cerebral peduncle (in the midbrain).

  (Note: the internal capsule is continuous with the cerebral peduncle.) For more scroll down!	Plane of sectioning:
  ("pre-tect." = pretectal area) U is the uncus.	optic tract

  The cerebral peduncle (aka crus cerebri or pes pedunculi) contains, among other things, the pyramidal (corticospinal and corticobulbar) tracts, and thus the output conduits for voluntary motor activity; thus a hemianopsia plus a hemiparesis (partial paralysis) -- or even just weakness -- would strongly suggest untoward events at work in the environs of the optic tract. Recall, the pyramidal decussation (crossing) occurs below the pons. Thus the motor involvement, like the visual field data, would pertain to the contralateral side of the central nervous system.
  Now the uncus is part of the olfactory system. Therefore, a lesion in or near the uncus can affect the sense of smell: olfactory agnosia (I can't smell no more, Doc!); or olfactory hallucinations (somebody vomit, Doc?).

• Injuries to the anterior temporal lobe can evoke psychomotor epilepsy: convulsive seizures sometimes accompanied by violent rage (possibly because of a damaged amygdaloid nuclear complex, which lies within the uncus).
  
• The hippocampus, rolled into the temporal lobe, is an important mediator of some short-term, working memory. Thus a hemianopsia plus recent loss of recall ability (with long-term memories intact) can also indicate a lesion near the optic tract.
• Now if one suspects damage to an optic tract, there are other useful colateral data, namely about pupillary light reflexes. Are they intact?
  
  
  • A lesion anterior to the branching of the brachium of the superior colliculus off the optic tract would prevent photic sensory data from reaching the pretectal area.
  • Intraocular reflexes could survive lesions posterior to the latter branching.
  • Thus information about the pupils and other intraocular reflexes (i. e., a thorough eye examination) can be useful in formulating a cogent hypotheses about visual field defects.

2. BITEMPORAL HEMIANOPSIA In bitemporal lesions, with the loss of both temporal crescents, the binocular fields shrink to produce what is commonly called, tunnel vision. The signals for the temporal fields are carried by fibers from the nasal hemiretinas; i. e., the crossed fibers. Thus the optic chiasm, where the crossed fibers (N + N') lie close together, is implicated in bitemporal hemianopsia. One source of such lesions is the close relationship between the optic chiasm and the pituitary gland in the underlying sella turcica:

Scroll for more --->

Hypophyseal tumors and pituitary hyperplasia can manifest themselves in bitemporal lesions. Consequences similar to hypophyseal involvement could conceivably obtain from increased hydrostatic pressure in cerebrospinal fluid within the hypothalamic recess of the 3rd ventricle. Appreciate, though, that in the latter situation, the crossing nasal fibers are still the mediators of the symptoms.

Circle of Willis

{back to homonymous scotoma}

The TS quadrant projects to the NI quadrant of the retina, served by inferior nasal fibers; these fibers hug the anterior-inferior margin of the optic chiasm and bow into the distal stump of the contralateral optic nerve, there to created the anterior genu (knee), before moving posteriorly into the optic tract.

In their seminal study of the primate visual pathways, Hoyt and Luis divided the optic chiasm into three strata:

The two investigators noted that macular fibers exist in all three strata; but most have been omitted here so as to focus attention on NI' fibers and thus the anterior genu in the inferior or lower stratum. (Note some artists represent a 'posterior genu' involving the SN' fibers. Hoyt and Luis found no such structure; none are represented here.)

left, superior, complete

Unilateral altitudinal field defects tend to involve one eye and most frequently the optic nerve. In the example, damage would probably be to the lower (inferior) fibers of the left eye -- both nasals and temporals. Bilateral altitudinal field defects may indicate a congenital condition (e. g., arterio-venus malformations) or a systemic disease. The critical feature in altitudinal hemianopsias is the extension of the field deficit(s) across the vertical meridian.

a. diffuse, unilateral

b. central, unilateral

c. homonymous, congruous

Figure a illustrates the kind of scotoma that would result from vascular lesions in the eye. The unilateral character of this defect suggests (but does not prove) that the pathology is in the vasculature of the involved eye. Bilateral diffuse scotomas point to systemic diseases (e. g., diabetes mellitus).
Figure b diagrams a scotoma that would occur if macular fibers were selectively damaged, as for example in retobulbar neuritis. Recall the organization of retinal ganglion cell fibers on the retina:

and how fibers of the papillomacular bundle lie on the lateral side of the retrobulbar portion of the optic nerve.
Figure c represents homonymous hemianopsia with scotomatous characteristics ('island of blindness'). The lesion is obviously posterior to the optic chiasm yet spares much of the pathway on the afflicted side. The probable site of the lesion is left side of the occipital pole -- where the upper and lower optic radiations converge and where central vision maps. Consistent with such field data would be pathological processes in posterior shoots of calcarine branches of the posterior cerebral artery.

A few parenthetical remarks about temporal and parietal lobe lesions:

• Lesions in the posterior temporal lobe can induce hearing deficits (and auditory hallucinations). The primary auditory cortex is on a subset of the superior temporal gyrus -- in the transverse gyri of Heschl (the auditory counterpart of the calcarine fissure in the visual pathways). The transverse gyri are buried within the relatively deep lateral (Sylvian) fissure.

  For a close look at Heschl's transverse gyri, scroll right-->

• As mentioned already, lesions of the anterior temporal lobe can account for olfactory deficits and hallucinations, as well as with psychomotor epilepsy.
  
