jurnal-progress1 (nearwork tdk)

ORIGINAL ARTICLE

Factors Related to the Progression of Myopia inSingaporean Children

SEANG-MEI SAW, MBBS, MPH, PhD, MD, F. JAVIER NIETO, MHS, PhD, JOANNE KATZ, ScD,OLIVER D. SCHEIN, MD, MPH, BRIAN LEVY, OD, MSc, and SEK-JIN CHEW, FRCS, PhD

Department of Community, Occupational, and Family Medicine, National University of Singapore, Faculty of Medicine, Singapore (S-MS), Departments of Epidemiology (FJN) and International Health (JK), Johns Hopkins University School of Hygiene and Public Health,

Baltimore, Maryland, Dana Center for Preventive Ophthalmology, Wilmer Eye Institute, Johns Hopkins University School of Medicine,Baltimore, Maryland (ODS), Global Biological and Clinical Research, Bausch & Lomb, Rochester, New York (BL), and Singapore Eye

Research Institute, Singapore (S-JC)

ABSTRACT: Purpose: To examine the possible factors related to the progression of myopia in Singapore children.Methods: One hundred fifty-three Singapore children aged 6 to 12 years were recruited to participate in a concurrentcohort study of the risk factors for the progression of myopia. Socioeconomic status, outdoor activity, and near-workactivity were documented in a face-to-face clinic interview. The changes in cycloplegic subjective refraction andautorefraction were ascertained with the use of a Nidek ARK 900 over a 2-year period. Results: The average rate ofprogression of myopia as measured by subjective refraction was 20.59 D per year (95% confidence interval 20.52,20.66). Younger children and children who were more myopic at the beginning (refractive error worse than 22.0 D)of the study had higher myopia progression rates. Conclusions: Myopia progression was faster for younger children andfor children who had more severe myopia at baseline. Socioeconomic status and near-work activity were not relatedto myopia progression. (Optom Vis Sci 2000;77:549–554)

Key Words: reading, subjective refraction, cohort, epidemiology

Myopia is a seemingly benign ocular disorder. However,severe myopia is associated with potentially blindingconditions such as pigmentary degeneration, retinal de-

tachment, myopic macular degeneration, cataract, and glaucoma.1

The economic costs of correction for myopia with spectacles andcontact lenses as well as the costs of optometry visits amount tobillions of dollars per year.2 Myopia is an especially importantpublic health problem in Asian countries such as Taiwan, Japan,Hong Kong, and Singapore, where myopia rates are high and havebeen increasing over the past few decades.3–5

Environmental and genetic factors have both been postulated tocause myopia.5–10 The near-work hypothesis is substantiated byanimal studies, which suggest that visual cues determine axiallength, and cross-sectional studies of near work and myopia.11–17

Several studies have found an association between socioeconomicstatus, education, academic achievement, and myopia.8–10

To address these issues, a cohort study was undertaken to deter-mine the progression of myopia in preteen myopic Singaporeanchildren and the possible factors related to the progression of my-opia in schoolchildren.

MATERIALS AND METHODS

Study Population

Three hundred eleven children aged 6 to 12 years were volun-teers recruited to participate in a randomized clinical trial of theeffect of rigid contact lenses on the progression of myopia in Sin-gapore children. A concurrent cohort study of the risk factors formyopia progression, which was nested in the randomized clinicaltrial, included 153 children who were randomized to the standardtreatment (spectacles). The children were volunteers from all partsof the island recruited by several press releases and from the Re-fraction Clinic of the School Health Services from March 1996 toMay 1997. The research protocol was approved by the Committeeon Human Research, Johns Hopkins University School of Hy-giene and Public Health. The Ethics committee, Singapore Na-tional Eye Center, also approved this study.

Eligible children had myopia of 21.0 to 24.0 D (sphere) withno other ocular or medical condition or previous contact lens wear.Children with astigmatism of more than 22.0 D were also ex-cluded from the study. Informed consent was obtained after the

1040-5488/100/7710-0549/0 VOL. 77, NO. 10, PP. 549–554OPTOMETRY AND VISION SCIENCECopyright © 2000 American Academy of Optometry

Optometry and Vision Science, Vol. 77, No. 10, October 2000

study was explained to each parent and verbal consent obtainedfrom each child during the first screening eligibility visit. Thelength of follow-up of the 153 children was 13 to 40 months.

