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Epidemiologic Reviews
Copyright © 1996 by The Johns Hopkins University School of Hygiene and Public Health
All rights reserved
Vol. 18, No. 2
Printed in U.S.A.
Epidemiology of Myopia
Seang-Mei Saw,1 Joanne Katz,2 Oliver D. Schein,3 Sek-Jin Chew,4 and Tat-Keong Chan 5
INTRODUCTION
holes and tears, as well as retinal detachment. Methods
of correction of myopia are not without complications,
including corneal infections due to contact lens wear
and corneal scarring and persistent corneal haze from
refractive surgery (5).
The public health and economic impact of myopia,
the most common eye condition in the world, is enormous. In the United States, the cost of correcting
refractive errors with spectacles or contact lenses is
estimated to be 2 billion dollars per year (6). The
military spends large amounts of money on pilot training, but pilots may not be able to continue flying if
they develop myopia. Thus, myopia is a condition with
social, educational, and economic consequences.
Over the past few decades, there has been an increase in the prevalence of myopia in some populations, leading to growing concern among the public
and the scientific community. The Chinese and Japanese appear to have had escalations of myopia rates.
There is no well established or universally accepted
treatment for the prevention of myopia onset or progression.
This review will summarize the descriptive epidemiology of myopia, possible risk factors for myopia,
and the interventions available to prevent the onset and
progression of myopia. The limitations of the existing
research will be addressed, as well as suggested directions for further research.
Myopia is the state of refraction in which parallel
rays of light are brought to focus in front of the retina
of a resting eye (1). Myopia is measured by the spherical power in diopters of the diverging lens needed to
focus light onto the retina, which can be expressed as
the spherical equivalent or refraction in the least myopic meridian (2, 3). The clinical correlates of myopia
include blurred distance vision, eye rubbing, and
squinting.
Myopia has been classified as either physiologic or
pathologic. Physiologic myopia occurs due to an increase in the axial diameter of the eye over that which
is attained by normal growth. Pathologic myopia is
caused by an abnormal lengthening of the eyeball, and
is often associated with thinning of the scleral wall (1).
Another classification is based on age of onset. Congenital, or infantile, myopia occurs at birth, with a
reported prevalence in the full-term newborn varying
from 0.0 to 24.2 percent. This variability is due to
technical difficulties in measuring refraction in newborns (4). School myopia occurs at approximately
7-17 years of age and stabilizes by the late teens or
early twenties. Both school and adult-onset myopia are
mainly the result of idiopathic causes, while congenital myopia is often associated with other abnormalities.
Severe myopia may be associated with myopic macular degeneration, cataract, glaucoma, peripheral retinal changes (such as lattice degeneration), and retinal
DEFINITION OF MYOPIA IN EPIDEMIOLOGIC
STUDIES
Different studies have adopted different definitions
of myopia. The most common definitions are a refractive error greater than 0.25 diopter and a refractive
error greater than 0.50 diopter. The lack of uniform
criteria has led to difficulties in comparing prevalence
rates in different studies. Cross-sectional and cohort
studies use different criteria for defining persons as
myopic. All studies should specify the definition of
myopia used and the range of refractive error of the
subjects in the study.
The accuracy and reliability of ophthalmologic and
refractive examinations is crucial in epidemiologic
studies. The "gold standard" for measurement of re-
Received for publication January 4,1996, and accepted for publication July 16, 1996.
Abbreviation: NHANES, National Health and Nutrition Examination Survey.
1
Department of Epidemiology, School of Hygiene and Public
Health, The Johns Hopkins University, Baltimore, MD.
2
Department of International Health, School of Hygiene and
Public Health, The Johns Hopkins University, Baltimore, MD.
3
Dana Center for Preventive Ophthalmology, Wilmer Eye Institute, The Johns Hopkins Hospital, Baltimore, MD.
4
Singapore Eye Research Institute, Singapore National Eye Center, Singapore.
5
Department of Ophthalmology, National University Hospital,
Singapore.
Reprint requests to Dr. Joanne Katz, Department of International
Health, School of Hygiene and Public Health, The Johns Hopkins
University, 615 North Wolfe Street, Baltimore, MD 21205.
175
176
Sawetal.
fractive error in children is cycloplegic refraction (1).
Cycloplegia is the act of paralyzing the muscles of
accommodation in the eye. Usually, cyclopentolate
hydrochloride eye drops are instilled, which provides
cycloplegia lasting for 1 hour. Cycloplegic refraction
is especially important in children and infants, as they
have strong accommodative responses which may lead
to "pseudomyopia" (7). However, often cycloplegic
refraction is not used for the diagnosis of myopia in
children and young adults. Thus, myopia rates may be
overestimated in the determination of refractive error
in these studies.
PREVALENCE AND DEMOGRAPHIC PATTERNS
There is considerable geographic variation in the
reported prevalence of myopia (table 1). It is difficult
to compare prevalence rates between countries based
on previous studies; the definitions of myopia are not
uniform, and refraction may have been performed
without cycloplegia. Prevalence studies are not all
population-based, with some studies being conducted
on convenient select groups of individuals with limited
generalizability. The prevalence of myopia varies with
time and the age of the study population.
