Understanding Visual Field Defects




CHAPTER OUTLINE



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  • OPTICS AND MEDIA



  • RETINA




    • Organization



    • Visual field defects



  • NERVE FIBER LAYER/OPTIC DISC




    • Organization



    • Visual field defects



  • THE CHIASM




    • Organization



    • Visual field defects



  • OVERVIEW OF RETROCHIASMAL DEFECTS




    • Homonymous visual field patterns



    • Congruous versus incongruous



    • Effect on visual acuity



  • OPTIC TRACT



  • LATERAL GENICULATE NUCLEUS




    • Organization



    • Visual field defects



  • OPTIC RADIATIONS




    • Organization



    • Visual field defects



  • OCCIPITAL LOBE




    • Organization



    • Visual field defects



  • KEY POINTS





INTRODUCTION



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This chapter explores how the organization of the eye and visual system dictates specific, recognizable patterns of visual field loss in disease. The principles to be discussed in this chapter generally apply to all forms of perimetry (eg, confrontation, automated perimetry, Goldmann perimetry, and other methods that were discussed in Chapter 2). In addition to the visual field examples given in this chapter, the reader is encouraged to look at additional examples of automated and Goldmann perimetry (referenced in this chapter) found throughout the book.



As discussed in Chapter 2, the visual field extends approximately 90° temporally, 60° nasally, 70° superiorly, and 70° inferiorly from the fixation point in each eye (see Figure 2–14). Thus, the area of vision in each eye is roughly oval, with more area temporal to the fixation point than nasal. Although we generally measure and display the visual field one eye at a time, it is important to remember that the visual fields from the right and left eyes actually overlap. Motor systems (discussed in Section III) keep both eyes trained on a common fixation point. Therefore, there is binocular representation of the visual field for 60° to the right and left of a common fixation point. Note that the most temporal portion of the visual field from 60° to 90° is seen by only one eye (Figure 3–1). This area of the visual field is known as the temporal crescent.




Figure 3–1.


Superimposed visual fields.

The visual fields from the right and left eyes overlap with binocular viewing. The area from 60° to 90° in the temporal field of each eye is seen only by one eye (the temporal crescent). Note the position of the blind spot for each eye (L = blind spot of left eye, R = blind spot of right eye).





The afferent visual system includes the eyes (optics, media, and retina), optic nerves, chiasm, optic tracts, lateral geniculate nuclei, optic radiations, and visual (occipital) cortex (Figure 3–2). Visual information is transmitted by bundles of axons that represent specific portions of the visual field. The axonal wiring diagram seems complex on the surface, but actually follows a logical design.




Figure 3–2.


Afferent visual system.

The major components of the afferent visual system are identified. The inset shows the relative position of the visual system in the brain.





In this chapter, the organization of each segment of the visual pathway is discussed, with emphasis on how the anatomic organization determines identifiable patterns of visual field loss in disease.




OPTICS AND MEDIA



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Although the neural pathways for vision begin in the retina, the afferent visual system begins where light rays first encounter the eye. The tear film, cornea, pupil, and lens comprise an optical system that focuses incoming light to form an image of the world on the retina. Light rays converge, forming an inverted and reversed image on the retina, with a nodal point located near the posterior pole of the lens. This inverted visual representation is generally maintained throughout the visual system. Therefore, lesions in temporal retina cause nasal visual field defects. Lesions that affect the inferior portions of the visual system (in the retina, optic nerve, or even visual cortex) cause superior visual field defects.



Uncorrected refractive errors or disturbances in the normally clear media (such as cataract) cause a general loss of sensitivity of the visual field. Discrete opacities in the anterior media, such as a corneal scar or cataract with a particular shape, do not create corresponding focal visual field defects. Such opacities are not “imaged” on the retina, but rather they decrease the amount of focused light reaching the retina and cause a general blur. This condition is analogous to the hill of vision remaining intact, but sinking a few feet into the “sea of darkness.” Thus, anterior segment disorders cause diffuse depression (Box 3–1) of thresholds in static perimetry (Figure 3–3). This is analogous to constriction of the visual field with Goldmann testing, so called because the isopters are concentrically smaller than normal.



BOX 3–1. GENERAL TERMS USED TO DESCRIBE VISUAL FIELD DEFECTS


Congruency: The degree to which homonymous visual field defects in the right and left eyes resemble each other. The visual fields from each eye in optic tract lesions are homonymous, but tend to be incongruous (defects have different shapes and depths of visual field loss). Occipital lobe lesions cause highly congruous homonymous visual field defects.


