Ординатура / Офтальмология / Английские материалы / Visual Fields Examination and Interpretation_Walsh_2011

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Although the first bowl perimeter was introduced in 1872 by Scherk, due to problems with achieving even illumination on the screen, it did not become popular. The version of the bowl perimeter introduced by Goldmann in 1945 became widely accepted and is a significant contribution to clinical perimetry. The Goldmann perimeter incorporated a projected stimulus on an illuminated bowl, with standardization of background illumination as well as size and intensity of the stimulus, and allowed effective use of both static and kinetic techniques. For these reasons, the Goldmann instrument has remained the clinical standard throughout the world until widespread acceptance of automated perimetry. Harms and Aulhorn4 later designed the Tübingen perimeter with a bowl-type screen exclusively for the measurement of static threshold fields, using stationary test objects with variable light intensity.5 While excellent threshold measurements were possible with this instrument, the time and effort involved in such measurements prevented this perimeter from becoming widely used.

Automated perimetry has progressed rapidly over the past few decades, largely to the credit of Fankhauser,6,7 Heijl and Krakau,8 Flammer et al.,9 and others. Automated perimeters have substantially enhanced the clinical practice due to accurate, standardized measurements of the visual function. Most of these instruments use computerized static threshold algorithms, which have proved invaluable for clinical research and patient care, and are very familiar to all ophthalmologists. Despite the complex statistical algorithms used by visual field analyzers, standardized quantitative results allow easy and practical application of statistical methods to measure early functional loss.8-11

The high sensitivity of the methods has required attention to the multiple psychological and physiologic variables that may affect measured thresholds. Automated perimetry has improved standardization by reducing variability in the examination technique. Of course, the test still depends on the reliability of the patient’s responses and may be affected by optic, neural, and psychological factors.

Static automated threshold perimetry is one of the most important tests in the care of glaucoma patients. Detailed measurements of the visual field can be obtained, and some uncertainties regarding diagnostic and therapeutic decision making in glaucoma can be diminished. On the other hand, the large volume of data introduces uncertainties. Differentiating long-term fluctuation (LTF) from progressive loss remains one of the greatest clinical challenges in visual field interpretation.11 This chapter is designed to help in developing skills in the selection of automated perimetric tests and to afford familiarity with the various printouts used by the Humphrey perimeter. Chapter 4 [cross-check] is dedicated to the use of automated perimetry in the care of patients with glaucoma.

3-3 PRINCIPLES OF FIELD TESTING

The human visual system has a relatively poor ability to estimate absolute magnitudes of light, mostly to the exceptional adaptation of that system. The human visual system has, however, a remarkable ability to perceive contrast (relative magnitudes

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of light). Thus, it is the differential light sensitivity of a stimulus against a constantilluminated background that is measured with static perimetry.

Light can be measured using various units. A point source with an output of 1 candela emits a total of 4 Π lumens. The illuminance on a surface is the number of lumens per square meter incident to that surface. Some of this energy is absorbed and transformed to another form of energy (heat), and another part is reflected and emitted as light, known as luminance.

The apostilb (asb) is the unit of measurement for the luminance of a perfectly diffusing surface that is emitting or reflecting 1 lumen per square meter (m2). Most commercially available perimeters generate light of varying intensities by interposing neutral density filters, graded in decibels (dB), over a maximally emitting bulb. Retinal locations of reduced sensitivity require brighter stimuli to reach threshold, represented by lower decibel values. Similarly, higher decibel threshold values represent more sensitive retinal locations (Figure 3-1). Each decibel equals 1/10th of 1 log unit. Thus, 10 dB equals 1 log unit or a 10-fold change in intensity, and 30 dB equals 3 log units or a 1000-fold change in intensity. The maximum bulb intensities vary; Goldmann and Octopus perimeters generate a maximum stimulus luminance (0 dB) of 1000 asb, while the Humphrey perimeter uses a 10,000-asb bulb (0 dB). Background luminance also varies; Humphrey Visual Field Analyzer uses 31.5 asb.