Sample Size Estimations

Assuming a Type I error rate of 0.05, power of 0.8, the samplesize required to detect a 25% difference in myopia progressionrates for children who are exposed to the risk factor compared withchildren who are not exposed was 130, and the SD for refractiveerror was 0.50 D.

Near-Work Activity Measured by theQuestionnaire

An in-person interview was conducted in either English or Chi-nese during the first screening clinic visit by the same examiner(SMS). The parents were interviewed exclusively in this study. Apretested questionnaire was used and the interview lasted about 15min (available from the authors). The response rate was 100%.The parents were asked to recollect the amount of time in hours perday spent on near-work activities on an average day. The types ofnear-work activity assessed include reading and writing, computeruse, and playing video games. Questions on the amount of nearwork during an average school term weekday, school term week-end, before examinations, and during the school holidays wereasked separately. The primary analysis was conducted with rawnear work hours using school weekday hours. Near work at schoolperformed by the 153 children was estimated with a school diaryfilled by out a subgroup of the 153 children (n 5 119) from morethan 100 different schools during one school session (about 6 h perday in the morning or afternoon). All parents of the 153 childrenwere interviewed, but only 119 participants completed the schooldiary. The average estimate of near-work activity for the 119 chil-dren of each age was used to estimate near-work activity for theremaining children of similar ages in the study. Thus, raw nearwork hours was obtained by adding the hours spent at home andthe hours spent at school on near work during a school weekday.

A secondary analysis was conducted with weighted near worktime. Weighted time was obtained using the scheme: total nearwork on school weekdays (home near work plus school near work)was weighted by 0.5, school weekends (home near work) by 0.2,examination time (home near work plus school near work) by 0.1,and school holidays (home near work) by 0.2. This weightingscheme was based on the proportion each time period contributedto the entire school year. For example, in a typical school year, theschool terms are altogether 8.5 mo (70%), examinations 1 mo(10%), and vacation time 2.5 mo (20%). For the 8.5-monthschool term (70% of the school year), weekdays amount to 5 of 7days and weekends to 2 of 7 days. Thus, 50% of the entire schoolyear is school weekday time, and 20% school weekend time. Ad-ditionally, the distance of paper from the eyes while the child waswriting was measured (in centimeters) using a ruler at the clinicsite.

Reproducibility of Near-work Activity Measures

The parents of 30 volunteer children in the cohort study wereasked to complete the questionnaire in the same fashion twice over

a 2-week period, and the intraclass correlation coefficient for totalnear-work activity was 0.87 (95% confidence interval,0.85–0.91).18 The parents of consecutive participants were se-lected over a 2-day period.

Other Lifestyle Factors

Information on other confounders such as sex, age, and outdooractivities (number of hours per week of outdoor activity) were alsoobtained. Socioeconomic status was evaluated by father and moth-er’s education (no formal education, primary, secondary, preuni-versity or diploma or tertiary); housing type (one- or two-roomapartment, three- or four-room apartment, five-room apartment,private housing); and total monthly family income (all categoricalvariables). Information on parental history of myopia was obtainedfrom the interview by asking one parent whether any of the child’sparents were near-sighted.

Refractive Outcomes

Cycloplegic refractive error measurements were documented ev-ery 6 months. The refractive examinations were conducted 30 minafter the third drop of 1% Cyclogyl Ophthalmic solution wasinstilled, whereby one drop of cycloplegic solution was adminis-tered at 5-min intervals for a total of 15 min. Myopia was recordedin increments of 0.25 D. The primary outcome is the change incycloplegic subjective refraction. Cycloplegic autorefraction mea-surements were also measured with the Nidek ARK 900 (NidekCo. Ltd, Tokyo, Japan), where the average of three measurementsper eye was taken. We used the Generalized Estimating Equations(GEE) to evaluate change in refractive error measurement (spher-ical equivalent) every 6 months. Additionally, we measured the rateof myopia progression (difference between final and initial refrac-tive error measurement) annualized to produce diopters per year ofchange in refraction. Even though the length of follow-up variedfor the 153 children, we included all 153 children in our studyanalysis.

