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1 From the Bristol Eye Hospital; and the 3 Department of Ophthalmology, University of Bristol, United Kingdom; and the 2 School of Health Policy and Practice, University of East Anglia, United Kingdom.
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Abstract |
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 Top Abstract IntroductionMaterials and MethodsResultsDiscussionReferences |
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METHODS. Two hundred twenty-two children less than 8 years of age were included. All children were examined by an orthoptist using the PR2000 without inducing cycloplegia. All children then underwent retinoscopy with cycloplegia by an examiner who was unaware of the results from the PR2000 examination.
RESULTS. The PR2000 gave a numerical reading for 90% of the children’s right eyes and the message "Out of range" for a further 5%. The readings underestimated the amount of hypermetropic or astigmatic refractive error found on retinoscopy by an amount proportional to the magnitude of the refractive error. Agreement with retinoscopy for the axis of astigmatism more than 0.75 D was moderately good (intraclass correlation coefficient [ICC] = 0.63). The PR2000 was more useful as a screener, especially for anisometropia for which it was 91% sensitive and 92% specific. The repeatability was good for sphere (ICC = 0.74), less so for astigmatism (ICC = 0.59), and better than the optometrist for anisometropia (ICC = 0.38). The presence of nonrefractive diagnoses and the age of the children examined made little difference in the screening results.
CONCLUSIONS. The PR2000 underestimated hypermetropic refractive errors when used without cycloplegia. However, it was at least as good a screening device as other similar instruments, especially when judged by its ability to detect anisometropia and the repeatability of the results.
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Introduction |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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The PR2000 Pediatric Refractometer (Topcon, Tokyo, Japan) was designed for use in young children and was included in a longitudinal study of infant visual development, as a potential tool for preschool vision screening.13 The purpose of this study was therefore to assess the accuracy and repeatability of readings obtained with the PR2000.
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Materials and Methods |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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The PR2000 is a video-enhanced, infrared photorefractor and was used in
accordance with the manufacturer’s instructions. The child was seated
either alone or on a parent’s lap, at a distance of 0.9 m from
the machine in a darkened room. Built-in flashing red and green lights
and a beeping sound were used to attract the child’s attention. The
operator observed the image of the child’s face in the video screen
and slid the machine forward or backward so that the pupil margins were
clearly in focus. Both eyes were assessed simultaneously, unless there
was an obvious strabismus, when monocular readings were taken. The
PR2000 produced printouts with the refraction for each eye expressed as
sphere, negative cylinder, and axis. There were confidence scores
indicating which readings were estimated by the software to be very
reliable (the average of at least three recordings; denoted by a star),
unreliable (denoted by brackets) or moderately reliable (single
readings only; no star, no brackets). The machine stored the images
used for calculation for each subject. The measurable range was -5.0 D
to +5.0 D for sphere and cylinder, in 0.25 steps. Estimations outside
this range were denoted by error codes <<+>> or <<->>. Images that
could not be analyzed by the software were denoted by the message
ERROR. Axis determination was in gradations of 5°. The PR2000 showed
warning messages of "Room Light up" or "Room Light down" if the
ambient light levels needed adjustment. These instructions were
followed on the rare occasions when they occurred. In accordance with
the manufacturer’s instructions, readings for sphere of -1.00
D or more have had 1.0 added, to take account of the child’s
accommodation to a target at 1 m. This results in a dead zone of
between 0.0 D and -1.25 D, in which range there are no readings. For
uniocular measures, only the right eyes from each individual have been
analyzed. The statistical methods used were regression analysis,
calculation of the intraclass correlation coefficient (ICC), the
unweighted 
statistic, the sensitivity and specificity, and the
positive (PPV) and negative (NPV) predictive values.
