Ординатура / Офтальмология / Английские материалы / Handbook of Glaucoma_Azuara-Blanco, Costa, Wilson_2001

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calculate the intra-test variability or short-term fluctuation (SF, see below).

In the SITA algorithm, threshold values and measurement errors of threshold values are continuously estimated during the test, using maximum posterior probability calculations in the visual field models. Initially, models are based on prior knowledge of visual fields (using a large database of normal and glaucomatous subjects), and are available before the actual test starts. During the test, staircase procedures are used to alter stimulus intensities at predetermined test point locations. Staircases are interrupted when measurement errors have been reduced to a certain level. This novelty is the main reason for the test time reduction. Test time is further reduced by the elimination of catch trials and through the use of a more effective timing algorithm.

Reliability indices

To assess the reliability of the test, ‘catch trials’ were developed to assess the steadiness of fixation, the incidence of false negative and false positive responses. These indices appear in the upper left-hand portion of the printout (Figures 6.8 and 6.9). The first versions of the Humphrey perimeter used the Heijl-Krakau method of fixation monitoring. The method includes the presentation of stimuli at the expected location of the blind spot, and assumes that, if the patient responds to such stimulus, a gaze error has occurred. The ‘gaze monitor’, available in recent models, records and charts gaze direction at the time of each stimulus presentation. Deviations of gaze are indicated by upward deflections in the tracing (at the bottom of the printout). When the gaze monitor is available, the blind spot technique does not need to be used. Numerous deflections indicate poor fixation.

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Figure 6.10 Full-threshold Humphrey’s 24–2 program in a patient with a high false-positive rate (‘trigger happy’). Observe that the grayscale is almost white due to abnormally high sensitivities. This test must be repeated.

False-positive errors occur when the patient responds when no stimulus is presented (Figure 6.10).

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False-negative trials involve the presentation of suprathreshold stimuli (9 dB more intense) at randomly selected visual field locations where the threshold had been previously determined. If the patient does not respond to this stimulus, a false-negative response is recorded (Figure 6.11). In general, with the Full Threshold algorithm, fixation losses higher than 20%, false-positive or false-negative responses higher than 33% are indicative of an unreliable examination. With SITA, tests with a false-positive or false-negative rates of more than than 15% should be interpreted with caution.

During a visual field examination using the SITA algorithm, catch trials are not done in the same way as described above. In fact, reliability indices are inferred by the SITA algorithm, which helps to reduce the test time.

Numeric grid

This is the first graph appearing in the upper left portion of the printout, below the reliability indices. The numbers represent the measured threshold of each

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Figure 6.11 Full threshold Humphrey’s 30–2 program in a patient with high false-negative rates due to fatigue. Note that the peripheral points (tested at the end of the exam) show poor sensitivities compared to the central points (tested at the beginning of the test), giving a typical cloverleaf pattern. This test must be repeated.

test location. When the Full Threshold algorithm is done, numbers in brackets represent points where the threshold was measured twice, to allow calculation of the short-term fluctuation (Figure 6.8). The SITA algorithm does not measure the short-term fluctuation, and thus does not show numbers in brackets (Figure 6.9).

Grayscale

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0.5% of normal subjects). The key to the representation of the probability symbols is shown near the bottom of the printout. The symbols increase in darkness as the deviation becomes more significant.

Global Indices

The global indices, displayed in the lower right corner of the printout, are derived from statistical treatment of the raw data to help summarize the information of all measured thresholds.

MD stands for mean deviation and represents the weighted average of the total deviation values. The values near the center receive more weight than those in the periphery. MD is a reflection of the general sensitivity of the field: it is close to zero in a normal examination, and negative when the sensitivity is reduced.

Pattern standard deviation (PSD) is a measure of the variability of the hill of vision, and is high whenever a localized defect is observed.

Short-term fluctuation (SF) measures the intratest variability, and is estimated from duplicate measurements at some locations. It is abnormally high in patients who do not perform well during the test due to fatigue or poor attentiveness. SF is used in combination with the PSD to calculate the corrected pattern standard deviation (CPSD), an index that attempts to remove the variability from the PSD that the patient showed while taking the test, revealing just the irregularity of the visual field due to pathological loss (Figure 6.8). Since the SITA algorithm does not perform duplicate measurements of threshold, its printout does not include the SF and CPSD (Figure 6.9). If any of the global indices fall outside the normal range, they are followed by a probability value (p < 10%, p < 5%, p < 2%, p < 1%, or p < 0.5%).

Glaucoma hemifield test

The visual field damage in glaucoma is frequently asymmetrical, with one of the hemispheres being damaged before the other. Hence, the glaucoma hemifield test (GHT) compares five areas of the superior and inferior hemifields (Figure 6.13). It is available in the 24-2 and 30-2 tests, and its result appears above the global indices. When at least one pair of areas shows a difference that exceeds that found in 99% of the normal population, an ‘outside normal limits’ message is displayed. When this difference exceeds that found in 97% of the normal population, a ‘borderline’ message is displayed. ‘General reduction of sensitivity’ appears when the general height is depressed to a level observed in less than 0.5% of the normal population, whereas ‘abnormally high sensitivity’ appears when the general height is higher than that found in 99.5% of the normal population. When none of the above conditions are met, the GHT is considered to be ‘within normal limits’.