Statistical Analysis

The major outcome of the analysis of change in refraction wasbased on mean equivalent sphere (sphere power 1 half cylinderpower). The mean refractive error (spherical equivalent) changeper year as measured by subjective refraction and autorefractionwas evaluated for children of different gender, age, age of firstspectacle wear, astigmatism, and different socioeconomic status.We studied the relationship between myopia progression and near-work activity using the raw, unweighted hours of near work, as wellas weighted time on near work. We wanted to study the longitu-dinal trend in refractive error, but because each visit is highlycorrelated, the generalized estimating equation (GEE) method wasused to model the outcome, which was the change in refractiveerror (spherical equivalent) at each 6-month visit (difference be-tween refractive error at each visit and the baseline refractive error)as a function of the risk factors.19 Only data from the right eye waspresented because the results from right and left eyes were similar.Both univariate and multivariate analyses were performed usingStata Version 5.0 (Stata Corporation, College Station, TX).

550 Myopia Progression in Singaporean Children—Saw et al.


RESULTS

The mean and median follow-up periods of the study for the153 children were 28 and 27.7 months, respectively. Childrenwith longer follow-up times had similar ages but were more likelyto be male. Table 1 shows the characteristics of the study popula-tion. There were 92 boys and 61 girls in the study. The mean ageof the children was 8.5 years. Thirty-nine (25.7%) of the children’sfathers had tertiary education and 51 (33.3%) lived in privatehousing. In contrast, 9.4% of Singapore men had tertiary educa-tion and 15.4% lived in private houses.20 One hundred and one(66%) of the fathers were myopic and 100 (65.4%) of the motherswere myopic. These prevalence rates were higher than the preva-lence rate of 38.7% in Singapore adults above 40 years.21 Themedian raw near-work activity was 5.8 h/day (range, 0.5 to 10.9).

Fig. 1 shows the distribution of the change in right eye subjec-tive refraction per year, whereby the rate of progression of myopiawas greater than 20.5 D per year in 88 (55.4%) children. Fig. 2shows the distribution of the change in right eye autorefraction per

year and 91 (57.2%) children had a rate of progression of myopiagreater than 20.5 D per year. The mean changes in subjectiverefraction per year and autorefraction per year for children withdifferent characteristics at baseline were presented in Table 2. Theaverage change in subjective refraction for total subjects was 20.59D per year (20.56 D per year in boys, and 20.66 D per year ingirls). Younger children had faster rates of myopia progression asmeasured by subjective refraction (p , 0.0001) and autorefraction(P , 0.0001). The average change in refractive error for childrenwhose initial refractive error was 22.0 D or more was 20.65 D peryear compared with 20.56 D per year for those whose initialrefractive error was less than 22.0 D. This was statistically signif-icant for the change in autorefraction readings (p , 0.0001). Thechanges in refractive error per year for children with different base-line astigmatism measures, incomes, and types of housing weresimilar.

Table 3 presents the regression coefficients and 95% confidenceintervals of the generalized estimating equation (GEE) models thatmodel the difference in refractive error at each 6-month visit andthe baseline visit with the different covariates of interest includingage, gender, raw near work, total weighted near work time, rawreading and writing hours, total weighted reading and writing

TABLE 1.Characteristics of 153 Singaporean children aged 6 to 12years who participated in the cohort study of the factorsrelated to the progression of myopia, 1996 to 1999

Mean 6 SD N (%)

Age at first clinic visit (years) 8.5 6 1.4Gender

Male 92 (60.1)Female 61 (39.9)

Father’s educationNo formal education 0 (0.0)Primary 11 (7.3)Secondary 50 (32.9)Preuniversity or diploma 52 (34.2)Tertiary 39 (25.7)

Mother’s educationNo formal education 3 (2.0)Primary 5 (3.3)Secondary 84 (55.3)Preuniversity or diploma 44 (29.0)Tertiary 16 (10.5)

HousingOne/two room apartment 2 (1.3)Three/four room

apartment46 (30.1)

Five room apartment 54 (35.3)Private housing 51 (33.3)

Total family income permonth (1.6 Sin$ 5 1

US$),2000 16 (10.5)2001–5000 64 (41.8).5000 73 (47.7)

Father with myopiaYes 101 (66.0)No 52 (34.0)

Mother with myopiaYes 100 (65.4)No 53 (34.6)

FIGURE 1.Change in right eye subjective refraction 6 spherical equivalent per yearfor the children in the study, Singapore, 1996 to 1999.

