Ординатура / Офтальмология / Английские материалы / Visual Fields Examination and Interpretation_Walsh_2011
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4
Automated Perimetry
in Glaucoma
HYLTON R. MAYER, MD, MARC L. WEITZMAN, MD,
AND JOSEPH CAPRIOLI, MD
4-1 INTRODUCTION
Clinical experience and multiple prospective studies, such as the Collaborative Normal Tension Glaucoma Study and the Los Angeles Latino Eye Study, have demonstrated that the diagnosis of glaucoma is more complex than identifying elevated intraocular pressure.1,2 As a result, increased emphasis has been placed on measurements of the structural and functional abnormalities caused by glaucoma. The refinement and adoption of imaging technologies assist the clinician in the detection of glaucomatous damage and, increasingly, in identifying the progression of structural damage. Because visual field defects in glaucoma patients occur in patterns that correspond to the anatomy of the nerve fiber layer of the retina and its projections to the optic nerve, visual functional tests become a link between structural damage and functional vision loss (Figure 4-1). The identification of glaucomatous damage and management of glaucoma require appropriate, sequential measurements and interpretation of the visual field.
4-2 GLAUCOMATOUS FIELD LOSS
Glaucomatous visual field defects usually are of the nerve fiber bundle type, corresponding to the anatomic arrangement of the retinal nerve fiber layer. It is helpful to consider the division of the nasal and temporal retina as the fovea, not the optic nerve head, because this is the location that determines the center of the visual field. The ganglion cell axon bundles that emanate from the nasal side of the retina generally approach the optic nerve head in a radial fashion. The majority of
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Figure 4-1. The nerve fiber layer anatomy of the retina. Damage to discrete bundles of nerve fibers, usually at the superior and inferior poles of the disc, gives rise to typical patterns of visual field loss in glaucoma. The classic nerve fiber bundle defects are the arcuate scotoma, nasal step, and temporal-sector defect. Diffuse damage to nerve fibers may also cause a diffuse depression of visual field sensitivity.
these fibers enter the nasal half of the optic disc, but fibers that represent the nasal half of the macula form the papillomacular bundle to enter the temporal-most aspect of the optic nerve.
In contrast, the temporal retinal fibers, with respect to fixation, arc around the macula to enter the superotemporal and inferotemporal portions of the optic disc. The origin of these arcuate temporal retinal fibers strictly respects the horizontal retinal raphe, temporal to the fovea. As a consequence of this superior-inferior segregation of the temporal retinal fibers, lesions that affect the superotemporal and inferotemporal poles of the optic disc, such as glaucoma, tend to cause arcuateshaped visual field defects extending from the blind spot toward the nasal horizontal meridian.
Tremendous variability characterizes the patterns of glaucomatous cupping, with some patients having fairly diffuse, concentric loss of the neuroretinal rim, while others have extremely localized loss. Glaucomatous visual field damage shows a similar variability, with some patients demonstrating early scattered involvement of the visual field, while others display dense localized defects. When only a single hemifield is involved, it is the superior hemifield 60% of the time.3,4 When patients have visual field loss in both the superior and the inferior hemifields, there is still often a detectable difference in the measured threshold on either side of the nasal horizontal meridian.
For the most part, clinicians familiar with the typical glaucomatous scotomas detected by manual perimetry recognize these same defects when demonstrated by automated perimetry. Seidel scotomas emenating from the optic nerve, nasal steps, and paracentral scotomas are commonly seen. When more advanced damage occurs, these smaller scotomas may coalesce to form superior and inferior arcuate
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scotomas, which may ultimately isolate a spared central and/or temporal island of vision. Temporal wedge defects, representing glaucomatous damage to the nasal nerve fibers, also respect the nerve fiber layer anatomy but are seen less than 5% of the time.5 Scotomas involving fixation (central scotomas) typically occur after advanced cupping occurs. While paracentral scotomas may be seen, classically in normal tension glaucoma, if central scotomas are identified, one should consider alternative diagnoses, such as retinal disease or nonglaucomatous optic neuropathies.6,7
Similarly, defects that respect the vertical meridian are not retinal nerve fiber bundle defects and are often caused by disease at or behind the optic chiasm. One defect well described with manual kinetic perimetry, which is less often seen with static testing, is the Seidel scotoma, an arcuate-shaped elongation of the physiologic blind spot. The hill of vision at the superior and inferior poles of the blind spot tends to be relatively flat. As a consequence, kinetic testing originating from the blind spot can easily mimic a Seidel scotoma if the stimulus speed is slightly too fast.
