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Strabismus
ISSN: 0927-3972 (Print) 1744-5132 (Online) Journal homepage: https://www.tandfonline.com/loi/istr20
Global and regional prevalence of strabismus:
a comprehensive systematic review and meta-
analysis
Hassan Hashemi, Reza Pakzad, Samira Heydarian, Abbasali Yekta,
Mohamadreza Aghamirsalim, Fereshteh Shokrollahzadeh, Fahimeh
Khoshhal, Mojgan Pakbin, Shahroukh Ramin & Mehdi Khabazkhoob
To cite this article: Hassan Hashemi, Reza Pakzad, Samira Heydarian, Abbasali Yekta,
Mohamadreza Aghamirsalim, Fereshteh Shokrollahzadeh, Fahimeh Khoshhal, Mojgan
Pakbin, Shahroukh Ramin & Mehdi Khabazkhoob (2019): Global and regional prevalence
of strabismus: a comprehensive systematic review and meta-analysis, Strabismus, DOI:
10.1080/09273972.2019.1604773
To link to this article: https://doi.org/10.1080/09273972.2019.1604773
Published online: 23 Apr 2019.
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Global and regional prevalence of strabismus: a comprehensive systematic
review and meta-analysis
Hassan Hashemi
a
, Reza Pakzad
b
, Samira Heydarian
c
, Abbasali Yekta
d
, Mohamadreza Aghamirsalim
e
,
Fereshteh Shokrollahzadeh
a
, Fahimeh Khoshhal
f
, Mojgan Pakbin
a
, Shahroukh Ramin
g
, and Mehdi Khabazkhoob
h
a
Noor Research Center for Ophthalmic Epidemiology, Noor Eye Hospital, Tehran;
b
Department of Epidemiology, Faculty of Health, Ilam
University of Medical Sciences, Ilam;
c
Department of rehabilitation science, School of Allied Medical Sciences, Mazandaran University of
Medical Sciences, Sari;
d
Refractive Errors Research Center, Mashhad University of Medical Sciences, Mashhad;
e
Eye Research Center, Tehran
University of Medical Sciences, Tehran;
f
Department of Pediatrics, Dezful University of Medical Sciences, Dezful;
g
Department of Optometry,
Shahid Beheshti University of Medical Sciences, Tehran;
h
Department of Medical Surgical Nursing, School of Nursing and Midwifery, Shahid
Beheshti University of Medical Sciences, Tehran
ABSTRACT
Purpose: Despite the importance of information on the prevalence of strabismus, which can be
effective in planning preventive and curative services, no study has addressed its prevalence
comprehensively. In this study, a systematic search was done to estimate the regional and global
prevalence of strabismus in different age and sex groups and factors affecting prevalence
heterogeneity.
Methods: A comprehensive and systematic search was done in different international databases,
including Web of Science, Scopus, PubMed, Embase, etc. to find published articles on the total
prevalence of strabismus and the prevalence of exotropia and esotropia. A binomial distribution
was used to calculate the prevalence and 95% confidence interval (CI). The Cochran’s Q-test and I
2
were applied to evaluate heterogeneity and a random-effects model was used to assess the
pooled prevalence. The Begg’s test was administered to investigate publication bias and finally,
a meta-regression method was applied to determine the factors affecting the heterogeneity
among studies.
Results: Of 7980 articles, 56 articles with a total sample size of 229,396 were analyzed. Many of
these articles (n = 14) were from the Regional Office for the Americas. The estimated of pooled
prevalence (95% CI) of any strabismus, exotropia, and esotropia was 1.93% (1.64–2.21), 1.23%
(1.00–1.46), and 0.77% (0.59–0.95), respectively. The heterogeneity in prevalence of strabismus
and its subtypes according to I
2
was above 95% (p value <.001 for all). Age had a direct effect on
heterogeneity in the prevalence of exotropia (b: 3.491; p: 0.002). Moreover, WHO region had
a significant direct effect on heterogeneity in the prevalence of strabismus (b: 0.482; p < .001) and
esotropia (b: 0.168; p: 0.027), and publication year had a significant direct effect on heterogeneity
in the prevalence of exotropia (b: 0.059; p: 0.045). Sample size and publication year did not have
any association with strabismus nor with other variables. There was no publication bias according
to the Begg’s test.
Conclusion: The prevalence of strabismus varies widely in the world. As for factors affecting
heterogeneity in the prevalence of strabismus, the results showed that age affected heterogeneity
in the prevalence of exotropia, WHO region affected heterogeneity in the prevalence of strabis-
mus and esotropia, and publication year affected heterogeneity in the prevalence of exotropia.
Information about the global prevalence of strabismus can help health care planners design
interventions and prioritize resource allocation.
KEYWORDS
Prevalence; strabismus;
exotropia; esotropia;
meta-analysis
Introduction
Strabismus is defined as any deviation of the binocular
alignment that can be the cause or the effect of poor
binocularity.
