RESEARCH ARTICLE
Myopia is associated with education: Results
from NHANES 1999-2008
Stefan Nickels 1ID , Susanne Hopf
2 2
ID , Norbert PfeifferID , Alexander K. Schuster
1
ID *
1 Center for Ophthalmic Epidemiology and Healthcare Research, Department of Ophthalmology, University
Medical Center of the Johannes Gutenberg-University Mainz, Mainz, Germany, 2 Department of
Ophthalmology, University Medical Center of the Johannes Gutenberg-University Mainz, Mainz, Germany
* alexander.schuster@uni-mainz.de
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a1111111111 Abstract
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Purpose
Myopia is increasing worldwide and possibly linked to education. In this study, we analyse
the association of myopia and education in the U.S. and investigate its age-dependency.
OPEN ACCESS
Citation: Nickels S, Hopf S, Pfeiffer N, Schuster AK
(2019) Myopia is associated with education: Methods
Results from NHANES 1999-2008. PLoS ONE 14 We conducted a secondary data analysis using the public use files from the cross-sectional
(1): e0211196. https://doi.org/10.1371/journal.
study National Health and Nutrition Examination Survey of the period from 1999 to 2008.
pone.0211196
19,756 participants aged 20 to 85 years were included with data on education and ophthal-
Editor: Der-Chong Tsai, National Yang-Ming
mic parameters (distance visual acuity, objective refraction and keratometry). Spherical
University Hospital, TAIWAN
equivalent, astigmatism, corneal power and corneal astigmatism were evaluated for an
Received: August 22, 2018
association with education using linear regression analysis with adjustment of potential
Accepted: January 9, 2019 confounders.
Published: January 29, 2019
Copyright: © 2019 Nickels et al. This is an open Results
access article distributed under the terms of the
Analysis revealed an association between spherical equivalent and educational level in the
Creative Commons Attribution License, which
permits unrestricted use, distribution, and univariate analysis (P < .001), and in the adjusted model (P < .001). Subjects who attend
reproduction in any medium, provided the original school to less than 9th grade had a mean spherical equivalent of 0.34 D, subjects with 9-
author and source are credited.
11th grade -0.14 D, subjects that finished high school -0.33 D, subjects with partial college
Data Availability Statement: Our statistical education -0.70 D, subjects that graduated from college or a higher formal education -1.22
analyses are based on the NHANES public use files
D. Subjects that graduated from college or above were -1.47 D more myopic compared to
of these survey cycles (https://wwwn.cdc.gov/
subjects that completed less than 9th grade school in the adjusted analyses. Astigmatism
nchs/nhanes/continuousnhanes/default.aspx, last
accessed 2017-12-11). and corneal curvature was not associated with education.
Funding: The authors received no specific funding
for this work. Conclusions
Competing interests: Susanne Hopf received travel Myopia is associated with higher education in the U.S. Our analysis shows that corneal cur-
expenses from Actelion Pharmaceuticals Germany
vature does not contribute to this association, therefore axial elongation or lens power are
GmbH, Norbert Pfeiffer receives financial support
by Novartis, Ivantis, Santen, Thea, Boehringer likely to contribute to myopia.
Ingelheim Deutschland GmbH & Co. KG, Alcon,
PLOS ONE | https://doi.org/10.1371/journal.pone.0211196 January 29, 2019 1 / 15
Myopia is associated with education in the U.S.
Sanoculis., Alexander K. Schuster receives Introduction
research funding by Novartis, Bayer Healthcare and
Heidelberg Engineering. There are no competing Myopia (short-sightedness) is a complex condition leading to blurred vision in distance. To
interests with regard to this submission. Financial overcome this, dispersing (minus-diopter) lenses or contact lenses are worn to create a sharp
support for other research projects are: Susanne image on the retina. Prevalence of myopia has rapidly increased during the last two decades,
Hopf received travel expenses from Actelion
especially in south-east Asia, but also in the western world [1–4]. Both environmental factors
Pharmaceuticals Germany GmbH, Norbert Pfeiffer
and genetic factors are thought to play a role in the pathogenesis of myopia [5–10]. Several
receives financial support by Novartis, Ivantis,
Santen, Thea, Boehringer Ingelheim Deutschland environmental factors are described to be related to prevalence and severity of myopia such as
GmbH & Co. KG, Alcon, Sanoculis., Alexander K. near work and lack of outdoor activity during childhood [11, 12], level of education [13, 14],
Schuster receives research funding by Novartis, and residence (urban vs. rural) [15]. Prior research on education and myopia mainly focuses
Bayer Healthcare and Heidelberg Engineering. This on Asian population [16–19] while less is known about the U.S. population.
does not alter the authors’ adherence to PLOS ONE
Therefore, the purpose of this study was to analyze the association of education with refrac-
policies on sharing data and materials.
tion in the U.S.