From data gathered on 7,401 persons aged 12-54
years in the National Health and Nutrition Examination Survey (NHANES) in 1971 and 1972, the prevalence of myopia in the United States was estimated to
be 25 percent (8). However, the exact criteria for
myopia in this survey were not clearly defined. This
population-based survey did not include cycloplegic
refraction (thereby probably overestimating the rate
and degree of myopia), and the nonparticipation rate
was 30 percent. A more recent population-based survey in Beaver Dam, Wisconsin, of 4,926 adults between the ages of 43 and 84 years showed a decreasing
prevalence of myopia with age, from 43.0 percent in
the age group 43-54 years to 14.4 percent in subjects
above the age of 75 years (9). Myopia was defined as
more than 0.5 diopter; however, there was no mention
of whether cycloplegia was used.
TABLE 1.
In Scandinavia, most of the studies were not
population-based (10). Myopia prevalence was reported to be 50.3 percent among 133 medical students
in Norway (11). In Sweden, the prevalence of myopia
among 2,616 Swedish conscripts aged 20 years was
8.9 percent. These studies defined myopia as more
than 0.25 diopter, and no cycloplegia was used. Approximately 20.5 percent of 21,000 Icelanders refracted with cycloplegia in 1975 were myopic, defined
as more than 0.5 diopter (10).
In Asia, there is currently a high prevalence of
myopia, especially among the Chinese and Japanese.
As early as the 1930s, Rasmussen (12) estimated a
prevalence of myopia of approximately 70 percent in
China; however, the refraction procedures were not
clearly described. A total of 4,000 schoolchildren aged
6-18 years were refracted with cycloplegia in an
island-wide survey in Taiwan in 1983. There was an
increasing prevalence of myopia with age, from 4
percent at age 6 years to 40 percent at age 12 years,
more than 70 percent at age 15 years, and more than
75 percent at age 18 years (13). Three studies carried
out in Singapore showed varying myopia rates: 24.9
percent in 10-year-old Chinese children, 63 percent in
university freshmen aged 19 years, and 82 percent in
medical students (14-16). However, the definitions of
myopia differed. Various surveys in India have found
myopia prevalences ranging from 6.9 percent to 19.7
percent (17, 18). The techniques used for refraction
and the definition of myopia used were often not
mentioned in the studies conducted in Asia.
In agricultural countries, the prevalence of myopia
has been low. There have been several populationbased studies. On the South Pacific island of Vanuatu,
788 Melanesian children aged 6-19 years were examined and refracted without cycloplegia. Only 2.9 percent were found to be myopic by 0.5 diopter or more
(19). In the Solomon Islands, an ophthalmic survey
conducted in 1966 found that only 0.8 percent of the
study population from the islands of Bougainville and
Malaita were myopic by 0.25 diopter or more. No
Summary of selected studies of myopia prevalence
oounuy
Solomon Islands
Vanuatu
Sweden
Iceland
Study
(ref. no.)
Verlee (20)
Grosvenor(19)
Str6mberg (cited by
Fledelius (10))
Sveinsson (23)
Populationbased?
Yes
Yes
No, conscripts
No, spectacle
Cycloplegic
refraction?
Myopia
definition
512
788
2,616
No
No
No
>0.25 diopter
>0.5 diopter
>0.25 diopter
1-69
6-19
20
21,000
Yes
>0.5 diopter
1-89
7,401
133
No
No
>0.25 diopter
12-54
21-33
4,000
Yes
Sample
size
Age
(years)
Prevalence
(%)
0.8
2.9
8.9
21
/MrtCI iftC
United States
Norway
Sperduto et al. (8)
Midelfart et al. (11)
Taiwan
Lin et al. (13)
Yes
No, medical
students
Yes
6-18
25
50.3
75 at age
18 years
Epidemiol Rev Vol. 18, No. 2, 1996
Epidemiology of Myopia
cycloplegic refraction was done on the 512 subjects
(20).
Myopia not only shows regional variation in prevalence but also exhibits country-specific differences in
secular trends as well. A possible reason for the increase in myopia rates in many countries is the increase in formal education, with more time being spent
on closeup work, in the past few decades. The prevalence of myopia has increased over the past several
decades in Singapore and Japan (21, 22). Similarly,
the prevalence of myopia in Iceland increased from
3.6 percent in 1935 to 20.51 percent in 1975 (23). The
Iceland study included the use of cycloplegic refraction and the same myopia definition of more than 0.5
diopter over the 50-year period.
Sex and race also affect the distribution of myopia.
The 1971 and 1972 NHANES data showed that prevalence rates were higher in females than in males and
higher in whites than in blacks in the United States (8).
Several other studies have found a slightly higher
preponderance of myopia in females (9, 21). Certain
ethnic groups, such as Asians and Jews, have a higher
prevalence of myopia, whereas Africans and African
Americans have a low myopia prevalence rate (8). In
Hawaii, the prevalence of myopia varies among the
different ethnic populations: 17 percent in Chinese, 13
percent in Koreans, 12 percent in Japanese, and 12
percent in Caucasians (24). In a Taiwanese survey,
where the eyes of children in one school were refracted under cycloplegia, the prevalence of myopia
among the purely aboriginal children was 13 percent,
compared with 30 percent in the Chinese children
(25).
The prevalence of myopia changes considerably
with age. Newborns are usually hyperopic. In subsequent years, the ocular axis elongates, with thinning of
the lens and flattening of the cornea, which leads to
emmetropia in children by age 8-10 years (22). When
myopia occurs, it usually develops between the ages of
6 and 14 years. Thereafter, the prevalence of myopia
remains relatively constant between the ages of 12 and
54, as reported in the US NHANES data (8). There is
a decreasing prevalence of myopia with increasing age
after age 40 years (9).
