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Astigmatism in a Countywide Preschool Population
Introduction
of Optometry, University of Waterloo, Waterloo, Ontario, Canada
Analysis
 It is generally held that astigmatic errors undergo emmetropization
during the early years of life. However, large-scale population data is
mostly limited to infant and school aged children.1,2,3
 There is a general pattern of against-the-rule astigmatism (AGR) in
infancy, which typically changes to with-the-rule (WTR) by school age.3
Studies of preschool children show that prevalence of AGR decreases
with age, while WTR increases.5
 With the recent development of more ‘child-friendly’ autorefractors, the
opportunity for population based measures has increased.
 Large screenings usually preclude the use of cycloplegic agents and
thus validation of measures is important.
Retinomax Preschool Screening
 The sphero-cylinder refractive error measurements were decomposed
into three independent components using a Fourier transformation. The
components are based on equivalent sphere (M), and two (Jackson
Cross) cylinder components (J0 and J45). For negative-cylinder refractive
measure (S, C x ) we used the transformation as follows:
C
c
c
M  S
J 0   cos 2 
J 45   sin 2 
2
2
2
 This transformation was used to determine the validity of using
Retinomax measures to describe the pattern of refractive errors found in
this preschool population. As the Fourier transpositions allows M,J0 and
J45 to be analysed seperately, simple over-accommodation would affect
equivalent sphere and not the astigmatic components.
 A non-parametric 95th percentile was deemed to be the limit of normal
astigmatism.
Measurment Validation
 Transposition from Fourier form back to sphero-cylinder form gives an
over-correction of (0.93, –0.09 x 137) for the mean differences in table 1.
 If added to the mean Retinomax readings, the over-correction would
correct them to that of the clinical retinoscopy. Note that cylindrical
adjustments are small.
 ANOVA split-plot design showed that population (preschool vs adult) has
a significant effect on the equivalent sphere component (p=0.0001).
Differences in the cylinder components are coming from the same source
in the 2 populations; likely in the fact that the Retinomax provides a more
accurate measure of small cylinders than the reported clinical measures.
Stratification
 No significant differences were found between three and four year old
age-groups in regards to astigmatism frequencies in the preschool
population.
 Astigmatism breakdown into WTR, AGR and oblique shows that most
cylinder magnitudes are between –1 and 0 D (figure 4).
W i th -th e
F
r
e
q
u
n
c
y
0 50 10 150 20 250 30
Of Statistics and Actuarial Science;
2School
Bobier
Figure 4. WTR, AGR, and oblique
astigmatisms for the pooled sample.
 Retinomax astigmatism measurments are valid, however equivalent
sphere measures are not. Problems with equivalent sphere seem to be
consistent with over-accommodation.
 Autorefractive measures of a population of preschool children were
taken as part of an annual screening of kindergarten registrants.6 Vision
screening tests were performed during the spring of 1999 (figure 1).
Retinomax Performance
Preschool Astigmatism
 Validation studies supported the use of the cylinder components and not
the equivalent sphere.
 The frequency distribution of all astigmatisms is given in figure 2. WTR
astigmatisms dominate this preschool population.
 Non-cycloplegic refractive error measures were taken using the
Retinomax Plus (Nikon Co) on 1162 participants of the study.
 In the preschool study, the equivalent sphere is significantly more
myopic for the Retinomax measures (table 1).
Figure 2. Frequency distribution of astigmatism for the preschool population.
- 4
A s ti g m a ti s m
- 4
Table 1. Differences in practitioner and Retinomax measures of M, J0 and J45
for 121 preschool children. Mean, standard deviation and significance level
of a paired t-test are shown.
Objective measure with Nikon Co. Retinomax Plus auto-refractor
Fail screening
M
J0
J45
Pass screening
• Referred to Eye Care Practitioner
• Retinoscopy taken for validation of Retinomax measures
• Data sent to Oxford County Public Health Unit
Table 2. Differences in practitioner and Retinomax measures of M, J0 and J45
for a) 167 adult study and b) 69 adult study. Mean, standard deviation and
significance level of a paired t-test are provided.
Retinomax Validation Studies
a)
 Preschool children who failed the screening were referred to an eyecare practitioner to receive a retinoscopy. In 23% of the 155 referrals,
cycloplegic refractions were conducted. Retinoscopies provided the
means for validation of the Retinomax measures.