Methods
The National Health and Nutrition Examination Survey (NHANES) is a representative survey
research program to assess the health and nutritional status of adults in the United States of
America (https://www.cdc.gov/nchs/nhanes/index.htm, last accessed 2017-12-11). Since 1999,
regular data collection is carried out of approximately 5000 persons from 15 areas, who are
examined in two-year periods. Data is raised via questionnaire-based personal interviews at
the participant’s home, followed by a visit of a mobile examination center. From 1999 to 2008,
NHANES evaluated the level of education and included an ophthalmic examination. Our sta-
tistical analyses are based on the NHANES public use files of these survey cycles (https://
wwwn.cdc.gov/nchs/nhanes/continuousnhanes/default.aspx, last accessed 2017-12-11). The
study design is cross-sectional; NHANES survey protocols were approved by the NCHS
Research Ethics Review Board (ERB).
We included all participants of the survey years 1999–2008 aged 20 or older with available
information on education and having had an ophthalmic examination. For the variable differ-
ence in corneal power between meridians, we excluded negative values (n = 61) and extreme
values above 10 diopters (n = 15), as they are highly suspected to be measurement or data
entry errors. We excluded all participants with prior refractive surgery (“Have you ever had
eye surgery to treat or prevent near-sightedness or myopia?”) (n = 350) or missing information
on prior refractive surgery (n = 24) and prior cataract surgery (n = 1,194) or missing informa-
tion on prior cataract surgery (n = 6). Finally, 19,756 right eyes of study participants were
available for analysis.
Education
Detailed educational attainment information was evaluated asking the question “What is the
highest grade or level of school you have completed or the highest degree have received?”.
Answer categories were
1 Less than 9th grade
2 9-11th grade (Includes 12th grade with no diploma)
3 High School grade/GED (General Education Development or Diploma) or equivalent
4 Some College or AA degree
5 College Graduate or above
7 Refused (set to missing)
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Myopia is associated with education in the U.S.
9 Don’t Know (set to missing)
. Missing
AA or Associate of Arts degree is an undergraduate academic degree awarded by colleges
usually after completion of a two-year course. GED means General Education Development or
Diploma; this certification provides that the test taker has United States or Canadian high-
school-level academic skills.
Ophthalmic data
Study participants took part in an examination of visual function. Objective refraction and kera-
tometry were examined using an autorefractor/keratometer (Nidek ARK-760A, Nidek Co. Ltd.,
Tokyo, Japan) in non-cycloplegic state. Presenting distance visual acuity was tested in both eyes
and, if available, conducted with participants’ own spectacle correction, if available. Objective
refraction (sphere, cylinder, axis) and keratometric data was obtained by taking the average of
three measurements. Corneal curvature was transformed into corneal power using a kerato-
metric index of r = 1.3375 and corneal power was averaged across the two meridians. Difference
in corneal power between meridians was defined as corneal astigmatism and the axis of the stee-
pest meridian was recorded. Myopia was defined as a spherical equivalent� -0.75 diopters [14].
In the analysis of differences in corneal power between meridians participants with negative
values and extreme values above 10 diopters were excluded. We furthermore excluded all par-
ticipants that reported a history of eye surgery including refractive and cataract surgery.
Demographic data
Age and sex were self-reported, as was ethnicity. Ethnicity was provided in the categories Mex-
ican American, other Hispanic, Non-Hispanic white, Non-Hispanic black, and other.
Statistical analysis
We included only right eyes in our analysis. Spherical equivalent (SE) was calculated as sphere
value plus half the cylindrical power. Visual acuity measures were transformed in the Snellen
equivalent to LogMAR [20], the category “20/200+” was set to 1.1 LogMAR. All variables were
controlled for outliers.
Following the approach of Thibos [21], who applied Fourier analysis to calculate astigma-
tism components, namely J0 and J45, for both refractive and corneal astigmatism as follows:
C
J0 ¼  � cos2a2
C
J45 ¼  � sin2a2
α is the cylindrical axis, and C is the cylinder power. J0 represents the power vector for the
cylinder power of the vertical (90˚) and horizontal (180˚) meridians. Positive values equal to
with-the-rule astigmatism, negative values to against-the-rule astigmatism. J45 reflects the cyl-
inder power of the oblique meridians (45˚ and 135˚).