The hill of vision may be mapped by using moving kinetic or stationary static stimuli. The visual threshold at a specific location in the retina is defined as the luminance at which 50% of the stimulus presentations are identified by the patient as seen. A patient undergoing a threshold examination may see only half of the presented stimuli. This can be a source of frustration to patients, who may feel they have performed poorly. Understanding the phenomenon requires knowing the bracketing strategy used to make threshold measurements at each location of

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the visual field. Full threshold strategy algorithms use a double crossing of the threshold. For instance, if the initial stimulus is subthreshold (not seen), intensity is increased in steps of 4 dB until the patient responds with a “yes” (seen). The stimulus intensity is then decreased in steps of 2 dB until the patient does not respond (not seen). The visual threshold is thereby crossed twice. Suprathreshold stimuli are brighter than the expected thresholds at given locations in the retina.

The Humphrey Visual Field Analyzer reports threshold values as the last seen stimulus using the 4-2 strategy (Figure 3-2). If the initial stimulus is suprathreshold, stimulus intensity is decreased by 4-dB steps until the threshold is crossed, then increased in 2-dB steps (the threshold again is doubly crossed). The choice of the initial stimulus luminance is explained below. With this strategy, accurate threshold estimates are achieved by presenting on average approximately five stimuli per test location. Stimulus presentations are not performed sequentially at a single location but are moved randomly throughout the entire visual field. This discourages cheating, because the patient does not know where to expect the next stimulus presentation.

3-3-1 Kinetic Perimetry. Kinetic perimetry uses a stimulus of constant size and intensity that moves from nonseeing to seeing areas of the visual field (Figure 3-3), that is, from the periphery toward fixation when outlining an isopter and from the center of the blind spot or a scotoma. Kinetic techniques are not very accurate for the examination of relatively flat areas of the visual field, requiring a sloping hill of vision in the tested area. Enlargement of the blind spot and Seidel’s scotoma (an arcuate-shaped elongation of the blind spot) have been described as early defects

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as well. Computerized kinetic perimetry was historically thought to be less reproducible than automated static perimetry because of the complex algorithms required. Nevertheless, recent studies suggest the opposite,12 and the reliability and reproducibility of the kinetic measurements have yet to be established. The optimal rate of movement of the target is about 4° per second,13 but a slower velocity of 2° per second may provide more reproducible results in some patients.14 A combination of central static threshold perimetry and peripheral kinetic perimetry has been used in glaucoma and neuro-ophthalmology patients.15 The optimal technique for examining the peripheral visual field that provides the most information in the shortest period of time has not yet been determined.

3-3-2 Static Perimetry. Standard automated static perimeter typically presents a projected stationary stimulus of known size, intensity, time, and location against a standard white background, also with known brightness level (Figure 3-5). Automated techniques of stimulus presentation are particularly suited to static measurements because the computer algorithms are relatively straightforward. Suprathreshold screening techniques may be used to make qualitative estimates of the visual field. Threshold measurements are required to obtain the quantitative data needed for the early diagnosis and careful follow-up of glaucoma patients.

3-3-2-1 Suprathreshold Techniques. Suprathreshold static perimetry is a technique in which a bright stimulus with an intensity that is above the anticipated threshold for the retinal area being tested and is expected to be seen in all parts of normal visual field. The locations at which the patient fails to recognize the target are noted as visual field defects. It is a quick way of detecting areas of blindness, usually within the central visual field. Stimulus intensity for suprathreshold perimetry may be constant over the entire field (Figure 3-6) or may correspond to the slope

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The static suprathreshold strategy is used as a screening test, to determine if visual function falls within or outside the normal range.8

3-3-2-2 Threshold Techniques. Computerized perimetry allows the performance of static threshold measurements in a short period of time under standardized conditions. Static threshold measurements are relatively sensitive to shallow depressions of the visual field when the locations of the stimuli are close together (Figure 3-8). This high sensitivity has led to an increased detection rate of early glaucomatous defects compared with manual techniques and has enhanced the ability to meaningfully compare successive visual field examinations. Manual perimeters can be used for quantitative threshold measurements but only by highly trained personnel.