1
If strabismus is not treated in a timely
manner in children, in addition to cosmetic conse-
quences, it may have a dramatic impact on their
learning and educational ability and impair their phy-
siological and psychological performance,
2,3
eventually
affecting their development and maturity.
4
For this
reason, investigation of the prevalence of strabismus,
in addition to clinical significance, is important from
the perspective of public health.
CONTACT Mehdi Khabazkhoob khabazkhoob@yahoo.com Department of Medical Surgical Nursing, School of Nursing and Midwifery, Shahid
Beheshti University of Medical Sciences, Tehran, Iran
Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/istr.
STRABISMUS
https://doi.org/10.1080/09273972.2019.1604773
© 2019 Taylor & Francis Group, LLC
Epidemiological data of the prevalence of strabis-
mus, especially from population-based studies, help to
determine the extent and burden of the disease and
plan appropriate preventive and curative services to
prepare health and screening infrastructures.
4,5
A number of large studies have reported the prevalence
of strabismus across the world, including the Multi-
Ethnic Pediatric Eye Disease Study (MEPEDS),
6,7
Baltimore Pediatric Eye Disease Study (BPEDS),
8
and
Strabismus, Amblyopia, and Refractive Error in
Singaporean Children Study (STARS) in Singapore,
2
as well as other studies in India
7,9–12
and Iran.
13–15
These studies have improved our knowledge of strabis-
mus in the world and provide policymakers with more
comprehensive local information.
16,17
Despite several studies across the world, our extensive
search showed no comprehensive study of the pooled
prevalence of strabismus and its epidemiological charac-
teristics and risk factors in children and adults.
Considering the medical, social, and psychological
impacts of this disease, there is an urgent need for relevant
information to designs plans for screening, early diagno-
sis, and timely intervention. The aim of the present study
was to present a systematic assessment of the sources to
provide an estimate of the global prevalence of strabismus
in different age and sex groups and determine the factors
affecting its variability.
Materials and methods
Search strategy and study selection
The results of this meta-analysis are presented accord-
ing to the Preferred Reporting Items for Systematic
Reviews and Meta-Analyses (PRISMA) guideline.
18
The protocol of the study was registered in the
International Prospective Register of Systematic
Reviews with CRD42019119961 code. Four interna-
tional electronic databases (Web of Science, PubMed,
Scopus, and Embase) were searched extensively and
systematically from inception to 29 September 2018 to
retrieve articles related to the prevalence of any stra-
bismus using its MeSH terms (Table 1). The PICO of
the study was as follows:
Population: child and adult
Intervention: none
Comparison: none
Outcome: prevalence of any strabismus, Exotropia
(XT), and Esotropia (ET))
The details of the search strategy, which was first
developed for Medline and then used for other data-
bases, are presented in Table 1. Google Scholar was
used to access the grey literature.
19
An expert was
consulted to access feature articles in the field of
strabismus.
The articles retrieved from each database were entered
into EndNote X6. After removing duplicates, screening
was done in three stages. First, the titles of the articles
were evaluated. Then, if the title was relevant, the abstract
was evaluated. If the abstract was deemed relevant, the
article was read in full. If the raw data of an article were
required, an email was sent to the corresponding author.
The above three steps were carried out by two raters (M.
KH. and R.P.) independently and any disagreement
between them was resolved by a third rater (H.H.).
Blinding and task separation were applied to the study
selection procedure. The kappa value for inter-rater
agreement was 87%.
Exclusion criteria
The inclusion criteria of this study were studies with
across-sectional design (population based and) and
surveys. Studies originating from the phase one of
large cohort studies with a cross-sectional design were
also included.
Since the aim of the study was to assess the preva-
lence of any strabismus in the general population, stu-
dies performed in certain groups like inpatients and
patients suffering from ocular or certain systemic dis-
eases (Down syndrome, etc.) were excluded from ana-
lysis. Moreover, cohort, follow-up and longitudinal,
retrospective, and hospital and clinic based studies,
conference reports, letters, editorials, commentaries,
reviews and case series also excluded.
Data extraction and quality assessment
All articles on the prevalence of strabismus in all age
groups published that reached the final stage were scruti-
nized and data such as the author’sname,
publication year, country, study design, participants’
characteristics including age, and sample size were
recorded. Finally, the prevalence and number of subjects
Table 1. Search strategy for MEDLINE (MeSH, Medical Subject
Headings).