To facilitate appropriate comparisons of the prevalence of myopia across different populations, studies
should be population-based, have similar definitions
of myopia, refract children under cycloplegia, and
report findings by age. This will allow researchers to
compare prevalence rates across geographic boundaries. Similarly, studies of secular trends in myopia
rates and the sociodemographic characteristics of myopia should have the same definitions of myopia and
should include refraction by cycloplegia.
Epidemiol Rev Vol. 18, No. 2, 1996
177
Incidence and progression of myopia
There is a lack of adequate data on the incidence of
myopia from population-based cohort studies. Over a
10-year period, the incidence of myopia among Israeli
pilots was 7.4 percent in 991 pilots with 20/20 vision
in each eye upon entry into the profession and 22.5
percent in 221 pilots with 20/25 vision in one eye upon
entry into the profession (26). The results of this study
are only generalizable to populations of pilots in Israel, who are varied ethnically (European, North African, Asian). This is also a very unusual definition of
myopia; it is unclear how 20/25 vision relates to refractive error.
Longitudinal studies have found that myopia stops
increasing earlier in females than in males, and that
mean cessation ages range from 14.44 to 15.28 years
for females and 15.01 to 16.66 years for males (27).
Lin et al. (28), however, showed that even after puberty, myopia continues to progress slowly, and the
increase in axial length is the main component in
myopia progression. Both Goss (29) and Chew et al.
(30) have reported that a greater amount of myopia at
the initial examination age is associated with a greater
rate of progression. In a study of Finnish schoolchildren by Parssinen and Lyyra (31), myopia progressed
faster in girls than in boys, in children with an earlier
age of onset of myopia, and in children who had more
severe myopia at initial examination. All of these
studies have a potential bias in that they examined
populations that self-referred for spectacle or contact
lens correction of myopia (i.e., perhaps only certain
types of people seek help when they first start to
become myopic, while others wait longer before seeking correction of their myopia).
RISK FACTORS FOR MYOPIA
Both environmental and genetic factors have been
associated with the onset and progression of myopia.
The use-abuse theory states that closeup work causes
myopia, as seen in the higher prevalence of myopia
among persons who are more highly educated and are
in white collar occupations. The genetic theory, on the
other hand, is based on the belief that natural individual variation in eye growth will produce myopia in
certain individuals (3). The mechanisms underlying
the environmental and genetic factors, and the nature
of the interaction between the two factors, is not
certain. Educational level, intelligence, certain personality traits, and socioeconomic status have all been
associated with myopia. Premature and lowbirth-weight infants have a higher risk of developing
myopia later in life (32-34). The effects of malnutrition and height on myopia are poorly substantiated
178
Saw et al.
(35-37). The strongest evidence for an environmental
cause is the effect of closeup work on the onset and
progression of myopia.
Family history
There is a greater prevalence of myopia in children
of myopic parents than in children of nonmyopic parents (38, 39). Genetic studies of myopia have mainly
been twin studies, pedigree studies, and studies of
familial correlation. Family studies by Sorsby et al.
(40) and Keller (41) demonstrated significant parentchild correlations. However, it is difficult to separate
hereditary factors from environmental factors such as
similar work patterns in parents and their children
(41). Initial cross-sectional results of the Orinda Longitudinal Study of volunteer schoolchildren showed
that before the onset of myopia, the children of myopic
parents had longer eyes, suggesting a possible hereditary predisposition to myopia. However, early environmental factors may also have led to longer eyes
(42). The role of heredity is postulated to be more
significant in persons with higher degrees of myopia.
In a study of 258 myopic patients, the percentage of
parents with myopia was 15 percent for those with
myopia of less than 1.00 diopter versus 55 percent for
patients with myopia of more than 7.00 diopters (43).
Different modes of Mendelian inheritance, including autosomal dominant, autosomal recessive, and
sex-linked, have been suggested by different authors
(44, 45). In a study conducted in Hawaii of 185 families with both parents of Japanese ancestry and of 192
families with both parents of European ancestry, segregation analysis was performed (46). The results
showed that there was little evidence for a Mendelian
mode of genetic inheritance.
Past twin studies have not defined the mode of
inheritance but have provided evidence to support the
heritability of myopia. Accurate classification of zygosity and the comparison of monozygotic and dizygotic twin populations of similar characteristics are
important considerations in the design of twin studies
(47). Similar results have been obtained from twin
studies conducted in the United Kingdom, Finland,
Taiwan, and Shanghai, where there were higher concordance rates of myopia in monozygotic twins than in
dizygotic twins (48-51). In a study of Chinese twin
pairs (52), there was a higher concordance rate of
myopia (92.2 percent) for monozygotic twins with
concordant close-work habits (differences of less than
1 hour per day spent studying and reading) than for
monozygotic twins (79.3 percent) with discordant
close-work habits. The authors concluded that there
was significant additive interaction between zygosity
and close-work habits.
The exact mode of inheritance and possible genetic
markers for myopia have not been identified. Not all
observations, such as the increase in myopia prevalence in Taiwan, Singapore, and Hong Kong, can be
explained solely by genetic causes. There may be an
interaction between genetic and environmental factors
wherein some individuals have a genetic predisposition such that they are more susceptible to environmental influences causing myopia. More conclusive
and well-designed studies of family pedigrees of individuals with high myopia that use genetic markers
associated with myopia must be conducted. The markers for collagen metabolism, intelligence, and retinal
neurotransmitters could provide clues to the location
of possible myopia genes.
