Citation: Signes-Soler I, Piñero DP, Murillo MI, Tablada S.
Prevalence of visual impairment and refractive errors in an urban area of
Mexico. Int J Ophthalmol 2019;12(10):1612-1617. DOI:10.18240/ijo.2019.10.14
·Investigation·
Prevalence
of visual impairment and refractive errors in an urban area of Mexico
Isabel Signes-Soler1,2, David P Piñero3,
Milagro Inés Murillo2, Silvia Tablada2
1University of Valencia, 46100
Burjassot, Valencia, Spain
2Vision Without Border, 03710 Calp, Alicante,
Spain
3Department of Optics, Pharmacology
and Anatomy, University of Alicante, Alicante 03690, Spain
Correspondence to: Isabel Signes-Soler. Calle Corbeta
6, 03710 Calp, Alicante, Spain. isabel@onvivim.com
Received:
Abstract
AIM: To determine the distribution of refractive errors in a school-age
population in Quintana Roo (Mexico) in the framework of an international
cooperation campaign for the prevention of blindness.
METHODS: A sample of 2647 school-age children (ranging from 5
to 14 years old) with a mean age of 9.1±1.9 years old were tested by trained
volunteers for distance visual acuity (VA) and refractive errors. The first
screening examination included uncorrected distance visual acuity (UDVA) and VA
with a +2.00 D lens. Inclusion criteria for a second complete cycloplegic eye
examination performed by an optometrist were UDVA <20/25 (0.10 logMAR or 0.8
decimal) and/or VA with +2.00 D ≥20/25.
RESULTS: A total of 633 (23.9%) children underwent the second
complete eye examination. Mean logMAR UDVA was 0.035±0.094 (range 1.00 to 0.00
logMAR) for the right eyes and 0.036±0.160 (range 1.00 to 0.00 logMAR) for the
left eyes. Bilateral amblyopia was found in 17 children (2.7% of refracted
eyes; 0.64% of the total). The main reason for visual impairment (VI) in the
sample analyzed was found to be refractive errors. In 12 children (1.9% of
refracted eyes; 0.45% of the total) the VI was bilateral and 9 (1.4% of
refracted eyes; 0.34% of the total) achieved a corrected distance visual acuity
of 20/25 or better in both eyes. Mean magnitude of sphere and refractive
cylinder was +0.20±0.96 D and -0.43±0.85 D in right eyes, and +0.24±1.08 and
-0.43±0.83 D in left eyes. The proportion of myopic eyes [standard equivalent (SE)
≤-0.50 D] was 4.6% of the whole sample (5290 eyes). The mean magnitude of
myopia was -0.84±3.44 D for the right eyes and -0.82±5.21 D for the left eyes.
The proportion of hyperopic patients (SE≥+2.00 D) was 2.4% (15/633), which
corresponded to 0.60% of the whole sample (32/5290 eyes). No statistically
significant correlation of age to manifest sphere or cylinder was found.
CONCLUSION: VI due to uncorrected refractive errors can be
easily corrected with glasses but it is still a burden to be treated. Myopia is
prevalent in this sample. More efforts towards correcting uncorrected
refractive errors are needed.
KEYWORDS: myopia; hyperopia; refractive error;
blindness; visual impairment
DOI:10.18240/ijo.2019.10.14
Citation: Signes-Soler
I, Piñero DP, Murillo MI, Tablada S. Prevalence of visual impairment and
refractive errors in an urban area of Mexico. Int J Ophthalmol
2019;12(10):1612-1617
INTRODUCTION
Uncorrected refractive errors (URE) constitute
an important major public health problem in the world and continue to be the
leading cause of visual impairment (VI)[1]. Global
estimation indicates that 122.5 million people suffer VI due to URE[2]. This fact has been found to have a major
social and economic impact, including a limitation in educational opportunities[3].
The cost of dealing with VI
resulting from URE is very low in proportion to the loss of productivity
associated with VI. It is estimated that the annual loss of global gross
domestic product due to distance VI caused by URE is US$ 202 000 million[4].