FIGURE 2.Change in right eye autorefraction 6 spherical equivalent per year for thechildren in the study, Singapore, 1996–1999.

Myopia Progression in Singaporean Children—Saw et al. 551


time, reading distance, outdoor activity, total combined familyincome, type of housing, and parental history of myopia. All mod-els adjust for age, gender, and parental history of myopia. Childrenwho were younger had higher rates of change in cycloplegic sub-jective refraction for the different visits. Children with myopicparents had larger changes in myopia rates. There were no statisti-cally significant associations between cycloplegic subjective refrac-tion change and total or raw near-work activity, reading and writ-ing, reading distance, total combined family income, or type ofhousing.

DISCUSSION

To our knowledge, this is one of the first concurrent cohortstudies conducted in Asia that studies detailed assessments of thedifferent environmental risk factors for the progression of myopia.There are many factors that may contribute to the different indi-vidual variations in myopia progression.22 Near-work activity maybe one factor related to myopia progression; objects that are viewednearby may cause the eye to elongate further to maximize thesharpness of images on the retina.12–13 A positive association hasbeen found for near-work activity and myopia in several cross-sectional studies.5, 15–17, 23–24 There have also been positive corre-lations between socioeconomic status and myopia.8–10

The average rate of progression of myopia as measured by cy-cloplegic subjective refraction in the right eye was 20.59 D per

year, and 20.60 D per year as measured by autorefraction. Theserates are slightly higher than the average rates of progression ofmyopia of 20.46 D in Hong Kong children aged 6 to 17 years,20.41 D per year in men and 20.46 D in women aged 6 to 15years in the United States, and 20.55 D per year in Finnish chil-dren aged 7 to 15 years.25–27 The age groups of the study popula-tions vary and measurements of myopia progression rates may notbe exactly comparable. Younger children were found to havehigher myopia progression rates.27–28 Children who were moremyopic at the beginning of the study were found to have morerapid myopia progression rates.29–30 Similar results were found byBraun et al., whereby younger children and children with morethan 1 D myopia at baseline were found to have faster myopiaprogression.31

Socioeconomic status of the family as denoted by family incomeor housing did not predict the rate of progression of myopia. Onlya small proportion of parents had completed primary education(elementary school) only (11 fathers and 5 mothers), thus ourstudy population has a rather uniform socioeconomic status.Therefore, our population may not be suitable for determiningpossible modest associations between socioeconomic status andmyopia progression. There was also no relationship between out-door activity and myopia progression.

Our study may not find a relationship between near-work ac-tivity and the progression of myopia for several reasons (other than

TABLE 2.Sociodemographic factors and the change per year of cycloplegic subjective refraction and autorefraction for 153children in our study, 1996 to 1999

Mean change per year forsubjective refraction

Mean change per yearfor autorefraction

Mean 6 SD Mean 6 SDTotal subjects 20.59 6 0.44 20.60 6 0.41Gender

Male 20.56 6 0.40 20.59 6 0.39Female 20.66 6 0.42 20.60 6 0.51

Age (years at the start of the study)7 and below 20.85 6 0.41 20.85 6 0.418 20.62 6 0.31 20.66 6 0.339 20.51 6 0.33 20.51 6 0.4010 20.32 6 0.42 20.32 6 0.3811 and above 20.36 6 0.24 20.20 6 0.29

Astigmatism (More than 0.5 D)Yes 20.60 6 0.42 20.57 6 0.42No 20.60 6 0.40 20.62 6 0.46

Initial refractive errorLess than 22.0 D 20.56 6 0.38 20.50 6 0.4422.0 D or more 20.65 6 0.44 20.70 6 0.41

