Prevalence of refractive errors among primary school children in a tropical area, Southeastern Iran

Abstract

Objective: To determine the prevalence of refractive errors among primary school children in Zahedan District, Southeastern Iran, as a tropical area.
Methods: In this cross sectional study, a total of 400 students were selected randomly using multi-stage sampling technique. Myopia was defined as spherical equivalent (SE) of −0.5 diopter (D) or more, hyperopia was defined as SE of +2.00 D or more and a cylinder refraction greater than 0.75 D was considered astigmatism. Anisometropia was defined as a difference of 1 D or more between two eyes. Cycloplegic refractive status was measured using auto-refractometer (Topcon 8800). Data were analyzed using SPSS, version 22 software program.
Results: Mean ± SD of SE was (1.71 ± 1.16) D. A total of 20 students [6.3%, 95% confidence interval (CI): 3.96%–9.64%] were myopic (≤ −0.5 D), 186 students (58.1%, 95% CI: 52.50%–63.56%) were hyperopic (≥ +2.00 D) and 114 students (35.6%, 95% CI: 30.43%–41.18%) were emmetropic. The prevalence of astigmatism (≥ 0.75 D) among students was 3.4% (95% CI: 1.82%–6.25%). Anisometropia of 1 D or more was found in 21.3% (95% CI: 16.98%–26.23%) of the studied population. The prevalence of refractive errors was higher among girls than boys (73.1% vs. 55.6%, P = 0.001), but it was not significantly different among different age groups (P = 0.790).
Conclusions: Refractive errors affect a sizable portion of students in Zahedan. Although myopia is not very prevalent, the high rate of hyperopia in the studied population emphasizes its need for attention.

Full text

Management and decision-making http://dx.doi.org/10.1016/j.apjtb.2015.10.008
Prevalence of refractive errors among primary school children in a tropical area,
Southeastern Iran
Monireh Mahjoob
1
, Samira Heydarian
1*
, Jalil Nejati
2
, Alireza Ansari-Moghaddam
2
, Nahid Ravandeh
2
1
Department of Optometry, School of Paramedical Science, Mashhad University of Medical Sciences, Mashhad, Iran
2
Health Promotion Research Center, Zahedan University of Medical Sciences, Zahedan, Iran
ARTICLE INFO
Article history:
Received 1 Sep 2015
Received in revised form 16 Sep 2015
Accepted 5 Oct 2015
Available online xxx
Keywords:
Refractive error
Myopia
Hyperopia
Astigmatism
Anisometropia
ABSTRACT
Objective: To determine the prevalence of refractive errors among primary school
children in Zahedan District, Southeastern Iran, as a tropical area.
Methods: In this cross sectional study, a total of 400 students were selected randomly
using multi-stage sampling technique. Myopia was defined as spherical equivalent (SE)
of −0.5 diopter (D) or more, hyperopia was defined as SE of +2.00 D or more and a
cylinder refraction greater than 0.75 D was considered astigmatism. Anisometropia was
defined as a difference of 1 D or more between two eyes. Cycloplegic refractive status
was measured using auto-refractometer (Topcon 8800). Data were analyzed using SPSS,
version 22 software program.
Results: Mean ± SD of SE was (1.71 ± 1.16) D. A total of 20 students [6.3%, 95%
confidence interval (CI): 3.96%–9.64%] were myopic (−0.5 D), 186 students (58.1%,
95% CI: 52.50%–63.56%) were hyperopic (+2.00 D) and 114 students (35.6%, 95%
CI: 30.43%–41.18%) were emmetropic. The prevalence of astigmatism (0.75 D)
among students was 3.4% (95% CI: 1.82%–6.25%). Anisometropia of 1 D or more was
found in 21.3% (95% CI: 16.98%–26.23%) of the studied population. The prevalence of
refractive errors was higher among girls than boys (73.1% vs. 55.6%, P= 0.001), but it
was not significantly different among different age groups (P= 0.790).
