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Comparison of Glaucomatous Visual Field Defects Using Standard Full Threshold and Swedish Interactive Threshold Algorithms
Donald L. Budenz, MD;
Paul Rhee, OD;
William J. Feuer, MS;
John McSoley, OD;
Chris A. Johnson, PhD;
Douglas R. Anderson, MD
Arch Ophthalmol. 2002;120:1136-1141.
ABSTRACT
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Objectives To compare the severity, size, and depth of glaucomatous visual field
defects using standard full threshold (FT), Swedish interactive threshold
algorithm (SITA) standard (SS), and SITA fast (SF) algorithms of the Humphrey
perimeter.
Methods A prospective observational case series of 77 patients with glaucoma
performed FT, SS, and SF 30-2 white-on-white testing programs on the same
day on 2 occasions for 1 month. The severity of defects was compared using
the mean deviation, pattern standard deviation, Advanced Glaucoma Intervention
Study, and Hodapp-Anderson-Parrish severity scores. The sizes of defects were
compared using the total number of abnormal points on the pattern deviation
plot that fit standard criteria for glaucomatous visual field defects. The
depths of the defects were compared using the sum of the threshold values
for points identified in the pattern deviation plot as fitting criteria for
glaucomatous defects.
Results The mean deviations were slightly better using the SS (-9.6 ±
7.1 dB) or the SF (-9.1 ± 6.7 dB) algorithm compared with the
FT algorithm (-10.3 ± 7.1 dB) (P<.005). There
were no significant differences in pattern standard deviations between SS
(8.6 ± 4.0, P = .08) and SF (8.1 ±
3.6, P = .19) compared with FT (8.3 ± 3.3),
although the pattern standard deviation was higher in SS fields compared with
SF fields (P<.001). Advanced Glaucoma Intervention
Study scores were slightly better when the SS (7.5 ± 5.6) or SF (7.2
± 5.4) algorithm was used compared with the FT algorithm (8.6 ±
5.4) (P<.001). The sizes of glaucomatous defects
were slightly larger using the SS (20.9 ± 10.7) algorithm compared
with the FT algorithm (19.2 ± 10.9) (P = .004)
but not the SF algorithm (20.0 ± 10.6) (P
= .11). The depth of defects measured by the SS (220.4 ± 108.0 dB)
and SF (219.8 ± 101.3 dB) algorithms was significantly shallower compared
with that measured by the FT algorithm (152.3 ± 79.1 dB) (P<.001). There were no significant differences in Hodapp-Anderson-Parrish
severity scores among algorithms (P = .12).
Conclusions Glaucomatous defects are measured significantly shallower using the
new SITA algorithms but are approximately the same size and severity compared
with FT measurements. Care should be taken when using threshold values to
compare glaucomatous defects in a patient when converting from FT to SITA
algorithms.
INTRODUCTION
VISUAL FIELD ANALYSIS and optic nerve visualization are critical indicators
used in the diagnosis and management of glaucoma.1
Full threshold (FT) white-on-white automated static perimetry2-3
is the gold standard for the diagnosis, grading, and detection of progression
of glaucomatous visual field defects in standard practice and in glaucoma
clinical trials research.4-9
However, the standard FT method for measuring the visual field is time-consuming
for patients and is subject to fatigue effect, which has been shown to result
in poorer results.10-12
This effect is more pronounced in patients with glaucoma.11, 13
The Swedish interactive threshold algorithm (SITA) is a new computer
program that was developed for the Field Analyzer II (Humphrey Systems, Dublin,
Calif) that reduces test-taking time. The SITA standard (SS) program has been
shown to reduce test-taking time by approximately 50%14-18
and the SITA fast (SF) program by approximately 70%.16, 18-20
Overall, the number of stimuli presented in the SITA algorithms is reduced
by approximately 29% in normal fields and 26% in glaucomatous fields.21 This is accomplished using a combination of techniques,
including the use of information about surrounding points, threshold values
in age-matched control subjects and patients with glaucoma at each location,
changes in the pacing of the test, elimination of retest trials for the 10
points used for the calculation of short-term fluctuation in the FT algorithm,
changing the way in which false-positive and false-negative reliability factors
are determined, and using a maximum likelihood procedure for estimating threshold.19, 21-22 The difference between
the SS and SF programs is that the SF program allows a higher measurement
error, permitting the determination of threshold to stop sooner than in the
SS program.19
Because of the time-saving benefits of SITA, it has been suggested that
this testing algorithm might replace the FT algorithm,14, 18
which has been the gold standard for detecting and following up glaucomatous
visual field defects for more than 15 years. However, few studies23-24 have compared the severity of glaucomatous
visual field defects using the different algorithms. Potential differences
between results of these 3 tests have implications for setting target intraocular
pressures for the treatment of glaucoma and for determining whether patients'
defects are progressing over time. The objectives of this study were to evaluate
the visual field defects in patients with glaucoma using the new SS and SF
algorithms vs the standard FT algorithm and to determine whether results from
these procedures can be compared in a patient during follow-up.
