Computerized Method of Visual Acuity Testing - University of Arizona

Computerized Method of Visual Acuity
Testing: Adaptation of the Amblyopia
Treatment Study Visual Acuity
Testing Protocol
PAMELA S. MOKE, MSPH, ANDREW H. TURPIN, PHD, ROY W. BECK, MD, PHD,
JONATHAN M. HOLMES, BM, BCH, MICHAEL X. REPKA, MD, EILEEN E. BIRCH, PHD,
RICHARD W. HERTLE, MD, RAYMOND T. KRAKER, MSPH, JOSEPH M. MILLER, MD,
AND CHRIS A. JOHNSON, PHD
● PURPOSE: To report a computerized method for determining
visual acuity in children using the Amblyopia
Treatment Study visual acuity testing protocol.
● METHODS: A computerized visual acuity tester was
developed that uses a programmed handheld device that
uses the Palm operating system (Palm, Inc, Santa Clara,
California). The handheld device communicates with a
personal computer running a Linux operating system and
17-inch monitor. At a test distance of 3 m, single letters
can be displayed from 20/800 to 20/12. A C program on
the handheld device runs the Amblyopia Treatment
Study visual acuity testing protocol. Using this method,
visual acuity was tested in both the right and left eyes,
and then the testing was repeated in 156 children age 3
to 7 years at four clinical sites.
● RESULTS: Test-retest reliability was high (r .92 and
0.95 for and right and left eyes, respectively), with 88%
of right eye retests and 94% of left eye retests within 0.1
Accepted for publication Aug 22, 2001.
From the Jaeb Center for Health Research (P.S.M., R.W.B., R.T.K.),
Tampa, Florida; Curtin University (A.H.T.), Bentley, Western Australia;
Mayo Clinic (J.M.H.), Rochester, MinnesotaUSA; Johns Hopkins University
(M.X.R.), Baltimore, Maryland; Retina Foundation of the SW
(E.E.B.), Dallas, Texas; National Eye Institute (R.W.H.), Bethesda,
Maryland; University of Arizona (J.M.M.), Tucson, Arizona; Devers Eye
Institute (C.A.J.), Portland, Oregon.
This work was supported by grants from the National Eye Institute
(EY11155, EY11751, EY13095) and Research to Prevent Blindness, Inc,
New York, New York, (J.M.H. as R.P.B. Olga Keith Wiess Scholar and an
unrestricted grant to the Department of Ophthalmology, Mayo Clinic,
Rochester, Minnesota).
Additional technical information about the Electronic Visual Acuity
Tester and the Amblyopia Treatment Study visual acuity testing protocol
application can be obtained from the lead author (pmoke@jaeb.org).
Reprint requests to PEDIG Data Coordinating Center, Jaeb Center for
Health Research, 3010 E. 138th Ave, Suite 9, Tampa, FL 33613.
Correspondence to Roy W. Beck, MD, Ph.D., Jaeb Center for Health
Research, 3010 East 138th Ave, Suite 9, Tampa, FL 33613; fax: (813)
975-8761; e-mail: rbeck@jaeb.org
logarithm of minimal angle of resolution (logMAR) units
of the initial test. The 95% confidence interval for an
acuity score was calculated to be the score 0.13
logMAR units. For a change between two acuity scores,
the 95% confidence interval was the difference 0.19
logMAR units.
● CONCLUSIONS: We have developed a computerized
method for measurement of visual acuity. Automation of
the Amblyopia Treatment Study visual acuity testing
protocol is an effective method of testing visual acuity in
children 3 to 7 years of age. (Am J Ophthalmol
2001;132:903–909. © 2001 by Elsevier Science Inc. All
rights reserved.)
VISUAL ACUITY IS A COMMON PRIMARY OUTCOME
measure in clinical studies. In multicenter clinical
trials, considerable effort is placed on the standardization
of acuity testing across multiple sites. To more
easily standardize measurement of visual acuity in clinical
trials and to provide a method to directly capture acuity
data electronically, we developed a computerized vision
tester called the Electronic Visual Acuity Tester.