URE are of importance for public
health and preventive actions are needed to manage the problem[5], as a high URE burden is associated to a lower
socioeconomic status[6]. Given the
potential life of a child, a refractive error at a young age may have a
lifelong impact[7]. Amblyopia secondary to URE in
childhood can lead to visual and also social, educational and economic problems
in adulthood[8].
Mexico has a high level of
development for the Latin American area with a human development index of (HDI)
of 0.774 (77/188)[9]. Quintana Roo is
one of the youngest states of Mexico and was rapidly developed as tourist
destination in the early 1970s. Despite the substantial growth in tourism,
Quintana Roo contributes only 1.34 percent to the national gross domestic
product, ranking it at 24 of 31 states. The rapid urbanization of the region
has led to a growth of the poor population in the cities, who come searching
for opportunities, public services and a place to live[10].
The American and the Caribbean regions have an overall prevalence of
blindness of 0.45/1000, although the available data for these region show a
wide disparity[11].
In this study, we evaluated the
distribution of the refractive errors in school-age children of the suburbs of
Cancun (Mexico). Although access to a vision specialist is available in the
area, the low income families cannot afford the expenses of a specialist
consultation plus the cost of the glasses as this must be covered by the
patient, which is impossible for most people living in this area. Volunteers
from the non-governmental organization “Vision Without Borders” invited by
local associations (“Manos de apoyo y vida” and “Embracing the word Mexico”)
screened school children, providing glasses to those needing them.
The aim of this study is to report
the prevalence of URE and VI in this specific region. There are few
publications regarding the prevalence of refractive error in children in
Mexico. A study evaluating the refractive error in different states of Mexico
did not include Quintana Roo[12].
SUBJECTS AND METHODS
Ethical Approval The principles outlined in the
Declaration of Helsinki were followed. Consent was obtained from the
parents for the study after explanation of the nature and possible consequences
of the study.
Sample Selection A total of 2942 children attending 4
primary schools in Cancun (Mexico) were enumerated, and of these 2647 (89.9%)
were included for eye examination in February 2014 because they were present on
the day of the examination. The schools were contacted in advance by local
people who informed them about the eye examination campaign.
All parents were informed prior to
the date of examination about the eye screening activity and that all children
attending the school would be examined during the two weeks of the program. The
study was cross-sectional, with the objective of evaluating the prevalence of
refractive errors and their impact on VI in the area.
Examination Protocol The protocol for examining the
children was divided into two parts. Visual screening was performed on all
subjects by a trained non-eye care group of volunteers guided by an
optometrist. This test included uncorrected distance visual acuity (UDVA) (E
Snellen chart at
The second revision was carried out
by three experienced optometrists on those children who fulfilled the criteria
and included the following: ocular motility, retinoscopy, cycloplegic and
subjective refraction and dilated fundus examination. Cycloplegic refraction
was measured 30min after instilling 2 drops of 1% cyclopentolate, each
administered 5min apart. Additionally, anterior segment integrity was explored
by means of a portable slit lamp.
The refractive errors were
classified according to the magnitude of the spherical equivalent (SE): sphere+1/2
cylinder. Myopia was classified as a SE ≤-0.50 D, hyperopia as a SE ≥+2.00 D,
and astigmatism equal or higher than 0.75 D (minus cylinder form was used)
[13].
Children were considered as myopic
if one or both eyes were myopic (including antimetropic patients), hyperopic if
one or both eyes were hyperopic as long as neither eye was myopic, and
emmetropic if neither eye was myopic nor hyperopic[14].
