Dyschromatopsias Associated with Neuro Ophthalmic Disease

People with congenital dyschromatopsias are frequently not aware that their color perception differs from those with normal, trichromatic color vision. Others have learned to adapt to their limited perception of certain colors. Thus, a person with faulty red/green color discrimination will often describe a dark green color as red. Some of these people know that they cannot properly identify colors accurately in certain ranges of hue and luminance. Patients with acquired dyschromatopsias are often unaware of the changes in their color perception, due to the subtle onset and gradual progression of the damage. Not infrequently, this allows major color vision disturbances to go unnoticed. Unilateral dyschromatopsias - if not associated with cataract - should be examined with tests of color saturation in the affected eye, since this is frequently the clinical presentation for an optic neuropathy. Other neuro-ophthalmic disorders (see ■ Table 6.1) should initiate a targeted search for an acquired dyschromatopsia.

Table 6.1. Signs and symptoms indicating that one should test for a specific type of dyschromatopsia

Signs and symptoms

Suspected diagnoses

Tests (a partial selection)

Monocular loss of acuity

Desaturation, or a red/green color deficit as a sign of optic neuropathy or a tritan (blue) deficit suggesting macular disease.

Color saturation screening test with the panel D-15 or a complete Farnsworth-Munsell 100-hue test

Bilateral loss of acuity or visual field loss

Chorioretinal dystrophies, toxic retinopathy, cerebral disease

Velhagen, Hardy-Rand-Rittler plates I and II, panel D-15, Roth 28-hue, Farnsworth-Munsell 100 hue

Nystagmus

Congenital nystagmus, albinism, cone monochromacy, blue cone monochromacy

Blue/yellow plates, Roth 28-hue, Farnsworth-Munsell 100-hue

Dyschromatopsia

Congenital red/green dyschromatopsia, toxic retinopathy or optic neuropathy, (particularly pronounced in patients with antecedent congenital red/green color deficits)

Ishihara-Plates, anomaloscopy, Farnsworth-Munsell 100 hue

Fundamentals of Color Vision

Intact color perception requires a high level of performance from the human visual system. The three types of cone photoreceptors with their absorption maxima at 580, 535, and 440 nm, and their postsynaptic neurons function together in a balanced process, integrating their signals to produce an extraordinary range of color discrimination. This is why even the smallest disturbances of retinal metabolism can produce large changes in color perception. Neural processing of the information gathered by the cones begins within the retina itself. The neural net of the retina contains opponent color neurons that interact with one another to encode several channels of color and brightness perception. The spectral properties of the cone pigments and the synaptic connections between the photoreceptors and downstream elements of the retinal neural net are organized into two primary opponent channels, a red/green channel and a blue/yellow channel. This is the physiologic basis for the detection of color contrast, which is separate and apart from the processing of brightness (luminance) perception. Numerous metabolic, toxic, and inherited retinal diseases can alter the photoreceptors, their synapses, the interneurons, bipolar cells, amacrine cells, and retinal ganglion cells. Changes in transmitter metabolism can alter the balanced state of photoreceptor interactions, resulting in a disturbance of color perception. For this reason, color vision deficits are often the presenting visual abnormality for many diseases.

Colors seen by a normal trichromat (an observer with normal contributions from all three cone pigments) can be organized into a three-dimensional space (called a color model), defined by hue, saturation, and brightness. The 3D model has the appearance of an oblate spheroid with a central vertical axis defining brightness, while planes orthogonal to the axis encode hue as the various loci around the circumference of the model (with the order of hues arranged to match that of the natural spectrum), and saturation loci along the radial lines from the central axis to the external surface of the model. Locations close to the central axis have low saturation (pale, gray, or pastel), while those close to the surface are highly saturated (have the maximum "purity" of the hue in question). If we section a plane through the model in a horizontal orientation (perpendicular to the vertical axis), the plane surface thus created contains many hues, all of identical brightness, as diagramed by the color triangle in ■ Fig. 6.1. Dyschromatopsias cause pathologic changes in the size and shape of the 3D color model. The affected colors (that are confused with one another) lie along straight lines drawn through the model. These are called color confusion lines. Nearly all clinical tests of color vision are meant to determine the extent of the confusion along one or more of these lines.

In area Vi of the visual cortex, the encoded (opponent) color channels that originate in the cell bodies of neurons in the lateral geniculate body provide the afferent flow of color information at the cortex. The various color opponents (blue/yellow and red/green) are segregated into vesicle-shaped regions of the cortex called blobs that are responsible for processing the afferent color information. The output of their processing is then sent on to higher visual centers that are responsible for higher levels of color perception (e.g., Area V4). Occasionally, cases of faulty cortical function have been associated with unusual types of dys-chromatopsias.