Total family income (1.6 Sin$ 5 1 US$),$2000 per month 20.49 6 0.42 20.58 6 0.41$2000 to $5000 per month 20.61 6 0.42 20.56 6 0.44.$5000 per month 20.63 6 0.41 20.62 6 0.46

HousingOne/two room apartment 20.49 6 0.17 20.80 6 0.20Three/four room apartment 20.49 6 0.44 20.54 6 0.45Five room apartment 20.74 6 0.41 20.70 6 0.42Private housing 20.57 6 0.37 20.51 6 0.45



the fact that such a relationship might not exist). An association ofnear-work activity and myopia may represent a threshold effectwhere, for example, virtually all the risk is assumed once the expo-sure (near-work activity) exceeds 3 or 4 h. Near-work activity mayalso be related to the onset rather than the progression of myopia.

This study conducted in Singapore has several unique advan-tages. Cycloplegic refractions were determined by both autorefrac-tion and subjective refraction in 153 myopic children. Further-more, the optometrists who performed the refractivemeasurements were masked to the different factors as measured inthe interview. There was no interviewer bias, because lifestyle fac-tors were quantified prospectively before progression rates of my-opia were ascertained. However, a limitation is that any changes innear work cannot be associated with changes in myopia progres-sion. The length of follow-up varied for each child. Children whoremained in the study for a longer period were more likely to bemale compared with children who left the study earlier. However,the length of follow-up for each child did not determine myopiaprogression, even after adjustment for gender. This study may havelimited generalizability, because the children participating in thestudy are volunteers in a clinical trial with different socioeconomicbackgrounds compared with the Singapore population. However,it is likely that the exposure-outcome relationship remains intact indifferent populations.

In summary, younger children and children with higher degreesof refractive error at baseline had faster myopia progression rates.

No definite relationship between socioeconomic status, near-workactivity, and myopia progression was found in preteen myopicSingapore children who participated in a myopia clinical trial.Migration studies, in which migrants may adopt certain lifestylesof the new country, may provide us with some information onboth the environmental and genetic factors that may contribute tomyopia. The incidence of myopia may be determined in a largecohort with a follow-up period of 3 to 5 years.

ACKNOWLEDGMENT

This study was supported by the Johns Hopkins Summer Epidemiology Pro-gram and Bausch & Lomb, Inc, Rochester, New York. Dr. Saw was supportedby the National Medical Research Council-Shaw Foundation Research Fel-lowship from October 1996 to May 1997.

Received January 19, 2000; revision received June 20, 2000.

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TABLE 3.Rate of change in cycloplegic subjective refraction readings as the response variable and the factors as the main exposuresof interest, adjusting for age, sex, and parental history of myopia using the generalized estimating equation (GEE model)in a cohort study of Singaporean children, 1996 to 1999 (n 5 153).

Regressioncoefficient

95% CIa 2-sided p

Age 6 years 0.19 0.13, 0.25 ,0.0001Gender

Males Reference ReferenceFemales 20.13 20.31, 0.05 0.16

Raw total near work 6 hours per day on school weekday 0.023 20.18, 0.063 0.27Weighted time on near work 6 hours per day 0.013 20.023, 0.048 0.49Raw total reading and writing 6 hours per day on school

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,$2000 Reference Reference$2000–$5000 20.049 20.36, 0.26 0.76.$5000 20.063 20.37, 0.24 0.69

Type of houseOne or two room apartment Reference ReferenceThree or four room apartment 0.32 20.45, 1.08 0.42Five room apartment 0.11 20.65, 0.88 0.77Private housing 0.36 20.41, 1.13 0.35

Number of parents with myopiaNone Reference ReferenceOne 20.38 20.64, 20.11 0.005Two 20.43 20.69, 20.18 0.001a 95% confidence intervals 6 CI based on the GEE robust estimates of the standard error.

Myopia Progression in Singaporean Children—Saw et al. 553


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Seang-Mei SawDepartment of Community, Occupational and Family Medicine

National University of Singapore16 Medical DriveSingapore 117597

Email: [email protected]



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