Constriction: In a visual field that is overall poorly sensitive, a given isopter in kinetic perimetry encloses a smaller area than normal. The isopter (and the visual field) is said to be constricted, analogous to the intact hill of vision partially sinking into the sea of darkness. Concentric constriction suggests a global, uniform effect on the visual field.


Depression: In static perimetry, a point in the visual field is said to be depressed if the threshold is below normal. Diffuse depression in static perimetry is analogous to concentric constriction in kinetic perimetric terms.


Hemianopia: (hemi = one-half + an = without + opia = vision) This term describes visual field defects that encompass half of the visual field (eg, to the right or to the left of the vertical meridian). Technically, this term could apply to a visual defect in one eye only, but it usually refers to homonymous defects (see below).


Heteronymous: (hetero = different, opposite) This term describes defects that are present in the visual fields of both eyes but on opposite sides of the vertical meridian. This term is of little practical use, as the descriptors bitemporal or binasal are sufficient alone to designate the pattern as heteronymous.


Homonymous: (homo = same) Defects that are present in the visual fields of both eyes and are on the same side of the vertical meridian are said to be homonymous. Use of this term implies that the defect is thought to be retrochiasmal. Homonymous defects are right or left (the side of visual space affected), may be quadrantic or hemianopic, and are usually described as congruous or incongruous (see previous definition). For example, a right optic tract lesion may cause a left incongruous homonymous defect, and a left occipital lesion may result in a right homonymous hemianopia.


Meridian: A line that passes through the fixation point of a visual field representation. The vertical and horizontal meridians are the most important in describing and interpreting visual field results.


Quadrantanopia: Similar to hemianopia, except only a quadrant is affected. Suggests a homonymous defect with respect of both the horizontal and vertical meridians (see Table 3–1).


Scotoma: (Greek, “darkness”) A focal area of decreased sensitivity that is surrounded by normal visual field, analogous to a pothole or crater in the hill of vision. A scotoma is absolute if the patient cannot see the brightest possible stimulus (of the testing device), or may be relative if some stimuli are seen. Deep and shallow are also used to describe scotomas with reference to the hill of vision. Scotomas can be described by their location (cecocentral, central, paracentral) or shape (arcuate, altitudinal, ring). More than one scotoma should technically be called scotomata, but modern usage allows scotomas.


Visual field: This term can mean one of two things: the general concept of an expanse of space that can be perceived by the eye; or an actual plot, drawing, computer readout, or map resulting from a visual field test.





Figure 3–3.


Effect of anterior media opacities on the visual field.

Opacities in the anterior segment are not imaged on the retina, but rather decrease the amount of focused light overall, causing a generalized depression. This patient with cataract and glaucoma had diffuse depression of the visual field (upper panel) that cleared following cataract surgery (lower panel). Observe that a small nasal step also present was successfully identified in the pattern deviation, even in the presence of diffuse depression (arrow). This printout selection from the Humphrey Visual Field Analyzer combines sequential visual fields in a format that allows easy comparison. The computer can also perform statistical comparisons of visual fields performed at different times.






RETINA



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ORGANIZATION



The retina collects the image formed by focused light rays, translating subtle shapes, shadows, and colors into an image map of electrical impulses. Its organization seems somewhat counterintuitive as the light-detecting cells—the photoreceptors—are buried in the deepest layers away from the incoming light (Figure 3–4). However, the photoreceptors play a metabolically demanding role in the creation of vision, and their location in the outer retina allows constant nurturing by the retinal pigment epithelium and the choroidal blood supply.




Figure 3–4.


Retinal organization.

A schematic cross-section of the retina. Note that the light must pass through several layers of the retina to get to the rods and cones. This arrangement places the metabolically demanding photoreceptors in proximity to retinal pigment epithelial cells and the high-flow choroidal circulation. Ganglion cells in the inner retina receive input from a number of photoreceptors, transmitted and modulated by bipolar cells. Horizontal cells interconnect and modulate adjacent photoreceptor cells, and amacrine cells interconnect and sample groups of bipolar cells. Ganglion cell axons travel over the surface of the inner retina as the nerve fiber layer to exit the eye as the optic nerve. (Modified from Haines D: Fundamental neuroscience. Philadelphia: Churchill Livinstone, 2006.)





Each retina contains more than 125 million photoreceptors, but only about one million axons leave the eye. This is because neurons in the middle retinal layer process the image information from the photoreceptors, conveying a refined signal to the ganglion cells in the inner retina. Ganglion cell axons flow over the innermost layers of the retina, converging at the neuroretinal rim of the optic disc to exit the eye as the optic nerve.



