Ординатура / Офтальмология / Английские материалы / Atlas of Glaucoma, Second Edition_Choplin, Lundy_2007
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92 Atlas of glaucoma
Damage to the optic nerve is not limited only to loss of large-diameter axons. There is considerable evidence that axonal loss occurs in bundles which may be visible ophthalmoscopically. Typical glaucomatous loss favors the superior and inferior poles of the disc. Areas of retina which have lost bundles of nerve fibers will manifest a loss of sensitivity compared to surrounding areas. Such areas of decreased
sensitivity are detectable on visual field testing; these scotomas and other defects in the mid-peripheral portion of the visual field begin to emerge as large numbers of axons are lost. Diffuse loss of axons may occur early in certain types of glaucoma or late in uncontrolled and progressive cases, resulting in generalized reduction in visual sensitivity. Other visual functions affected by loss of nerve fibers
Figure 8.1 Testing contrast sensitivity. One method of testing contrast sensitivity requires the patient to discern the orientation of a pattern of stripes of increasing spatial frequency (more lines per unit area or degree of visual angle) and decreasing contrast. The top of the figure is an example of decreasing contrast between the test object and the background; the bottom of the figure illustrates a series of stimuli showing increasing spatial frequency, i.e. the pattern of stripes gets ‘tighter’ as the patient looks from left to right, making it harder to determine the orientation of the stripes. Tests involving these types of stimuli determine grating acuity.
Figure 8.2 Decreased contrast sensitivity. Glaucoma may cause a decrease in contrast sensitivity in the absence of visual field defects or reduction in visual acuity. This figure is an example of decreased contrast sensitivity as determined by the Vectorvision system. The left side of the figure illustrates a reduction in sensitivity, particularly at the higher spatial frequencies (12 and 18 cycles per degree) in a patient with newly diagnosed open-angle glaucoma. The left eye, which has higher intraocular pressure, is worse. The gray area on the chart represents normal data. The right side of the figure illustrates normalization of the curves following institution of medical therapy and reduction in intraocular pressure.
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Psychophysical and electrophysiological testing in glaucoma 93 |
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include contrast sensitivity (Figures 8.1 and 8.2), |
determine the presence of abnormalities, and can |
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temporal contrast sensitivity, multifocal visual-evoked |
easily be followed over time for change. Statistical |
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potentials, color vision, scotopic retinal sensiti- |
packages have been developed to help in the inter- |
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vity, flicker perimetry, flicker visual-evoked poten- |
pretation of quantitative visual field data, both for |
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tial, color pattern-reversal visual-evoked potentials, |
determining abnormality in a single visual field |
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visual-evoked potentials following photostress, |
examination and for determining the significance |
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regional retinal visual acuity and foveal acuity. Loss |
of observed changes in a series of visual fields meas- |
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of foveal acuity usually occurs late in the course of |
ured over time. Newer testing algorithms such as |
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the disease but may occasionally occur early. |
the Swedish Interactive Thresholding Algorithm |
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Purely objective tests of visual function, such |
(SITA) can shorten test time by 50%, resulting in |
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as electro-oculography, electroretinography and visual- |
less patient fatigue while maintaining excellent |
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evoked potentials lack specificity for glaucoma. |
sensitivity and specificity. Although the examples in |
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Other objective tests, such as multifocal visual- |
this chapter were obtained with the full threshold |
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evoked potentials, flicker visual-evoked potential, |
algorithm (and illustrate the types of visual field |
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color pattern-reversal visual-evoked potentials and |
defects that may be found in patients with glaucoma), |
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pattern-evoked electroretinographic responses, have |
SITA has become widely accepted as the ‘standard’ for |
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so far proved to have limited clinical applicability |
computerized automated perimetry. Other researchers |
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due to equipment requirements (e.g. cost, complex |
have investigated the combination of the known |
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engineering, lack of commercial availability), lack |
objective effects of optic nerve disease on color vision |
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of familiarity to clinicians, complexity of interpre- |
with the familiar types of subjective automated |
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tation, and ease of performance. |
perimetry to determine if a blue stimulus on a yellow |
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background will detect earlier defects than standard |
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VISUAL FIELDS IN GLAUCOMA |
white-on-white perimetry. This has been named |
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‘short-wavelength automated perimetry’, or ‘SWAP’. |
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Although helpful in detecting early loss, SWAP may |
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Of the subjective tests currently available, visual |
be affected by media opacities such as cataracts. |