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Results |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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Thirteen children (6%) registered ERROR and 10 children (5%) registered "Out of range," for the right eye. Of the remaining 199 right eye readings, 136 (68%) were "confidence 1, reliable," 39 (20%) were "confidence 2, moderately reliable," and 24 (12%) were "confidence 3, poor reliability." Readings (confidence 1–3) for both eyes were obtained for 189 children. Four children (2%) had error readings in both eyes. There was no difference in the magnitude of the differences between PR2000 and retinoscopy for any of the three confidence scores, for either spherical refractive errors (analysis of variance [ANOVA], df = 2, F ratio = 1.77, P = 0.17) or for astigmatism (ANOVA, df = 2, F ratio = 2.04, P = 0.13). Therefore, the data are presented for confidence readings 1 through 3 combined for the right eye (n = 199) for uniocular measures of accuracy, and for the children with confidence results 1 through 3 in both eyes (n = 189) for the results concerning anisometropia and screening.
Spherical Refractive Error
The PR2000 and cycloplegic retinoscopy readings for the spherical
refractive error of the right eye were compared, and Figure 1
shows the differences between the two measurements plotted against the
mean of the two measures. The mean difference between retinoscopy and
the PR2000 was 1.16 ± 1.52 D. As Figure 1
shows, the differences
between the two measures were related to the mean magnitude of the
hypermetropic refractive error (r = 0.67,
r2 = 0.45, P <
0.001).
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statistic (95% confidence interval [CI]) for
agreement between the PR2000 and cycloplegic retinoscopy was =
0.33 (0.23–0.42). For 32 cases in which the PR2000 estimated the
astigmatism as more than 0.75 D, the agreement with the optometrist was
better ( = 0.68, 0.45–0.90). The ICC for actual measurements
of the axis was 0.20 for all 199 cases but increased to 0.63 for the 32
cases with more than 0.75 D astigmatism.
Children with and without Nonrefractive Diagnoses
The proportion of error readings in the right eye was 3 (5%) of
58 for the children with nonrefractive diagnoses, compared with 10
(7%) of 164 for the children with refractive errors only
(P = 0.54, Fisher’s exact test). The proportion of
out-of-range values was 6 (10%) of 58 for the children with
nonrefractive diagnoses, compared with 4 (2%) of 164 for the children
with refractive errors only (P = 0.02, Fisher’s exact
test). Of the 189 children with results (confidence 1–3) for each eye,
there were 35 children with nonrefractive problems (31 with strabismus,
2 with lens opacity, 1 with corneal scarring, and 1 with pseudophakia).
The mean differences between the PR2000 and the retinoscopy results
were larger for the children with nonrefractive problems than for the
children with refractive errors alone. The differences were 1.8 ±
1.9 D versus 1.0 ±1.4 D for sphere, 0.29 ± 0.57 D versus
0.13 ± 0.62 D for astigmatism and 0.09 ± 0.83 D versus
0.008 ± 0.83 D for anisometropia. The only statistically
significant difference between the two groups was in the results for
sphere (ANOVA, df = 1, F ratio 7.8, P =
0.006). The differences were still related to the mean quantity
measured, for all parameters in the children with refractive errors
only and for the measurement of spherical error in the children with
nonrefractive problems (ANOVA, df = 1, P <
0.001 in all cases).
Accuracy of the PR2000 for Different Age Groups
The 189 children with full results were classified into age
groups: up to 36 months (n = 83), 36 to 59 months (n
= 45), and more than 59 months (n = 61). The mean
differences between the spherical refractive errors, measured by
retinoscopy, compared with those measured with the PR2000 for the three
age groups were; 0.84 ± 1.3 D, 1.3 ± 1.7 D, and 1.5 ±
1.6 D for children aged less than 36 months, 36 to 59 months, and more
than 59 months, respectively (ANOVA, df = 2, F ratio =
3.8, P = 0.02). The only statistically significant
individual difference was between the results for children aged less
than 36 months compared with those for children aged more than 59
months (P = 0.02). The corresponding ICCs for agreement
between the PR2000 and retinoscopy for sphere were 0.35, 0.61, and
0.59, respectively. The range of the retinoscopic errors for children
aged up to 36 months was -1.25 to +7.5 D; for children 36 to 59
months, -4.5 to +7.75 D; and for children above 59 months, -5.5 to
+6.7 D.