Conclusions: Refractive errors affect a sizable portion of students in Zahedan. Although
myopia is not very prevalent, the high rate of hyperopia in the studied population
emphasizes its need for attention.
1. Introduction
Refractive errors affect a large proportion of world's popu-
lation, regardless of sex, age and ethnic group
[1]
. We can easily
diagnose, measure and correct these refractive errors with
spectacles or other refractive corrections to achieve normal
vision. If, however, refractive errors are not corrected or the
correction be inappropriate, they may become a major cause
of visual impairment and even blindness
[2]
. The estimate of
visual disability due to uncorrected refractive errors is a public
health concern. It is reported that more than 12 million
children in the age group 5–15 years are visually impaired due
to uncorrected or inadequately corrected refractive errors
[3]
.
Refractive errors can impose a heavy financial burden on the
society
[4]
. School children are considered a high risk group
because uncorrected refractive errors can negatively affect
their learning abilities and their mental and physical health
[5,6]
.
To address the issue of visual impairment in children, the
World Health Organization recently launched a global initiative,
VISION 2020-The Right to Sight. Their strategy for the elimi-
nation of avoidable visual disability and blindness includes the
correction of refractive errors
[7]
. So, in order to provide an early
detection and initiate early treatment, a professional based
screening program for all school-aged children is recom-
mended. In recent years, a number of surveys have been done
among students and elderly subjects in Iran
[8–14]
. These studies
suggest that the prevalence of hyperopia in Iran is high, so
further studies in different parts of the country should be done
to determine the role of factors such as race, genetics and
even environment.
*Corresponding author: Samira Heydarian, Department of Optometry, School of
Paramedical Science, Mashhad University of Medical Sciences, Mashhad, Iran.
Tel: +98 9113547406
E-mail: opt_heydarian@yahoo.com
Peer review under responsibility of Hainan Medical University.
Foundation Project: Supported by Zahedan University of Medical Sciences
(Grant No. 91-506).
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Please cite this article in press as: Mahjoob M, et al., Prevalence of refractive errors among primary school children in a tropical area, Southeastern Iran, Asian Pac J Trop Biomed (2015), http://dx.doi.org/
10.1016/j.apjtb.2015.10.008

The objective of this study was to determine the prevalence
and pattern of refractive errors among primary school children in
the age group of 7–12 years of both sexes in Zahedan District,
Southeastern Iran.
2. Materials and methods
2.1. Study area
This cross sectional study was carried out among primary
school children of Zahedan District in 2012. Zahedan is the
capital of Sistan and Baluchestan Province, located in south east
of Iran and known as a tropical area (Figure 1)
[15]
.
2.2. Optometric research
We conducted this study based on Refractive Error Study in
Children protocol
[5]
. To maintain the comparability of our
results with other studies, consistent definitions for refractive
errors were used. However, there were some differences in
methods. One of these differences was that the target
population in our study was school children who greatly
affected the generalizability of the results, because it failed to
cover school age children who not attended school. However,
it seemed that in Zahedan, the majority of primary school-age
children were in school. The other difference was that our
sample was drawn from children between 7 and 12 years, rather
than 5–15 years of age.
The sample size was calculated using a single proportion
formula. Assuming an estimated refractive error prevalence rate
of 24.6%, a precision of 0.05 diopter (D) and 95% confidence
interval (CI) (Z1-
a
/2 = 1.96), the total sample size was calcu-
lated as 288 students
[8]
. Due to the possibility of loss of
samples, we increased the sample size to 400 students. The
multi-stage sampling technique was used to select random
samples of school children. Zahedan has two administrative
districts. In each district, schools were listed according to public
and private ownership. Four private and four public schools
were randomly selected (2 boys' centers and 2 girls' centers,
from each group). Lists of Grade 1 to Grade 5 students from
each selected school were obtained and then random selection of
the students was carried out by using a simple table of
randomization. A total of 25 students were selected from each
school, based on their academic grade. A total of 400 students
were selected, among which 320 students (80%) participated in
the study, including 160 boys and 160 girls with age ranging
from 7 to 12 years.
