Ординатура / Офтальмология / Английские материалы / Visual Fields Examination and Interpretation_Walsh_2011
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Figure 3-12. Seed-point and short-term fluctuation locations. The initially thresholded locations are the four primary seed-point locations (circled), one per quadrant 9° from the horizontal and vertical meridians. These four locations are tested twice and used to calculate starting values for threshold measurements in the neighboring threshold. Throughout the course of the fullthreshold test, threshold values in standard locations (circled and squared) are measured twice to calculate the short-term fluctuation. Additional locations (in parentheses) may be thresholded twice if the result of the first threshold differs substantially from age-matched expected values. These additional locations are not used to calculate STF.
Some patients are attentive during the initial four central quadrant location threshold determination process and then rapidly fatigue, with deficient further responses. This results in a characteristic cloverleaf-shaped field (Figure 3-13).
3-4-4 Fixation Monitoring. Throughout the performance of the test, periodic assessment of patient fixation and level of alertness is made to ensure correspondence of projected stimuli to the correct part of the retina. To monitor fixation, the Humphrey Visual Field Analyzer uses the gaze tracker and the Heijl-Krakau blind spot– monitoring technique.27,28 The latter technique assumes that if stimuli projected on the blind spot location (which has a diameter of ≈5-7°) are seen, then the fixation is poor. If patient does not respond to those stimuli, fixation is assumed to be central— which is not always the case. A high number of fixation losses may result from wandering fixation but may also result from a displaced blind spot or from many false-positive responses. High-plus lenses tend to shift the blind spot toward fixation (Figure 3-14), while myopic correction moves the blind spot peripherally (Figure 3-15). These optical effects can be minimized with the use of contact lenses during the test.
Figure 3-13. Cloverleaf field. During the four primary seed-location threshold process, the patient responded with threshold sensitivities of 25 to 31 db. Throughout the remainder of the test, the patient quickly fatigued or fell asleep, with no additional responses seen through the rest of the test. The falsenegative errors indicate that patient was not responding to the bright light in the original four locations. No response was produced when the blind spot was tested, resulting in the deceptively low rate of fixation losses.
Figure 3-14. High-plus lens artifact. A patient was tested with a +12 spectacle correction. The high-plus lens shifted the blind spot toward fixation. There also appears to be a peripheral ring scotoma. When the patient was retested with a contact lens, there were no fixation losses and the peripheral scotoma disappeared.
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Figure 3-15. High-minus lens artifact. A highly myopic patient was tested with spectacle correction. The high-minus lenses shifted the blind spot away from fixation.
Gaze tracking is a system that records the patient’s fixation during the test. Deviations from the fixation are displayed on the screen and the printout. The small downward markings on the graph indicate the system’s inability to locate a reflection from the eye, with large downward marking indicating that the patient blinked. The upward markings indicate deviation from the fixation target, with the larger markings showing greater magnitude of the deviation.
Another way to reduce the percentage of fixation losses is to instruct the technician to re-plot the blind spot if high fixation losses are detected early in the test.29 The computer then executes a short subprogram that presents densely packed stimuli in the region of the expected blind spot until the actual blind spot is mapped.
The technician’s description of patient fixation, based on the observation of the patient’s eye through the video monitor, is also extremely valuable in detecting pseudo–loss of fixation. The absence of a low patient-reliability indicator should not assure the examiner that the test is error-free. Consider the patient who falls asleep at the machine. Clearly, that patient is unlikely to respond to stimuli presented in the blind spot despite poor fixation.
False-positive errors are tested by periodic withholding of a stimulus projection, although the faint noise of a stimulus presentation is retained. False-positive responses tend to indicate anxious, “trigger-happy” patients. The rate of falsepositive responses can often be improved if the visual field technician coaches patients to respond only when they are certain that they have seen the stimulus.
False-negative errors are tested by projecting a 9-dB suprathreshold stimulus in a region already thresholded. Failure to respond to this markedly suprathreshold stimulus indicates patient fatigue. False-negative errors are less influenced by coaching; however, the visual field technician should ensure that the patient is awake and consider giving the patient a short break. False-negative errors are produced both by patient inattentiveness and by a diseased, easily fatigued visual system. If the threshold results are markedly reduced, the machine may not generate
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suprathreshold stimuli. This may lull the inexperienced visual field technician into a false sense of patient responsiveness.
3-4-5 Threshold Testing. Throughout the performance of the Humphrey programs, if a threshold result differs significantly from age-matched normal values, the location will be retested to confirm this deviation. Performance of a complete full threshold program 30-2 in a patient with fairly normal vision required approximately 550 questions, taking about 15 minutes per eye. This was quite fatiguing to the patient, and current SITA algorithms could perform the same test within 4 minutes.