One application developed for the Electronic Visual
Acuity Tester is the automation of the Amblyopia Treatment
Study visual acuity testing protocol. This protocol
was developed to facilitate the standardization of visual
acuity testing in clinical trials of pediatric eye disease
involving children in the 3-year-old to 7-year-old age
range. 1 Its design represents a compromise between testability
and precision for acuity testing of children. In a
study of 178 children in which this acuity testing method
was implemented by manually displaying a sequence of
single-surrounded HOTV optotypes on the Baylor-Video
Acuity Tester (Medtronic Xomed Solan Ophthalmics,
Jacksonville, Florida), 1 we found that 67% of the 21
© 2001 BY ELSEVIER SCIENCE INC. ALL RIGHTS RESERVED.
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3-year-olds, 87% of the 60 4-year olds, and 96% of the 72
5- to 7-year olds successfully completed the testing of both
eyes. Test-retest reliability was high (r .82), with 93% of
retests within 0.1 logMAR units of the initial test. The
95% confidence interval (CI) for an acuity score was
calculated to be the score 0.125 logMAR units. For a
change between two acuity scores, the 95% confidence
interval was the difference 0.18 logMAR units.
Our experience in using this acuity testing protocol on
the Baylor-Video Acuity Tester in the Amblyopia Treatment
Study 1 (a randomized multicenter clinical trial
comparing part-time patching to atropine treatment for
moderate amblyopia) identified several reasons for a computerized
version of the testing protocol. First, some
members of our Pediatric Eye Disease Investigator Group
were unable to participate in our amblyopia trials, because
they did not have a Baylor-Video Acuity Tester and future
availability was limited. Second, acuity testers required
considerable training to minimize errors resulting from
manual administration of the testing protocol on the
Baylor-Video Acuity Tester. Third, presentation of surrounded
optotypes larger than 20/125 on the Baylor-Video
Acuity Tester required moving to a shorter testing distance
where patient movement toward or away from the monitor
has a proportionally greater effect on visual acuity measurement
accuracy than it does at longer testing distances.
Fourth, automation of the protocol could potentially reduce
the variability in how the testing procedure is
conducted in a multicenter study.
Herein, we describe the development of the Electronic
Visual Acuity Tester, its application to the Amblyopia
Treatment Study visual acuity protocol, and the results of
a study that assessed test-retest reliability.
METHODS
THE ELECTRONIC VISUAL ACUITY TESTER CONSISTS OF A
handheld device that uses the Palm operating system
version 3.5.0 (Palm, Inc, Santa Clara, California) and a
personal computer running a Linux operating system and
the Super VGA Graphics Library version 1.4.3. The
handheld device and personal computer are connected by
a serial cable (Figure 1). Stimuli are high-contrast blackand-white
letters with luminance of 85 to 105 candelas/m 2
and contrast of 98%. Both HOTV and Sloan letter sets are
available for testing. Single letters are presented, framed
with crowding bars that are spaced either a half-letter
width or full-letter width around the letter. With a 17-inch
monitor at 1600 by 1200 resolution, letters can be displayed
from 20/800 to 20/12 at a test distance of 3 m.
Letters are rendered on the monitor by manipulating the
individual points on the display screen, known as pixels
(picture element). Letter sizes are executed by translating
octave steps to the number of pixels for a given stroke
width, beginning with three pixels for a 20/12 letter.
FIGURE 1. Electronic Visual Acuity Tester. The Electronic
Visual Acuity Tester consists of a handheld device that uses the
Palm operating system (Palm, Inc, Santa Clara, California)
connected by a serial cable to a personal computer running a
Linux operating system. Both HOTV and Sloan letter sets are
available for testing. With a 17-inch monitor at 1600 by 1200
resolution, single letters can be displayed from 20/800 to 20/12
at a test distance of 3 m.
Programs on the personal computer render the letters,
provide serial communication with the handheld device,
and store test results. Programs on the handheld device
provide prompts for the tester, send display instructions to
the personal computer, and transmit test results to the
personal computer for storage. The size of each letter
presentation can either be controlled by the tester or can
be determined based on the patient’s responses by a
program on the handheld device. All programs are written
in the C programming language. In our development, we
have used a Palm handheld device.