The study protocol was approved by the Ethical Committee
of Zahedan University of Medical Sciences. Our study was
conducted according to the tenets of the Declaration of Helsinki.
Informed written consent was obtained from the parents/legal
guardians after a detailed explanation of the study. Assents from
selected school children were also obtained. After acquiring their
consent, they completed a questionnaire on personal information
and family history. Students who refused visual acuity assess-
ment or eye examination and with any previous ocular surgery
or ocular pathology including any strabismus, corneal opacity,
cataract and retinal pathology which were recognized in initial
ophthalmic examination, were excluded from the study.
Those students included in the study underwent a complete
initial ophthalmic examination. Slit lamp biomicroscopy (Model
BQ 900: Haag Streit, Bern, Switzerland) and ophthalmoscopy
(Ophthalmoscope, Heine Beta 200, Germany) were done for all
the students. Then, an optometrist began optometric examinations
with testing visual acuity at 6 m in good lighting conditions using
Snellen eye chart for each eye separately, according to standard
protocol. Cycloplegic refractive status was measured for all the
participants using auto-refractometer (Topcon 8800), 45 min after
instilling 2 drops of 1% cyclopentolate with 5 min apart.
Spherical equivalent (SE) was used for calculations of
refractive error. The SE was derived by adding the spherical
component of refraction to half of the cylindrical component.
Myopia was defined as an SE of at least −0.5 D and hyperopia
was defined as an SE of +2.00 D or more. Emmetropia was
defined if neither eye was myopic or hyperopic. Astigmatic
students were a cylinder refraction of 0.75 D or more in at least
one eye, which was recorded with a negative sign. Anisome-
tropia was defined as a difference in SE of at least 1.0 D between
right and left eyes. As SEs in the right and left eyes were highly
correlated (Spearman correlation: r= 0.719, P= 0.000), we
presented data for only the right eye.
Data were analyzed using SPSS, version 22 software pro-
gram. Percentage and 95% CI were used to describe the prev-
alence of refractive errors. Spearman Chi-squared test was
applied for qualitative data. P<0.05 was considered statistically
significant.
3. Results
Of the 400 primary school children approximately, 320 were
included with approved consent forms from their parents/
guardians with a response rate of 80%, from which 50% were
girl and 50% were boy. The age of the included school children
ranged from 7 to 12 years with a mean ± SD of (9.11 ± 1.62)
years and there was no significant differences between the
mean ± SD age of girls (9.06 ± 1.59) and boys (9.16 ± 1.64)
(P= 0.606).
Mean ± SD of SE was 1.71 ± 1.16 ranged from −2.5 to +8 D
overall, 1.68 ± 1.19 ranged from −2.25 to +8 D in boys and
1.75 ± 1.13 ranged from −2.5 to +4 D in girls (P= 0.181). The
overall prevalence of refractive errors among school children
was 64.4% (95% CI: 58.83%–69.58%), higher among girls than
boys (73.1% vs. 55.6%, P= 0.001), but it was not significantly
different among different age groups (P= 0.790). Tables 1 and 2
show the prevalence of different types of refractive errors based
on sex and age. As these tables shown, 20 students (6.3%, 95%
CI: 3.96%–9.64%) were myopic (−0.5 D), 186 students
(58.1%, 95% CI: 52.50%–63.56%) were hyperopic (+2.00 D),
and 114 (35.6%, 95% CI: 30.43%–41.18%) were emmetropic.
Turkmenistan
Tehran
Persian Gulf
Oman Sea
Pakistan
Afghanistan
Chabahar
Nikshahr Sarbaz
Saravan
Khash
Zahedan Study area
Iranshahr
Sistan &
Baluchistan
Iraq
Turkey
Caspian Sea
Figure 1. Map of Sistan–Baluchestan Province, located in Zahedan Dis-
trict, Southeastern Iran [16].