We computed mean and standard deviation for continuous variables showing an approxi-
mately normal distribution. Absolute and relative frequencies were computed to describe cate-
gorical variables. For continuous variables of interest, we used weighted linear regression
models, and for categorical data we conducted weighted logistic regression analyses. As uni-
variate analysis, one model was built for each of the following outcome variable (spherical
PLOS ONE | https://doi.org/10.1371/journal.pone.0211196 January 29, 2019 3 / 15
Myopia is associated with education in the U.S.
equivalent, J0-vector and J45-vector of astigmatism, corneal power, corneal J0- and corneal J45-
vector) with level of education as independent variable. Multivariable regression models
included age, sex and NHANES survey cycle. For spherical equivalent, we performed an addi-
tional model with adjustment for corneal power. Additional models were adjusted for ethnicity
and income as well, as well as for vitamin D levels as a proxy for outdoor activity and sunlight
exposure [22, 23]. We performed a sensitivity analysis restricted to participants older than 30
years, since some participants might be still under exposure of education between 20 and 30
years of age. We additionally performed regression models stratified by ethnicity and addition-
ally restricted to US-born participants.
To account for the complex survey design, we followed the NHANES analytical guidelines
including combined sample weights for the analyses [24]. Based on the primary sampling
units and strata, the variance estimation uses Taylor Series Linearization. In the regression
models stratified by ethnicity, we used linear regression models without consideration of the
design structure, since numbers in primary sampling units and strata became too small. We
used R version 3.3.0 with Rstudio version 1.0.136 and the packages nhanesA (version 0.6.5),
ggplot2 (version 2.1.0), survey (version 3.34), knitr (version 1.20), MASS (7.3–45), and table-
one (version 0.8.1). P-values should be considered as a continuous measure of evidence and
should be cautiously interpreted, given the exploratory character of this analysis. To allow rep-
lication of our statistics, we provide the source code on github (https://github.com/snickels/
nhanes_edu_se).
Results
Sample description
The mean age of our analysis sample (n = 19,756) was 46.6 ± 16.8 years and 10,174 (51.5%)
were female. 21.7% where Mexican Americans, 5.9% other Hispanics 47.8% non-Hispanic
Whites and 20.7% non-Hispanics Blacks, 3.9% had another ethnicity.
With respect to level of education, 2,457 (12.4%) did reach less than 9th grade, 3,275
(16.6%) 9-11th grade (includes 12th grade with no diploma), 4,805 (24.3%) high school grad/
General Education Development or Diploma (GED) or equivalent, 5,397 (27.3%) some college
or AA degree and 3,822 (19.3%) college graduate or above. Subjects with less than 9th grade
had a mean spherical equivalent of 0.34 D, subjects with 9-11th grade -0.14 D, subjects that fin-
ished high school -0.33 D, subjects with some college education -0.70 D, subjects that gradu-
ated from college or a higher formal education -1.22 D.
Prevalence of myopia was present in 16.8% of subjects with less than 9th grade, 23.5% with
9-11th grade, 28.6% in subjects that finished high school, 35.4% in subjects with some college
education and in 45.0% of those that graduated from college or a higher formal education (Fig
1). These proportions seemed to be relatively constant in the age of 20 to 60 years with only
increasing myopia in subjects with education of less than 9th grade (Fig 2), in older study par-
ticipants myopia was less prevalent. Data of general and ocular parameters are reported in
Table 1. In the excluded participants with self-reported refractive surgery, there were more
individuals with a higher education compared to the study sample (S1 Table).
When stratified by ethnicity, non-Hispanic Whites and participants of other ethnicity had
the highest percentage of both college graduate and myopia (S2 Table).
Association analysis
Linear regression analysis revealed an association between myopic spherical equivalent and
higher level of education in the univariate analysis (Table 2), and in the model adjusted for
age, sex and NHANES examination cycle (Table 2). Our analysis showed a monotonic increase
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Myopia is associated with education in the U.S.
Fig 1. The distribution of myopia (right eyes;� -0.75 D) by level of education in the NHANES 1999–2008.
https://doi.org/10.1371/journal.pone.0211196.g001
of myopia with higher level of education. The association was still present when additionally
adjusting for ethnicity and corneal power, and also when restricting the analysis to participants
older than 30 years (S3 and S5 Tables).
Logistic regression analysis showed that myopia� -0.75D is associated with higher level of
education. The higher the level of education, the more likely the study participants had a
Fig 2. Prevalence of myopia (right eyes;� 0.75D) by level of education and age in the NHANES 1999–2008.
https://doi.org/10.1371/journal.pone.0211196.g002
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Myopia is associated with education in the U.S.