SUBJECTS AND METHODS
The human subjects subcommittees of the institutional review boards
of the University of Miami, Miami, Fla, and the University of California,
Davis, approved this study. Patients older than 17 years with known glaucoma,
defined as characteristic cupping of the optic nerve and glaucomatous visual
field defects in at least 1 eye, regardless of intraocular pressure level,
were invited to participate. Subjects were required to be experienced visual
field takers, having been tested on 2 or more prior occasions using the Humphrey
visual field analyzer. Subjects were excluded if the visual acuity in the
eye to be tested was less than 20/40. If both eyes had glaucoma and met inclusion
criteria, the eye to be tested was selected by the investigator before initiation
of the study. An attempt was made to perform testing on eyes with a wide range
of visual field defects based on evaluation of prior FT testing.
After obtaining written informed consent, all subjects underwent the
following visual field testing protocol during 3 visits on separate days within
1 month of each other. At each visit, a standard FT test using program 30-2
and a size III white stimulus on a white background (31.5 apostilbs or 10
candelas/m2) was performed using the Humphrey Field Analyzer I
perimeter. Calculations of the total and pattern deviation plots and global
indexes (mean deviation and corrected pattern standard deviation) were performed
using StatPac version 9.31 (Humphrey Systems). With the Humphrey Field Analyzer
II, SS and SF tests using program 30-2 and a size III white stimulus on a
white background were performed. Calculations of the total and pattern deviation
plots and global indexes (mean deviation and pattern standard deviation [PSD])
were performed using StatPac for SITA version A10.1 (Humphrey Systems). The
order of testing was alternated between subjects to equalize fatigue effects
among testing algorithms. However, the order of tests was kept constant for
each patient during all testing sessions. The subject's best-corrected distance
refraction and age-appropriate near-add power were placed in the lens holder,
and the same prescription was used at each session. Subjects were required
to take at least a 15-minute rest between visual field tests. Tests using
a particular algorithm were performed with the same visual field machine,
and each subject had the same visual field technician for all tests. Patients
using topical miotics were required to be tested using miotics at all visits,
and every attempt was made to perform testing at a consistent time after instillation
of the miotic.
Visual fields with any abnormal reliability factor (fixation losses
>33%, false-positive responses >33%, or false-negative responses >33%) were
excluded. If a subject failed to produce 2 complete sets of reliable fields
using any 1 algorithm within 1 month, they were excluded from the study. To
be included in the study, both FT fields had to meet one of the following
minimal criteria for glaucomatous visual field defects: glaucoma hemifield
test results outside normal limits, corrected pattern standard deviation with
a probability less than 5%, or a cluster of 3 or more points in the pattern
deviation plot in a single hemifield (superior or inferior) with a probability
less than 5%, one of which must have a probability level less than 1%.25-26 The first 2 complete sets of fields
to meet inclusion criteria were used for analysis.