In the peripheral retina, a single ganglion cell receives information from a thousand or more photoreceptor cells, whereas a ganglion cell in the fovea may receive information from only a few, or even a single photoreceptor. This ratio of photoreceptors to ganglion cells, as well as the greater concentration of ganglion cells in the macula, accounts for the much greater spatial discrimination of the fovea compared to the peripheral retina. In essence the “pixels” (receptive fields) are much finer in the center and rather coarse in the periphery. Most of the photoreceptors in the macula are cones, with cone density decreasing rapidly toward the periphery. Rod photoreceptors are virtually absent in the fovea, increasing and reaching a peak in concentration in the midperiphery, then decreasing again more peripherally (Figure 3–5). The cone receptors are most sensitive in bright light and the rods operate best in dimmer light. Therefore, patients who depend mainly on rod function (cone dystrophies or other causes of central scotomas) avoid brightly lit conditions (hemeralopia), and those patients with rod dysfunction (retinitis pigmentosa affecting rods) have night blindness (nyctalopia).




Figure 3–5.


Distribution of rods and cones in the retina.

There are approximately 7 million cones in the human retina, virtually all of which are concentrated in the macula. There are about 120 million rods, in greatest concentration about 15° to 30° from the foveola. (Data from Osterberg G: Topography of the layer of rods and cones in the human retina. Acta Ophthalmol [suppl 6]:8, 1935.)





VISUAL FIELD DEFECTS



Later in this chapter we will discuss how the organization of the visual system produces natural “straight-edged” boundaries along the vertical or horizontal meridians, or shape constraints defined by the sweeping nerve fiber layer. Lesions in the deep (outer) retina have no such constraints, and typically produce more nebulous shapes, defined primarily by the shape of the lesion. For example, in the predominantly rod dystrophy of retinitis pigmentosa, the central macula (mostly cones) is spared, with a donut-shaped ring scotoma in the midperiphery (Figure 3–6) that corresponds to a relatively higher density of rods (as illustrated in Figure 3–5).




Figure 3–6.


Outer retinal disease: retinitis pigmentosa.

A 75-year-old woman described a lifelong gradual decline in the vision of both eyes, saying that objects seemingly disappear and reappear from view. She was diagnosed with retinitis pigmentosa 15 years earlier—a diagnosis she shares with her mother and maternal grandfather. (A) Fundus photograph demonstrates slight disc pallor, arteriolar narrowing, retinal pigment epithelial (RPE) hypopigmentation, and pigment clumping. (B) RPE pigmentation forming “bony spicules” is seen in the retinal midperiphery. (C) Goldmann visual fields show a “ring scotoma” with relative sparing of the central visual field and far periphery. Objects are easily “lost” in the scotoma with movement in the visual field or with changes in fixation. No respect of the vertical or horizontal meridian is present. Because the inner retinal layers are not affected, the visual field defect does not follow patterns dictated by the nerve fiber layer. The pattern of visual field loss in this patient with disease primarily affecting the rods corresponds to the relatively greater concentration of rods in the midzone of the retina (see Figure 3–5). (D) The bright-flash electroretinogram (ERG) is a flat line, demonstrating that this is a disorder of photoreceptors and the outer retina (compare to normal ERG in Figure 2–28B).





In the macular area the receptive fields are small; deep retinal lesions generally produce focal visual field defects of a corresponding shape (Figure 3–7). However, outside of the macula the receptive field increases in size; focal lesions do not cause discrete corresponding scotomas, but rather a more ill-defined depression. For example, the multiple lesions produced from peripheral panretinal photocoagulation cause global constriction of the visual field, rather than hundreds of small scotomas. Lesions in the retina are generally visible with the ophthalmoscope, unlike lesions in the more posterior visual pathways.




Figure 3–7.


Visual field defect with focal macular disease.

A 45-year-old woman has reactivation of a macular histoplasmosis chorioretinal lesion. (A) The lesion is located just superior and nasal to the foveola (crosshairs) in the left eye. (B) The visual field defect corresponds precisely to the location and extent of the visible lesion. Observe that the visual field plot is presented as it would be seen by the patient’s left eye. The optics of the eye project an inverted image of the world on the retina. Thus, the physiological blind spot is seen on the left side (in the temporal visual field), and the lesion projects a scotoma just inferior and temporal to fixation.