1. Strabismus [text word] OR Strabismus [MeSH term]
2. Esotropia [text word] OR Esotropia [MeSH term]
3. Exotropia [text word] OR Exotropia [MeSH term]
4. 1 OR 2 OR 3
5. Prevalence [text word] OR Prevalence [MeSH term]
6. Frequency [text word] OR Frequency [MeSH term]
7: 5 OR 6
8: Cross sectional studies [text word] OR Cross sectional studies [MeSH term]
9: observational studies [text word] OR observational studies [MeSH term]
10: 8 OR 9
11: 4 AND 7 AND 10
2H. HASHEMI ET AL.
with any strabismus, XT, and ET and the criteria applied
to investigate strabismus were extracted and entered into
an Excel sheet. It should be noted that the term “any
strabismus”in this study was used to define any type of
strabismus, including vertical, horizontal, exotropia, eso-
tropia, etc. reported in the selected studies . Moreover,
phoria was not considered for prevalence of any
strabismus.
The STROBE checklist was used for quality assess-
ment. This checklist contains 22 questions to assess the
methodological aspects of descriptive and cross-sectional
studies.
20
To categorize the studies, each question was
assigned a score of 0 or 1 (the total score of each article
ranged from 0 to 22). Accordingly, the articles were
divided to 5 categories: very low quality (less than 20%
of the maximum score), low quality (20–40% of the max-
imum score), moderate quality (40–60% of the maximum
score), high quality (60–80% of the maximum score), and
very high quality (more than 80% of the maximum score).
Definition of variables
Different definitions of strabismus were used in the
selected studies but all of them are part of following
definitions which were ordered form high to low sensi-
tive definition.
1 = Manifest –far or near –with or without glasses
2 = Any Tropia –far or near- with or without glasses
3 = Constant or Intermittent ≥10 prism diopters
As for age categorization, according to the expert’s
opinion, the studies were divided to two categories:
studies conducted in children and adolescents (below
20 years) and studies performed in adults (above 20
years). The latest WHO regional grouping, including
six regions of African Regional Office (AFRO),
American Regional Office (AMRO), Eastern
Mediterranean Regional Office (EMRO), European
Regional Office (EURO), South-East Asia Regional
Office (SERO), and Western Pacific Regional Office
(WPRO), was also used to categorize countries.
Statistical analysis
The Stata version 14 (College Station, Texas) was used
for statistical analysis. The prevalence of strabismus was
extracted from the studies. For studies that did not
report these data, if the sample size and number of
subjects with strabismus were available, the prevalence
and 95% CI were calculated using a binomial distribu-
tion. It should be mentioned that since strabismus had
a low prevalence, the metaprop command was used to
merge the results.
21
The Cochran’s Q-test was applied
to assess between-study heterogeneity and I
2
was used
to quantify it. According to the Higgins classification,
an I
2
> 0.7 indicated high heterogeneity. To estimate
the pooled prevalence of strabismus, a fixed-effects
model was applied if I
2
< 0.7 and a random-effects
model was used if I
2
> 0.7. Finally, a meta-regression
analysis was employed to examine the effect of age,
sample size, publication date, strabismus criteria, and
geographical region on heterogeneity between studies.
22
Also the Metabias command was used to detect pub-
lication bias.
23
The level of significance was set at 0.05
for all tests.
Results
In total, 7957 articles were retrieved from databases and
23 were retrieved from other sources. After removing
duplicates, 6039 articles were independently screened
by two raters. Assessment of titles and abstracts
resulted in exclusion of 4861 and 891 articles, respec-
tively. Then, the remaining 287 articles were read in
full, and 231 articles were excluded in this stage. The
corresponding authors of three articles were contacted
to obtain the required data but no response was
received; therefore, these articles were also excluded
from analysis.
Finally, final analysis was performed in 56
studies
1,2,4–15,24–65
with a total sample size of 229,396
participants. Figure 1 shows the flow diagram of study
selection and Table 2 presents the characteristics of the
selected articles.
The highest and lowest number of articles was
related to AMRO (n = 14) and AFRO (n = 3), respec-
tively. The oldest study included was published in
the year 1986
52
and the most recent articles were pub-
lished in. 2018
26,45
The minimum and maximum age
range of the studies was 6–72 months and 20–93 years,
respectively. Four articles of moderate quality, 23 were
of high quality, and 29 were of very high quality. Since
heterogeneity for strabismus, XT and ET according to
I
2
was above 95%, a random-effects model was used to
estimate the pooled prevalence.
Estimate of pooled prevalence of Strabismus, XT
and ET in overall and in World Health Organization
regions
Figure 2 shows the estimated pooled prevalence of
strabismus, XT, and ET in total and also according to
the gender, and age group. The estimated pooled pre-
valence and 95% CI of strabismus, XT, and ET was
1.93% (1.64–2.21), 1.23% (1.00–1.46), and 0.77%
(0.59–0.95), respectively. According to the WHO
region, the highest and lowest pooled prevalence (95%
STRABISMUS 3
CI) of strabismus was seen in AMRO (2.86%,
2.07–3.65) and AFRO (0.42%, 0.29–0.55), respectively.
Moreover, the highest and lowest pooled prevalence of
XT was seen in WPRO (1.54%, 0.58–2.51) and AFRO
(0.14%, 0.06–0.23) and the highest and lowest pooled
prevalence of ET was seen in EURO (2.17%, 1.13–3.21)
and AFRO (0.13%, 0.05–0.21), respectively (Figure 3).