The severity of visual defects was graded using 2 standard grading systems,
the Advanced Glaucoma Intervention Study (AGIS) severity scale6
and the Hodapp-Anderson-Parrish (HAP) grading scale27-29
(Table 1). The mean deviation,
a global index that reflects the overall depression in the visual field, was
compared. Also, the PSD, a global index that reflects the amount of localized
(rather than diffuse) depression of the visual field, was compared for the
3 algorithms.
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Table 1. HAP Visual Field Severity Score*
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The size of glaucomatous defects was determined by counting the number
of points in the pattern deviation plot that fit the following criteria for
minimal abnormality25-26: cluster
of 3 or more points on the pattern deviation plot in a single hemifield (superior
or inferior) with a probability less than 5%, one of which must have a probability
level less than 1%. Points that were on the edge of the 30° field, except
for the 2 points at the far nasal positions above and below the horizontal
meridian, were excluded because of high variability in these points.26 The depth of glaucomatous defects was determined
by adding the threshold values for points identified as belonging to a glaucomatous
scotoma in the pattern deviation plot, as already outlined.
All values were averaged for each individual patient for the same test,
except for the HAP severity score, a categorical variable, which was treated
separately for the first and second fields rather than averaged. Comparisons
of continuous variables were performed using repeated-measures analysis of
variance for the 3 testing algorithms, followed by paired t tests for comparison of any 2 algorithms. P
values in the text are derived from paired t tests,
while those in the tables are derived from repeated-measures analysis of variance.
Pairwise McNemar nonparametric tests for correlated samples were used to compare
HAP severity scores.
RESULTS
Seventy-seven glaucomatous eyes of 77 patients met the inclusion criteria.
Forty (52%) of the 77 subjects were men and 37 (48%) were women. The mean
(±SD) age of the subjects was 68.0 (±10.6) years (range, 38-84
years).
Table 2 provides a summary
of the overall severity of glaucomatous visual field defects for the 3 algorithms
using 3 measurements: mean deviation, PSD, and AGIS score. Mean deviation
was worse in the FT fields compared with those of either SITA algorithm, and
SS had a slightly worse mean deviation compared with SF (P<.005 for all comparisons). There were no significant differences
in PSD between SS (P = .08) and SF (P = .19) compared with FT, although the PSD was higher in SS fields
compared with SF (P<.001). Advanced Glaucoma Intervention
Study scores were significantly lower when the SS and SF algorithms were used
compared with the FT algorithm (P<.001), indicating
that, using this grading scale, overall visual field severity was somewhat
less with the newer algorithms.
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Table 2. Severity of Glaucomatous Visual Field Defects*
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Table 3 provides a distribution
of severity classification using the HAP severity scale for the 3 algorithms.
By design, all patients were required to have a visual field defect on FT
but not necessarily on SS and SF testing. There were no significant differences
in HAP severity scores among algorithms (FT vs SS, P
= .19; FT vs SF, P = .80; and SS vs SF, P = .09).
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Table 3. Classification of Glaucomatous Visual Field Defects Using
the HAP Criteria*
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A comparison of the size and depth of glaucomatous defects is shown
in Table 4. Using the 3 algorithms,
the sizes of glaucomatous defects were within 1 or 2 points of each other.
The mean size of the defect on the SS algorithm was about 1.7 points larger
than on the FT algorithm
(P = .004), but there was
no difference in size comparing SF with FT (P = .11). The defects on the SS
and SF fields were significantly shallower than on the FT fields
(P<.001). Figure 1 is an
example of a glaucomatous visual field defect that is similar in size using
the 3 algorithms but shallower when measured with the SITA algorithms vs FT.
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Table 4. Size and Depth of Glaucomatous Visual Field Defects*
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Figure 1. Glaucomatous visual field defect
measured with full threshold (top), Swedish interactive threshold algorithm
(SITA) standard (middle), and SITA fast (bottom) in the same patient on the
same day. The supranasal defect outlined in the pattern deviation plot is
the area under consideration. There are 15, 16, and 19 points in the area
of abnormality for the 3 fields, respectively, with total threshold values
of 221 dB, 317 dB, and 338 dB, respectively. This is a typical example showing
similar defect sizes but with shallower defects measured with SITA compared
with full threshold algorithms. GHT indicates Glaucoma Hemifield Test; FL,
fixation losses; FN, false-negative responses; FP, false-positive responses;
MD, mean deviation; PSD, pattern standard deviation; SF, short-term fluctuation;
and CPSD, corrected pattern standard deviation.