3-5 SINGLE TEST PRINTOUT
The single field printout from Programs 30-2 and 24-2 of the Humphrey perimeter contains a large amount of data, with various analyses presented in multiple ways (Figure 3-16). Familiarity with the overall organization and the derivation of the plots and indices greatly facilitates interpretation of the printout, which can be conveniently divided into the following sections:
1.Test Selection and General Information, located at the top
2.Reliability Indices, located at the top and at the left
3.Numeric Results (dB), located in the second row, at the left
5.Grayscale Results, located in the second row, at the right
4.Total Deviation, a plot located in the third row, at the left
5.Pattern Deviation, a plot located in the third row, in the middle
6.Global Indices, located in the third row, at the right
7.Glaucoma Hemifield Test, located above the Global Indices
8.Probability Symbols and Gaze Graph, located at the bottom of the printout
9.Practice Information, is located on the right side at the bottom of the printout
3-5-1 Test Selection (and General Information). Located at the top of the printout, the general information section displays important data about the individual patient as well as particular test variables. Included here are program name (e.g., Central 24-2 Threshold Test), patient name and patient birth date (removed from the present illustration), stimulus size, background illumination, blind spot check size, threshold strategy (SITA Standard in this example), fixation target type, test time and date, optical correction, pupil diameter (optional), and Snellen acuity (optional). Patient age is calculated from the date of birth and test date information. Many of these variables can significantly affect the raw and calculated data and can be invaluable in interpreting results. For example, a miotic pupil or incorrect refraction can reduce threshold values, while an incorrectly entered birth date will create erroneous age-compared deviations.
3-5-2 Reliability Indices. Located below and at the left of the general information section, the reliability data display the number of fixation losses to characterize stability of the patient’s gaze directed at the fixation target. The false positive errors score is designed to identify the “trigger-happy” patients, who press the response
Figure 3-16. Sample single field printout demonstrating glaucomatous field loss. (A) The important areas of the single field printout are labeled. This patient has a superior arcuate scotoma with a superior fixation loss and a superior nasal step. (B) Because the seventh-most sensitive total deviation value is −1 dB (only slightly depressed), the pattern deviation plot is similar to the total deviation plot.
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button even when stimulus is not seen. The false-negative errors indicate patient fatigue or inattention. Questions asked can be a rough indicator of the consistency of the patient’s responses. The final items of information displayed in the reliability indices section are test time and the optional foveal sensitivity. If fixation losses reach 20% or if false-positive or false-negative errors reach 33%, a double X is placed next to the percentage to attract the reviewer’s attention. Similarly, the message low patient reliability may be displayed above the reliability indices section.
3-5-3 Numeric Results and Grayscale Results (Raw Data). The two largest images on the printout, located to the right of the reliability indices, are the raw data plots. These images are displayed in numeric and gray tone format, with the actual threshold values in decibels. The testing algorithm would repeat threshold testing in the locations where the initially obtained values significantly deviated from the age-matched normal data. The grayscale plot is extrapolated from the numeric plot, and although it implies uniform sampling of the 30° field, in reality less than 1% of this area is actually tested. The grayscale plot remains useful to alert the examiner to problem areas and is an effective way of demonstrating visual field results to the patient.
3-5-4 Total Deviation. Since the introduction of the Humphrey perimeter, the manufacturer has upgraded the machine with increasingly sophisticated statistical analysis packages. The main package of these, STATPAC, allows comparison of the raw threshold data against age-matched normal values at each location; accompanying probability symbols indicate the significance of any abnormality. These plots are displayed in the lower-left portion of the printout, both numerically and with probability symbols. The P values take into account the wider range of normal values as the distance from fixation increases.30 For example, a value of 23 dB located 9° superiorly and nasally from the fixation (Figure 3-16) demonstrates that this reduced sensitivity is highly significant (P < 5% [.05]) compared with the agematched normal subjects, while the same 23 dB value located 24° superotemporally to fixation may is only significant. The examiner should keep in mind that statistical significance does not always mean clinical significance.
3-5-5 Pattern Deviation. Patterns of visual field loss can be divided into generalized depression, which uniformly affects the entire field by a similar amount, and localized (“scotomatous”) loss, which is frequently more diagnostic. STATPAC modifies the total deviation plots in an attempt to display any superimposed pattern of localized loss that is hidden under generalized depression. This is done by correcting the deviation of the seventh-highest threshold location within the Program 24-2 test grid to zero deviation and “adjusting” the entire field by that value. The threshold value of this seventh-most elevated location has been termed the general height value, although it is not routinely displayed on the printout. In the situation when the entire visual field is diffusely reduced in sensitivity by 7 dB, with an underlying moderate superior arcuate defect of an additional 12 dB, the pattern deviation plot will be reduced by those 7 dB and displayed in the pattern deviation plot, where that relative depression will become more apparent. A probability analysis is again displayed on these adjusted deviation values.