Table 1. Characteristics of the NHANES 1999–2008 sample with age, sex, education and refraction (right eye) available, by level of education (n = 19,756).
Education Less Than 9th 9-11th High School Grad/GED or Some College or AA College Graduate or
Gradea Gradea Equivalenta degreea abovea
n 2457 3275 4805 5397 3822
Age 53.69 (17.02) 45.38 (17.17) 46.51 (17.30) 44.10 (16.20) 46.67 (15.32)
Female sex 1159 (47.2) 1668 (50.9) 2439 (50.8) 2965 (54.9) 1947 (50.9)
Family income to poverty indexa 1.47 (1.02) 1.82 (1.33) 2.47 (1.49) 2.90 (1.56) 3.93 (1.39)
Ethnicitya:
Mexican American 1524 (62.0) 915 (27.9) 816 (17.0) 770 (14.3) 265 (6.9)
Other Hispanic 241 (9.8) 244 (7.5) 226 (4.7) 298 (5.5) 159 (4.2)
Non-Hispanic White 403 (16.4) 1001 (30.6) 2625 (54.6) 2865 (53.1) 2552 (66.8)
Non-Hispanic Black 219 (8.9) 1029 (31.4) 982 (20.4) 1253 (23.2) 598 (15.6)
Other 70 (2.8) 86 (2.6) 156 (3.2) 211 (3.9) 248 (6.5)
Ocular characteristics
Sphere [D] -0.12 (1.74) -0.58 (2.03) -0.76 (2.20) -1.11 (2.42) -1.62 (2.68)
Cylinder [D] 0.92 (0.83) 0.87 (0.86) 0.86 (0.79) 0.81 (0.78) 0.80 (0.67)
Spherical Equivalent [D] 0.34 (1.66) -0.14 (1.91) -0.33 (2.11) -0.70 (2.32) -1.22 (2.62)
Spherical Equivalent < = -0.75D 413 (16.8) 771 (23.5) 1372 (28.6) 1912 (35.4) 1721 (45.0)
(%)
Spherical Equivalent (%)
>-0.75 D 2044 (83.2) 2504 (76.5) 3433 (71.4) 3485 (64.6) 2101 (55.0)
-0.75/-3 D 357 (14.5) 578 (17.6) 956 (19.9) 1224 (22.7) 912 (23.9)
-3/-6 D 47 (1.9) 144 (4.4) 324 (6.7) 491 (9.1) 583 (15.3)
< = -6 D 9 (0.4) 49 (1.5) 92 (1.9) 197 (3.7) 226 (5.9)
Distance glasses (%) 830 (33.8) 1161 (35.5) 2208 (46.0) 2682 (49.7) 2333 (61.0)
Refractive astigmatism >0.5 D 1340 (54.5) 1577 (48.2) 2474 (51.5) 2560 (47.4) 1897 (49.6)
(%)
With-the-rule astigmatism 628 (25.6) 601 (18.4) 1033 (21.5) 1003 (18.6) 775 (20.3)
Against-the-rule astigmatism 225 (9.2) 405 (12.4) 583 (12.1) 637 (11.8) 450 (11.8)
Oblique astigmatism 751 (30.6) 960 (29.3) 1394 (29.0) 1574 (29.2) 1130 (29.6)
Refractive Astigmatism J0 -0.11 (0.53) -0.02 (0.54) -0.06 (0.51) -0.04 (0.50) -0.06 (0.45)
Refractive Astigmatism J45 0.05 (0.30) 0.03 (0.29) 0.00 (0.28) 0.00 (0.27) -0.01 (0.25)
Keratometric power [D] 43.63 (1.55) 43.62 (1.55) 43.67 (1.55) 43.71 (1.57) 43.68 (1.55)
Corneal astigmatism [D] 0.98 (0.81) 1.03 (0.84) 0.95 (0.75) 0.95 (0.75) 0.88 (0.61)
Corneal Astigmatism >0.5 D (%) 1545 (63.2) 2125 (65.2) 3021 (63.0) 3430 (63.7) 2320 (60.7)
With-the-rule corneal 265 (10.8) 161 (4.9) 290 (6.0) 272 (5.1) 239 (6.3)
astigmatism
Against-the-rule corneal 709 (29.0) 1228 (37.7) 1790 (37.3) 2105 (39.1) 1329 (34.8)
astigmatism
Oblique corneal astigmatism 828 (33.9) 1067 (32.7) 1438 (30.0) 1626 (30.2) 1182 (31.0)
Corneal Astigmatism J0 0.18 (0.62) 0.30 (0.53) 0.26 (0.52) 0.28 (0.50) 0.23 (0.44)
Corneal Astigmatism J45 0.08 (0.40) 0.07 (0.36) 0.03 (0.36) 0.02 (0.32) 0.01 (0.31)
Visual acuity [logMAR] 0.20 (0.25) 0.15 (0.22) 0.13 (0.20) 0.11 (0.19) 0.09 (0.16)
aself-reported;
AA: Associate of Arts degree, undergraduate academic degree awarded by colleges usually after completion of a two-year course; GED: General Education Development
or Diploma, certification that provides that the test taker has United States or Canadian high-school-level academic skills.
https://doi.org/10.1371/journal.pone.0211196.t001
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Myopia is associated with education in the U.S.