Monireh Mahjoob et al./Asian Pac J Trop Biomed 2016; ▪(▪): 1–42
Please cite this article in press as: Mahjoob M, et al., Prevalence of refractive errors among primary school children in a tropical area, Southeastern Iran, Asian Pac J Trop Biomed (2015), http://dx.doi.org/
10.1016/j.apjtb.2015.10.008

Mean ± SD of cylindrical power was (−0.045 ± 0.280) D.
The prevalence of astigmatism (0.75 D) among students was
3.4% (95% CI: 1.82%–6.25%), from which 70% (95% CI:
35.37%–91.91%) was the rule astigmatism and 30% (95% CI:
8.09%–64.63%) was oblique astigmatism. There was no sex and
also age difference with regard to the prevalence of astigmatism
(P= 0.056, P= 0.486, respectively).
Anisometropia of 1 D or more was found in 21.3% (95% CI:
16.98%–26.23%) of the studied population and the differences
between girls and boys (P= 0.585), and different age groups
(P= 0.902) were not significant.
4. Discussion
The prevalence of different types of refractive errors among
school children has already been evaluated in various studies
during the past years
[8–11,17,18]
. In any discussion of the
prevalence of refractive errors, we must consider that the
prevalence varies widely from one geographical, racial or
occupational group to another. Factors such as types of
studied populations, different definitions, and methods of
measurement (cycloplegia or non-cycloplegia), patient's age
and ethnic differences could be responsible for these differences.
In this cross sectional study, we assessed the prevalence of
refractive errors among primary school children in Zahedan.
Several surveys with a similar methodology have been per-
formed in different part of Iran. In 2002, the prevalence of
different types of refractive errors in those younger than 15 years
of age was studied in Tehran, which reported that the prevalence
of myopia and hyperopia based on cycloplegic refraction was
7.2% and 76.2%, respectively
[19]
. In 2006–2007, a similar study
was carried out in Mashhad and their results showed that the
prevalence of myopia and hyperopia was 2.4%, and 87.9%,
respectively
[10]
. Rezvan et al. evaluated the prevalence of
refractive errors among school children in Northeastern Iran
and reported that the prevalence of hyperopia in Northeastern
Iran was higher than that of some countries
[8]
. In consistent
to other reports from Iran, the prevalence of hyperopia in our
study was higher than myopia
[8–11]
.
The prevalence of astigmatism has varied in different studies
in different populations. In the current study, we have found a
low prevalence of astigmatism (3.4%) and a result was lower
than that reported by other studies
[8–11]
. We know that the race-
related factors are among the reasons of the difference in the
prevalence of astigmatism worldwide
[20,21]
. However, similar to
other studies, our results showed that the prevalence of
astigmatism did not change significantly with age, but with-
the-rule astigmatism was more prevalent than against-the-rule
or oblique astigmatism. The high prevalence of with-the-rule
astigmatism in primary school children confirms the effect of
age on astigmatism axis
[22–24]
.
The prevalence of anisometropia according to a cut-point of 1
D or higher is different in school age populations. In our study,
anisometropia was found in 21.3% of the students which was
higher than previous studies
[9–12]
. However, since
anisometropia may disturb binocular vision, its correction
particularly in younger subjects is very important.
We also found higher prevalence of refractive errors among
girls compared to boys, which confirms the results of previous
studies in Iran
[10–12]
. Similar findings were also reported from
Saudi Arabia, Qatar, India and Ghana
[16,25–27]
. Moreover,
some studies have demonstrated an equal prevalence among
two sex groups
[8–11,28]
.
In this study, no association was found between
age and refractive errors, though other studies stated otherwise
and a difference may be attributed to the 5 year of age range
[11,22–24]
.