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Figure 6.13 Graphic illustration of the five zones analysed in the glaucoma hemifield test (superior hemifield).

Interpreting the visual field

An automated perimetry printout presents an intimidating combination of numbers, graphs, and indices. The examiner needs to understand the meaning of each graph and index in order to interpret the available information correctly. Initially, there are some general guidelines that need to be followed:

a)A normal visual field does not exclude the presence of glaucoma (since visual field defects appear late in the disease), and a typical nerve fiber bundle defect is not necessarily indicative of glaucoma since other conditions such as optic disc drusen, branch retinal vein occlusion, or other optic nerve or chorio-retinal lesions may result in a similar defect (Figure 6.14).

b)The first visual field examination should be interpreted with caution. A learning effect is expected, and some patients may require two or more fields before a reliable examination can be made.

c)To characterize a localized visual field defect, two conditions need to be fulfilled: the defect should affect three or more adjacent and non-peripheral

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Figure 6.14A and 6.14B Fundus photograph and visual field of a patient with an inferior hemispheric retinal vein occlusion. The visual field shows a superior field loss, undistinguishable from one caused by advanced glaucomatous damage.

points in the pattern deviation plot, and it needs to be confirmed by subsequent examinations (Figure 6.15).

d)The same principles apply to the diagnosis of progression. Some degree of change in threshold sensitivity is expected between one examination and another due to long-term fluctuation (Figures 6.16 and 6.17).

e)Visual fields should be interpreted in association with the clinical information. As a subsidiary examination, it should not be evaluated as an independent tool.

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Figure 6.15 The locations with loss of sensitivity are mainly peripheral (see pattern deviation plot), and could not be confirmed in repeated examinations.

With these general guidelines in mind, it is now necessary to apply a systematic approach when facing the visual field printout. The authors’ preference includes the following sequence of events:

a)Check the patient’s data: refraction (the test should be done using near-vision correction after the age of 40 or after cataract surgery; large refractive errors may induce artifacts), pupil size (ideally greater than 3 mm but, if smaller than 3 mm, at least unchanged from the previous examination), and duration of the test.

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Figure 6.16 Humphrey’s printout overview of a patient followed from 1990 to 1992. This is a typical example of long-term fluctuation. Note that there is an apparent improvement in the third visual field, which disappears in the last examination. In fact, the last test is similar to the initial visual field.

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Figure 6.17 Humphrey’s glaucoma change probability analysis of a patient followed from 1990 to 1994. The black triangles refer to points where the probability of not having a deterioration is less than 5%. The graph in the right superior area shows a significant reduction in MD (from –3.92 dB in 1990 to –9.25 dB in 1994).

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b)Check the reliability indices, observing if the test can be considered reliable.

c)Look at the grayscale, which will allow the visualization of gross defects.

d)Observe the numerical graph to check if suspected areas have low thresholds.

e)Analyse the total deviation probability plot to look for diffuse loss of sensitivity.

f)Look for localized visual field defects (clusters of abnormal points) in the pattern deviation probability plot.

g)Analyse the global indices, particularly the MD (to evaluate the general sensitivity), and the CPSD in full-threshold perimetry, or the PSD if SITA is done (which may confirm the impression of a localized defect).

h)Check the GHT.

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i)Compare the visual field to the clinical information.

j)Rule out possible artifacts (e.g. ptosis, edge of the lens rim, inexperienced patient).

The identification of a dense, well-established visual field defect is not a difficult task. The recognition of mild glaucomatous visual field loss, however, requires the establishment of some criteria, among which the following are the most commonly used:

a)Three or more non-edge, adjacent points in the pattern deviation probability plot with p < 5%, one of them with p < 1%.

b)CPSD (or PSD) with p < 5%.

c)GHT outside normal limits.

Other examples of computerized perimetry are shown in Figures 6.18–6.21.

Other forms of perimetry

Short-wavelength automated perimetry (SWAP)

This technique is a modification of automated static threshold perimetry and is available on newer Humphrey models. SWAP uses a yellow background and size V, blue projected stimuli to test the blue cones. The blue cone system is slower and has a low visual acuity (about 20/200). As a consequence, the stimulus is perceived as fuzzy, and the test is more difficult and time-consuming. Uncorrected refractive errors have less of an effect on the thresholds determined by SWAP, but lens opacities tend to result in profoundly depressed fields that are difficult to interpret. SWAP, which comes with statistical analysis software designed to compare the examination with a normative database, has been shown to perform better than standard white-on-white perimetry in the detection of

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Figure 6.18 Dense, complete superior scotoma in a patient with an inferior notch. Note that

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