3-3-3 Frequency-of-Seeing Curves and Fluctuations. Perimetry is a subjective psychophysical test requiring the patient’s cooperation, effort, and communication. As in any diagnostic test, the response to a specific question has an associated error. The threshold of the differential light sensitivity represents the brightness of the stimulus so dim that is seen only on 50% of repeated presentations. The frequency-of-seeing curve is a useful to emphasize the importance of probability in estimating a location’s threshold (Figure 3-9) and may be generated by repeated testing at a single location. Figure 3-9 indicates that these probabilities can never be 0% or 100% because of the influence of false-positive and false-negative responses, respectively. The slope of the curve is correlated to threshold deviation from age-appropriate normal values at a particular location. Thus, areas of high retinal sensitivity tend to test with high reproducibility, while locations with abnormally reduced sensitivity have a shallower

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Figure 3-9. Frequency-of-seeing curve. As the intensity of the stimulus increases (abscissa), the probability of seeing the stimulus increases (ordinate). The threshold is defined as the intensity that is seen 50% of the times it is presented. This implies that thresholds can only be estimated, not measured. The probability of seeing the stimulus is never 0% and never 100% because of false-positive or false-negative answers.

slope of the frequency-of-seeing curve, which is associated with greater uncertainty (Figure 3-10).16,17

Fluctuations in measurements of a physiologic parameter produce variation of a test result. Careful measurements can reveal those fluctuations in visual field thresholds. Bebie et al.18 described several components of this fluctuation: short-term fluctuation (STF) and LTF. STF is the variation of responses that occurs over the performance of a single test and may be caused by a combination of the instability of the threshold being tested and the level of cooperation and attentiveness of the patient.19 STF was calculated for full threshold algorithms (not often used now) of the Humphrey Visual Field Analyzer by measuring threshold values in 10 locations twice during the course of a given test. LTF is the fluctuation between tests, which occurs over days, months, or years. The causes of LTF are not well established. Possible reasons include fluctuations in intraocular pressure and in age and those

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analysis of the patterns of glaucomatous damage and is more time efficient than full threshold strategies, utilizing standard 24-2 or 30-2 patterns to assess the visual field. It significantly minimizes test time without reduction of data quality. Two versions of SITA are currently available: SITA Standard and SITA Fast. SITA Standard takes approximately half the time to complete compared with the full threshold program. SITA Fast takes about half the time of the FASTPAC algorithm and was found to be useful in pediatric population,24 although SITA Fast has somewhat increased variability compared with SITA Standard.25 SITA requires significant computer power, and therefore it is available only on the newer Humphrey Visual Field Analyzers.

SITA is based on visual field modeling that uses frequency-of-seeing curves for glaucoma and healthy patients. During the SITA test, a computer also produces an “information index,” which discontinues testing when threshold reaches a preselected level. The SITA algorithm also makes individual adjustments to patient response time, allowing the patient to be in charge of the test. After measurements are complete, the program performs additional calculation of all thresholds measured and produces estimates of false-positive and false-negative errors, displaying results as percentages. Average time reduction by SITA Standard depended on the severity of glaucomatous stage. No significant time difference exists for advanced glaucoma fields, whereas normal fields using SITA are performed in half of the time of full threshold strategy. The reduction of test time reduces the patient fatigue, allowing for more frequent visual field examinations and subsequent early detection of early glaucoma or progressing visual field damage.26

3-4-2 Foveal Threshold. Measurement of the foveal sensitivity is an option that, if selected, occurs at the very beginning of the test. This option should generally be left on, as it takes very few stimulus presentations and provides information about a very valuable portion of the visual field. The patient is asked to maintain gaze on an illuminated diamond that is projected inferior to the standard central fixation target used throughout the remainder of the test. The initial stimulus intensity is 30 dB, and the regular 4-2 bracketing strategy is used to determine foveal sensitivity. Once this portion of the test is completed, the fixation diamond is removed and the patient is asked to fixate on the central target.

3-4-3 Initial Values. Important time savers are used to reduce the numbers of stimuli necessary to estimate the threshold level and somewhat shorten the full threshold test. Starting points for threshold determinations usually depend on results from already tested primary locations, because a location’s threshold result is statistically correlated with its neighboring location’s threshold value. The Humphrey Visual Field Analyzer initially tests four seed locations, one per quadrant located 9° from the horizontal and vertical meridians. The initial stimulus intensity at these four seed locations is 25 dB and the full 4-2 strategy is used. Threshold in each location is measured twice, and the results from these four seed locations are used to determine the starting stimuli in adjacent areas (Figure 3-12). Threshold results from these adjacent areas are, in turn, used to determine their neighbor’s starting locations until the entire test is completed.