The prevalence of refractive errors was
calculated on the assumption that eyes with normal or near-normal vision (VA≥0.8)
were emmetropic. This hypothesis was made considering the fact that subjective
refraction data were not available for this type of eyes. According to WHO
definitions, a logMAR UDVA between 0.5 and 1.0 (between 0.05 and
We define amblyopia as a difference
of two lines or more between the two eyes or a corrected distance visual acuity
(CDVA) of 20/30 or worse. The term VI comprises category 0 for mild or no VI (VA≥0.3),
category 1 for moderate VI (0.3≥VA≥0.1), category 2 for severe VI (0.1≥VA≥0.05),
categories 3, 4 and 5 for blindness and category 9 for unqualified VI[16]. We considered VI as a logMAR UDVA between 0.5 and
1.0 (0.05 and 0.3 decimal), which includes moderate and severe VI.
Refraction Notation The spherocylindrical refractions
obtained were converted to vectorial notation using the power vector method
described by Thibos and Horner. With this procedure, any spherocylindrical
refractive error can be enunciated by 3 dioptric powers: M, J0 and J45,
with M being a spherical lens equal to the SE of the given refractive error,
and J0 and J45 two Jackson crossed cylinders equivalent
to the conventional cylinder. These numbers are the coordinates of a point in a
three-dimensional dioptric space (M, J0, J45). The length
of this vector is a measure of the whole blurring strength B of a
spherocylindrical refractive error.
In accordance with the power vector method, manifest
refractions in conventional script notation [S (sphere), C (cylinder) × φ
(axis)] were converted to power vector coordinates and overall blurring
strength (B) by the formulas: M=S+C/2; J0=(–C/2) cos (2 φ); J45=(–C/2)
sin (2 φ); and B=(M2+J02+J452)1/2.
Statistical Analysis Data analysis was performed using
the software SPSS for Windows version 19.0 (IBM, Armonk, NY, USA). Mean,
standard deviation (SD) and range for each of the parameters were calculated.
Normality of data samples was confirmed by the Kolmogorov-Smirnov test. The
degree of correlation between different clinical variables was assessed using
the coefficient of correlation (Pearson or Spearman depending on whether the
condition of normality could be assumed). Correlations were considered to be
statistically significant when P-value was <0.05.
RESULTS
A total of 2647 children (aged
5-14y) with a mean age of 9.1±1.9 years old were screened. Of them, a total of
633 (23.9%) who had a UDVA <20/25 (0.10 logMAR or 0.8 decimal) and/or VA with
+2.00 D ≥20/25 underwent a second full eye examination by an optometrist. The
gender distribution of the total sample was even, with 51.2% of males. The 40%
of subjects were aged 8 or younger. According to the Kolmogorov-Smirnov test,
UDVA and CDVA for right and left eyes were not normally distributed (P<0.001).
Likewise, refractive data (P<0.001) were also found to be not
normally distributed in our sample.
Table 1 shows the number of
school-age children screened, the number of children refracted by optometrists
and the glasses prescribed in each school.
Table 1 Schools visited in the
screening campaign performed and the number of children tested in each one
|
School name |
Children screened |
Children with VA≥20/25 |
Refracted by optometrist |
Glasses prescribed |
|
Año Del Centenario |
764 |
562 |
202 |
26 |
|
Enrique Estrella Oxte |
846 |
610 |
236 |
29 |
|
Pedro Balado |
622 |
461 |
161 |
23 |
|
Diego Rivera |
710 |
484 |
226 |
29 |
|
Missing data |
-297 |
-105 |
-192 |
0 |
|
Total |
2645 |
2012 |
633 |
107 |
Visual Outcomes Mean logMAR UDVA was 0.035±0.094
(range 1.00 to 0.00 logMAR) for the right eyes and 0.036±0.160 (range 1.00 to
0.00 logMAR) for the left eyes. Mean logMAR VA with +2 D was 0.036±0.094 (range
1.00 to 0.00 logMAR) for both eyes of the total sample of children.
An UDVA of 20/25 (0.1 logMAR, 0.8
decimal) or better and 20/200 (1 logMAR, 0.1 decimal) or worse was found in the
better eye in 94% and 0.3% of eyes, respectively. VI was found in 23 (0.5%)
right eyes and 28 (0.7%) left eyes of the total.