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field testing remains the mainstay, and the use of |
Also, it is a somewhat more lengthy and tiring test |
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automated perimetry has allowed the development |
for the patient. Use of the SITA testing algorithm |
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of standardized tests for obtaining quantitative |
for SWAP visual fields may address this issue. |
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measurements (Figures 8.3–8.26). Such measure- |
Visual field defects in glaucoma are summarized in |
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ments can be compared to known normal values to |
Table 8.2. |
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Table 8.2 Visual field defects in glaucoma. Visual field defects in glaucoma are well known. None of the defects that can occur in glaucoma is 100% specific; any defect that respects the horizontal meridian may occur in any optic nerve disorder. The interpretation of any visual field defect with regards to a differential diagnosis of disorders that can produce it must be made with regard to the entire clinical picture of the patient – intraocular pressure level, optic nerve appearance, family history and other risk factors. This table lists the visual field defects that occur in glaucoma, and indicates the frequency with which those defects have been observed to be initial defects
Type of defect |
Glaucoma patients manifesting this as |
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their initial defect (%) |
Increasing scatter (fluctuation) |
Probably all |
Diffuse depression, i.e. increased threshold |
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Paracentral defects |
41 |
Nasal steps |
54 |
Arcuate enlargement of the blind spot |
30 |
Arcuate scotomata not connected to the blind spot |
90 |
Nerve fiber bundle defects |
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Altitudinal defects |
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Temporal wedge defects |
3 |
Central and temporal islands |
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End-stage (temporal island only) |
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94 Atlas of glaucoma
Figure 8.3 The normal visual field and methods to ‘map’ it. The visual field defines all that is visible to one eye at a given time. It has been likened to an ‘island’ or ‘hill’ of vision in a ‘sea of blindness’. The job of visual field testing is to draw a map of the island of vision. The top of the figure represents the three-dimensional structure of a normal island of vision. The fovea is the 0,0 point on the x and y axes. The z axis represents the height of the island of vision at any point x,y above the ‘sea’, and is equivalent to the sensitivity of the retina at that point. Two different methods are available for drawing a map of the island. The middle of the figure represents the island as viewed from above as drawn by isopter perimetry, such as the tangent screen or Goldmann perimeter. An isopter may be thought of as the boundary of a retinal area within which all points have equal or greater sensitivity to those at the boundary. Each curved line, equivalent to the lines on the top figure, thus represents an isopter boundary. The lines are determined by moving test objects of fixed size and intensity from areas of nonseeing towards the center until the patient indicates it has been seen, thus giving this type of testing the name ‘kinetic’ perimetry. The smaller circles indicate areas determined by smaller and/or dimmer stimuli. By comparing the isopter locations and shapes to known normals, visual field defects can be determined. The bottom of the figure represents a ‘slice’, or profile of the island of vision through any meridian. It is determined by varying the intensity of a stimulus of fixed size at each point along the meridian until threshold has been determined. This type of testing has been termed ‘static’ perimetry, since the object does not move to determine threshold, and is typified by the Octopus perimeters and the Humphrey Field Analyzer (Zeiss-Humphrey Inc., San Leandro, CA). Testing multiple meridians and putting them together will give the three-dimensional picture of the island. Comparing the measured sensitivities to known normals allows for the detection of defects. Since quantitative information is generated (i.e. sensitivity values), statistical techniques can be applied for determining abnormalities and significant changes over time.
Psychophysical and electrophysiological testing in glaucoma 95
Figure 8.4 The visual field as measured by automated static perimetry. Modern automated static perimeters determine retinal threshold at an array of points and can display the results in a variety of ways. The user determines what points to test and what strategy to use to test them. This figure displays the results of a 30-2 threshold test from the Humphrey Field Analyzer. The test consists of an array of 76 points centered around the fovea with a spacing of six degrees and offset from the axes by three degrees. The numerical grid at the center top represents the retinal threshold expressed in decibels for each test point. Since the decibel scale is a relative scale representing attenuation of the maximum available stimulus intensity, high numbers (above 30 dB, depending upon age) represent good sensitivity (greater attenuation dimmer stimulus greater sensitivity). The ‘graytone’ display on the upper right is a graphical representation of the threshold values, with lighter symbols used for areas of better sensitivity and progressively darker symbols used to represent decreasing sensitivity. The plot on the middle left, labeled ‘total deviation’, is an array of the differences of the patient’s measured threshold values from those exhibited by age-corrected normals, and the lower plot symbolically shows the probability of obtaining the value exhibited by the patient in the reference population. The pattern deviation plot represents a software correction applied to the field for any factors that affect all the points, allowing focal defects to be more readily displayed. A probability plot is displayed for the pattern deviation as well. This figure is an example of a visual field displaying no defects in a patient with mild elevation of intraocular pressure and normal-appearing optic nerves.