Estimate of pooled prevalence of strabismus, XT
and ET based on sex and age group
Figure 2 also presents the pooled prevalence according
to age (above and below 20 years) and sex (male and
female). Since some studies have reported the total
prevalence of strabismus and its subtypes, it was not
possible to retrieve the data for age and sex.
1,64,66
Therefore, a smaller number of studies were included
to estimate the pooled prevalence according to age and
sex. According to the findings of this study, the esti-
mated pooled prevalence (95% CI) of strabismus, XT,
and ET was 1.78% (1.56–2.01), 1.07% (0.86–1.29), and
0.73% (0.56–0.91) in subjects below 20 years and 3.29%
(0.22–6.80), 4.60% (3.75–5.59), and 0.50% (0.21–0.80)
in subjects above 20 years, respectively. The estimated
pooled prevalence of strabismus, XT, and ET was 1.83%
(1.31–2.35), 1.54% (0.57–2.51), and 0.30% (0.09–0.51)
in women and 1.84% (1.28–2.39), 1.52% (0.11–2.93),
and 0.46% (0.07–0.85) in men, respectively.
Heterogeneity and meta-regression
The Cochran’sQ-test of heterogeneity showed
a significant between-study heterogeneity (p value
<.001 for all). The value of heterogeneity for strabis-
mus, XT, and ET according to the I
2
index was
98.2%, 97.5%, and 96.4% respectively, indicating
high levels of heterogeneity. Table 3 presents the
results of univariate meta-regression analysis.
According to the results, age had a significant direct
relationship only with XT (b: 3.491; p: 0.002); in
other words, the prevalence of XT in subjects above
20 years was 3.49% higher compared to subjects
below 20 years on average. Moreover, there was
a significant difference in the prevalence of strabis-
mus and ET between the 6 WHO regions. On aver-
age, there was a 0.48% difference in the prevalence of
strabismus (p < .001) and a 0.16% difference in the
prevalence of ET (p: 0.027) among WHO regions. In
other words, the prevalence of strabismus in SERO
and EURO was 0.48% higher than its prevalence in
AFRO and EMRO, respectively. Moreover,
publication year had a significant positive
Figure 1. Flowchart of study and inclusion in studies with systematic review and meta-analysis.
4H. HASHEMI ET AL.
Table 2. characteristics of the studies included in this meta-analysis.
Author Design Year Country WHO region Age/Sex Group SS
Number of event (prevalence %)
QOS Definitionstrabismus XT ET
MEPEDS
6
CSS 2008 USA AMRO 6–72 months 6014 149 (2.47) 85 (1.41) 29 (0.48) VH 2
Ajaiyeoba et al.
24
CSS 2006 Nigeria AFRO 4–24 yrs. old 1144 3 (0.26) 2 (0.17) 1 (0.9) VH NA
Al Fara et al.
25
CSS 1992 KSA EMRO 6–20 yrs. old 3590 17 (0.47) NA NA VH NA
Alsaqr et al.
26
CSS 2018 KSA EMRO 12–20 yrs. old 998 19 (1.90) NA NA VH 2
Azonobi et al.
27
CSS 2009 Nigeria AFRO 2–16 yrs. old 7288 32 (0.44) 10 (0.14) 10 (0.14) VH NA
Bigyabati et al.
4
CSS 2016 India SERO 5–15 yrs. old 1700 2 (0.11) NA NA VH NA
Chen et al.
5
CSS 2016 China WPRO 6–72 months 5667 320 (5.65) 259 (4.57) 43 (0.76) VH 2
Female 2648 149 (5.63) 119 (4.49) 18 (0.68)
Male 3019 171 (5.66) 138 (4.57) 25 (0.83)
Chia et al.
2
CSS 2010 Singapore WPRO 6–72 months 3009 24 (0.80) 20 (0.66) 3 (0.10) VH 2
Female 1431 10 (0.70) 8 (0.56) 2 (0.140)
Male 1561 14 (0.90) 12 (0.77) 1 (0.06)
Garcia et al.
1
CSS 2004 Brazil AMRO 6–20 yrs. old 1015 29 (2.85) 20 (2.16) 6 (0.59) VH 2
Dey et al.
28
CSS 2017 India SERO 6–15 yrs. old 208 2 (0.96) NA NA VH NA
Diniz et al.
29
CSS 2007 Brazil AMRO 0–12 yrs. old 525 45 (8.57) NA NA M NA
Ertekin et al.
30
CSS 2016 Turkey EURO 5–13 yrs. old 1938 43 (2.22) NA NA H 2
Faghihi et al.
13
CSS 2012 Iran EMRO 14–18 yrs. old 1133 17 (1.50) 10 (0.88) 5 (0.44) H NA
Fischbach et al.