Color Vision Tests

Although it makes sense to use quantitative tests of color vision (e.g., with the Farnsworth-Munsell 100-hue test) when following patients being treated with chloroquine or similar drugs, this is a time consuming, tedious and therefore expensive test that requires the close attention of a trained assistant for periods of up to an hour. Simpler and quicker tests are available to be used when there is a suspicion of an optic neuropathy. Tests of color saturation perception (e.g., color discrimination along one line of color confusion at constant hue and brightness) are strongly correlated with optic nerve disease. Quick tests are needed, given the high incidence of optic neuropathies.

The number of available color vision tests exceeds the necessary repertoire by far. It makes sense to choose a small group from among the tests that are explained in the following section, building one's own little "tool box."

Testing the Discrimination of Color Saturation

Light seen by an eye with an optic neuropathy appears darker than when seen with the contralateral (normal) eye. Colors fade, lose their vibrancy, and appear darkened. This can be detected simply by using the red cap from a bottle of a mydriatic preparation. With both eyes open, the bottle cap is shown to one eye or the other by alternating monocular occlusion. Patients with macular disease in the early stages do not see any change in the saturation (richness) of the red color. At the onset of a serous elevation of the macula (usually monocular at first), the patient will note a striking difference in the appearance through one eye as compared with the other. Using a red test object, one can simultaneously screen for defects in luminance perception at the long-wavelength (red) end of the color spectrum.

Confusion Line Cie

Fig. 6.1. The International Commission on Illumination (Commission Internationale de l'Eclairage - CIE) chromaticity diagram (a) is bordered by the locus of spectral colors (curvedline) and the violet (straight) line that connects the two ends of the spectral locus curve. The diagram represents a plane section through the color space (of the human eye), orthogonal to the axis of luminance, meaning that all colors represented in this section of color space are of equal brightness. The triangular shape arises from the three cone types of varying spectral sensitivity that define the limits of color space in all three directions. If all three cone types are stimulated to a proportionally balanced extent, the resulting sensation is a colorless white (marked by the region with the white point). If one of the cone types is missing, the colors that depend on that

Fig. 6.1. The International Commission on Illumination (Commission Internationale de l'Eclairage - CIE) chromaticity diagram (a) is bordered by the locus of spectral colors (curvedline) and the violet (straight) line that connects the two ends of the spectral locus curve. The diagram represents a plane section through the color space (of the human eye), orthogonal to the axis of luminance, meaning that all colors represented in this section of color space are of equal brightness. The triangular shape arises from the three cone types of varying spectral sensitivity that define the limits of color space in all three directions. If all three cone types are stimulated to a proportionally balanced extent, the resulting sensation is a colorless white (marked by the region with the white point). If one of the cone types is missing, the colors that depend on that cone type for distinction from other colors will appear identical, called color confusion. In protan and deutan color deficits, colors that vary in their content of red versus green appear more alike, while in tritan deficits, colors that differ by their relative content of blue versus yellow become indistinguishable. Such colors lie along the protan (b) deutan (c), or tritan (d) lines of confusion. Pseudoiso-chromatic plates use colors taken from a series of points along one of the confusion lines. In acquired dyschromatopsias caused by neuro-ophthalmic diseases, signal interactions between the inputs of the three cones types are weakened. This causes an increasingly large, colorless area surrounding and expanding from the white point in the direction of the affected interaction

Note

Hue discrimination and brightness comparisons are subjective tests. Careful observers with normal vision will occasionally notice spontaneous variations in color when comparing one eye with the other, yet responses are not always precise, and there are frequent false-

positive responses. Variations in light adaptation when comparing one eye with the other - which can be the case immediately after monocular opthalmoscopy -will cause transient periods of interocular color and brightness disparities.

Pseudoisochromatic Plates

^ Definition

Isochromatic means very similar or identical color appearances. Pseudoisochromatic means that observers with color deficiencies will see foreground and background of figures as indistinguishable (equally colored and equally bright), and will be unable to see the patterns, while those with normal trichromatic vision will see the embedded characters.

The principle of the color plates can be diagrammed by placing the various shades of color in the International Commission on Illumination (Commission Internationale de l'Eclairage - CIE) color triangle (see ■ Fig. 6.1). The lines are loci of isochromatic stimuli for those with color deficiencies, and the various spots or elements of the image are chosen to fall along these lines. While the color contrast is evident to the normal trichromat, the color-deficient observer will not be able to see the figures as distinct from their background. The embedding of figures in a chaotic mixture of brighter or darker, and smaller or larger elements means that the only way to differentiate correctly a figure from its background is by color differences (■ Fig. 6.2).