 A clinical population provided two adult samples (n1=167, n2=69) where
both Retinomax and retinoscopy refractive error measures were taken.
by
comparison
with
M
J0
J45
Mean
-0.39
0.09
-0.04
SD
SL
0.73 0.0001
0.38 0.002
0.23 0.03
b)
M
J0
J45
Mean
-0.37
-0.05
-0.04
SD
SL
0.63 0.0001
0.28 0.19
0.15 0.03
0
lin d e r
M
- 3
- 2
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- 1
0
lin d e r
Fre q
- 4
- 3
- 2
- 1
0
M a g n it
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uy
d lin
e
d e r
M
Conclusions
 Oxford County preschool children astigmatisms can be broken down
into 58% WTR, 30% AGR and 12% oblique.
1
2
W
3
it
h -
4
t
h
Population Norm
 The cumulative density of the cylinder magnitudes was used to
determine the 95th percentile, which was found to be -1.25D (figure 3).
 The norm for this population’s cylinder measure was -1.25D.
Prescribing patterns of eye-care practitioners show a threshold of
e - R u le ,
2 =
Ag
prescribing which falls close to this norm
(1.50D for 4 to
7 years 8). a
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Acknowledgements
The assistance and advice of the members of the Oxford County Board
Figure 3. Cumulative density of the cylinder magnitudes showing the 95th
of Health and of Drs Barbara Robinson, Carloyn Machan and Melanie
percentile to be -1.25D.
Campbell at the School of Optometry, University of Waterloo is greatly
appreciated.
We
thank Welch
Allyn and NSERCn
Canada s
for financial
Cu m u l a t
i v
e
De
i t
support of this project.
C
u
m
l
a
t
i
v
e
D
n
s
y
0. 0.2 0.4 0.6 0.8 1.0
• Data sent to Waterloo
• Data Analysis
determined
 Equivalent sphere differences between retinoscopy and the Retinomax
in the adult population were smaller and less variable (Table2).
y
- 1
 No significant differences in cylinder frequencies when age group
stratified the population.
0
 Differences in the astigmatic components and their variation were similar
to those found in the preschool population (Table 1 and 2).
Clinical Results collected by Oxford County Board of Health
was
W
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=
6
7
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T
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2
1 =
Discharged from the study
 Retinomax performance
retinoscopic measures.
Mean SD
SL
0.889 1.483 0.0001
0.003 0.268 0.91
-0.045 0.161 0.003
F
r
e
q
u
n
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y
0 20 40 60 80
Preschool Child enters vision screening study
 The cylinder components, show smaller biases and less variation in the
differences between retinoscopy and Retinomax measures.
- 2
C
C
Figure 1. Flow Chart of the Oxford County Vision-Screening Programme.
- 3
A g a i n s t-th eO
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References
- 4
1. Baldwin WR. in Rosenbloom AA, Morgan MW, editors. Principles and practice of pediatric
optometry. Philadelphia: J.B. Lippincott Company; 1990; Ch6, Refractive status of infants and
children. p. 104-52.
2. Banks MS. Infant refraction and accommodation. Int Ophthalmol Clin 1980;20(1):205-32.
3. Hirsch MJ. Hirsch MJ, Wick RE, editors. Vision of Children. Philadelphia: Chilton; 1963; The
refraction of children. p. 150-2.
4. Abrahamsson M, Fabian G, Sjostrand J. Changes in astigmatism between the ages of 1 and 4
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5. Gwiazda J, Scheiman M, Mohindra I, Held R. Astigmatism in children: Changes in axis and amount
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9 5 t
h
Pe r c e
n
t
ile
a
t 1
D
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B, Bobier
WR, Martin E, Bryant
L. Measurement
of the
validity of.
a2
preschool 5
vision
screening program. Am J Pub.Health 1999;89(2):193-8.
7. Thibos LN, Wheeler W, Horner D. Power vectors: an application of fourier analysis to the
description and statistical analysis of refractive error. Optom Vis Sci 1997;74(6):367-76.
- 3
- 2
- 1
0
8. Miller JM, Harvey EM. Spectacle prescribing recommendations of AAPOS Members. J Pediatr
Ophthalmol
&a
Strab 1998;35(8):51-2.
C y lin d e r
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u d e