31
CSS 1993 USA AMRO 6–7 yrs. old 854 11 (1.29) NA NA H NA
Female 434 5 (1.38) NA NA
Male 420 6 (1.43) NA NA
Friedman et al.
8
CSS 2010 USA AMRO 6–71 months 2298 60 (2.61) 31 (1.35) 28 (1.22) H 2
Fu et al.
32
CSS 2014 China WPRO 10–16 yrs. Old 2151 108 (5.02) NA NA H 2
Garvey et al.
33
CSS 2010 USA AMRO 4.87–9.42 yrs. Old 594 9 (1.51) 1 (0.17) 2 (0.34) H 2
Female 454 8 (1.76) NA NA
Male 455 4 (0.88) NA NA
Gronlund et al.
34
CSS 2006 Sweden EURO 4–15 yrs. old 143 5 (3.50) 1 (0.70) 4 (2.80) M 2
Gupta et al.
9
CSS 2009 India SERO 6–16 yrs. Old 1561 39 (2.50) NA NA H NA
Female 748 24 (2.21) NA NA
Male 813 16 (1.97) NA NA
Gupta et al.
10
CSS 2013 India SERO 6–16 yrs. old 9067 76 (0.84) NA NA VH NA
Female 3282 33 (1.01) NA NA
Male 3338 43 (1.29) NA NA
Hashemi et al.
15
CSS 2017 Iran EMRO 0–20 yrs. old 879 19 (2.16) 12 (1.37) 7 (0.80) H 2
20–93 yrs. old 2369 121 (5.10) 109 (4.60) 12 (0.51)
Female 1830 77 (4.21) 4 (0.22) 8 (0.44)
Male 1418 63 (4.44) 4 (0.28) 6 (0.42)
Hashemi et al.
35
CSS 2015 Iran EMRO 7 yrs. old 3675 92 (2.50) 47 (1.28) 16 (0.44) H 2
Female 1757 22 (1.25) NA NA
Male 1918 42 (2.19) NA NA
Lanca et al.
36
CSS 2014 Portugal EURO 6–11 yrs. old 672 27 (4.02) 12 (1.79) 14 (2.08) VH 2
Lithander et al.
37
CSS 1998 Oman EMRO 6–12 yrs. old 6292 55 (0.87) 15 (0.24) 26 (0.41) VH 2
Lu et al.
38
CSS 2008 China WPRO 6–14 yrs. old 1084 27 (2.49) 23 (2.12) 4 (0.40) VH 2
Female 439 14 (3.19) 14 (3.19) 0 (0)
Male 645 13 (2.01) 9 (1.40) 4 (0.62)
McKean-Cowdin et al.
7
CSS 2013 USA AMRO 6–72 m 3036 103 (3.39) 43 (1.42) 56 (1.84) VH 2
Nepal et al.
39
CSS 2003 Nepal SERO 5–16 yrs. old 1100 18 (1.64) NA NA H NA
Female 595 10 (2.01) NA NA
Male 505 8 (1.58) NA NA
Ohlsson et al.
40
CSS 2003 Mexico AMRO 12–13 yrs. old 1035 24 (2.32) NA NA H NA
Rajavi et al.
41
CSS 2015 Iran EMRO 7–12 yrs. old 2417 55 (2.28) 31 (1.28) 24 (0.99) VH 3
Reddy et al.
42
CSS 2006 Malaysia WPRO 7–12 yrs. old 1214 30 (2.47) NA NA VH NA
Schaal et al.
43
CSS 2016 Brazil AMRO 1–12 months 1852 30 (1.62) 6 (0.32) 21 (1.13) VH 2
Male 821 12 (1.46) NA NA
Female 1031 18 (1.76) NA NA
Schimiti et al.
44
CSS 2001 Brazil AMRO 6–12 yrs. old 13471 114 (0.85) 35 (0.30) 73 (0.54) M NA
Sandfeld et al.
45
CSS 2018 Denmark EURO 4.5–7 yrs. old 445 7 (1.57) NA NA H 2
Sharma-1 et al.
12
CSS 2017 India SERO 5–16 yrs. old 5918 24 (0.41) NA NA M 2
Female 2456 12 (0.49) NA NA
Male 3462 12 (0.35) NA NA
Sharma-2 et al.
12
CSS 2017 India SERO 6–15 yrs. old 1265 10 (0.79) NA NA H NA
Female 755 5 (0.66) NA NA
Male 510 5 (0.98) NA NA
Sherpa et al.
46
CSS 2011 Nepal SERO 0–15 yrs. old 466 2 (0.43) NA NA H 2
Singh et al.
47
CSS 2017 India SERO 5–15 yrs. old 4838 13 (0.27) NA NA H 2
Uddin et al.
48
CSS 2017 Bangladesh SERO 4–6 yrs. old 900 7 (0.78) NA NA H 2
Turacli et al.