NERVE FIBER LAYER/OPTIC DISC



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ORGANIZATION



From ganglion cells throughout the retina, axons travel in the nerve fiber layer of the inner retina and converge to form the optic disc, positioned approximately 15° nasal to the optical center of the eye (foveola). The path from each of the ganglion cells scattered throughout the retina to the optic nerve is not necessarily a straight line; the high sensitivity and discrimination of the foveola is preserved by routing all axons around the foveola on their journey to the optic disc. Even the middle and inner retinal layers are seemingly pulled radially from the foveola to minimize any potential interference of a finely focused image on the sensitive foveola. This pattern of axonal routing creates a curious road map of axons in the inner retina: Axons originating temporal to the foveola must arch above or below the foveola. The horizontal temporal raphe is thus created because all ganglion cells above the level of the foveola send their axons arching superiorly, and those below this horizontal line send axons inferiorly. Axons from ganglion cells on the nasal side of the disc can travel directly to the optic disc, creating a radial pattern (Figure 3–8). Axons from ganglion cells between the optic disc and the foveola also have a relatively straight path. The highly concentrated ganglion cells in the macula create a concentrated sheaf of nerve fiber layer entering the temporal disc: the papillomacular bundle.




Figure 3–8.


Patterns of visual field loss: optic nerve/nerve fiber layer.

Nasally, the nerve fibers have a relatively straight course to the optic disc, but temporally, the nerve fibers arch above and below the foveola. Interruption of these axons produces distinct patterns. Note that the visual field defects in A–E can occur above or below the horizontal meridian, although only one example is shown. (A) Nasal step (see Figures 4–13B, 4–18D, 4–26B). (B) Arcuate scotoma (see Figure 3–9). (C) Altitudinal visual field defect (see Figures 4–10, 4–26C, 6–3A). (D) Temporal wedge (see Figure 3–10). (E) Cecocentral scotoma (see Figures 4–32, 4–33, 4–34, 4–35).





VISUAL FIELD DEFECTS



Diseases affecting axons in the optic nerve, optic disc, or nerve fiber layer (inner retina) demonstrate patterns of visual field loss that parallel the nerve fiber arrangement, frequently with sharp borders that respect the horizontal meridian (Table 3–1). Because the axons converge on the optic disc, many optic-nerve-related visual field defects connect with, or point toward, the blind spot. Common patterns of optic nerve disease that reflect the organization of the nerve fiber layer include nasal steps, arcuate defects, altitudinal defects, cecocentral scotomas, and temporal wedges.




TABLE 3–1.

RESPECT OF THE VERTICAL AND/OR HORIZONTAL MERIDIAN





Nasal Steps


Nasal step defects are caused by optic nerve disorders that affect the long, arching axons that originate temporal to the macula, entering the disc superiorly or inferiorly. A nasal step may begin as a small depression above or below (and respecting) the horizontal meridian in the nasal visual field (see Figure 3–8A). Although a nasal step does not actually connect to the blind spot, progression of the visual field defect advances along an arcuate path that points toward the blind spot, eventually forming an arcuate scotoma. Nasal steps are so common in optic neuropathies (including glaucoma) that most visual-field testing strategies pay extra attention to the nasal visual field.



Arcuate Scotomas


Arcuate scotomas reveal the sweeping path of those axons that arch around the foveola. They may be broad, as in the extension of a nasal step toward the blind spot (see Figure 3–8B), or well-defined narrow arches framing the fixation point (Bjerrum scotoma). Other variations include an extension of the blind spot (or “baring of the blind spot” in Goldmann terms) along the arcuate path (Siedel scotoma), and isolated scotomas in the arcuate bundle (Figure 3–9).




Figure 3–9.


The spectrum of arcuate scotomas.

All of the visual fields shown are from the same patient over time, with primary open-angle glaucoma (this patient’s optic discs can be seen in Figure 4–38). (A) October 2008: 24-2 Humphrey visual field shows an extension of the blind spot superiorly (Seidel scotoma). (B) August 2009: The extension of the blind spot has extended in an arcuate fashion superior to fixation (Bjerrum scotoma). (C) November 2010: Further progression of the scotoma, now more altitudinal. The apparent respect of the vertical meridian in the grayscale presentation (left) is potentially misleading—the scotoma actually extends across the vertical meridian, as seen in the total deviation plot (right). (D) November 2011: The scotoma is clearly altitudinal now, with absolute respect of the horizontal meridian, extending across to the nasal visual field. (E) The left eye in this patient has maintained a broad arcuate scotoma, approaching a complete altitudinal defect.





Altitudinal Defects

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Jan 2, 2019 | Posted by in OPHTHALMOLOGY | Comments Off on Understanding Visual Field Defects
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