Since disorders of color perception can also change brightness sensitivity, separate groups of plates must be used for the detection of congenital versus acquired dys-chromatopsias.

A group of pseudoisochromatic plates has been developed for recognition of the most common forms of congenital color vision deficiencies (e.g., Ishihara plates for the detection of congenital red/green color deficiencies). There are also plates specifically designed for the detection of acquired dyschromatopsias (e.g., the Velhagen-Brosch-mann plates, or the SPP 2-plates according to Ishikawa), which in part have pseudoisochromatic stimuli for detecting deficiencies of blue perception. Both types of plates should be available for use.

Color Sorting Tests

The subject being tested is asked to sort colored elements in a continuous sequence of hues that together form a circle in the color triangle, surrounding the white point at the center (■ Fig. 6.3). For suspected congenital and for most of the pronounced color vision deficiencies, the saturated color tests may be used, while for acquired and for subtle disorders, mostly desaturated (pale) colors are used. For initial testing one can use the Farnsworth panel D-15 test (in the

Spp Plates According Ishikawa

Fig. 6.2. a Pseudoisochromatic plates for the detection of red/ green dyschromatopsias (Ishihara). The colors of the optotypes and their backgrounds are taken from one of the respective lines of color confusion. Normal trichromats will be able to see the contrast between optotypes and surround, but those with the respective dyschromatopsia will not. (The figures shown here are for instructional purposes only and are not suited to the testing of color vision.) b Pseudoisochromatic plates for the detection of a tritano-pic reduction in blue perception (Velhagen, Broschmann). The colors are all taken from a tritan confusion line, and will be difficult to distinguish by an eye that has poor blue/yellow discrimination

Fig. 6.2. a Pseudoisochromatic plates for the detection of red/ green dyschromatopsias (Ishihara). The colors of the optotypes and their backgrounds are taken from one of the respective lines of color confusion. Normal trichromats will be able to see the contrast between optotypes and surround, but those with the respective dyschromatopsia will not. (The figures shown here are for instructional purposes only and are not suited to the testing of color vision.) b Pseudoisochromatic plates for the detection of a tritano-pic reduction in blue perception (Velhagen, Broschmann). The colors are all taken from a tritan confusion line, and will be difficult to distinguish by an eye that has poor blue/yellow discrimination saturated or the desaturated form, according to Lanthony) or the more quantitative Roth 28-hue test (also available in both saturated and desaturated forms). For a more precise, quantitative determination, the Farnsworth-Munsell 100-hue test with 85 colored caps can be used.

Color Test

Fig. 6.3. The CIE chromaticity diagram with the locations of the colored test caps used in the panel D-15 color ordering test. These colors fall on an oval contour that surrounds the neutral white point. The distances between cap colors and the white point are proportional to the color saturation of the cap colors. The lines of color confusion cross the test's color circle at varying angles. Patients with normal color perception will order the caps correctly, while those with dyschromatopsias will show sorting errors that fall parallel to lines that are approximately tangential to the color circle

Fig. 6.3. The CIE chromaticity diagram with the locations of the colored test caps used in the panel D-15 color ordering test. These colors fall on an oval contour that surrounds the neutral white point. The distances between cap colors and the white point are proportional to the color saturation of the cap colors. The lines of color confusion cross the test's color circle at varying angles. Patients with normal color perception will order the caps correctly, while those with dyschromatopsias will show sorting errors that fall parallel to lines that are approximately tangential to the color circle

Note

Since subjects with altered brightness sensitivity can sometimes rely on achromatic brightness differences (varying levels of gray) to sort a color sequence, they can sometimes produce a correct sequence in spite of having a significant color vision deficiency. For this reason, it is especially important to use a standardized illumination (e.g., the Macbeth table lamp or light boxes with normative light "C" [6,775 K] or "D65" [6,500 K]) for all color testing with reflective sorting elements. The luminance level should be 500 to 1,000 lux.

For each cone type there is a group of lines of color confusion that converge on one another at a single point (■ Fig. 6.4). The position of this point is different for each of the cone types. Patients fail to differentiate those colors that fall along one of the color confusion lines in the color triangle. Given the orientation of the axis of confusion determined by this test, one can deduce the nature of the patient's color vision disorder. Transpositional errors that lie parallel to the color confusion lines for a protan defect indicate a disturbance of the long-wavelength red cone mechanism, an orientation of transpositional errors paral-

Lanthony Desaturated
Fig. 6.4. a The color confusion lines of congenital color defects (e.g., for a protanopia) have an orderly pattern and are very nearly parallel to one another. b In acquired disturbances of color vision (e.g., Stargardt's disease) errors of color confusion can develop at many different angles

lel to the deutan group of color confusion lines is consistent with a disorder of the medium-wavelength green cone mechanism, and sorting errors that fall on lines parallel to the tritan family of lines suggests a disturbance of the short-wavelength blue cone mechanism. The severity of the disorder can be quantified by the number and size of the transpositional errors, calculated as a sum of the errors.