The handheld device programs provide the option for
customized entry of patient identifiers and demographic
data before testing. During testing, the handheld screen
shows the current letter presentation, asks whether the
subject gave a correct or incorrect response, and prompts
for confirmation of the correct/incorrect selection. When
confirmed, the handheld device instructs the personal
computer to write the response to disk and then render the
next letter presentation. At each phase transition, the
tester is given instructions for continuing the test. Upon
test completion, the subject’s final acuity score is displayed.
The Amblyopia Treatment Study visual acuity testing
protocol 1 was converted to a C program on the Electronic
Visual Acuity Tester. The testing protocol, which is
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described in Table 1, consists of the presentation of
single-surrounded optotypes in four steps: a screening
phase, a first threshold determination (phase 1), reinforcement
phase, and a second threshold determination (phase
2). For the testing of young children, the HOTV letter set
with half-width crowding bars is used. For older children
and adults, the Sloan letter set can be used. The letter
displayed on the monitor is randomly determined with the
stipulation that there be no sequential repeated letters.
We verified the accuracy of the computer program in
adhering to the Amblyopia Treatment Study visual acuity
testing protocol by performing 500 simulated tests. For 400
tests, score sheets from actual testing using the acuity
TABLE 1. Amblyopia Treatment Study Visual Acuity Testing Protocol
Single letters with surround bars are presented in four phases: screening, phase 1 (first threshold determination), reinforcement, and
phase 2 (second threshold determination).
● In phase 1 and phase 2, up to four single letters are sequentially presented at each logMAR level* that is tested.
● A level is considered to be passed if three of three or three of four letters are correct and “failed” if two letters at a level are missed.
● Testing of a level stops as soon as criteria are met for either “pass” or “fail.”
Screening
Starting from either 20/100 or 20/400, single letters, in sequential descending logMAR sizes, are shown until one is missed.
1. Tester selects 20/100 or 20/400 size letter to present as starting point (depending on the expectation of visual acuity level based on
previous testing or a pretest).
2. If response is correct, letter at next smallest logMAR level is presented and testing continues sequentially with one letter per logMAR
level through 20/20 until there is an incorrect response.
3. If starting point was 20/100 and response is incorrect at either 20/100 or 20/80, screening is restarted at 20/400.
4. If screening starts at 20/400 and response is incorrect, testing continues sequentially with one letter per logMAR level through 20/800
until there is a correct response.
● If 20/800 is missed, 20/800 becomes the starting level for phase 1.
Phase 1
Starting two logMAR levels above the missed level in screening, the smallest logMAR level at which three of three or three of four letters
are correctly identified is determined.
1. Up to four single letters are sequentially presented two logMAR levels above the level missed in screening
● Exception: if 20/20 was correct in screening, phase 1 starts at 20/30.
● Exception: if 20/800 or 20/640 was missed in screening, phase 1 starts at 20/800.
2. If first tested level is failed, testing continues at sequentially larger logMAR levels until a level is passed.
● If 20/800 is failed, phase 1 ends, the reinforcement phase is omitted, and 20/800 is retested in phase 2.
3. If first tested level is passed, testing continues at sequentially smaller logMAR levels until a level is failed.
Reinforcement Phase
In order to get the child whose attention is drifting back on track, three letters larger than the phase 1 threshold are sequentially
presented.
1. Starting three levels larger than the level missed in phase 1, three successively smaller single letters are presented.
● Exception: if the level failed in phase 1 is 20/500 or 20/640, three 20/800 letters are shown for reinforcement.
● Note: the reinforcement phase responses do not contribute to the visual acuity score, and even if the responses are incorrect the test
proceeds to phase 2.
Phase 2
The last level missed in phase 1 is retested and if “passed,” testing continues until a level is “failed.‘
1. Up to four single letters are sequentially-presented at the last level missed in phase 1.
2. If the level is failed, testing stops.
3. If the level is passed, testing continues at sequentially smaller logMAR levels until a level is failed.
Final Visual Acuity Score
The visual acuity score is the smallest logMAR level passed in phase 1 or phase 2.