Amblyopia was found in 27 right eyes
(2.1% of refracted eyes, 1% of the total) and in 28
left eyes (2.2% of refracted eyes, 1% of the total). Bilateral amblyopia was
found in 17 children (2.7% of refracted eyes, 0.64% of the total).
Refractive errors were found to be
the main reason for VI in the sample analyzed. Of the 23 children with VI
(23/2645, 0.87% of the total) in the right eye and 28 (28/2645, or 0.86%) in
the left eye, 16 (69.5%) and 19 (67.8%) right and left eyes achieved a CDVA of
20/25 or better with refractive correction.
In 12 children, the VI (12/633 or
12/2645) was bilateral and 9 of these achieved a CDVA of 20/25 or better in
both eyes. VI was also caused by retinal problems 2/23 (8.7%) in right and 2/28
(7.1%) left eyes respectively, and unknown causes in 5/23 (21.7%) right and
7/28 (25%) left eyes.
Refractive Outcomes Table 2 summarizes the refractive
data in conventional format as well as in vector notation. Mean magnitude of
sphere and refractive cylinder was +0.20±0.96 D and -0.43±0.85 D in right eyes,
and +0.24±1.08 and -0.43±0.83 D in left eyes.
Table 2 Summary of the refractive
outcomes in conventional and vector notation.
|
Refractive parameters |
Right eye, mean (SD) |
Right eye (range) |
Left eye, mean (SD) |
Left eye (range) |
|
Sphere (D) |
+0.20 (0.96) |
-4.00 to +12.00 |
+0.24 (1.08) |
-3.25 to +12.50 |
|
Cylinder (D) |
-0.43 (0.85) |
-5.00 to 0.00 |
-0.43 (0.83) |
-5.00 to 0.00 |
|
SE (D) |
-0.01 (0.89) |
-5.00 to +11.25 |
+0.02 (1.02) |
-3.50 to 11.50 |
|
J0 |
+0.17 (0.43) |
-1.75 to +2.46 |
+0.17 (0.42) |
-1.00 to 2.35 |
|
J45 |
-0.01 (0.13) |
-0.98 to +0.88 |
-0.01 (0.11) |
-0.87 to +0.64 |
|
B (D) |
+0.56 (0.85) |
0.00 to 11.27 |
+0.57 (0.96) |
0.00 to 11.54 |
The proportion of myopic eyes (SE≤-0.50
D) was 19.6% (124/633) for the right eyes and 18.9% (120/633) for the left eyes
of the refracted children which corresponded to 4.6% of whole sample (244/5290
eyes). If we consider myopia as a SE≤-0.75 D, then the mean data for both eyes
was 12.8% (81/633) and of the total sample 3.1% (162/5290). Mean magnitude of
myopia was -0.84±3.44 D for the right eyes and -0.82±5.21 D for the left eyes.
The percentage of myopia in the sample according to these further definitions
was: SE≤-1.00 D 9.6% (61/633) for the right eye and 9.1% (58/633) for the left
eye. A SE≤-3.00 D was found in 0.6% (4/633) in both eyes.
The proportion of hyperopic patients
(SE≥+2.00 D) was 2.4% (15/633) for the right eyes and 2.7% (17/633) for the
left eyes of the refracted children which corresponded to 0.60% of the whole sample
(32/5290 eyes). If we define hyperopia as SE≥+1.00 D, then the total percentage
would be 1.7% (90/5290).
If we define myopia as SE≤-0.50 D (244
eyes) and hyperopia as SE≥+1.00 D (90 eyes), then the incidence of refractive
errors was 6.3%.
Refractive astigmatism of 0.75 D or
more was present in 22.3% (141/633) of the right eyes (5.3% of the whole
sample) and in 23% (146/633) of left eyes (5.5% of the whole sample).