Table 2. The association of spherical equivalent with education in separate models in the NHANES 1999–2008.
Crude analysis (n = 19,756) Adjusted modela (n = 19,756) Adjusted modelb
(n = 19,704)
Education Estimate in diopter [95% CI] P value Estimate in diopter [95% CI] P value Estimate in diopter P value
[95% CI]
Less Than 9th Grade Reference - Reference - Reference -
9-11th Grade -0.45 1.02e-07 -0.23 0.001 -0.25 < 0.001
[-0.60; -0.30] [-0.36; -0.10] [-0.39; -0.11]
High School Grad/GED or Equivalent -0.68 1.87e-12 -0.49 2.22e-09 -0.48 9.44e-09
[-0.83; -0.52] [-0.63; -0.35] [-0.62; -0.34]
Some College or AA degree -1.07 < 2e-16 -0.80 < 2e-16 -0.79 < 2e-16
[-1.22; -0.92] [-0.93; -0.66] [-0.92; -0.65]
College Graduate or above -1.65 < 2e-16 -1.47 < 2e-16 -1.46 < 2e-16
[-1.83; -1.48] [-1.63; -1.30] [-1.62; -1.29]
All models calculated with consideration of the study sample structure;
aresults from the multivariable linear regression models adjusted for age, sex, survey cycle;
badditionally adjusted for corneal power;
AA: Associate of Arts degree, undergraduate academic degree awarded by colleges usually after completion of a two-year course; GED: General Education Development
or Diploma, certification that provides that the test taker has United States or Canadian high-school-level academic skills; CI: confidence interval
https://doi.org/10.1371/journal.pone.0211196.t002
myopic refractive error (Table 3). Adjustment for potential confounders did not alter this find-
ing (Tables 3 and S4), as well as restricting to participants older than 30 years (S6 Table).
Additional adjustment for vitamin D levels as a proxy for outdoor activity had only minor
influence on estimates (S3 and S4 Tables). In the analysis stratified by ethnicity, the effect esti-
mates were similar across most ethnicities except for the Non-Hispanic Black and the other
ethnicity group, in which the estimates for the association of education with myopia were
higher (S7 and S8 Tables). We additionally restricted to US-born participants, assuming a
more uniform exposure to the US education. The effect for the Non-Hispanic black and the
“other” group were higher than for other ethnicities (Mexican American, Non-Hispanic
Table 3. The association of myopia (� -0.75 D) with education in separate models in the NHANES 1999–2008.
Crude analysis (n = 19,756) Adjusted modela (n = 19,756) Adjusted modelb (n = 19,704)
Education Odds ratio [95% CI] P value Odds ratio [95% CI] P value Odds ratio P value
[95% CI]
Less Than 9th Grade Reference - Reference - Reference -
9-11th Grade 1.42 0.002 1.27 0.02 1.30 0.02
[1.15; 1.74] [1.04; 1.57] [1.05; 1.61]
High School Grad/GED or Equivalent 1.87 1.64e-09 1.71 9.55e-08 1.71 2.39e-07
[1.57; 2.23] [1.43; 2.03] [1.42; 2.04]
Some College or AA degree 2.66 3.96e-16 2.32 3.56e-13 2.34 1.59e-12
[2.22; 3.19] [1.94; 2.78] [1.93; 2.83]
College Graduate or above 4.05 < 2e-16 3.74 < 2e-16 3.81 < 2e-16
[3.37; 4.86] [3.12; 4.48] [3.15; 4.60]
All models calculated with consideration of the study sample structure;
aresults from the multivariable logistic regression models adjusted for age, sex, survey cycle;
badditionally adjusted for corneal power;
AA: Associate of Arts degree, undergraduate academic degree awarded by colleges usually after completion of a two-year course; GED: General Education Development
or Diploma, certification that provides that the test taker has United States or Canadian high-school-level academic skills; CI: confidence interval.
https://doi.org/10.1371/journal.pone.0211196.t003
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Myopia is associated with education in the U.S.
white) but the precision of the estimates was low due to the small sample size (S9 and S10
Tables).