Education and intelligence
Several cross-sectional studies in Denmark, Israel,
the United States, and Finland have shown a higher
prevalence of myopia among individuals with higher
educational levels (53-56). Other studies have shown
an association between myopia and intelligence and
socioeconomic status (57-60).
Refractive error and intelligence have been compared in various studies, with inconsistent results. Positive associations were found in Ohio and in Auckland,
New Zealand, when the California Test of Mental
Maturity and the Otis Self-Administered Test, respectively, were used to evaluate intelligence (59, 60).
However, no relation was found when the StanfordBinet Test was used in Ohio or when the Raven Matrix
Test was used in Auckland (59, 60).
Ashton (61), in Hawaii, measured the effects of both
closeup work and intelligence on the onset of myopia.
Although no association between myopia and closeup
work was found, a relation between school achievement and myopia was noted. The results of this study
may have been affected by the crude measure of
closeup work (number of books and magazines read
per month), refraction without the use of cycloplegia,
and the cross-sectional nature of the study.
Questions about the validity of intelligence testing
and the omission of information on other confounding
factors, such as closeup work, socioeconomic status,
and educational level, limit conclusions from previous
studies of intelligence and myopia (57-60). Hirsch
(59) noted that intelligence test scores could be influenced by the amount of reading a child does or that a
more intelligent child might read more and thus become more myopic. Educational level and intelligence
are strongly related to amount of closeup work and are
probably not independent risk factors but surrogates
for closeup work.
Epidemiol Rev Vol. 18, No. 2, 1996
Epidemiology of Myopia
Closeup work
Closeup work encompasses tasks of high accommodative demand, such as reading, writing, computer
work, and close television viewing. It has been suggested that the side-to-side movement of the eyes
during reading has a different effect on myopia than
does close work without similar eye movement, such
as sewing (31). The incidence of myopia increases at
the time children start attending school, and this suggests that closeup work may be a cause of the development of myopia (62). The increase in myopia prevalence observed in Hong Kong, Taiwan, Japan, and
Singapore over the past few decades suggests an environmental risk factor, since the gene pool has not
changed. There has been an increase in educational
attainment over the past several decades, with an accompanying increase in myopia incidence, in countries such as the United States (63). However, these
observations have generally been ecologic rather than
epidemiologic. An increased prevalence of myopia is
observed in certain occupations, such as microscopy,
sewing, and carpet weaving, that require a large
amount of time spent in closeup work (64). However,
it is difficult to separate cause from effect; the study of
persons in select occupational groups who spend large
amounts of time on close work may be part of a
selection process whereby individuals with myopia
may prefer these occupations. Further evidence for the
close-work hypothesis is the higher prevalence of myopia among college graduates, with a higher number
of new cases in the college years, compared with other
adults in the same age group (65). In 1964, Sato (66)
noted a higher incidence of myopia among US graduates after they studied Chinese in universities.
In the native populations of the Arctic regions of
Alaska and Canada there has been a notable increase
in myopia in the younger generation. There is little
parent-child correlation in refractive error, but a
sibling-sibling correlation now exists. The prevalence
of myopia was much higher in young persons compared with older individuals among Alaskan Eskimos,
Canadian Inuit, members of a Labrador community,
Yupik Eskimos, and American Indians in Ontario (6771). The increase in myopia incidence in Arctic regions has coincided with the establishment of compulsory schooling after World War II and with an increase
in exposure to closeup work. Intermarriage with
whites could also be contributing to a genetic change
in the predisposition to myopia. However, a homogenous change in refractive error in different populations
suggests that intermarriage is unlikely to be contributing substantially to the rising incidence of myopia
and, more importantly, Caucasians do not have very
high rates of myopia. Thus, closeup work has been
Epidemiol Rev Vol. 18, No. 2, 1996
179
implicated as a risk factor for the onset of myopia (8).
The mechanisms for myopia onset and progression
may be similar, and the association between closeup
work and myopia progression can provide evidence
for the causation of myopia onset.
Cross-sectional prevalence studies
Cross-sectional studies conducted in Newfoundland
and Hong Kong have found positive associations between closeup work and the prevalence of myopia
(72-74). The odds ratio for myopia in subjects who
attended school in the Hong Kong study was 1.7 (95
percent confidence interval 1.0-3.0). However, refraction was measured without cycloplegia in these studies. The measures of closeup work were crude and
were obtained from questions on the amount of reading and writing done. The effects of different types of
closeup work, such as reading or watching television,
and variations in levels over time were not assessed.
Moreover, the studies did not account for variations in
the amount of closeup work by age, or the distances
used for various tasks.
An interesting study was conducted in Israel in
which orthodox schoolboys of identical ethnic background had a myopia prevalence of 81.3 percent, as
compared with 27.4 percent among boys from general
schools (myopia was defined as more than 0.50 diopter; cycloplegic refraction could not be performed on
all subjects) (75). The authors of this study attributed
this increased myopia in orthodox males to their
unique study habits, and to the fact that the printed
letters in the commentaries studied may be as small as
1 mm in height. In addition, there was a large difference in the amount of time spent reading and writing
at school. The girls in the orthodox schools had rates
of myopia comparable to those of girls in the nonorthodox schools. Again, the big difference was in the
amount of closeup work, which was much less for
girls than for boys in the orthodox schools. However,
individual estimates of the amount of closeup work
were not obtained.