Repeatability of the PR2000 and of Cycloplegic Retinoscopy
The results estimating the repeatability of the PR2000 and of
cycloplegic retinoscopy by the optometrist are shown in Table 1
. The ICC values suggest that the PR2000 was less repeatable for
sphere, spherical equivalent, and astigmatism but was more repeatable
for anisometropia than was retinoscopy. However, the differences in
measures between visits are smaller for the PR2000 than for
retinoscopy, except those relating to astigmatism. This discrepancy
between the ICC and the magnitude of the differences is likely to be
because the ICC is affected by the range of values, which was greater
for the retinoscopy data than for the PR2000 data. The ICC for axis
determination was 0.92 for the PR2000 and for retinoscopy was only
0.04. However, of the nine cases in which there was retinoscopic
astigmatism on both occasions, six axes agreed exactly: one differed by
20°, one by 85°, and one by 175°. Of the eight cases in which the
PR2000 detected astigmatism on both visits, seven axes agreed within
10° and 1 by 40°. The unweighted statistics for the
repeatability of classification of the axis of astigmatism (with the
rule, against the rule, or oblique), were = 0.75 for the
PR2000 and = 0.64 for the optometrist.
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Accuracy of the PR2000 as a Screening Tool
Receiver operating characteristic (ROC) curves demonstrating the
sensitivity and specificity of the PR2000, using different cutoff
points for referral, are shown in Figures 3
4
and 5
(for 189 children with full data). When screening is conducted
for a relatively rare condition, cost considerations may necessitate
using a very specific referral cutoff for the screening test, to avoid
large numbers of overreferrals. Table 2
shows the results that were obtained, either when the sum of
sensitivity + specificity was the greatest of those tested, or when the
specificity was required to be 95% or greater. The sensitivities
decline, but the PPV values increase, when the specificity is required
to be at least 95%. Screening for anisometropia was more accurate than
screening for hypermetropia or astigmatism in both situations.
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Discussion |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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Many of the children had other diagnoses besides suspected refractive error. The differences between the PR2000 and retinoscopy were larger for the children with nonrefractive diagnoses, although this was only statistically significant for the measurement of spherical refractive error. This might be expected, because many of these children had strabismus and may have a strong tendency to overaccommodate, which would mask any hypermetropic error in noncycloplegic conditions. However, the screening results for the PR2000 were similar whether the children with other problems were included or excluded.
There was no consistent effect of the age of the child on the accuracy of the PR2000. Although the differences between the PR2000 and the retinoscopy results for sphere were smaller for the youngest children (aged <36 months) than for the oldest children (aged >59 months), the ICC for agreement with retinoscopy was only 0.35 for the youngest children, whereas that for the other two categories was approximately 0.60. This apparent disagreement may be because the range of refractive errors was smaller in the youngest children. Because examination by the PR2000 requires very little cooperation from the child except brief fixation on the target, it is not surprising that age made little difference in these results. The PPV values for screening the younger children were less than those for the older two groups (Table 3) , but the prevalence of target conditions was also lower in this group, which would reduce the PPV.
The ROC curves and the data in Tables 2 and 3 indicate a more accurate profile for the PR2000 when screening for anisometropia than when screening for spherical or astigmatic errors. This might be expected, because both eyes were measured simultaneously. There are few data in the literature with which to compare these data, but a custom-built device detected both of two cases of more than 2.0 D anisometropia, whereas four infants with anisometropia between 2.0 and 1.5 D were missed, in a sample of 97 infants. These data suggest a lower sensitivity than found for the PR2000.3
The data in Tables 2 and 3 for hypermetropia and astigmatism also compare well with the results of screening obtained using other devices in children without cycloplegia. For the group using a custom-built device, the sensitivity was 85% and specificity only 53%,when screening for hypermetropia in excess of 3.25 D,14 compared with 80% and 82%, respectively, in the present study (Table 2) . Another group, using the VPR-1 (Clement Clarke, Harlow, UK), found that the sensitivity was only 12%, but with 99% specificity, when screening for hypermetropia of 4 D or more, whereas for astigmatism of more than 1.75 D, sensitivity was 54% and specificity was 62%.10 The equivalent percentages in this study were 94% sensitivity and 75% specificity (Table 2) . In two recent studies on the MTI photoscreener (Medical Technologies, Riviera Beach, FL), there was marked variability between different operators in the accuracy of grading the pictures.12 15 In one of these studies, no individual grader achieved sensitivity and specificity both above 70%.15 In the present study the PR2000 achieved this for all target conditions (Table 2) . The Retinomax autorefractor (Nikon, Tokyo, Japan) had a sensitivity of 70%, specificity of 95%, PPV of 79%, and NPV of 92%, respectively, for hyperopia of 3.5 D or more in children aged 9 to 36 months.8 These values suggest slightly greater sensitivity than the present study (Tables 2 3) . The percentages for astigmatism of more than 2 D detected with the Retinomax were 59% sensitivity and 95% specificity, with PPV of 71% and an NPV of 92%,7 which again are slightly more sensitive but otherwise very similar to the results in the present study (Tables 2 3) .