96 Atlas of glaucoma
Figure 8.5 Asymmetrical visual field loss. Visual field loss occurs in two ways. The entire visual field may be diffusely affected, causing a loss of sensitivity at all points, manifested as lower threshold values. Many factors acting on the field can produce diffuse loss, including incorrect refraction at the time of the test so that the patient was not properly focused on the bowl, media opacities such as cataract which reduce the amount of light entering the eye, small pupils, inattentiveness and false-negative responses, and diffuse optic nerve damage. This set of visual fields illustrates a difference in mean sensitivity between the two eyes, indicative of asymmetric damage.
The right eye, the same as Figure 8.4, shows no defect and the mean deviation when compared to age-corrected normals is 0.04 dB. The left eye shows no significant focal defect, but a mean deviation of –1.58 dB, indicating a mild overall reduction in sensitivity, not only compared to the reference population but also more importantly when compared to the fellow eye. The intraocular pressures were 16 mmHg in the right eye and 23 mmHg in the left. In addition, there was a mild increase in the cup/disc ratio in the left eye. This mild reduction in sensitivity in the left eye would not usually be considered clinically significant, except when all the data are considered. It is consistent with mild diffuse depression and with early glaucomatous damage and consequently therapy was started in the left eye.
Psychophysical and electrophysiological testing in glaucoma 97
Figure 8.5 Continued.
98 Atlas of glaucoma
Figure 8.6 Diffuse depression. This is another example of diffuse depression occurring in a patient followed for many years with increased intraocular pressure which had been under treatment. The ocular media are clear, the patient’s pupils were dilated for the examination, he was refracted following dilatation to insure the proper distance lens, and the full 3.00 add was used to make sure he was properly focused at the test distance. The mean deviation is 6.50 dB, a value expected to occur in less than 0.5% of the age-corrected normals (i.e. 99.5% of the normals had higher values than this patient), consistent with diffuse optic nerve damage.
Psychophysical and electrophysiological testing in glaucoma 99
Figure 8.7 Fluctuation. The results of psychophysical tests, such as visual field testing, are subject to a certain degree of variability. Indeed, threshold is defined as that stimulus intensity that has a 50% probability of being seen. This in itself will give rise to varying results as points are retested. Test–retest variability in static perimetry is measurable, has known normal values and has clinical significance. Although an unreliable patient who does not know how to perform the test will show variability in results, unreliability is measured in other ways (e.g. fixation losses, false-pos- itive and false-negative responses), and a large spread in repeat measures in an otherwise reliable field has other significance. It has been shown that, as retinal sensitivity decreases, the variability of threshold in that region increases. It has also been shown that increasing fluctuation may precede the development of a visual field defect, thus giving its measurement particular clinical importance. This figure illustrates ten points (circled) that always have threshold measured twice. The difference of each measurement from the average value is squared, then the squares are summed, averaged across the field and the square root taken. Other factors are applied to account for point location in the field, and the result expressed as the short-term fluctuation value, or ‘sf’. This patient has angle-recession glaucoma with intraocular pressure in the low 30s. The field demonstrates diffuse depression, but more importantly an increased sf value of 3.36, expected in less than 2% of the reference population. The high value is derived from two points in the superior arcuate area – one showing measurements of 35 and 25 dB and the other 28 and 20 dB. These large differences in repeat measurements (10 and 8 dB, respectively) point to disturbed portions of the visual field that will most likely go on to develop paracentral and arcuate defects.
100 Atlas of glaucoma
Figure 8.8 Isolated paracentral defects. As indicated in Table 8.2, isolated paracentral defects occur as the initial glaucoma defect in about 40% of patients. This patient shows an isolated defect in the superior paracentral region of 22 dB below normal. Note also the wide fluctuation in repeat measurements of this point (13 dB and then 0 dB). Untreated intraocular pressure was in the upper 20s in this eye.
Psychophysical and electrophysiological testing in glaucoma 101
Figure 8.9 Asymmetry across the horizontal meridian. Asymmetry across the horizontal meridian is an important sign of optic nerve disease. The glaucoma hemifield test is a software option as part of the statistical analysis package for the Humphrey Field Analyzer that compares the differences of mirror-image clusters of points on opposite sides of the horizontal from the reference population to determine asymmetry. This is an example of an asymmetric disturbance in the inferior portion of the visual field of a glaucoma patient with an abnormal glaucoma hemifield test. This patient shows an early inferior arcuate enlargement of the blind spot. Note also from the probability map of the pattern deviation that threshold values of 19 dB (or more) occur in the periphery of the superior hemifield in 95% of the age-cor- rected normals, but a value of 19 dB would be expected in the inferior arcuate area less than 0.5% of the time.