Cohort studies
Parssinen et al. (76) reported a faster rate of progression of myopia among children who spent a
greater amount of time on closeup work. Refractive
error was measured annually with cycloplegia. A
questionnaire was designed to determine the amount
of time spent on closeup work to the nearest half hour,
with information obtained on closeup work done on
both weekends and school days, as well as details on
reading distance.
180
Sawetal.
Occupational studies
Rapid industrialization and modernization has led to
many workers' spending more time on closeup work
with video display terminals and to children's using
video display terminals for computer-aided instruction
and video games, as well as increased television
watching (77). Studies of persons in occupations involving long hours of closeup work, such as textile
workers and visual display unit workers, show that the
prevalence of myopia is higher in these occupations
(78, 79). These studies often compare groups of people
with different educational levels and socioeconomic
status; such comparisons are difficult.
There is a growing belief that both genetic and
environmental factors, such as closeup work, play a
part in the onset of myopia. Refraction is possibly a
product of both genetic and environmental factors,
with the environment modifying the genetically determined development of the eye.
Biologic theories for closeup work
The growing eye of a child is sensitive to visual cues
that could determine axial length and whether the eye
grows in the direction of myopia or hyperopia (80).
There are several theories which attribute closeup
work to the increase in axial length that causes myopia. One of the most widely held theories is the accommodation theory, wherein there is an increase in
pressure in the posterior part of the eye during accommodation which is poorly resisted by the sclera, resulting in increased ocular length (63). Although intraocular pressure plays a role in normal eye growth
during development, there has been no documented
increase in intraocular pressure in myopic eyes. Nonetheless, defective accommodation may cause retinal
image defocus, which is increasingly regarded as a key
factor in myopia development (81).
Animal research showed that monkeys whose vision
was restricted to a distance of 18 inches (46 cm) by
drapes became myopic, and cage-reared animals had a
higher prevalence of myopia than their wild counterparts (82). This supports the association of closeup
work with increased accommodation and myopia. Experiments by Raviola and Wiesel (83) showed that
monkeys with unilaterally surgically closed eyelids
who were reared in lighted environments developed
axial myopia in the closed eye and none in the other,
open eye. This could be due to visual form deprivation, as animals with sutured lids who are reared in the
dark do not become myopic. Another theory (84)
postulates that the printed page provides an impoverished stimulus for nonfoveal retinal neurons, which
have large receptive fields. Posterior poles of chicks
show a greater role in myopia development than equatorial areas. Recent studies have shown strong evidence that objects viewed nearby may cause the eye to
elongate further than it does during normal growth to
maximize the sharpness of images on the retina. This
growing eye thus elongates and becomes myopic (85).
In a study by Hung et al. (86) in Houston, Texas,
refractive errors such as myopia were induced in monkeys by lenses. There was resultant compensating eye
growth that reduced the effect of refractive errors
produced by the spectacle lenses. This experiment
supports the hypothesis that lens wearing affects the
growth of the eye and that myopia progression may be
hastened by focusing on close objects when wearing
minus lenses, but this has not been demonstrated in
humans. Further research is needed to bridge the gap
between animal models and human eye physiology.
Other risk factors
Other risk factors that have been explored as possibly contributing to myopia onset and progression include prematurity, low birth weight, height, personality, and malnutrition. There is strong evidence for a
link between prematurity and low birth weight and
myopia, but unconvincing evidence for any association between myopia and height, personality, or malnutrition.
Past studies have reported a greater prevalence of
myopia later in life in premature infants as compared
with full-term infants (32-34). Myopia is especially
common in premature infants with retinopathy of prematurity, which is caused by excessive exposure to
oxygen during the first few weeks of life (1).
Eye size may be linked to body stature, with taller
individuals having longer axial lengths. Several studies have shown that myopic individuals are taller than
nonmyopic individuals (35, 87). However, this difference is often explained by a difference in socioeconomic status. A Finnish case-control study by Teikari
(35) showed that myopic persons were significantly
taller than nonmyopic persons. However, refractive
status was not directly examined, and height information was obtained indirectly from a questionnaire.
There have been several studies which investigated
the association between personality and myopia. Early
studies showed that myopic individuals may be more
introverted, reflective, self-confident, dominant, and
sedentary than nonmyopic individuals (57, 58), while
other studies, such as a cross-sectional study by
Bullimore et al. (88), did not find any association
between personality and myopia. These personality
attributes of myopic individuals may be associated
with other risk factors such as intelligence and large
amounts of time spent on closeup work.
Epidemiol Rev Vol. 18, No. 2, 1996
Epidemiology of Myopia
There is no evidence that specific vitamin deficiencies are associated with myopia (57). The evidence for
nutritional causes for the onset of myopia has been
unconvincing, as past studies showing an association
have had methodological limitations. Studies in African tribal people and Lebanese Arab infants showed
that malnourished individuals had higher myopia rates
(36, 37). However, only limited conclusions may be
made, as the cross-sectional studies do not allow direct
analysis of the temporal nature of the relation and
there may be more proximal causes of myopia that are
associated with nutrition that have not been examined.
In addition, there is a question as to why there would
be an increase in myopia in Singapore, Taiwan, Japan,
etc., at a time when people's diets were improving (in
terms of calories and protein content). If there is any
association, the attributable risk is probably very
small.