*logMAR levels (Snellen equivalents): 20/800, 20/640, 20/500, 20/400, 20/320, 20/250, 20/200, 20/160, 20/125, 20/100, 20/80, 20/63,
20/50, 20/40, 20/32, 20/25, 20/20, 20/16, 20/12
protocol that was conducted on the Baylor-Video Acuity
Tester in the Amblyopia Treatment Study 1 were used to
enter responses on the handheld device in the same order
as the real testing to verify that the proper testing sequence
was followed and that the final visual acuity score was the
same. This was supplemented by entering responses for 100
additional tests from a Baylor-Video Acuity Tester score
sheet that was completed to simulate all possible sequences
of letter size presentations.
A test-retest reliability study was conducted at four
clinical sites to evaluate the Amblyopia Treatment Study
visual acuity testing protocol using single-surrounded
HOTV letters on the Electronic Visual Acuity Tester.
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Subjects were 3 to 7 years old. Informed consent for
study participation was obtained from a parent or
guardian. First the right and then the left eye of each
child was tested and, after a short break, the right and
left eyes were retested by the same examiner. The visual
acuity score on each test was recorded as a logMAR
equivalent. A matching card containing the HOTV
letters was used so that the child could point to a letter
rather than verbalize a response.
There was no formalized training for the vision testers.
However, each was familiar with the acuity testing protocol
and conducted acuity testing using the Electronic
Visual Acuity Tester at least once before the study to
become familiar with the use of the handheld device.
Only children who completed the testing of both eyes were
included in the test-retest analysis. Frequency distributions of
the differences in visual acuity scores between the first and
second test of each eye were constructed, and Pearson
correlation coefficients were computed separately for the right
and left eye data (the eye-specific results were slightly
different, so they are reported separately). The correlation
between the difference in test-retest score and the
age of the subject also was assessed with the Pearson
correlation coefficient. Test-retest reliability was also
assessed using the method of Bland-Altman, 2 plotting
the test-retest difference versus the mean test-retest
score. Similar analyses were conducted for the interocular
difference data. The standard error of measurement
for the test was computed separately for each eye
and then averaged for reporting and establishing a 95%
confidence interval for an acuity score. 3 A 95% confidence
interval for a change in an acuity score from a
baseline value was computed by two methods; one based
on the standard error of measurement from the first test
and one based on the standard deviation of the testretest
differences. These were calculated separately for
right and left eyes and then averaged. The confidence
interval for the interocular difference was based on the
standard deviation of the test-retest difference in interocular
difference. Statistical analyses were performed
using SAS (personal computer version 8.01).
RESULTS
ONE HUNDRED FIFTY-SIX CHILDREN WERE INCLUDED IN THE
study. One site contributed data on 95 children (61%) and
the remaining three secondary sites data on 61 children
(39%). The mean age of the children was 5.6 1.4 years;
51% were males and 78% were Caucasian. Seventy-three
of the children (47%) had no ocular cause for decreased
acuity in both eyes, 60 (38%) had amblyopia in one eye,
and the rest had other specified causes for decreased acuity.
Most of the amblyopic eyes had only a mild decrease in
visual acuity (78% were 20/50 or better). The distributions
TABLE 2. Baseline Characteristics of Children in the
Reliability Analysis
Baseline Characteristic N 156
Age in years (mean SD)
Age in years n (%)
5.6 1.4
3 22 (14%)
4 39 (25%)
5 37 (24%)
6 24 (15%)
7 34 (22%)
Male gender, n (%)
Ethnicity, n (%)
80 (51%)
Caucasian 122 (78%)
African American 17 (11%)
Hispanic 3 (2%)
Asian 5 (3%)
American Indian 2 (1%)
Mixed 2 (1%)
Other 5 (3%)
Developmental delay, n (%)
Clinical diagnosis, n (%)
Right eye
11 (7%)
Normal* 106 (68%)
Uncorrected refractive error 6 (4%)
Amblyopia 31 (20%)
Other
Left eye
13 (8%)
Normal* 101 (65%)
Uncorrected refractive error 5 (3%)
Amblyopia 35 (22%)
Other
Visual acuity in initial test
15 (10%)
† (median [quartiles]
in logMAR units)
Right eye 0.1 (0.0, 0.3)
Left eye 0.1 (0.0, 0.3)
Previously tested with isolated surrounded
HOTV letters, n (%) ‡
93 (60%)
*”Normal” includes corrected refractive error.