49
CSS 1995 Turkey EURO 6–12 yrs. old 23810 60 (0.25) NA NA H NA
Wedner et al.
50
CSS 2000 Tanzania AFRO 7–19 yrs. old 1386 7 (0.51) NA NA H 2
Williams et al.
51
CSS 2008 UK EURO 7 yrs. old 7538 173 (2.30) NA NA H 2
Woodruff et al.
52
CSS 1986 Canada AMRO 6 yrs. old 6080 242 (3.98) 78 (1.28) 164 (2.70) VH 2
Yekta et al.
53
CSS 2010 Iran EMRO 7–17 yrs. old 2683 49 (1.83) 31 (1.16) 15 (0.60) VH 2
Yekta et al.
54
CSS 2017 Iran EMRO 6–15 yrs. old 1130 21 (1.86) NA NA VH 2
Female 520 7 (1.35) NA NA
Male 610 15 (2.46) NA NA
(Continued)
STRABISMUS 5
Table 2. (Continued).
Author Design Year Country WHO region Age/Sex Group SS
Number of event (prevalence %)
QOS Definitionstrabismus XT ET
Yekta et al.
55
CSS 2016 Iran EMRO 4–6 yrs. old 3628 44 (1.21) 44 (0.17) 25 (0.22) VH 2
Female 1758 12 (0.68) 1 (0.06) 4 (0.23)
Male 1870 32 (1.71) 5 (0.21) 4 (0.21)
Hashemi et al.
56
CSS 2012 Iran EMRO 6–17 yrs. old 1551 31 (1.99) NA NA VH 2
Ying et al.
57
CSS 2014 USA AMRO 3–5 yrs. old 3837 153 (3.98) NA NA VH 2
Yoon et al.
58
CSS 2011 ROK WPRO 20–70 yrs. old 11048 168 (1.84) NA NA VH 1
3–20 yrs. Old 3416 63 (1.52) NA NA
Female 7947 119 (1.50) NA NA
Male 6517 98 (1.50) NA NA
Drover et al.
59
CSS 2008 Canada AMRO 1.6–11.6 yrs. old 946 41 (4.33) NA NA VH 2
Cotter et al.
60
CSS 2011 USA AMRO 6–72 months 5121 NA 61 (1.19) 48 (0.94) VH 2
Faghihi et al.
14
CSS 2011 Iran EMRO Female 942 39 (4.14) NA NA H 2
Male 1208 25 (2.07) NA NA
6–21 yrs. old 2150 67 (3.12) 45 (2.09) 19 (0.88)
Fan et al.
61
CSS 2011 China WPRO 3–6 yrs. old 1424 28 (1.97) 23 (1.62) 5 (0.35)
Nirmalan et al.
11
CSS 2003 India SERO 1 months –15 yrs. old 1065 4 (0.38) NA NA VH NA
Robaei D et al.
62
CSS 2006 Australia WPRO 12 yrs. old 2353 64 (2.72) 27 (1.15) 21 (0.89) VH 2
Robaei D et al.
63
CSS 2006 Australia WPRO 6 yrs. old 1739 48 (2.76) 14 (0.81) 26 (1.50) H 2
Sitompul et al.
64
CSS 2017 Indonesia SERO 1–70 yrs. old 667 1 (0.1) NA NA H NA
Matsuo et al.
65
CSS 2007 Japan WPRO 1.5–3 yrs. old 33929 22 (0.06) 11 (0.03) 7 (0.02) H NA
CSS = cross-sectional study; KSA = Kingdom of Saudi Arabia; UK = united kingdom; ROK = republic of Korea; USA; united states of America; NA = not
available; QOS = Quality of study; VH = very high; H = high; M; moderate.
Coding for definition: 1 = Manifest or latent –far or near –with or without glasses; 2 = Any Tropia –far or near- with or without glasses; 3 = Constant or
Intermittent ≥10 prism diopters
MEPEDS: Multi-Ethnic Pediatric Eye Disease Study
Figure 2. Pooled prevalence and 95% CI of strabismus, exotropia, and esotropia in total and according to age, and sex subgroup.
The diamond marks illustrate the estimate of pooled prevalence.
6H. HASHEMI ET AL.
relationship with heterogeneity in the prevalence of
XT but its significance level was borderline (b: 0.059,
p: 0.045) (Figure 4). Sample size and publication year
did not have any correlation with strabismus nor
with other variables. Table 3 shows the status of
other variables.
Figure 3. Pooled prevalence and 95% CI of strabismus, exotropia, and esotropia according to WHO region. There were no data for
pooled estimation of esotropia and exotropia in SERO.
STRABISMUS 7
Publication bias
The results of the Begg’stest showed no publication
bias for strabismus (z score: 0.85; p: 0.396), XT (z score:
0.60; p: 0.548), and ET (z score: 0.85; p: 0.396).
Discussion
It is very important to diagnose and treat strabismus in
early stages to achieve the maximum best binocular
vision.
