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Autorefractors in Preschool Screenings
W.R. Bobier, L. Cowen, C. Machan, M. Lane, M. Parks, D. Wintermeyer, B. Robinson
School of Optometry, University of Waterloo, Waterloo, Ontario, Canada
Introduction
Hand-held autorefractors such as the Retinomax (Nikon Co) have been suggested
for use in preschool screenings. Best results appear to be when cycloplegia is
used.1,2 For large scale preschool screenings however, non cycloplegic measures
are required. It is not clear if hand held autorefractors can control accommodation
sufficiently in preschool children.3 We look at the performance of two autorefractors,
(Figs. 1 &2) the Retinomax Plus (Nikon Co) and two DAV prototypes (SureSight,
Welch Allyn Co) employed in a County wide vision screening programme for
preschool children. The DAV prototype uses a more remote working distance (40cm)
than the Retinomax.
Child fails one or more
screening criteria
Child passes screening
• Referred to Eye Care Practitioner
• Specific clinical measures taken
• Form completed and sent to Oxford County
Public Health Unit
• Clinical Results collected by Oxford County Board of
Health
• Clinical data sent to the University of Waterloo and
combined with screening data
• Data Analysis
The mean equivalent sphere (M) and cylinder components (J0, J45) for the
practitioners’ retinoscopy were 0.71D, 0.13D and -0.01D respectively (Table 1). The
difference between the retinoscopy and the Retinomax and DAV prototypes are shown
in Table 2. It can be seen that the equivalent sphere of the Retinomax is significantly
less hyperopic with respect to the retinoscopy. However, cylinder values show no
significant departure.
Variable N
M
88
Jo
88
J45
88
A separate investigation of the two instruments was conducted on an older population
with mean age 38.7 years, range (7 to 78). Subjects who gave prior consent were
selected from a clinical population at the School of Optometry, University of Waterloo.
A total of 167 subjects were tested. All retinoscopic measures were reviewed from the
clinic record and classified in terms of close agreement with the subjective refraction.
Cases where the discrepancy exceeded 0.50D were not included. Instrument
performance was determined by comparison with retinoscopic measures taken from
the practitioners’ reports.
Figure 2: THE WELCH ALLYN DAV2 PROTOTYPE
Preschool Study
A screening program, targeting all kindergarten registrants, is conducted annually by
the Oxford County, Health Unit (Ontario, Canada).4 Parental consent is obtained for
inclusion of each child in the study. During the spring of 1999, vision-screening tests
were performed on 1180 children (mean age= 46.9 months, range= 39 to 62). Of
these, 369 children were referred to eye care practitioners who were most
predominantly optometrists.
Screening was primarily based on subjective measures of visual and stereo acuity.
The pass/fail criteria was 6/6 acuity (single letters) and 100 sec stereoacuity
(SteroFly, Titmus Optical Co). When a child was referred, the eye-care practitioner
reported specific ocular-visual findings to the county health unit.
Figure 3 outlines the screening and data collection.The autorefractors were
introduced as part of the screening but did not affect the pass/fail decision. The sole
exception was for hyperopes showing +2D or more with the Retinomax. The
refractive measures of the two instruments were compared to the retinoscopic
measures reported by the eye-care practitioners.Only data with sufficiently high
confidence levels were accepted. The initial analysis was conducted from a sample
of 92 children referred, followed by an analysis on an increased sample of 155
children. Of the 92 initial children, 19 received a cycloplegic refraction.
Data Analysis
Data analysis was conducted by decomposing sphero-cylinder values using a Fourier
series representation that makes refractive errors more amenable to traditional
statistical analysis.5 Specifically, the sphero-cylinder components are decomposed
into independent terms representing the equivalent sphere, (M) and two crosscylinders components one at axis 0 (J0) and the other at axis 45( J45).
The negative sphero-cylindrical format is specified as (S, C x ), (where C is a
negative number) and is transformed as follows:
C
M S
2
C
J 0  cos2 
2
C
J 45   sin 2 
2
Instrument differences (bias) could be determined by subtraction from the clinical
retinoscopy. Statistical significance was determined using a paired t-test.
Mean
0.71
0.13
-0.006
SD
1.05
0.32
0.10
Min
-2.75
-0.5
-0.56
Max
6.5
1.38
0.43
0.257 -0.291
0.35
-0.777
156
DAV2
-1.39
0.55
-0.83
-1.11
004
J0
J45
Instrument Variable N
Retinomax
DAV2
M
Jo
J45
M
Jo
J45
M
Jo
J45
88
88
88
69
69
69
16
16
16
Mean Std Dev Prob>|T|
0.73
-0.008
-0.02
-0.04
0.26
-0.29
-1.39
0.55
0.07
1.38
0.28
0.15
1.24
0.76
0.29
0.80
0.92
0.16
Retinomax
0.07
Table 5. Over Correction to match instrument with retinoscopy. Conversion
of the differences in M, J0, J45 back to negative sphero-cylinder form.