NEEDS FOR FURTHER EPIDEMIOLOGIC
RESEARCH
Currently, there is no conclusive evidence for any of
the myopia risk factors postulated above. Most of the
observed associations have come from cross-sectional
studies. There are very few cohort studies that have a
sufficient sample size, accurate measurement of risk
factors, adjustment for possible confounding factors,
and measurement of the different refractive components in myopia development, which include refraction by cycloplegia, axial length, and corneal curvature. There is a need for further well-designed
epidemiologic studies to provide us with information
on risk factors for myopia onset and progression.
From our assessment of the available literature, we
must make inferences about the relative importance of
the different risk factors in order to set directions for
further epidemiologic research. It appears that there is
an hereditary component of myopia, as seen in the
many familial correlation, twin, and pedigree studies
that have been conducted. However, the exact mode of
interaction between genetic and environmental factors,
the relative contribution of genetic factors as opposed
to environmental factors, and the nature of the genetic
markers is unknown. Time trends showing increased
myopia rates in many countries point to an environmental cause for myopia. The most important environmental risk factor for myopia appears to be closeup
work, for which several cross-sectional and cohort
studies have shown an association. Other risk factors,
such as intelligence, academic achievement, socioeconomic status, and educational level, are possible surrogates for closeup work. Myopia also varies with age,
sex, race, and gestational age at birth. All of the above
factors are potential confounders and should be meaEpidemiol Rev Vol. 18, No. 2, 1996
181
sured and appropriately adjusted for in studies examining the association between myopia and closeup
work. There is no consistent evidence for height, personality, or malnutrition as risk factors for myopia.
INTERVENTIONS
Visual corrective aids, such as spectacles and contact lenses, are established methods of correcting the
defective distant vision arising from myopia. However, to date, there has not been any convincing or
widely accepted method of preventing the onset of
myopia or retarding the progression of myopia in
humans.
A variety of different methods to reduce the onset
and progression of myopia have been described. These
methods include visual training, biofeedback training,
the use of bifocal spectacles, contact lenses, the instillation of atropine eyedrops, the instillation of betablocker eyedrops, lowering of the intraocular pressure,
and surgery (89). Unfortunately, most of the results
published have had limited validity. Some of the early
intervention trials did not have a control group for
comparison. Many clinical trials did not include randomization, thus allowing for selection bias by the
investigators and participants. Furthermore, the treatment groups were not comparable with regard to measured confounding factors. The sample size and length
of follow-up were often insufficient. In addition, large
numbers of dropouts were common, and a difference
in myopia progression among subjects lost to
follow-up may have led to biased conclusions. Masking of subjects is almost impossible, and it is difficult
to mask the technicians who refract the subjects with
regard to intervention status. The trials discussed here
are limited to those that utilized controls, as shown in
table 2.
Bifocal spectacles
Bifocal spectacles have been postulated to slow the
progression of myopia by reducing accommodative
demand. Clinical trials on the effects of bifocals are
often not randomized, and there is no conclusive evidence for the effect of bifocals in the slowing of
myopia progression (90). In 1975, Oakley and Young
(91) conducted a clinical trial which assigned bifocals
to volunteers and spectacles to subjects who refused to
wear bifocals. The study population of 156 Native
Americans and 441 Caucasians aged 6-21 years was
followed for 2-4 years, and an average of three cycloplegic refractions were performed. The results
showed a significant difference in the rate of progression of myopia of -0.04 diopter in the bifocal group
compared with —0.51 diopter in the control group. No
182
Saw et al.
TABLE 2.
Clinical trials of interventions to decrease the rate of progression of myopia
Study
(ref. no.)
Oakley and Young (91)
Bifocal lenses
Goss and Grosvenor
(92)
Bifocal lenses
Grosvenor et al. (93)
Bifocal lenses
Parssinen et al. (76)
Bifocal lenses
Stone (94)
Contact lenses
Andreo (95)
Hydrophilic contact
lenses
Gas-permeable contact
lenses
Grosvenor et al. (97)
Result
Intervention
Perrigin et al. (98)
Silicone-acrylate contact
lenses
Bedrossian (99)
1 % atropine eyedrops
Kaoetal. (102)
1 % atropine ointment
Hosaka (104)
Labetalol and timolol
eyedrops
Jensen (105)
0.25% timolol maleate
Limitations
Significant difference in annual rate of
myopia progression of -0.12 diopter
in the bifocal group compared with
-0.38 diopter in the control group
No significant difference in myopia
progression between different groups
No significant difference in myopia
progression between different groups
No significant difference in myopia
progression between different groups
Significant difference of annual myopia
progression of 0.1 diopter in contact
lens wearers compared with 0.36
diopter in spectacle wearers
No significant difference in myopia
progression in different groups
Significant difference in annual myopia
progression of 0.14 diopter in the
contact lens group versus 0.40
diopter in the spectacle group
Significant difference in annual myopia
progression of 0.16 diopter in the
contact lens group compared with
0.51 diopter in the spectacles group
No myopia progression in 74% of
atropine treated eyes versus 4%
of untreated fellow eyes
Significant difference in myopia
progression of 0.17 diopter in the
atropine group compared with 0.75
diopter in the control group
Significant difference in myopia
progression between eyes treated
with labetalol and placebo but no
difference for eyes treated with timolol
and placebo
No statistically significant difference in
myopia progression
masking was done, and this could have led to investigator bias wherein favorable refractive measurements were made in the bifocal group. An analysis of
three studies by Grosvenor et al., Roberts and Banford,
and Goss showed decreased rates of progression of
myopia in patients with convergent strabismus who
wore bifocals, but no difference in rates in patients
with no strabismus or divergent strabismus who wore
bifocals (92). The Grosvenor and Goss (90) bifocal
study of 112 myopic patients from three optometry
practices in the central United States showed no statistically significant difference in the rate of progression of myopia of —0.44 diopter per year for wearers
of single-vision spectacles and —0.37 diopter per year
for wearers of bifocals. The treatment assignment was
not randomized, and refractive measurements were
No randomization; investigators
measuring outcome not
masked
No randomization; refractive
outcomes from medical
records
Large number of dropouts
No randomization; refraction
measured without cycloplegia
No randomization
No randomization
No randomization; large number
of dropouts
Fellow eye used as control
No randomization
No randomization; small sample
size
Small sample size
elicited from past medical records. A randomized clinical trial (93) in Houston, Texas, placed subjects into
three groups consisting of children wearing singlevision lenses, +1.00 diopter added bifocals, or +2.00
diopters added bifocals based on a table of random
numbers. The mean increase of myopia in the 124
participants was —0.34 diopter per year for the singlevision subjects, —0.36 diopter per year for the +1.00
diopter added bifocal subjects, and —0.34 diopter per
year for the +2.00 diopters added bifocal subjects.