• A generalization about temporal lobe lesions: the more anteriorly situated, the more "quadrantic" the corresponding field defects tend to be.
• Lesions high in the parietal lobe tend to produce inferior quadrantanopsias. (Note though, that the situation is not the mirror image of the temporal lobe; there is nothing in the medial radiations comparable to the loop of Meyer.)
• lesions of the superior part of the parietal lobe on the dominant side of the patient's brain (usually the left hemisphere) could affect cognitive language activity, such as writing. In the non-dominant parietal lobe, lesions can impair awarenss associated with sensory input. ("Where did I put my left foot, Doc?'')
• Wernicke's receptive speech aphasia may be a concomitant of damage to optic radiations in the parieto-occipital area. Wernicke's aphasia is marked by meaninglessness in the content of the speech (in contrast to Broca's expressive aphasia where the the ability to speak is impared).

  Broca's area is at the posterior end of the inferior frontal gyrus; Wernicke's area is on the supramarginal and angular gyri at the junction of the parietal and temporal lobes.

7. Nasal Hemianopsia

Here, the labelled internal carotid artery has been pried away to expose the lateral edge of the optic chiasm. A hardened internal carotid artery impinging upon the optic chiasm at this location could induce a nasal defect (by damaging the temporal fibers on that side.) The TI fibers lie laterally in the chiasm; the TS move deeper into the chiasm, aiming for the medial side of the optic tract, but conceivably vulnerable to pressure from the lateral direction. In the example presented -- binasal hemianopsia -- the field defect would be consistent with atherosclerosis affecting the intracranial portions of both internal carotid arteries. Note also, internal hydrocephalus with swelling of the narrow 3rd ventricle could force the chiasm against snug internal carotid arteries, pulsations in which could translate to and induce degeneration in fibers of the lateral chiasm.

QUIZZES

1____ 2___ 3___ 4___ 5___ 6___ 7___ 8___

Choices: A. BITEMPORAL HEMIANOPSIA (heteronymous) B. BINASAL HEMIANOPSIA (heteronymous) C. RIGHT HOMONYMOUS HEMIANOPSIA, with macular sparing D. COMPOUND HEMIANOPSIA -- total left eye blindness plus right superior temporal quadrantanopsia E. QUADRANTANOPSIA, HOMONYMOUS RIGHT SUPERIOR TEMPORAL, congruous F. LEFT HOMONYMOUS HEMIANOPSIA, incomplete, incongruous, with macular splitting G. CENTRAL SCOTOMA, homonymous, congruous H. LEFT ALTITUDINAL HEMIANOPSIA For the answer to quiz 1, click!

Harris' "Triagram" of the Visual Pathways

Dr. Randy Harris constructed this Triagram, or 3-D diagram, as a tactile aid for blind students in neuroanatomy. The color coding was to assist sighted instructors. The temporal crescents are representd in brighter hue and smoother texture than the binocular sector of the given quadrant. The coding was maintained for representatives of the retina and the fiber pathways through the optic tracts. {back to narrative}

ANSWERS

For a simple key, click The complete answers (for review purposes): 01. LEFT HOMONYMOUS HEMIANOPSIA, incomplete, incongruous, with macular splitting 02. BITEMPORAL HEMIANOPSIA (heteronymous) 03. RIGHT HOMONYMOUS HEMIANOPSIA, with macular sparing 04. COMPOUND HEMIANOPSIA -- total left eye blindness plus right superior temporal quadrantanopsia 05. CENTRAL SCOTOMA, homonymous, congruous 06. LEFT ALTITUDINAL HEMIANOPSIA 07. QUADRANTANOPSIA, HOMONYMOUS RIGHT SUPERIOR TEMPORAL, congruous 08. BINASAL HEMIANOPSIA (heteronymous) Back to quiz 1 (click)!

QUIZ 2 Visual field diagrams: left is left; black is blind.

A. This illustrates	(true or false): 01___binasal hemianposia 02___bitemporal hemianopsia 03___heteronymous field defects 04___effects of a contralateral post-chiasmal lesion 05___effects of an ipsialateral post-chiasmal lesion
B. This diagram	(true or false): 06___exhibits macular sparing 07___is consistent with a post-chiasmal lesion on the right side of the brain 08___is consistent with a post-chiasmal lesion on the left side of the brain 09___illustrates incompleteness 10___is a heteronymous hemianopsia 11___is a scotoma
C. These defects would be highly consistent with:	12___none of the facts of normal human anatomy 13___an aneurysm of the right anterior cerebral artery at its base 14___a rupture at the origin of the left anterior cerebral artery 15___an aneurysm in the left anterior communicating artery at its origin in the circle of Willis Click for key of Quiz 2

ENDNOTES

Halstead, W. C., et al. Sparing and Nonsparing of 'Macular' Vision Associated with Occipital Lobectomy in Man. Archives of Ophthalmology, vol. 24, pp. 948-966, 1940
{back to narrative}

Hoyt, W. F. and Luis, O. The primate chiasm: details of visual fiber organization studies by silver impregnation techniques. Arch. Opthalmol. 70:69-85, 1963.

*LGB versus LGN. In humans and other primates, the lateral geniculate body, LGB, is a composite nuclear complex and is homologous to both the dorsal and ventral lateral geniculate nuclei (LGN) of lower mammals. The ventral LGN is homologous to the magnocellular layers of the primate LGB and the ventral LGN with the parvicellular layers. 'LGB' is used here because of its greater inclusiveness than 'LGN' with reference to humans; but both terms are widely employed and virtuallty interchangeably.

1 F
2 A
3 C
4 D
5 G
6 H
7 E
8 B

(Back to answers for Quiz 1)

Key to Quiz 2