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Figure 2, Figure 3, and Figure 4
provide graphic representations of scotoma depths when FT, SS, and SF fields
were compared with each other. These graphs show that most glaucomatous defects
appear shallower when measured with the SITA algorithms compared with FT,
but not when SITA fields are compared with each other. There was a statistically
significant correlation between the defect depths for the FT compared with
the SS fields (r2 = 0.47, P<.001) and for the FT compared with the SF fields (r2 = 0.47, P<.001), although
less than half of the variance in the SITA fields was explained by the FT
fields. For this reason, regression analysis would not be expected to provide
an accurate conversion between field types.
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Figure 2. Scattergram comparing the depth
of glaucomatous defects measured with Swedish interactive threshold algorithm
(SITA) standard vs full threshold testing. The higher the number of decibels,
the shallower the visual field defect. Most fields have defects that are shallower
on SITA standard than on full threshold testing (r2
= 0.47).
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Figure 3. Scattergram comparing the depth
of glaucomatous defects measured with Swedish interactive threshold algorithm
(SITA) fast vs full threshold testing. The higher the number of decibels,
the shallower the visual field defect. Most fields have defects that are shallower
on SITA fast than on full threshold testing (r2 =
0.47).
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Figure 4. Scattergram comparing the depth
of glaucomatous defects measured with Swedish interactive threshold algorithm
(SITA) fast vs SITA standard testing. The higher the number of decibels, the
shallower the visual field defect. There is no significant difference in defect
depth between these 2 algorithms (r2 = 0.35).
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COMMENT
Two important factors that go into deciding whether a glaucomatous visual
field defect is worsening are a change in the size and a change in the depth
of an existing defect. In this study, the size of the defects as seen on the
pattern deviation plots was similar across the 3 algorithms, with a mean difference
in the number of affected points of 0.8 to 1.7. Although this was statistically
significant, the small difference is not clinically meaningful. In contrast,
the depth of glaucomatous defects, expressed in decibel threshold values,
was significantly shallower when measured with the SITA algorithms, both statistically
and clinically.
Heijl and associates24 performed a retrospective
review of 31 patients with glaucoma who had performed several FT and SS tests
on different occasions. The time between tests compared was not reported.
The authors found no difference between the size of glaucomatous defects as
measured on the total deviation or pattern deviation plots, but found that
defects were shallower when measured by SS compared with FT. The authors concluded
that new baseline visual fields are desirable when switching from one algorithm
to another, but that "larger prospective investigations are desirable to confirm
these preliminary findings."24 Bengtsson and
Heijl23 performed a prospective study similar
to ours and found that the number of significantly depressed points was greater
for SS than for FT or SF.
Sharma and colleagues16 compared results
of FT and SS testing and found no significant difference in defect depths
between the 2 algorithms. When converting from FT to one of the newer SITA
algorithms, the size of defects does not change beyond the 2 points that were
found to diagnose progression in the Normal-Tension Glaucoma Study.4 However, using the newer algorithms, the depth of
defects measures about 70 dB (46%) shallower. Therefore, defects on SITA testing
may appear less severe compared with FT fields if the actual threshold values
are compared, a method for detecting glaucomatous progression that is highly
sensitive and specific when baseline and follow-up field tests are done with
the same algorithm.4
In the present study, several methods were used to characterize the
overall severity of visual field defects. The HAP severity scale is a categorical
scale that factors in the size of the glaucomatous defect, depth of the defect,
and proximity of the defect to fixation.27
This scale can be helpful in classifying glaucoma severity to set target intraocular
pressure goals. Results of FT and SITA tests did not differ substantially
using the HAP scale, indicating that the classification of defects as mild,
moderate, or severe is similar between algorithms and does not require modification.