| Pearl

For congenital color deficiencies the errors tend to be aligned in highly ordered groups, with a high reproducibility and converging on a single point in the color triangle. Acquired dyschromatopsias more commonly produce transpositional errors aligned toward two or even all three of the corners (see ■ Fig. 6.4). Additionally, the level of illumination has an influence. While cone dystrophies tend to yield increasing transpositional errors as luminance levels are increased, deficits associated with maculopathies and optic neuropathies tend to worsen as the luminance levels are decreased.

Note

Many patients will show a significant learning effect during the administration of these tests. It is best to verify abnormal results rather than accept the results of a single test. Errors that are parallel to the tritan axis (which is roughly tangential to the upper and lower segments of the color/circle in the panel D-15 test, i.e., color caps 2-6 and 9-14) are particularly prone to be the result of careless mistakes and can closely mimic the appearance of a blue deficit in color vision.

Anomaloscopy

An exact classification of a red/green color dyschromatop-sia can be determined with a spectral instrument called the anomaloscope. The subject to be tested is shown a semicircle with a mixture of red and green spectral lights that he/she can change, and he/she is then asked to adjust the brightness of the mixture to match that of a yellow semicircle (the two semicircles abut one another to form a full circle) so that the two halves of the circle appear to be most nearly alike in color and brightness. (This is the so-called Rayleigh equation; see ■ Fig. 6.5.) Normal subjects can produce a match with only a narrow range of mixtures, while subjects with red/green dyschromatopsias can produce matches of the same kind over a much larger range of red/green mixtures.

The evaluation form (according to Pitt, ■ Fig. 6.6) and the procedural and testing data forms (■ Fig. 6.7) are designed to assist with proper completion of the test, recording of the results, and calculation of the anomaly quotient (AQ). As a rule, normal trichromats can find a match by adjusting a mixture of red and green lights in one half of the circle to match the brightness of a spectral yellow light in the other half circle, such that the two halves appear equal in hue and intensity. (A normal subject's equation typically averages 40 scale units and must lie between values of 36.5 and 43.8 scale units to be considered normal. This corre-

Anomaloscopi

Fig. 6.5. Rayleigh spectral matching in anomaloscopy. The subject is asked to fix attention on a circular test field that must subtend a visual angle of 2°. The upper half contains the additive mixture of green and red spectral lights, 549 and 666 nm, while the lower half is the comparison yellow light at 589 nm. The red and green components of the mixture in the upper half are continuously variable in small steps (0 to 73 or 0 to 100), with the scale value of 0 indicating a completely red-free stimulus, and the scale value of 73 (or 100) is a stimulus devoid of green spectral light. The scaling of 0 to 73 was used in the original anomaloscope developed by Nagel. The value for the proportioned mixture that matches the appearance of the spectral yellow light for eyes with normal trichromacy is 40 (36.5 to 43.8), when using a scaling of 0 to 73, but will have a scale value of 50 for those instruments that use a scaling of 0 to 100. The brightness of the comparison yellow light is continuously variable; the norm for most testing is a setting of 15 units ±2. Color-neutral adaptation with a gray/white light is maintained throughout the testing by using the neutral field about 50% of the time, switching from color to neutral and back to color again at 3-s intervals. Anomaloscopes for determining the Rayleigh equation must match the wavelengths bandwidth, visual angle of the test-field, and must have a luminance value according to DIN 6160

sponds to an AQ of 0.7 to 1.4.) A subject with a congenital, X-linked red and/or green sensory deficit, depending on the type, will show a higher red or green component needed to produce a match, than will a normal subject. The values chosen by the test subject with a protanomaly will be found to parallel the protan axis when recorded in the Pitt diagram, while the values chosen by a subject with a deu-teranomaly will be arrayed along lines parallel to the deu-tan axis (■ Fig. 6.6). Dichromats - i.e., protanopes and deuteranopes - when adjusting the red and green components find acceptable matches (meaning of equal brightness and color) to the spectral yellow light with varying mixtures of red and green across the entire scale of values from 0 to 73 (■ Fig. 6.6). The differentiation of protanopia from deuteranopia is made based on the differences in the brightness distribution of the matches for the two types of dyschromatopsia. In subjects with achromatopsia, where there is a loss of both the red and green cone inputs, brightness perception is determined by the rods. Red light appears very dark, which can be inferred from the marked

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  • mary
    What causes ophthalmic artery traverses the optic nerve?
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    Why color deffict in optic neuropathy?
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