Ординатура / Офтальмология / Английские материалы / Visual Fields Examination and Interpretation_Walsh_2011
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Figure 3-28. Follow-up demonstrates individual test results with Deviation from Baseline and Progression Analysis point-by-point.
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3-8-1 Short-Wavelength Automated Perimetry (SWAP). Testing a subgroup of ganglion cells, called midget ganglion cells, that are sensitive to blue stimuli may detect loss of visual function at an earlier stage of glaucoma than with standard automated perimetry.26 Short-wavelength automated perimetry (SWAP) presents a large Goldmann size V, short-wavelength blue targets on a bright yellow background.39 SWAP defects appear to represent early glaucomatous damage and may indicate significant change in visual function before it becomes apparent on standard white-on-white visual fields. Several longitudinal studies have demonstrated the ability of blue-on-yellow perimetry to predict the development of glaucoma in ocular hypertensive patients,40,41 as well as which patients with early glaucomatous visual field loss are most likely to progress.42 Other studies have demonstrated a significant relationship between structural optic nerve damage and SWAP visual field defects.43 However, the test is influenced by age and cataracts, and stringent statistical analysis in interpreting the results is necessary.44,45 The SWAP is currently available on the newer Humphrey Visual Field Analyzers, incorporating the efficiency of the SITA algorithm with the 24-2 testing pattern, to decrease testing time from 12 minutes to less than 4 minutes.46,47 SITA-SWAP has been shown to demonstrate a higher prevalence of visual field deficits, suggesting earlier detection of glaucomatous changes, compared with standard automated perimetry.42,48 The printout is very similar to the standard HFA printout.
3-8-2 Frequency Doubling Perimetry. Frequency doubling illusion is the base for the frequency doubling technology (FDT) perimetry.49 Each test stimulus is a series of white-and-black bands flickering at 25 Hz.50 FDT perimetry is thought to be mediated by a subset of the large-diameter ganglion cells that project to the magnocellular visual pathway.51 These cells are presumed to be sensitive to motion and contrast and more vulnerable to glaucomatous damage,52 although this view has been questioned by some.53,54 The FDT is a portable and relatively inexpensive tool1 with a short testing time, making it a very useful screening device.55 However, FDT perimetry was reported to be less sensitive to visual field damage associated with neurologic disorders compared with standard automated perimetry.56 Sensitivity to FDT was found to be reduced in the second tested eye57—this effect is accounted for in the perimeter’s normative database.58 The original FDT perimeter tested only 17 points over the central 20° of the visual field. The second-generation FDT, Matrix, uses 69 smaller stimuli, utilizing 24-2 and 30-2 strategies to allow for early detection of glaucoma.59 The Matrix printout is organized similarly to a printout from the Humphrey Visual Field Analyzer.
3-8-3 High-Pass Resolution Perimetry. High-pass resolution perimetry (HPRP), or ring perimetry, is presumed to selectively test the parvocellular system.60 The stimuli used in HPRP are rings of different sizes with dark borders and bright centers projected at different locations on the screen, creating average luminance of the stimulus equal to the luminance of the background. The results of the test are presumed to correspond to the density of ganglion cells.1 In glaucoma patients, HPRP is comparable to standard perimetry in sensitivity and specificity.61 Some studies suggested that HPRP is a viable test for detection of glaucomatous visual field damage.62
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3-8-4 Tendency-Oriented Perimetry. Tendency-oriented perimetry (TOP) is a fast strategy algorithm available on Octopus perimeters.63 It also uses a computational approach to estimate threshold values by extrapolating information from surrounding test points. Compared with SITA Fast, the mean testing time for the TOP strategy was slightly over 2½ minutes, compared with approximately 4 minutes for SITA Fast.64 However, the TOP algorithm may not always accurately estimate visual field defects.65
3-9 LEARNING EFFECT AND ARTIFACTS
The results of many psychophysical tests improve as the subject gains more experience performing the test. The learning effect in automated perimetry seems to be small in most patients who have had experience with manual perimetry.66 Some patients, however, may demonstrate a dramatic improvement in the second test compared with the first.67 Occasionally, patients continue to improve over the initial three, four, or (rarely) five automated fields. The variability of test results may decrease significantly with experience. Whenever possible, a patient who is new to perimetry should undergo several test sessions, to establish a baseline for subsequent comparisons. The magnitude of the learning effect can be reduced by an attentive, thoughtful technician who takes the time to explain the examination thoroughly to the patient.
Several factors can produce abnormal-appearing field printouts without necessarily affecting the reliability indices. Other causes may contribute to unreliable results yet not be apparent on the printout. Test areas that are noisy, have confounding stray light, or provide uncomfortable chairs may contribute to patient fatigue or inconsistent responses. Some of the clinically important artifacts are discussed in the sections that follow.
3-9-1 Miosis and Mydriasis. Pupil size of less than 3 mm, particularly if combined with lens opacity, tends to produce a diffuse depression of the visual field. For this reason, the pupil size should be recorded on each test, and the influence of pupil size should be considered when a field change is detected. The technician usually enters the pupil size into the general information section, so that it is available to the clinician. Newer versions of automated perimeters can automatically record the size of the pupil.