Adults
Table 1. Means of M, J0 and J45 components, for eye-care practitioner, right eye
of the Preschool sample.
Figure 3. Flow Chart of Oxford County Vision Screening Program
Adult Study
-0.04
M
Preschool Children
DAV1
Figure 1: THE NIKON RETINOMAX PLUS
DAV1
Measurement
Discharged from
the study
• Screening Data
sent to Waterloo
• Data Analysis
0.734 -0.008 -0.022
Negative SpheroCylinder form
S
C

(D)
(D)
()
0.76 -0.047 124
Results
Pre School Child enters vision screening study:
• Subjective tests of visual and stereoacuity: Pass/Fail
• Objective measure with Welch Allyn SureSight (DAV) auto-refractor (data only)
• Objective measure with Nikon Co. Retinomax Plus auto-refractor: fail > +2.00 D Sphere
Fourier Form
0.0001
0.78
0.17
0.81
0.006
0.0001
0.0001
0.03
0.13
Table 2. Differences in means between practitioner and instrument
measurements of M, J0 and J45 (right eye). Paired t-test significance
levels for the preschool children.
DAV prototypes are seen to be more hyperopic with respect to the retinoscopy.
Further, the J0 component in both DAV prototypes is significantly different from those
of the practitioner while only DAV1 has a significant J45 component. Final analysis of
the larger sample of 115 showed no significant changes from the smaller sample.
DAV1 and DAV2 measures were taken on 137 of the children. The DAV1 and DAV2
prototype measures could not be pooled since analysis of variance testing showed
that the prototypes were significantly different in measures of M (F=7.01, p=0.001)
and J45 (F=35.17, p=0.0001).
Counts of equivalent spheres that are 1 D greater or less than those of the practitioner
for the initial sample (n=88) are given in Table 4. As expected with the bias for the
Retinomax was less hyperopic and the DAV prototypes were more hyperopic in both
eyes.
Instrument Eye
Count > 1D
Count < -1D
Total
More Myopic More Hyperopic
Retinomax Right
29
4
88
Left
22
3
87
DAV1
Right
7
12
69
Left
8
17
67
DAV2
Right
1
9
16
Left
0
9
16
Table 4. Count of the measurements that are greater than or less than 1 Diopter
from the practitioner measurements of equivalent sphere M (e.g.
Mpractitioner – Mretinomax)
Conversion of the instrument bias defined by M, J0 and J45 back into sphero-cylinder
form is shown in Table 5. This represents the over- correction required to match the
instrument with the retinoscopy.
The results for the Retinomax and DAV2 show the adult sample to be hyperopic in
Table 6. Retinomax measured 16% of the equivalent spheres less than -1D,
whereas Dav2 measured 65%. Percentages greater than 1D were 4% and 2%
respectively. J0 components are significantly different from the practitioners’
measures, but only Retinomax has a significant J45 component. An analysis of
variance, split-plot design was used to test the hypothesis that variation in the
preschool and adult populations was equivalent when considering population as a
factor. Population does have a significant effect however, only when considering the
equivalent sphere component (F=168.07, p=0.0001). This would imply that
differences in cylinder components are not coming from an effect of the population.
Instrument Variable
Retinomax
DAV2
M
Jo
J45
M
Jo
J45
N
Mean
167
167
167
167
167
167
-0.39
0.09
-0.04
-1.27
0.16
-0.001
Std Dev Prob>|T|
0.73
0.38
0.23
1.23
0.71
0.23
0.0001
0.002
0.03
0.0001
0.004
0.95
Table 6. Mean difference in refractive error components between the
practitioner and Retinomax and the practitioner and DAV2 in an adult
sample.
A recent study of 69 adult subjects showed that a third SureSight prototype (DAV3)
had much smaller biases with respect to the retinoscopy (M= -0.58; J0 = 0.04; J45=
0.01). Only the equivalent sphere difference was significant.
Conclusions
Retinomax measures of refractive errors in preschool children are more myopic than
clinical retinoscopy. We conclude that the close working distance of the Retinomax
induces variable instrument myopia in non-cyclopleged preschool children despite a
‘fogging’ target. This is not found in the adult sample. However, the Retinomax
provides a robust measure of astigmatism in preschool children.
The DAV prototypes have reduced over-accommodative effects. This is likely due in
part to their 40 cm working distance. However, they show similar variances in
measures. The cylinder discrepancy in the early versions has been significantly
improved in the latest prototype. The accommodative behaviour of the preschool
children appears to lead to considerable variance in autorefractor and possibly
clinical measures.
Acknowledgements
The support of the Oxford County Public Health Board and the assistance of Ms.
Kathleen Hill are greatly appreciated. This research was funded by a grant from the
Welch Allyn Co.
1
2
3
4
5
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