The differences in the rates were not statistically significant. There was a large number of dropouts, with
only 124 of the 207 subjects remaining in the study
after 3 years. In Finland, a randomized clinical trial in
which children aged 8-13 years were assigned to the
use of bifocal lenses, continuous use of single-vision
Epidemiol Rev Vol. 18, No. 2, 1996
Epidemiology of Myopia
spectacles, or use of single-vision spectacles only for
distant vision showed no significant difference between rates of progression in the three groups (76).
Contact lenses
Rigid contact lenses have been used in several clinical trials, as it is postulated that these lenses retard
myopia progression by causing corneal flattening. One
of the first studies to assess' the possible effects of
contact lenses on the rate of progression of myopia
was conducted by Stone in the London Refraction
Hospital, where 120 children were followed for 5
years (94). However, the subjects were not randomized into contact lens and spectacle groups, and myopia was measured with noncycloplegic refraction. The
increase in myopia among the contact lens wearers
was 0.10 diopter per year as compared with 0.36
diopter per year for the spectacle wearers. Andreo (95)
studied a small sample of 56 patients who were wearing spectacles or hydrophilic contact lenses over a
period of approximately 12 months, and the results
showed no statistically significant difference in the
rates of progression between the two groups. As with
Stone's study, the subjects were not randomized to the
two different groups. A study by Grosvenor et al. (96)
used gas-permeable contact lenses in 100 myopic children and compared them with another nonrandomized
age-matched group of spectacle-wearers. They found
an increase in myopia of 0.14 diopter in the contact
lens group compared with 0.40 diopter in the spectacle
group in this nonrandomized study. Grosvenor et al.
noted that upon discontinuation of contact lens wear,
myopia progression increased. However, the reduction
in myopia progression was not accounted for entirely
by corneal flattening as measured by the keratometer.
The researchers concluded that the keratometer did not
provide an accurate assessment of corneal flattening
from contact lens wear (97). However, another Houston study (98) fitted 100 children with silicone-acrylate contact lenses and made comparisons with 20
spectacle-wearing children matched by age and initial
amount of myopia over a 3-year period. The myopia of
the contact lenses wearers progressed at a statistically
significantly slower rate of 0.16 diopter per year, compared with 0.51 diopter per year in the spectacle wearers. However, there was a large dropout rate, with only
56 of the original 100 children fitted with contact
lenses remaining in the study at the end of 3 years, and
there was no randomization of treatment assignments.
Atropine eyedrops
Another putative method of myopia control is the
daily instillation of a long-acting cycloplegic agent
Epidemiol Rev Vol. 18, No. 2, 1996
183
such as atropine to decrease ocular accommodation.
Several past clinical trials did not randomize subjects,
and dropout rates were high. The findings were often
equivocal and inconclusive (99-101). Bedrossian's
study (99) involving 75 subjects aged 7-13 years used
the other eye as a control. Bedrossian found that 112
of the 150 atropine-treated eyes had no change or a
decrease in myopia, whereas in the control eyes, only
four had no change or a decrease in myopia. Kao et al.
(102) studied the effect of 1 percent atropine ointment
on the progression of myopia in Taiwanese schoolchildren with myopia of more than —0.5 diopter. A total
of 40 schoolchildren received 1 percent atropine ophthalmic ointment in both eyes every night for the
duration of 1 year; 40 similarly myopic schoolchildren
wearing spectacles but not receiving atropine treatment served as controls. The authors found that 51.3
percent of the treated group showed no progression of
myopia, and only 10 percent showed progression of
greater than 0.5 diopter. By contrast, in the control
group, 12.5 percent showed no myopia progression
and 62.5 percent showed progression of greater than
0.5 diopter.