The occurrence of purely generalized field loss in glaucoma is controversial.8-10 Because many common conditions, such as cataract, miosis, refractive error, and patient fatigue, can cause an elevated mean deviation (MD) with a normal pattern standard deviation (PSD) and corrected pattern standard deviation (CPSD), these elements should be carefully considered before attributing generalized depression to glaucoma. Rarely, patients present with asymmetric intraocular pressure, asymmetric cupping, no confounding factors, and asymmetric MD values without localized scotomas (Figure 4-2). As a general rule, normal patients tend to test with MD asymmetry less than 2.0 dB 95% of the time on a single test. A 1.5-dB difference trend over two fields is equally significant, and a difference as small as 1 dB may be meaningful if reproducible over four fields.11
Several studies have suggested that normal-tension glaucoma defects are more localized and are closer to fixation compared with primary open-angle glaucoma defects.12-13 This theory has not been unequivocally proved, with critics alleging bias based on presenting factors because patients with primary open-angle glaucoma are more likely to be diagnosed on a basis of elevated intraocular pressure and may often be detected at an earlier stage of field loss compared with patients with normal-tension glaucoma.14
4-3 AUTOMATED PERIMETRY OPTIONS
For more than a decade, the full threshold testing strategy was the most widely used automated perimetry algorithm. Advantages of the full threshold test include numerous stimuli presented at each testing point in the field, theoretically offering a more refined estimation of the threshold value. The full threshold testing algorithm also offers a variety of reliability indicators, including estimations of short-term fluctuation (STF) within the test itself. Finally, many landmark studies regarding glaucoma diagnosis and management used full threshold automated perimetry, establishing it as a gold standard in automated static perimetry. Disadvantages
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of the full threshold strategy include longer testing times, which can decrease patient comfort and test reliability. Patient fatigue during full threshold testing may also artificially depress sensitivities and overestimate glaucomatous damage or progression.
Evolutions in testing algorithms involved adjustments in the grid size from 30-2 to 24-2, or from manipulation of the starting sensitivity or frequency of points crossing the threshold value, such as in fast threshold, FASTPAC. While FASTPAC can shorten test times 35-40% compared with full threshold strategies, it is thought to underestimate the size and density of scotomas. Humphrey field machines now offer the Swedish interactive threshold algorithm (SITA) that has
A
Figure 4-2. Generalized depression on a full threshold test. The intraocular pressure is in the high 20s in the left eye and in the high teens in the right eye, with asymmetric cupping. There is 2.45 dB of asymmetry in the mean deviation between the two eyes with otherwise normal fields. This can be taken as evidence of early glaucomatous generalized depression. (A) Field chart of left eye. (B) Field chart of right eye. (C) Fundus photograph of right eye. (D) Fundus photograph of left eye.
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B
C D
Figure 4-2. (Continued)
largely supplanted previous testing strategies. SITA uses a proprietary software to more rapidly and accurately identify threshold values. In comparison with full threshold tests, SITA tests are 50% shorter, typically lasting about 5 minutes per eye. SITA has been extensively evaluated against full threshold strategies and is widely accepted to be equivalent to the “gold standard.”15 SITA tests have been
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applied to large multicenter prospective studies, allowing more direct comparison to clinical applications and improving longitudinal data.16,17 Advanced analytical software is available to assist in identifying glaucomatous defects, progression, and trends (see Section 4-5).
SITA is also available in a more streamlined testing algorithm, SITA Fast, which decreases testing time (70% faster than full threshold) by reducing the number of times the threshold limit is checked.18 In comparison with SITA, many believe the advantages in shorter test times are offset by the reduction in the quality of the test data. Advances in testing algorithms, software, and hardware are improving efforts to increase patient comfort by decreasing test duration while maintaining or improving reliability and reproducibility.
4-4 EVALUATION OF A SINGLE TEST
Regardless of the method used to obtain them, visual field measurements should not be interpreted in a vacuum. It is almost always preferable to integrate other clinical data, such as refractive status, corneal clarity, condition of the crystalline lens, intraocular pressure, appearance of the optic nerve head, estimation of the patient’s vascular status, and assessment of the patient’s general health. An analysis of specific risk factors and potential side effects should be included in decisions about therapy. Before diagnostic and therapeutic decisions based on visual field information are made, patient reliability, artifacts, and other confounding variables must be considered. The Advanced Glaucoma Intervention Study (AGIS) and The Collaborative Initial Glaucoma Treatment Study (CIGTS) demonstrated that visual field defects on one test often fluctuated on subsequent follow-ups, and patients often demonstrated wide STFs and long-term fluctuations (LTFs).19,20
4-4-1 Patient Reliability. Determination of patient performance is an important initial step in visual field interpretation. A number of routinely measured elements can supply an estimate of patient reliability and are generally displayed with the fields:
1.Test duration
2.Number of fixation losses
3.Rate of false-positive answers
4.Rate of false-negative answers
5.STF, or the fluctuation of repeated threshold measurements within the same test. This parameter is present on full threshold programs and is not evaluated in the more recent SITA tests.