1
Prevalence studies are of clinical and public
health significance since knowledge of the extent of
a disease is essential for providing treatment
infrastructures. There is a great variation in the preva-
lence of strabismus according to age, sex, etc. However,
regardless of these groupings, the pooled prevalence of
strabismus, XT, and ET was 1.93%, 1.23%, and 0.77%,
respectively. In other words, 106–221, 100–146, and
59–123 in 10,000 population had strabismus, XT, and
ET, respectively. Although the pooled prevalence of this
disease may seem low, it should be noted that strabis-
mus develops in early years of life and, if untreated,
may result in different sensorial adaptation disorders,
including retinic correspondence anomaly and amblyo-
pia, which have lifelong effects on the quality of life of
the patients.
1
Although some studies have investigated the preva-
lence of strabismus and its subtypes in different ethnic
groups,
6–8,60
but none of them haven’t estimated the
pooled prevalence in the worldwide. Therefore, it is not
possible to compare our results with other studies.
However, the results of this study showed that the pre-
valence of strabismus varied in different countries and
WHO regions; for example, the highest prevalence of
strabismus was seen in AMRO and EURO and this dif-
ference was not due to the diagnostic criteria. Table 2
presents the prevalence of strabismus in different ethni-
cities and geographical regions. The prevalence of strabis-
mus was higher in western countries and white people
compared to Asian ethnicities like Chinese and Indian
peoples. Caution should be practiced when comparing the
prevalence of strabismus according to the geographical
region because variations in the results of the studies may
might simply reflect differences in the methodology and
age range of the study populations. Regardless of the
Table 3. Results of the univarate metaregresion analysis on the hertogenisity of the determinants.
Variables
Strabismus XT ET
Coefficient p-value Coefficient p-value Coefficient p-value
Age 1.376
(−0.546 to 3.299)
0.157 3.491
(1.456 to 5.525)
0.002* −0.259
(−1.562 to 1.044)
0.686
WHO region 0.482
(0.275 to 0.688)
<0.001* 0.060
(−0.200 to 0.322)
0.637 0.168
(0.020 to 0.315)
0.027*
Sample size −0.001
(−0.001 to 0.001)
0.301 −0.001
(−0.001 to 0.001)
0.173 −0.001
(−0.001 to 0.001)
0.210
Publication years −0.001
(−0.057 to 0.055)
0.974 0.059
(0.001 to 0.117)
0.045* −0.026
(−0.064 to 0.011)
0.167
Sex (Female = 0) −0.034
(−0.994 to 0.926)
0.943 −0.158
(−2.890 to 2.573)
0.897 0.085
(−0.411 to 0.583)
0.696
Definition criteria 0.397
(−1.515 to 2.309)
0.676 −0.237
(−2.410 to 1.935)
0.823 0.076
(−1.270 to 1.424)
0.907
QOS 0.031
(−0.641 to 0.704)
0.926 0.323
(−0.334 to 0.982)
0.322 0.086
(−0.307 to 0.481)
0.654
XT: Exotropia; ET: Esotropia.
*: Significance
Coding of Age: 1 = lower than 20 years old; 2 = upper than 20 years old
Coding of WHO region: 1 = AFRO; 2 = SERO; 3 = EMRO; 4 = EURO; 5 = WPRO; 6 = AMRO
Coding of QOS (Quality of study): 1 = very low; 2 = low; 3 = moderate; 4 = high; 5 = very high
Coding for definition: 1 = Manifest or latent –far or near –with or without glasses; 2 = Any Tropia –far or near- with or without glasses; 3 = Constant or
Intermittent ≥10 prism diopters
Figure 4. The relationship between the prevalence of exotropia
with publication year study by means of meta-regression. Size
of circles indicates the precision of each study. There is sig-
nificant relationship with respect to Prevalence of exotropia and
publication year study. Prevalence of exotropia has been
approximated increased during years of study in this survey.
8H. HASHEMI ET AL.
above factors, many studies have reported the role of
environmental factors and ethnic-racial differences,
screening programs,
13
and even the dominant refractive
error in strabismus prevalence differences; for example, it
has been suggested that the high prevalence of strabismus
in white people could be due to the high prevalence of
hyperopia,
2
The high prevalence of strabismus in AMRO
in this study and the high prevalence of hyperopia in
western countries in other studies
16
could support this
hypothesis.
Similar to other studies
1,5,6
, our study also showed that
XT was the most common subtype of strabismus as the
prevalence of XT and ET was 1.23% and 0.77%, respec-
tively. Our study also showed a difference in the prevalence
of the dominant strabismus subtype between WHO
regions; for example, ET had a higher prevalence in
AMRO and EURO while XT was the dominant subtype
in Asia, especially WPRO, confirming the results of several
previous studies.