The AGIS scale6 is an ordinal scale used for
judging glaucomatous progression. The AGIS score was 1.1 and 1.3 points higher
(worse) when patients were tested with the FT algorithm compared with the
SS or SF algorithms, respectively, which means that care should be taken in
judging progression based on AGIS scores when changing tests in follow-up
visual fields in patients with glaucoma.
The present study showed a slight but statistically significant difference
in mean deviation scores, with the SS and SF testing producing 0.7 dB and
1.2 dB, respectively, better mean deviations than FT testing. Other studies17, 24 have also shown marginally (approximately
1 dB) better values for mean deviation scores using the SITA algorithms compared
with FT. Using the same study design as ours, Bengtsson and Heijl23 failed to show a difference in mean deviation between
algorithms in patients with glaucoma. Given the results of these studies,
there appears to be little, if any, difference between the FT and SITA algorithms
in mean deviation scores.
In the present study, PSD values were not significantly different, suggesting
that this factor may be compared if follow-up fields are obtained with a SITA
test in a patient who has previously undergone testing with FT. This confirms
results obtained by other investigators.17, 24
Pattern standard deviation is one of several factors used in assessing progression
of glaucoma; an increasing PSD value is a sign of increasing localized field
loss, by far the most common pattern of glaucomatous progression. Pattern
standard deviation may worsen through the early and middle stages of the disease,
but may decrease as the entire field becomes abnormal and many locations are
homogeneously close to 0 dB. Corrected pattern standard deviation, which is
the PSD corrected for short-term fluctuation, is not available in the SITA
algorithms and cannot be compared.
The most clinically significant difference between algorithms appears
to be in the depth of glaucomatous defects, which are 46% shallower when measured
with the SITA algorithms compared with FT. Shorter perimetric tests seem to
result in better sensitivity of the visual field, and this difference is accentuated
in areas that are abnormal, according to our results. Because progression
of visual fields is often judged based on change in threshold sensitivity
within or adjacent to a preexisting glaucomatous defect,4
care should be taken in using changes in threshold values when converting
from FT to SITA perimetry. Although defective visual fields progress by enlargement,
areas with reduced sensitivity commonly progress by showing deepening of the
abnormality. Therefore, when practical, it may be best to compare follow-up
visual fields with fields performed with the same algorithm. To benefit from
the advantages of SITA, it may be valuable to establish a new baseline field
using SITA performed around the same time as the follow-up field done with
FT. This may provide the best follow-up information for comparison with previous
visual fields and enables switching to one of the new, shorter algorithms.
In the future, software may be available that permits comparison of older
and newer algorithms in a patient.20 Until
then, the clinician must use judgment and expect a certain offset in threshold
values to occur when switching from a baseline series of FT fields to a fresh
set of fields performed with one of the SITA algorithms. A deeper defect with
SITA, or one that is the same, may be a sign of some progression and need
further evaluation with a subsequent FT test to be sure that the field is
not progressing.
In summary, the current study demonstrates that glaucomatous visual
field defects appear similar in size with the new SITA algorithms compared
with FT on pattern deviation plots, but are much shallower with the SITA algorithms.
Standard severity scales yield similar results across algorithms.
AUTHOR INFORMATION
Submitted January 17, 2002; final revision received April 17, 2002;
accepted May 16, 2002.
This work was supported by an unrestricted grant from Research to Prevent
Blindness, New York, NY. Dr Johnson received research support from Humphrey
Systems.
Corresponding author: Donald L. Budenz, MD, Bascom Palmer Eye Institute,
Department of Ophthalmology, University of Miami School of Medicine, 900 NW
17th St, Miami, FL 33136.
From the Bascom Palmer Eye Institute, Department of Ophthalmology,
University of Miami School of Medicine, Miami, Fla (Drs Budenz, Rhee, McSoley,
and Anderson and Mr Feuer), and Department of Ophthalmology, University of
California, Davis, Sacramento (Dr Johnson). Dr Johnson is now affiliated with
Devers Eye Institute, Portland, Ore.
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