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3-5-6 Glaucoma Hemifield Test. Another strategy to analyze the result of the visual field test is to compare localized decreased threshold values in corresponding mirror image areas of the superior and inferior hemispheres.31 In the Humphrey Visual Field Analyzer, it is called the Glaucoma Hemifield Test (GHT). This method allows a simple but clinically useful analysis of visual field changes in glaucoma patients. The software produces the GHT result by dividing each of the upper and lower halves of the field into five mirror-image zones; the outer edge loci, temporal loci, and loci around the blind spot are excluded, enabling the GHT to be performed on either Program 30-2 or Program 24-2 (Figure 3-17). Each zone is subsequently scored according to its pattern deviation values, and each upper zone is then compared with the corresponding lower zone. In addition, a general height of the field is determined by analyzing the most normal region of the field. The GHT uses a large normal database to calculate the significance of differences between the two hemispheres31 and has been shown to significantly improve the ability to separate between normal and glaucoma fields.32 The results of the GHT consist of five categories, which are displayed above the global indices, using plain language message: “Outside Normal Limits,” “Borderline,” “General Reduction of Sensitivity,” “Abnormally High Sensitivity,” and “Within Normal Limits.”31 The GHT “Outside Normal Limits,” used together with the pattern deviation probability plot, has been shown to provide high sensitivity and specificity for detecting early glaucomatous visual field changes.33
The five possible GHT messages and their derivations are as follows:
1.Outside Normal Limits This message results if either of these conditions is present: (a) any upperversus lower-sector pair difference is greater than that found in 1% of the normal population or (b) any upper-lower pair sum differs at the 0.5% normal population level.
Figure 3-17. Glaucoma hemifield test zones.83
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Borderline This message is displayed if the criteria for outside normal limits |
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are not reached, but an upper-lower difference is present that occurs in less |
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than 3% of the normal population. |
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General Reduction of Sensitivity This message occurs if the criteria for |
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outside normal limits are not met, but the general height calculation finds |
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the most normal region of the field to be below the 0.5% normal population |
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level. Borderline and general reduction of sensitivity messages can result |
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simultaneously (Figure 3-18). |
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Abnormally High Sensitivity This message is displayed when the general |
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height calculation concludes that the best 15% of the field exceeds expected |
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values for 99.5% of the normal population. If these conditions are present, the GHT will not display any of the other four messages (Figure 3-19).
Figure 3-18. Generalized depression. In this patient with cataract, with 20/40 visual acuity, the foveal sensitivity is reduced. The abnormal mean deviation, normal pattern standard deviation, and clean pattern deviation probability plot all indicate uniform field loss.
Figure 3-19. Abnormally high sensitivity. Both the patient and the technician did not seem to understand what was supposed to be happening during this test, resulting in a false nasal defect on the pattern deviation plot.
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5.Within Normal Limits This message is displayed if the criteria for the above four conditions are not met.
3-5-7 Global Indices. In the lower-right corner of the single field printout, STATPAC displays two global indices, which describe the entire visual field using averaged numeric values:
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MD (mean deviation) is an average value of overall deviation from the |
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expected results within the same age group of normal visual fields. This |
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value reflects the average height of the entire hill of vision. Negative values |
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represent depression. MD is relatively insensitive to localized defects and is |
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strongly affected by generalized decrease of sensitivity, such as cataracts. |
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PSD (pattern standard deviation) represents the unevenness of the surface |
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of the hill of vision. It is calculated by taking a location-weighted standard |
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deviation of all the threshold values. PSD is insensitive to the overall average |
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height and is strongly affected by localized defects. |
STF and corrected pattern standard deviation (CPSD) are not used by current SITA algorithms. Of historical interest, STF is the standard deviation of repeated threshold of the 10 predefined locations. STF increases in inconsistent patients. This increase may be due to poor patient cooperation or attention, but STF also tends to increase in scotomatous areas, particularly at their borders. CPSD was used as an attempt to better represent the surface of the hill of vision by accounting for the influence of STF.
STATPAC provides probability values for each global index value compared with age-matched normal subjects. For example, if the MD value is accompanied by a P < 5% symbol, the MD of the field is depressed by an amount greater than that found in 99.5% of the same-age normal population.
Global indices generally correlate with visual field shape. For example, uniform generalized depression, as seen in cataract, tends to produce an increasingly negative MD. A small localized scotoma may have only slightly decreased MD and a high PSD (Figure 3-20). A dense moderate-size scotoma typically produces an elevation in both MD and PSD (Figure 3-21).
3-6 CUSTOM TESTS
Several alternative tests to Program 30-2 with the full threshold strategy exist. These aim to save test time or provide more detailed information regarding specific regions of the visual field. Increasing the number of test locations and the precision with which they are tested does not necessarily provide a more accurate picture of the visual field. Lengthy tests become fatiguing to the patient and may result in greater variability of responses. A number of strategies have been devised in an attempt to shorten the test and reduce the number of tested points while still providing an accurate representation of the visual field. These strategies are described in the sections that follow. Multiple other tests are available with the Humphrey perimeter, and although they are rarely used, they are available in the Humphrey Visual Field Analyzer Main Menu.