Neither an association was present between corneal power with level of education
(Table 4), nor for refractive astigmatism. For corneal astigmatism, only subjects graduated
from college or a higher formal education had a slightly lower astigmatism (Table 5). Oblique
corneal and refractive astigmatism (J45) was less present in subjects with higher educational
level (S11 and S12 Tables).
Discussion
Our analysis provides data on the refractive error and corneal properties, namely corneal
power and corneal astigmatism in adults aged 20 to 85 years with respect to their level of edu-
cation in a population based setting in the U.S. It indicates that a higher level of education is
linked to myopic refractive error, while astigmatism and corneal power are not associated.
Despite differences in the educational system, similar results with respect to myopic refrac-
tive error have been reported in other population-based and cohort studies [25–37]. Hyper-
opia in adults has been proven to be associated with lower educational qualifications in the U.
K. and the strength of the association increases with the level of refractive error in both direc-
tions [34].
Mirshahi et al. reported that individuals who graduated after 13 years of school were more
myopic (median: -0.5D) than those graduating after 10 years (-0.2D), after 9 years (+0.3D) or
never finishing secondary school (+0.2D) [13]. Similar relationships were found in our data
analysis, showing a stepwise increase of myopia level and prevalence with higher education in
the U.S.: i.e. persons graduating from college were -1.46D more myopic than persons finish
school education with less than 9th grade. Similar, prevalence of myopia (� -0.75D) was 45%
in persons graduating from college was more prevalent than in subjects with some college edu-
cation (35%), in subjects that finished high school (28%), in those with 9-11th grade education
(24%) and those with less than 9th grade (17%). The U.S. prevalence data of myopia are in line
with the E3 data from Williams et al. reporting higher prevalence of myopia in individuals
with higher education (36.6%) compared to those who completed secondary (29.1%) or pri-
mary education (25.4%) [14]. The myopic shift seems to persist when attending university:
Kinge et al. reported that university students underwent a -0.5D shift in refractive error over a
3-years observational period, mainly due to vitreous chamber elongation [38].
Table 4. Association analysis of corneal power and corneal astigmatism with level of education in the NHANES 1999–2008.
Corneal power
Crude analysis (n = 19,704) Adjusted modela (n = 19,704)
Education B [95% CI] P value B [95% CI] P value
Less Than 9th Grade Reference - Reference -
9-11th Grade -0.03 [-0.13; 0.08] 0.65 -0.06 [-0.16; 0.04] 0.22
High School Grad/GED or Equivalent 0.07 [-0.04; 0.18] 0.21 0.04 [-0.06; 0.14] 0.43
Some College or AA degree 0.10 [0.00; 0.20] 0.05 0.04 [-0.05; 0.14] 0.39
College Graduate or above 0.07 [-0.03; 0.16] 0.18 0.03 [-0.06; 0.13] 0.49
Results from the multivariable linear regression models adjusted for age, sex, survey cycle, and with consideration of the study sample structure.
All models calculated with consideration of the study sample structure;
aresults from the multivariable linear regression models adjusted for age, sex, survey cycle;
AA: Associate of Arts degree, undergraduate academic degree awarded by colleges usually after completion of a two-year course; GED: General Education Development
or Diploma, certification that provides that the test taker has United States or Canadian high-school-level academic skills.
https://doi.org/10.1371/journal.pone.0211196.t004
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Myopia is associated with education in the U.S.
Table 5. The association of refractive and corneal astigmatism with education in separate models in the NHANES 1999–2008.
refractive astigmatism corneal astigmatism
(> 0.5 D cylinder power) (> 0.5 D cylinder power)
Crude analysis (n = 19,756) Adjusted modela (n = 19,756) Crude analysis (n = 19,704) Adjusted modela (n = 19,704)
Education Odds ratio [95% CI] P value Odds ratio [95% CI] P value Odds ratio [95% CI] P value Odds ratio [95% CI] P value
Less Than 9th Grade Reference - Reference - Reference - Reference -
9-11th Grade -0.19 0.0004 0.00 0.98 0.02 [-0.15; 0.20] 0.79 -0.05 0.57
[-0.29;-0.09] [-0.11; 0.12] [-0.23; 0.13]
High School Grad/GED or -0.03 0.65 0.13 0.03 -0.07 0.34 -0.12 0.11
Equivalent [-0.14; 0.09] [0.02; 0.25] [-0.22; 0.08] [-0.27; 0.03]
Some College or AA degree -0.16 0.01 0.07 0.27 -0.05 0.56 -0.12 0.12
[-0.28;-0.04] [-0.06; 0.20] [-0.20; 0.11] [-0.27; 0.03]
College Graduate or above -0.07 0.26 0.08 0.18 -0.17 0.03 -0.22 0.01
[-0.20; 0.05] [-0.04; 0.21] [-0.33; -0.02] [-0.38; -0.07]
All models calculated with consideration of the study sample structure;
aresults from the multivariable logistic regression models adjusted for age, sex, survey cycle;
AA: Associate of Arts degree, undergraduate academic degree awarded by colleges usually after completion of a two-year course; GED: General Education Development
or Diploma, certification that provides that the test taker has United States or Canadian high-school-level academic skills.