This study was conducted to provide baseline data on the
prevalence rate of refractive errors among primary school chil-
dren in Zahedan, Iran. Based on the results of the study,
refractive errors affected a fairly large portion of students in this
area. Primary school education is the most critical educational
years since children achieve basic literacy and numeracy during
this period. Because of the importance of good visual acuity in
this age group, the health system should give priority to iden-
tifying affected students and correcting their refractive errors.
Periodic screening in schools should be performed, and teachers
and their parents should be educated about the effects of
uncorrected refractive errors on the learning abilities and
development of children.
Conflict of interest statement
We declare that we have no conflict of interest.
Table 1
Prevalence of different types of refractive errors based on age.
Age Myopia
[n(%)]
95% CI Hyperopia
[n(%)]
95% CI Astigmatism
[n(%)]
95% CI Anisometropia
[n(%)]
95% CI Emmetropia
[n(%)]
95% CI
7 2 (3.3) 0.57%–12.36% 37 (60.7) 47.31%–72.66% 1 (1.6) 0.09%–9.98% 11 (18.0) 9.76%–30.39% 22 (36.1) 24.46%–49.44%
8 4 (5.3) 1.72%–13.80% 47 (62.7) 50.69%–73.34% 1 (1.3) 0.07%–8.21% 14 (18.7) 10.94%–29.68% 24 (32.0) 21.96%–43.90%
9 4 (6.9) 2.23%–17.55% 32 (55.2) 41.62%–68.03% 4 (6.9) 2.23%–17.55% 15 (25.9) 15.65%–39.29% 22 (37.9) 25.81%–51.65%
10 6 (10.7) 4.43%–22.55% 33 (58.9) 45.01%–71.63% 1 (1.8) 0.09%–10.82% 12 (21.4) 12.02%–34.80% 17 (30.4) 19.16%–44.26%
11 3 (9.1) 2.38%–25.47% 15 (45.5) 28.53%–63.40% 2 (6.1) 1.06%–21.62% 8 (24.2) 11.74%–42.63% 15 (45.5) 28.53%–63.40%
12 1 (2.7) 0.14%–15.81% 22 (59.5) 42.20%–74.80% 2 (5.4) 0.94%–19.54% 8 (21.6) 10.42%–38.66% 14 (37.8) 22.94%–55.21%
Table 2
Prevalence of different types of refractive errors based on sex.
Gender Myopia
[n(%)]
95% CI Hyperopia
[n(%)]
95% CI Astigmatism
[n(%)]
95% CI Anisometropia
[n(%)]
95% CI Emmetropia
[n(%)]
95% CI
Girl 14 (8.8) 5.05%–14.52% 102 (63.7) 55.74%–71.09% 8 (5.0) 2.34%–9.95% 36 (22.5) 16.44%–29.91% 44 (27.5) 20.89%–35.21%
Boy 6 (3.8) 1.53%–8.35% 84 (52.5) 44.49%–60.39% 3 (1.8) 0.49%–5.82% 32 (20.0) 14.27%–27.21% 70 (43.8) 36.00%–51.80%
Monireh Mahjoob et al./Asian Pac J Trop Biomed 2016; ▪(▪): 1–43
Please cite this article in press as: Mahjoob M, et al., Prevalence of refractive errors among primary school children in a tropical area, Southeastern Iran, Asian Pac J Trop Biomed (2015), http://dx.doi.org/
10.1016/j.apjtb.2015.10.008

Acknowledgments
This article is a part of the results of the fifth author's thesis
for achievement of BSc degree in Optometry from Zahedan
University of Medical Sciences, Iran. The authors wish to thank
the staff of primary schools of Zahedan District for their coop-
eration. This study was financially supported by Zahedan Uni-
versity of Medical Sciences (Grant No. 91–506).
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Monireh Mahjoob et al./Asian Pac J Trop Biomed 2016; ▪(▪): 1–44
Please cite this article in press as: Mahjoob M, et al., Prevalence of refractive errors among primary school children in a tropical area, Southeastern Iran, Asian Pac J Trop Biomed (2015), http://dx.doi.org/
10.1016/j.apjtb.2015.10.008