If a pupil in a patient undergoing visual field testing is less than 3 mm, as a result of miotics, neuro-ophthalmologic disease, or age-related miosis, it should be routinely dilated for perimetry. Mydriasis has less influence on the visual field, although pupillary dilatation may reduce peripheral threshold sensitivity with automated perimetry.68,69 Although decreased pupil size should have little effect on a patient’s perception of a stimulus, because background and stimulus are affected equally, significant miosis may depress central and peripheral threshold sensitivities and exaggerate field defects.70 Refractive error can change considerably after pupil dilation, particularly in younger patients, who may require a dilated refraction prior to testing. One study used neutral density filters to reduce the retinal illumination
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the equivalent of decreasing the pupillary diameter in half, which reduced the mean threshold with two automated perimeters by 1.1 to 1.7 dB.71 Another study has also shown that pilocarpine significantly worsened the visual field global indices, such as MD and PSD.72
3-9-2 Media Opacities. Any media opacity, such as a cataract, posterior capsular fibrosis, or corneal scar, can cause a generalized depression of the field result (see Figure 3-17). Prediction of this effect by Snellen acuity is often poor. The patient with 20/25 acuity and 3+ nuclear sclerosis may have 5 dB or more of generalized depression apparent on the MD index. Cataracts produce glare and change the intensity of the stimulus. Therefore, a cataract can cause or exaggerate central or peripheral field defects, which could be mistaken for the development or progression of glaucomatous field loss. Even minimal light scattering, as may be caused by an early cataract that has a relatively insignificant effect on visual acuity, may significantly influence visual field results. After cataract extraction, eyes with glaucoma may have improvement of foveal sensitivity and visual field scores and sometimes even a reversal of a partial or complete scotoma.73 However, cataract surgery can also reveal mild and moderate field defects masked by cataracts.74,75 Reduced clarity of the ocular media from other causes, such as a corneal disturbance, a cloudy posterior lens capsule, or vitreous opacities, may also affect the visual fields. Applanation tonometry prior to automated perimetry was not found to have a detrimental effect on the visual field results.76
3-9-3 Eyelid and Nose Effects. Acquired ptosis of a mild degree is very common in older adults. While it may not affect Snellen acuity, a drooping upper eyelid can produce a superior arcuate–type defect or a superior nasal step. The visual field technician can be instrumental in recognizing this artifact, which can often be improved by taping the eyelid up during the test. Improper head positioning by the visual field technician can also cause the nose to produce an inferonasal defect.
3-9-4 Refractive Errors. Improper refractive correction could cause the projected stimulus to be out of focus on the retina, and the stimulus may not be detected by the patient. Refractive errors primarily influence the central field.77 With a usual size-III stimulus, spherical refractive errors of less than 1 diopter may not need to be corrected, because error in measurement is going to be only slightly more than 1 dB of general reduction of sensitivity.78 Myopia of less than 3 diopters also may not require correction, although high myopic errors can create areas of retinal blur, called refraction scotomas, which appear as a vertical wedge–type defect and may be confused with glaucomatous field loss. Those can usually be eliminated with an appropriate refractive correction. Hyperopia has a greater influence on perimetric results, especially for the central field, and even small hyperopic refractive errors can significantly alter threshold sensitivity.77-79 In general, patients over age 30 require a near correction added to the distance refraction. Spherical add depends on an accommodative amplitude, for which Humphrey Visual Field Analyzer uses age data to aid in determining the appropriate correction. A contact lens provides
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the best correction for the aphakic and highly myopic80,81 eyes. Astigmatic error of more than 1 diopter should be corrected in full.
3-9-5 Corrective Lens/Frame Artifacts. A spectacle lens placed too far from the tested eye can produce a ring-type scotoma appearance. Poor lens centering can produce an arcuate-appearing defect. Frame/corrective lens artifacts tend to be perfectly semicircular, or circular and dense (Figure 3-29). They can be minimized with proper spectacle placement or eliminated entirely with contact lens testing.
Figure 3-29. Lens holder artifact. This patient was tested without a corrective lens, while the lens holder was moved away from the patient face toward the center of the bowl, demonstrating absolute loss in a circular pattern inferiorly. The very sharp falloff in sensitivities above the horizontal meridian with an absent nasal step strongly suggests an artifactual scotoma, which was confirmed when the field appeared normal after the lens holder was moved out of the way.
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3-10 ROLE OF THE VISUAL FIELD TECHNICIAN
An attentive, knowledgeable technician is of critical importance to obtaining highquality test results.82 Patients must be instructed on what to expect during the field test, stressing the fact that at least half of the stimuli are not supposed to be seen, and the ones that are seen will barely visible, thereby allowing the proper performance to be achieved. Patients must be given feedback throughout the test on maintaining fixation and proper position, minimizing false-positive and falsenegative responses, and how long the test will last. A good visual field technician is vigilant in providing a comfortable, quiet location for reliable results, as well as recognizing the need for pupil dilation. In addition, the visual field technician should be able to minimize the effects of ptosis and lens artifact. It may also be helpful to let the patient know that the machine will not present an additional stimulus until the button is released, providing the patient with a break to stretch and to become more comfortable and attentive to the test.
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