† Includes both normal and abnormal eyes.
‡
Child had previous visual acuity test using isolated surrounded
HOTV letters, although not necessarily this protocol.
of clinical diagnosis and visual acuity were similar in right
and left eyes (Table 2).
In right eyes, the correlation between the initial and the
retest visual acuity scores was 0.92, with 88% of the retest
scores within 0.1 logMAR units of the initial test score
(0.1 logMAR units represents 1 logMAR line on an acuity
chart). In left eyes, the correlation was 0.95, with 94% of
retests within 0.1 logMAR units of the initial test score
(Figure 2). Data plotted as the test-retest difference versus
the mean test-retest acuity suggested that test-retest reliability
did not vary with the level of visual acuity (Figure
3). In the 60 eyes with amblyopia, the correlation between
the initial and the retest visual acuity scores was 0.96, with
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FIGURE 2. Distribution of test-retest differences in acuity scores (N 156). A negative difference indicates that the repeat acuity
test score was worse than the initial score. Eighty-eight percent of right eyes and 94% of left eyes were within 0.1 logarithm of
minimal angle of resolution (logMAR) units on retest. The 95% confidence interval for an individual acuity score was calculated
to be the score 0.13 logMAR units. The 95% confidence interval for an individual test-retest difference was calculated to be the
score 0.19 logMAR units.
FIGURE 3. Bland-Altman 2 plot of test-retest difference versus average test-retest acuity (N 156). Data plotted as the test-retest
difference versus the mean test-retest acuity suggested that test-retest reliability did not vary with the level of visual acuity. A
negative difference indicates that the repeat acuity test score was worse than the initial score.
93% of the retest scores within 0.1 logMAR units of the
initial test score. The test-retest reliability appeared similar
across the age range of the subjects in the study (correlation
between the difference in test-retest acuity scores and
age 0.02 for right eyes and 0.07 for left eyes, Figure 4).
More eyes had a better rather than a worse acuity on the
retest, and the mean retest acuity was slightly better than
the initial acuity (in right eyes, 33% better versus 18%
worse, mean difference 0.022 0.10, P .01; and in
left eyes, 33% better versus 17% worse, mean difference
0.017 0.09, P .02). This better retest acuity score was
present for younger (3 to 4 years old) and older (5 to 7
years old) children, for children who had and who did not
have prior experience with HOTV testing, and for eyes
with amblyopia (data not shown). There was no tendency
for subjects in whom both eyes were classified as normal to
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FIGURE 4. Test-retest difference versus age in years (N 156). Test-retest reliability appeared similar across the age range of the
subjects in the study. The correlation between the difference in test-retest acuity scores and age 0.02 for right eyes and 0.07
for left eyes. A negative difference indicates that the repeat acuity test score was worse than the initial score.
FIGURE 5. Distribution of test-retest differences in interocular difference scores (N 156). A negative difference indicates that
the interocular difference on the repeat testing was larger than the interocular difference on the initial testing. The 95% confidence
interval for an individual interocular difference was calculated to be the interocular difference 0.18 logMAR units. The 95%
confidence interval and for an individual interocular difference difference between two tests was calculated to be the interocular
difference 0.25 logMAR units.
have a better acuity score in the left eye than in the right
eye (N 73; mean interocular difference 0.0 0.11 in
both the initial test and repeat test).
The 95% confidence interval for an acuity score was
calculated to be the score 0.13 logMAR units (based on
the standard error of measurement of 0.069). For a change
between two acuity scores, the 95% confidence interval
was the difference 0.19 logMAR units (when based on
either the standard error of measurement or on the 0.096
standard deviation of the test-retest difference).
The interocular difference test-retest correlation was
0.89. The retest interocular difference was within one
logMAR level (0.1 logMAR units) of the initial interocular
difference in 81% (Figure 5). The 95% confidence
interval for the interocular difference was calculated to be
the interocular difference 0.18 logMAR units and for a
908 AMERICAN JOURNAL OF OPHTHALMOLOGY
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change in interocular difference to be the difference
0.25 logMAR units.