The retinoscopy results were very repeatable, except those for anisometropia, for which the ICC was surprisingly low. However, this repeatability study was small, and the results should be interpreted cautiously. Although generally less repeatable than retinoscopy, the results for the PR2000 (Table 1) compare favorably with those given for the VPR-1, with which the mean difference between visits for assessment of spherical refractive error was 0.12 ± 1.1 D and for astigmatism was 0.2 ± 1.6 D.9 The repeatability reported for the Retinomax was also worse than that found in the present study for sphere, with a mean difference between refractions of 0.05 ± 1.33 D. The results were similar to those in the present study for astigmatism; however, with a mean difference between refractions of 0.04 ± 0.45 D.7 Autorefractors used in adults with pseudophakic eyes can be more repeatable: 0.04 ± 0.30 D for sphere and 0.02 ± 0.36 for astigmatism.16
Cycloplegic retinoscopy was the gold standard against which we judged the results from the PR2000. The results may differ from those that would have been obtained when comparing the PR2000 results with those of subjective refractions in subjects without cycloplegia. The use of cycloplegic drops introduces further variability. It is possible that the use of a variable gold standard reduced the apparent accuracy of the PR2000. Alternatively, if cycloplegia had not been fully induced, in many children it would have led to an increased estimate of the sensitivity of the PR2000 for hypermetropia. These caveats also apply to the other studies quoted in which cycloplegic retinoscopy was used,3 4 7 8 9 12 14 15 and therefore comparisons may still be made between the different devices tested.
In our institution, the most experienced retinoscopist was the senior hospital optometrist, who performed the refractions in this study, but different institutions may have other professionals who perform most of the retinoscopy. Many of the children were selected from hospital clinics, and the results therefore cannot be generalized to normal populations. Similarly, the results apply to a predominantly white population and may not be applicable in areas in which there is a much higher prevalence of children with heavily pigmented eyes. The PR2000 also provides an estimate of ocular alignment based on the corneal reflections. This feature was not assessed, because the PR2000 was not used in this respect in our longitudinal study. An earlier prototype of the PR2000 (the PR1100) was validated in this regard and found to be reliable but to have poor agreement with the results of a prism cover test.17
The current results suggest that the PR2000 provides repeatable results, has a more accurate screening profile than some devices in the literature (the VPR-1 and the MTI) and a comparable (although slightly less sensitive) profile to that of the Retinomax for sphere and astigmatism. However, the most accurate results were obtained when screening for anisometropia, and few comparable data were found for the other devices. Anisometropia may be labile in childhood,18 and the nature of the relation between anisometropia and strabismus or amblyopia is as yet unclear.19 Nevertheless, anisometropia is strongly associated clinically with strabismus and/or amblyopia.20 When used in our main population study in children less than 37 months of age (results presented separately), the PR2000 was more sensitive than was visual acuity testing, especially when detecting straight-eyed amblyopia, which is commonly associated with anisometropia. In conclusion, the PR2000 was a reliable device, most accurate when assessing anisometropia but with sufficient sensitivity and specificity for other refractive errors to be of potential use in research or screening programs.
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Acknowledgements |
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Footnotes |
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Submitted for publication April 14, 1999; revised October 6, 1999; accepted November 8, 1999.
Commercial relationships policy: N.
Corresponding author: Cathy Williams, Dept. of Ophthalmology, Bristol Eye Hospital, Lower Maudlin Street, Bristol, BS1 2LX, UK. cathy.williams{at}bristol.ac.uk
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References |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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