No statistically significant
correlation of age with manifest sphere (right eye: r=-0.06, P=0.15;
left eye: r=-0.02, P=0.55) or SE (right eye: r=-0.07, P=0.07;
left eye: r=-0.04, P=0.29) was found. Likewise, no significant
correlation of age with manifest cylinder was found either (right eye: r=-0.02,
P=0.63; left eye: r=-0.04, P=0.29).
DISCUSSION
The South American population
encompasses a widely diverse group of nations, with some of them suffering
significant social differences in the population. In the current study, we have
evaluated the prevalence of VI in a child population of Mexico in the suburbs
of Cancun (Quintana Roo). This area is especially relevant because a total of 7%
of the population live in conditions of extreme poverty according to the annual
report of Sedesol (Secretariat of Social Development, Mexico)[17], with no access to specialized eye care services. To
our knowledge, this is the first study reporting the outcome of a massive
screening visual campaign in this area. Screening programs bring to light an
increased knowledge of vision disorders and can help us to manage them[18].
Distribution of Refractive Data According to a Meta-analysis that
reviewed 163 articles on refractive error, the prevalence of child myopia SE≤-0.50
D in the Americas was found to be 8.4%, a lower number compared to the
prevalence of 19% in our study[19]. In 2003, the
prevalence of myopia was studied in a child population aged 12 to 13 years old from
Monterrey (Mexico). The authors report a prevalence of myopia SE≤-0.50 D of
44%, whereas bilateral myopia was present in 37% of children in comparison to the
prevalence of myopia in our study of 19%. In the total sample, high myopia
SE≤-5.00 D was found in 1.4%[20]. This difference
may be explained in part by the difference in the age of the children included
in each study, with older children included in the Monterrey study. When our
results were analyzed for the subgroup of patients of 12 to 13 years old from
our sample, the prevalence of monocular and bilateral myopia (SE≤-0.50 D) was
29.5% and 27.9%, respectively. These values were closer to those reported by
Villarreal et al[20] in an age-matched
child population from Monterrey (Mexico).
Concerning the SE, we found in our
sample a mean value of -0.01±0.89 D and +0.02±1.02 D in right and left eyes,
respectively. These mean values contrast with those found by our research group
using the same methodology in other rural areas of different countries
(Paraguay, -0.25±1.44 D; Kenya, -0.32±1.36 D)[21-22]. Likewise, Choong et al[23] found in a Malaysian young population a mean
binocular subjective refraction of -0.62±2.51 D (95%CI -1.07 to -0.16), which
is also a higher myopic outcome than our mean SE. The result obtained in our
sample may seem contradictory considering the global outcome reported in
scientific journals of the myopic epidemy around the world[24].
However, it should be considered that definitions of myopia may differ between
studies. Likewise, the prevalence of refractive errors varies significantly
when rural and urban populations are compared, suggesting that environmental
factors are crucial in the distribution of the refractive errors in a specific
population. This has been also confirmed after a careful analysis of the
peer-review literature on epidemiology of refractive errors[18].
Mean refractive vector parameters
were more hyperopic than those reported in other studies (J0:
0.17±0.43 D, J45: -0.01±0.13, B: 0.56±0.85 D, right eye; J0:
0.17±0.42 D, J45: -0.01±0.11, B: 0.57±0.96 D, right eye)[21-22]. In Paraguay, our research group
found more negative values for astigmatic power vector components (J0:
-0.08±0.70 D, J45: -0.02±0.29, B: 1.07±1.25 D)[21],
as well as in a rural population of Kenya[22].
Prevalence of Myopia and
Hyperopia Myopia and hyperopia definitions
vary from one paper to another. Some authors prefer to define myopia as SE≤-0.50
D and hyperopia as a SE>+0.50 D[25-26].
However, a bilateral myopia of -0.50 D can prevent a child from seeing the
blackboard, whereas a hyperopia of +1.50 D cannot. In our sample, we found a
proportion of eyes with SE≤-0.50 D of 19.6% and 18.9% for right and left eyes,
respectively (4.6% of the total). The total prevalence is lower than the
proportion of 21% found in Los Angeles for children who underwent cycloplegia
in a study using the same definition of myopia but in younger children
(preschool children of 3-5 years old)[27].