Intraocular pressure reduction using betablocking agents
Intraocular pressure could be an important mediator
of scleral stress, causing axial elongation of the eyeball and resultant myopia (103). On the basis of this
hypothesis, pharmacologic agents which lower the intraocular pressure may have an effect in retarding the
progression of myopia. Hosaka (104) conducted a
small study in which 20 Japanese children aged 6-14
years were treated with 0.25 percent timolol maleate (a
beta-blocker) twice daily, another 50 subjects were
treated with 0.5 percent or 0.25 percent labetalol eyedrops (another beta-blocker) twice daily, and other
subjects were treated with placebo. With a short
follow-up period of only 2-5 months, Hosaka found a
statistically significant difference in the progression of
myopia between the labetalol-treated eyes and the eyes
treated with placebo, whereas no such difference was
found in timolol maleate-treated eyes and placebotreated eyes. Jensen (105), in a preliminary report
published in 1988, studied the effect of timolol maleate in the control of myopia in 9- to 12-year-old
schoolchildren in Denmark. A total of 159 schoolchildren were randomly allocated to one of three groups:
a control group, a group with bifocal spectacles, and a
group with 0.25 percent timolol eyedrops instilled
twice daily. Timolol maleate was found not to have
any statistically significant effect in slowing the progression of myopia in these schoolchildren (106).
Thus, it can be inferred that there has been no conclu-
184
Saw et al.
sive evidence that beta-blocking agents help to retard
myopia progression.
The available interventions are limited by their side
effects, and there has been inconclusive evidence from
present intervention studies. Atropine instillation may
occasionally result in side effects such as atropine
dermatitis, allergic reactions to atropine, and chronic
pupillary dilation leading to cataract, and it has been
reported that the myopia tends to resume at a faster
rate once the eyedrops are withdrawn (107). Furthermore, the compliance rate is low, as the individual has
to instill eyedrops daily over long periods of time and
is unable to read without bifocals if the drops are
instilled in both eyes. Beta-blocking agents need to be
instilled in the eye daily, with possible side effects and
a low compliance rate. The results of clinical trials
using beta-blocking agents have not been conclusive.
Bifocals do not cause much discomfort for wearers.
However, the randomized trials of bifocals have not
showed any slowing of myopia progression. There
was some slowing of myopia progression with the use
of contact lenses, but the trials were not randomized.
Future research should be directed at interventions
such as the use of rigid gas-permeable contact lenses,
with the emphasis on well-designed randomized clinical trials with adequate sample sizes and accurate
refractive measurements.
CONCLUSIONS
Myopia is an ocular condition with a high prevalence in many parts of the world. The relative contribution of genetic and environmental factors to the
development and progression of myopia is not fully
understood (108). There are several questions that
remain unanswered. To what extent does closeup work
contribute to the increased prevalence of myopia in
Japan, Taiwan, Hong Kong, Singapore, and the United
States? Is the difference in myopia prevalence in different ethnic groups due primarily to genetic factors or
to environmental influences? How much of myopia is
genetically determined, and how do environmental
factors alter the onset and progression of myopia? Is
closeup work an equal risk factor for both the onset
and the progression of myopia? Is the age of onset of
myopia important? Are there different risk factors for
high and low myopia?
Recent studies have shown that a family history of
myopia and closeup work are the two strongest risk
factors. The relation between closeup work and genetic factors, as well as the interaction between these
two variables, should be further studied. It has been
suggested that in populations with little exposure to
closeup work, genetic factors play an important part in
the development of myopia, while in populations
where closeup work is common, there is a high prevalence of myopia and genetic factors do not have a
large influence (52).
Over the past few decades, epidemiologic studies
have been mainly cross-sectional in nature, with poor
documentation of the temporal relation between risk
factors and myopia. Confounding variables were not
examined, refraction was measured without cycloplegia, and the different components of refraction, such as
axial length and corneal curvature, were not measured
directly. The definition of myopia has varied widely,
sample sizes have been insufficient, and longitudinal
follow-up has been poor. Well-designed concurrent
cohort studies with accurate instruments for measuring
closeup work, other risk factors, and refractive outcomes will provide us with further insights into the
environmental causes of myopia. Closeup work is
difficult to quantify, and much more study is needed to
obtain precise estimates of amounts and types of
closeup work and the environmental conditions under
which closeup work is done. Future studies may examine the effects of reading English and Chinese
characters, as well as the direction of eye movements,
whether vertical or right-to-left. Tools for closeup
work assessment, mainly questionnaires and diaries,
may be administered repeatedly over different time
periods in order to document seasonal variations in
closeup activities that result from factors such as
school examinations or vacations. In children with
active accommodation reflexes, refraction with cycloplegia is essential. The availability of instruments for
biometric measurements of the eye will enable us to
better understand mechanisms of myopia onset and
progression.
Twin and familial correlation studies have supported the hypothesis of a genetic component of myopia causation. However, the exact mode of inheritance is uncertain, and marker studies have been few.
Further research should be directed at linkage-analysis
studies and the identification of myopia gene markers.
A better understanding of the risk factors for myopia
would enable better public health interventions, such
as health education efforts, to advise the public about
the types and circumstances under which closeup work
could accelerate myopia onset and progression. Cohort
studies examining the effects of changes in lighting,
types of closeup work, distance from reading material,
or type sizes could provide a basis for specific closeup
work interventions in the future. Potential interventions for the prevention of the onset and progression of
myopia should be subjected to rigorously performed
randomized clinical trials.
Epidemiol Rev Vol. 18, No. 2, 1996
Epidemiology of Myopia
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