https://doi.org/10.1371/journal.pone.0211196.t005
This impact of education on myopic refractive error also seems to be affected by year of
birth. Williams et al. reported that the association of education with myopia is dependent on
age (respective decade of birth): while in the 1920s only 25% of subjects with higher education
(leaving school at age of� 20 years) were myopic, in the 1950s and 1960s about 40% of sub-
jects with this educational level were myopic [14]. The former National Health and Nutrition
Examination Survey from 1971 to 1972 revealed a prevalence of 25% myopic individuals aged
12 to 54 years and prevalence rates increased with educational level [39]. In contrast, Liang
et al. reported for rural Chinese that higher level of education was linked to myopia in�50
years of age, but not in younger subjects (30 to 49 years) [40]. In these younger subjects in the
Handan region, hours of reading were associated with myopia. In our study however, myopia
prevalence in the different educational levels were relatively constant in the age of 20 to 60
years, only subjects with education of less than 9th grade showed a clear increase. In older
study participants (60 to 90 years), myopia was less prevalent. Other studies from Asia
reported a larger effect of education on myopia than we observed. In an Indian population,
higher education was associated with a nearly two-fold increased risk for high myopia. A
South Korean study [41] reported a two-fold increased risk for myopia and a study from Sin-
gapore a four-fold increased risk for myopia associated with a University degree [18]. A recent
meta-analysis provided evidence for stronger effects of high education in individuals of Asian
ancestry than in European ancestry [8]. We were not able to sufficiently explore the subsample
of Asian descent in our analysis. The category “Non-Hispanic Asian” was not available until
NHANES survey cycle 2011–12: In our analysis, participants of Asian descent are summarized
together with other ethnicities in the “other” category, and this might have diluted the effect.
Another explanation might be differences between Asian and US-American education sys-
tems. We have therefore performed a sensitivity analysis with U.S. born study participant
receiving most likely their school education in the U.S. In addition, different patterns of out-
of-school activities [42] might lead to differences between Asian and US-American education
systems with respect to myopia.
Rather the environmental factors of education play a role than cognitive function of the
myopic subjects. A recent population-based analysis by Mirshahi et al. reported that myopia
PLOS ONE | https://doi.org/10.1371/journal.pone.0211196 January 29, 2019 9 / 15
Myopia is associated with education in the U.S.
itself is independent of cognitive function after adjusting for level of education [43]. In contrast,
IQ test results and years in education were related to myopia in Danish conscripts [44]. The
SCORM (Singapore Cohort study Of Risk factors for Myopia) revealed nonverbal IQ, reading
and school grades as risk factors for myopia and high/higher myopia in children [45, 46].
The combination of good educational outcome and extra classes after school is related to
higher prevalence of myopia as shown in an analysis of the PISA2009 data [47]. The impact
of education on refractive error has been proven in countries with > 90% myopes such as
Korea [48].
Astigmatism
In our study, cylindrical power of refraction was not associated with level of education. While
astigmatism with respect to with/against the rule (J0-vector) was not associated, oblique astig-
matism (J45-vector) decreased with increasing education. Literature shows different findings
for the relationship between astigmatism and education. Hashemi et al. reported an inverse
association of educational level and astigmatism in rural areas of Iran [49], Rim et al. found
decreased prevalence of astigmatism in highly educated Korean subjects [26], while other stud-
ies did not find an association of astigmatism with level of education [28, 32].
Corneal power
Similar to literature, we did not find a relationship between corneal power and level of educa-
tion. Jonas et al. reported that corneal curvature in adults is not associated with level of educa-
tion in the Beijing Eye Study and the Central India Eye and Medical Study [50]. Comparable
findings were reported by Lim et al. [51] or Lee et al. [52]
Strengths and limitations
The strength of the NHANES study lies in the large sample size and the standardized popula-
tion-based study design. The adjustment for different covariates allows to precise estimates. A
limitation in this study is the use of non-cycloplegic refraction date. The analysis done may
have underestimated some hyperopes, which is why we performed a sensitivity analysis includ-
ing only myopic study participants and did find comparable associations as for the total study
cohort.