However, as previously mentioned, the prevalence of myopia found in our series
was higher than those values reported in other studies performed in the
Americas[20,28-29].
Carter et al[28] reported a prevalence
from 1.2% to 1.4% according to the ethnicity in a study performed in Asuncion,
Paraguay. Galvis et al[29] found that the
prevalence of myopia was higher in urban areas compared to rural ones in
Colombia. For 15 year olds the prevalence of myopia SE ≤-0.50 D was 14.7%,
lower than that found in our study. In contrast, the prevalence of myopia
obtained in our series is lower than values reported in European[30-31] and Asian countries[32-33].
Besides some differences in the way
of reporting refractive errors between studies, other factors such as exposure
to risk factors may account for this. One of these factors is time spent
outdoors[34] which may be a critical issue for
this higher prevalence of myopia in the Mexican area evaluated. This hypothesis
should be confirmed in future studies on the prevalence of myopia in Mexico.
One additional finding confirming a differential behavior compared to European
and Asian countries is the absence of correlation between age and refractive
errors, with no higher levels of myopia concentrated in older groups of
children.
Concerning, the proportion of hyperopes,
defined as an SE≥+2.00 D, this was 0.6% in our total sample (2.5% of the
refracted eyes). This percentage was slightly higher than that reported by our
research group in a rural area of Paraguay[21]
(0.2%) using the same definition of hyperopia, and lower than that found in
another rural area of Kenya (4%)[22].The
prevalence of hyperopia in our population was consistent with that reported in
Asian countries[35-36], but lower
than prevalence reported in other American countries[29].
Specifically, a recent Meta-analysis showing global and regional estimates of
prevalence of refractive errors, indicated that the estimated prevalence pool
(EPP) in children with hyperopia was 4.6% (95%CI: 3.9-5.2). Specifically, the
EPP of hyperopia ranged from 2.2% in South-East Asia to 14.3% in the Americas[19].
Prevalence of Visual Impairment Refractive error was the main reason
for VI in the sample studied, as in other rural areas of other countries
evaluated by our research group[21-22].
This confirms that most VI in the area evaluated is avoidable only by
prescribing and providing spectacle correction. This contrasts with the
outcomes reported in other low income countries, with ocular comorbidities such
as glaucoma being one of the main factors associated to VI. Alabi et al[37] identified in schoolchildren from the Ogun State of
Nigeria that the most common ocular morbidities associated to VI were
refractive errors 39.7%, high/asymmetrical vertical cup-to-disc ratio
(suggestive of glaucomatous optic neuropathy) 33.5%, allergic conjunctivitis
19.2%, corneal opacity 2.7% and lenticular opacity 2.2%.
Regarding amblyopia, the percentage
reported in our study (around 1%) was consistent with that reported in other
Asian[38] and American countries[21]. Xiao et al[39]
using data from the multi-country refractive error study in children found that
the prevalence of amblyopia varied with ethnicity and was highest in Hispanic
children (1.43%), of this only 0.17% being bilateral, compared to our sample
with 0.64% of bilateral amblyopia.
Limitations and Final
Conclusions This study has some drawbacks, such
as its design. A population-based study would have been a more suitable design.
However, given the lack of information about the distribution of refractive
errors in the area evaluated, we believe that our data can be of interest for
the scientific community.
In conclusion, the main cause of VI
in schoolchildren from the Quintana Roo region of Mexico was the presence of
refractive errors, with a minimal incidence of ocular comorbidities and
amblyopia. The prevalence of refractive errors was consistent with global
estimates, showing a higher proportion of myopes than hyperopes. However, the
prevalence of refractive errors in the sample evaluated was lower than those
values reported in other low-income countries.
ACKNOWLEDGEMENTS
Conflicts of Interest: Signes-Soler
I, None;
Piñero DP, None; Murillo MI, None; Tablada S, None.
REFERENCES