DISCUSSION
WE HAVE DEVELOPED A PERSONAL COMPUTER–BASED VIsual
acuity testing system controlled by a handheld device
running the Palm operating system and programmed to run
the Amblyopia Treatment Study visual acuity testing
protocol. Our results indicate that this testing procedure
has very good reliability for the measurement of visual
acuity in children 3 to 7 years old. These results are similar
to those we obtained when the Amblyopia Treatment
Study acuity testing protocol was evaluated with manual
administration on a Baylor-Video Acuity Tester. 1 In our
Baylor-Video Acuity Tester study, testability ranged from
67% in 3-year-olds to 96% in 5- to 7-year-olds. Testability
should not differ between the Baylor-Video Acuity Tester
or Electronic Visual Acuity Tester, because from the
child’s perspective, the testing procedure is identical
whether it is conducted on the Baylor-Video Acuity Tester
or on the Electronic Visual Acuity Tester. Two of our four
sites did evaluate testability with the Electronic Visual
Acuity Tester and in developmentally normal children
found it to be 85% in 27 3-year-olds, 94% in 35 4-yearolds,
and 100% in 71 5- to 7-year-olds.
We were surprised that the mean acuity was slightly
better in both eyes on the repeat test compared with the
initial test, particularly because this phenomenon was not
related to either younger age or to lack of prior experience
with HOTV testing and because there was no suggestion
that the left eyes tested better than the right eyes (testing
order was right eye, left eye). In reviewing our data from
the Baylor-Video Acuity Tester reliability study, we found
a similar phenomenon for right eyes but not for left eyes.
These findings suggest that there might be either a small
learning effect or increased comfort level that can lead to
a slightly better acuity on repeat testing. Regarding the
clinical importance of a small improvement of visual acuity
on repeat testing, the results of both this study and our
prior Baylor-Video Acuity Tester study indicate that we
can be reasonably certain of a real difference only if it is
two or more logMAR levels. With this requirement, the
learning/comfort effect, which appears on average to be on
the order of one-fourth of a logMAR level, is likely to be
inconsequential in the interpretation of whether a true
change in acuity has occurred.
In the clinical management of children with amblyopia,
considerable weight is given by some clinicians to the
interocular difference. As we found in the Baylor-Video
Acuity Tester reliability study, 1 we again found that the
test-retest reliability of the interocular difference was
slightly lower than that of the visual acuity measurements
themselves. This was true even when the analysis was
limited to the children with unilateral amblyopia (data not
shown).
Our results compare favorably with the few studies that
have evaluated the reliability of visual acuity testing
methods in children. For line HOTV acuity testing,
Harvey and associates 4 reported test-retest rank correlation
coefficients of 0.81 for 36 3.5-year-olds and 0.85 for 48
4.5-year-olds as part of the multicenter Cryotherapy for
Retinopathy of Prematurity Study. However, 10% of retests
differed from the initial test by more than 3 logMAR
lines. Using the Glasgow acuity cards, McGraw and
associates 5 reported that 95% of retests were within one
logMAR level of an initial test in 68 visually normal
children who had a mean age of 5.3 years.
In summary, we have developed a computerized method
for visual acuity testing and have successfully adapted the
Amblyopia Treatment Study visual acuity testing protocol
to test children with this system. For a clinical trial, the
advantages of using this computerized method of testing
over manual testing include better standardization of the
testing procedure across multiple sites, decreased training
requirements for the technicians administering the test,
ability to test acuity up to 20/800 with single-surrounded
optotypes at a 3-m test distance, and ability to directly
capture the testing data electronically without the need to
manually record every response on a score sheet. The
primary disadvantage is cost when compared with manual
testing using eye charts; the cost of the components of the
system (personal computer, monitor, handheld device) is
about $750. In addition to the Amblyopia Treatment
Study visual acuity protocol, we are currently developing
an adaptive testing strategy for testing older children and
adults using the Electronic Visual Acuity Tester as an
alternative to the Early Treatment Diabetic Retinopathy
Study method which has been the gold-standard for
measuring visual acuity in clinical trials.
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