We excluded all participants with self-reported refractive surgery for myopia, as we were
not able to identify the refractive error before treatment. These participants showed a higher
education compared to the study group and by excluding them, a bias might have been
introduced.
In addition, we were not able to include information on presence of cataract nor genetic
factors as potential risk factors. Nevertheless, Mirshahi et al. showed that environmental fac-
tors including education play the major role while genes have less impact on myopia [13],
accounting for less than 5% of the variance of refractive error [53]. In the past, two types of
high myopia were suspected based on dependency on genetic background and age of onset
that also differed in associations with education as well: in the non-adult study groups, myopia
increased with higher degree of education, while not in the adult participants of the Beijing
Eye Study and the Central India Eye and Medical Study [54]. This finding may be driven by
the underlying educational status of the population that is different between industrialised and
newly industrialised countries.
PLOS ONE | https://doi.org/10.1371/journal.pone.0211196 January 29, 2019 10 / 15
Myopia is associated with education in the U.S.
Conclusion
In conclusion, myopia is associated with higher school and college education in the U.S. Our
analysis shows that corneal curvature does not cause this association, therefore other parame-
ters may be regarded as underlying factors for myopia in higher educated persons.
Supporting information
S1 Table. Comparison of the NHANES 1999–2008 sample without and with self-reported
refractive surgery for near sightedness (excluded from analysis).
(PDF)
S2 Table. Characteristics of the NHANES 1999–2008 sample with age, sex, education and
refraction (right eye) available, by ethnicity (n = 19,756).
(PDF)
S3 Table. The association of spherical equivalent with education in separate models in the
NHANES 1999–2008, with additional adjustment.
(PDF)
S4 Table. The association of myopia (� -0.75 D) with education in separate models in the
NHANES 1999–2008, with additional adjustment.
(PDF)
S5 Table. Sensitivity analysis: The association of spherical equivalent with education in
separate models in the NHANES 1999–2008, restricted to participants aged 30 years and
older.
(PDF)
S6 Table. Sensitivity analysis: The association of myopia (� -0.75 D) with education in sep-
arate models in the NHANES 1999–2008, restricted to participants aged 30 years and
older.
(PDF)
S7 Table. The association of spherical equivalent with education in different ethnicities in
the NHANES 1999–2008.
(PDF)
S8 Table. The association of myopia (� -0.75 D) with education in different ethnicities in
the NHANES 1999–2008.
(PDF)
S9 Table. The association of spherical equivalent with education in different ethnicities in
the NHANES 1999–2008, restricted to US-born participants.
(PDF)
S10 Table. The association of myopia (� -0.75 D) with education in different ethnicities in
the NHANES 1999–2008, restricted to US-born participants.
(PDF)
S11 Table. Association analysis of refractive astigmatism with level of education in the
NHANES 1999–2008.
(PDF)
PLOS ONE | https://doi.org/10.1371/journal.pone.0211196 January 29, 2019 11 / 15
Myopia is associated with education in the U.S.
S12 Table. Association analysis of corneal astigmatism (J0 and J45 vector) with level of
education in the NHANES 1999–2008.
(PDF)
Acknowledgments
We are very much obliged to all participants of NHANES and to all members of the NHANES
team for planning and conducting the study. We would like to thank for the very helpful com-
ments on the webpages about the analysis of available survey data using R (http://www.asdfree.
com, https://github.com/ajdamico/asdfree) and David J Friedman (Wilmer Eye Institute of
Johns Hopkins University School of Medicine) for his comments which further improved our
manuscript.
Author Contributions
Conceptualization: Stefan Nickels, Alexander K. Schuster.
Data curation: Stefan Nickels.
Formal analysis: Stefan Nickels, Alexander K. Schuster.
Funding acquisition: Norbert Pfeiffer.
Investigation: Susanne Hopf, Alexander K. Schuster.
Methodology: Stefan Nickels, Alexander K. Schuster.
Project administration: Alexander K. Schuster.
Resources: Norbert Pfeiffer, Alexander K. Schuster.
Validation: Susanne Hopf, Norbert Pfeiffer.
Visualization: Alexander K. Schuster.
Writing – original draft: Stefan Nickels, Susanne Hopf, Alexander K. Schuster.
Writing – review & editing: Stefan Nickels, Susanne Hopf, Norbert Pfeiffer, Alexander K.
Schuster.
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