1,7,15,36,40,51,63,67
Studies have shown
a higher prevalence of esotropia in western countries, espe-
cially in white people and Caucasians,
1,7,15,36,40,51,63,67
while
exotropiahasahigherprevalenceinblackpeopleand
Asians, including the Chinese, Indian, and Iranian
populations.
7,15
It should be noted that variation in the
prevalence of strabismus subtypes in different parts of the
world could be due to different reasonssuchasethnicity,
genetics, environmental factors, and even other unknown
factors. Some review studies showed that the prevalence of
exotropia was sometimes higher in African and lower in
Europe, and attributed the reason to the distance from the
Equator.
68,69
However, one of the most important reasons
for the difference in the prevalence of strabismus subtypes
may be the dominant refractive error
13
such that the higher
prevalence of XT than ET in Asians can be due to the
higher prevalence of myopia in this population.
2
On the
other hand, ET has a stronger correlation with hyperopia
70
because subjects suffering from ET need more accommo-
dation to compensate hyperopia.
35
Studies in Japan
65
and
Korea have shown an increase in the ratio of XT to ET over
the past decade
71
duetoanincreaseintheprevalenceof
myopia in the world
16
and a decrease in the prevalence of
ET due to early diagnosis of intermittent ET with provision
of hypermetropic glasses,
8
indicating the important role of
refractive errors.
Despite several studies about strabismus, there is still
inconsistency about the strabismus trend change with age.
Studies in children aged 6–72 months,
6
36–72 months,
5
and 30–72 months
7
have shown a higher prevalence of
strabismus in older children. Kvarnström et al.
67
studied
children aged 1–12 years and reported that age was an
important determinant of the difference in the prevalence
of strabismus between different age groups. In this study,
the highest prevalence of strabismus was seen in children
4 years of age followed by a decreasing trend thereafter,
which was attributed to decreased prevalence of hypero-
pia with age
67
and early detection of the disease at lower
ages.
13
However, the results of other studies do not sup-
port this finding
15,30,36
; for example, Hashemi et al.
15
studied subjects aged 3 to 93 years and rejected any
difference in the prevalence of strabismus with age. The
results of this study and several other studies indicate that
age does not affect the prevalence of strabismus and
variations in different populations may be due to other
factors like genetic and environmental differences.
7
There are different reports of an inter-gender differ-
ence in the prevalence of strabismus and its subtypes.
Some studies have shown a higher prevalence of stra-
bismus in girls and have attributed the reason to the
higher prevalence of hyperopia in this gender.
14
However, several other studies have rejected any inter-
gender difference.
5,6,15,35
Our study pooled the data of
56 studies and showed no inter-gender difference in the
prevalence strabismus, XT, and ET. In other words,
inter-gender differences observed in other studies may
be due to other factors such as the age distribution of
the study population, ethnicity, and genetic and envir-
onmental factors. Lack of difference in the prevalence
of strabismus in multi ethnic studies confirms this
finding.
6–8
According to Vision 2020, many countries have
embarked on different programs for early detection
and treatment of some ocular disorders, including
strabismus.
72
In this regard, although we expected
a decrease in its prevalence in the recent three decades,
the results of meta-regression showed no change except
for a borderline significant increase in the prevalence of
XT. In other words, it seems that efforts to control the
disease are not yet at a scale to make a difference and
there is a need for more systematic screening programs,
especially in developing countries where screening pro-
grams are less efficient.
13
There were concerns over publication bias due to the
large number of exclusion criteria; therefore, we inves-
tigated publication bias in the reported prevalence of
strabismus, XT, and ET but the results showed no bias.
Hence, the results were robust to publication bias.
This study suffered some limitations. First, we were
willing to include all studies investigating the prevalence
of strabismus, XT, and ET but we had to exclude many of
them due to differences in their design and population
that impeded merging their results. Second, few studies
were published from some continents and therefore it
was not possible to provide a more robust estimate
according to the WHO region. However, our study had
some strong points. For example, this study presented an
estimate of the prevalence of strabismus, XT, and ET in
STRABISMUS 9
the global population and in WHO regions for the first
time. Our extensive search resulted in the retrieval of
a large number of articles of which 56 articles with
a total sample size of 229,396 subjects entered the final
analysis, which provided a sufficient statistical power.
Moreover, for the first time in the past three decades,
we also calculated the prevalence of strabismus, XT, and
ET in the age group above 20 years as a neglected age
group in most ophthalmologic studies.
Conclusion
The findings of this systematic review showed a higher
prevalence of strabismus in some countries and ethni-
cities, especially western countries and white people.
This study revealed that 1 in every 50 people had
strabismus, which severely affects their quality of life.
The results showed no significant difference in the
prevalence of strabismus between different age and
sex groups. Information about the global prevalence
of strabismus can help health care planners design
proper interventions and prioritize resource allocation.
Conflict of Interest
No conflicting relationship exists for any author.
Funding
This project was supported by Noor Research Center for
Ophthalmic Epidemiology.
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