6.Number of stimuli. This parameter is present on full threshold programs and is not reported in the SITA tests.
Also important is the operator’s ability to comment on or score patient cooperation and alertness during the examination. Maintaining accurate fixation and alertness depends on both the patient and the technician.
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The standard limits for acceptable false-positive and false-negative answers have been set at 33%, and for fixation losses, at 20%. If these limits are exceeded, the value is highlighted and a warning message is displayed. These messages should not necessarily prompt the examiner to discard the entire field but should result in a review of possible causal factors.
4-4-1-1 Test Duration. It is not uncommon for a normal subject with prior visual field experience to complete an SITA Standard Humphrey Program 24-2 in less than 5 minutes. Both lack of experience and localized scotomas increase test time. Nevertheless, it is unusual for reliable patients to require more than 10 minutes to complete Program 24-2. (Full threshold test times for 24-2 and 30-2 should take about 9 and 12 minutes, respectively.)
4-4-1-2 Fixation Losses. Fixation losses, as measured by blind spot checking, can occur with wandering gaze but can also result from a displaced blind spot or from a high rate of false-positive answers (see Section 3-4-4). Wandering gaze may result in peripheral threshold values as high as foveal sensitivity values. Further, an acceptable rate of fixation losses does not ensure that fixation was good (see Section 3-4-4 for an explanation of this apparent paradox). Evaluation of the quality of fixation can be best determined by a dedicated and attentive operator, but gaze tracking software enables a graphic record of fixation at the bottom of the Humphrey Visual Field printout (Figure 4-3).
4-4-1-3 False-Positive Responses. High false-positive values (Figure 4-4) tend to occur if the patient repeatedly responds when no stimulus is presented and correlate with patient response to subthreshold stimuli. If a false-positive response occurs during a blind spot check, a fixation loss is recorded. False-positive responses tend to create the false impression that the field contains high sensitivity. The graytone display may reveal regions without any gray stippling (decibel values higher than 41 dB), which are clearly nonphysiologic. These are known as white scotomas or a moth-eaten visual field.
Figure 4-3. Humphrey visual field analyzer software graphically displays variations in gaze for a semiquantitative record of visual fixation stability; a smooth line indicates steady fixation. Patients A and B both scored 3/14 fixation losses, but Patient B had more and larger eye movements.
Figure 4-4. High false-positive errors. Although the testing parameters identified only 6% false-positive results, note the frequent midperipheral threshold values greater than 41 dB, which produce white scotomas. The general height correction of the total deviation by approximately 17 dB in this case produces a markedly abnormal pattern deviation plot. Because more than 15% of the test locations contain abnormally high sensitivity values, the glaucoma hemifield test (GHT) displays abnormally high sensitivity. In addition, the mean deviation is positive.
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Frequent false-positive responses are seen in the total deviation plot as scattered positive numbers. These are not flagged on the total deviation probability plot, because they are not reduced below the 5th percentile. The pattern deviation values, however, are affected differently. Because the instrument adjusts the entire field by a value that sets the seventh-highest locus on the total deviation plot to 0, loci where the patient responded appropriately may become markedly reduced. These may reach statistically significant levels and be highlighted by probability symbols. Frequent false-positive responses tend to affect the global indices as well. MD becomes positive and the GHT alerts the interpreter that more than 15% of the test locations contain abnormally high sensitivity values (Figure 4-5).
Figure 4-5. High false-positive errors superimposed on glaucomatous defects in a full threshold test. This field, although identified as low patient reliability, still provides evidence of nerve fiber bundle defects. Despite the high rate of fixation losses and falsepositive errors, there is a recognizable superior arcuate defect ending in a superior nasal step. Because less than 15% of the locations in the total deviation plot are elevated above the 99.5% percentile, the GHT does not display abnormal high sensitivity and correctly identifies the field as outside normal limits. The inferior arcuate defect suggested on the pattern deviation plot may be partly an artifact based on the 6-dB correction of the total deviation plot by the general height index.