Icidece ad Progressio of Astigmatism i Sigaporea Childre Louis Tog, 1,2 Seag-Mei Saw, 1,2,3,4 Yu Li, 2 Kee-Seg Chia, 3 David Koh, 3 ad Doald Ta 1,2,4 PURPOSE. This study ivestigated the icidece ad progressio, as well as factors associated with chages i astigmatism i school childre. METHODS. This was a prospective cohort study. Childre 7 to 9 years of age, of Chiese, Malay, ad Asia Idia ethicity, were examied aually over a 4-year period. Cycloplegic autorefractio was performed. A questioaire was used to evaluate risk factors for icidece ad progressio of astigmatism. RESULTS. The cumulative 3-year icidece rate of astigmatism was 33.6% (cylider power of 0.5 D or worse) or 11.5% (cylider power of 1.0 D or worse). Myopic childre had a higher icidece rate of astigmatism tha omyopes (P 0.001). The mea J0 chage per year was 0.012 D (95% CI: 0.007 0.018), whereas J45 did ot show a sigificat chage each year (mea, 0.001 D per year). Chiese childre had greater worseig of J0 per year (P 0.001). Girls also had sigificatly greater progressio of J0 tha did boys (P 0.001). Similarly, myopia at baselie (P 0.001) ad the hours of computer use (P 0.049) were associated with a greater progressio rate of J0. J0 teded to improve i childre with compoud hyperopic astigmatism, worse i childre with compoud myopic astigmatism, ad remai stable i mixed astigmatics. CONCLUSIONS. Although there was miimal progressio of astigmatism i school age childre (0.44 0.53 D) over this period of follow-up, icidet cases of astigmatism ( 1.0 D) were ot ucommo. The progressio rate of astigmatism was affected by the ethicity, presece of myopia, axis, ad subtype of astigmatism. (Ivest Ophthalmol Vis Sci. 2004;45:3914 3918) DOI:10.1167/iovs.04-0492 Astigmatism is a importat refractive coditio i childre 1 because it imposes cosiderable optical defocus at all viewig distaces durig the years of ocular maturatio. Although debatable, 2 astigmatism is a possible cause of meridioal amblyopia. 3 I additio, there is evidece of difficulty of correctig high astigmatism i cliical practice, leadig to rejectio of eye wear i childre, with a decrease i classroom performace as a result of usatisfactory visio. 4 From the 1 Sigapore Natioal Eye Ceter, the 3 Departmet of Commuity, Occupatioal, ad Family Medicie, ad the 4 Departmet of Ophthalmology, Natioal Uiversity of Sigapore, Sigapore; ad the 2 Sigapore Eye Research Istitute, Sigapore. Supported by Sigapore Eye Research Istitute Grat MG/97-04/ 005 ad Natioal Medical Research Coucil Grat 0695/2003. Submitted for publicatio May 4, 2004; revised Jue 1 ad Jue 23, 2004; accepted July 1, 2004. Disclosure: L. Tog, Noe; S.-M. Saw, Noe; Y. Li, Noe; K.-S. Chia, Noe; D. Koh, Noe; D. Ta, Noe The publicatio costs of this article were defrayed i part by page charge paymet. This article must therefore be marked advertisemet i accordace with 18 U.S.C. 1734 solely to idicate this fact. Correspodig author: Louis Tog, Sigapore Natioal Eye Ceter, 11 Third Hospital Aveue, Sigapore 168751; louistog@hotmail.com. The axes of astigmatism differed betwee races, 5 suggestig a heritable compoet. A more recet study showed the importace of domiat iheritace i astigmatism. 6 Compared with myopia, the proportio of the variace of astigmatism explaiable by geetic factors appeared to be relatively lower. 6,7 I ifats, the severity of astigmatism was reduced from 1 to 3 years. 8 10 A study of astigmatism from ifacy to late childhood showed that the mea cylidrical power remaied stable betwee 7 ad 9 years of age, but icidece rates of astigmatism durig school age were ot reported. 11 The temporal chages i astigmatism withi a populatio over a few decades had bee studied, 12 but these studies had ot bee performed withi a cohort. Therefore, the chage i astigmatism costituted the least studied aspect of refractive error developmet i school age childre. Oe populatiobased icidece study ivolvig a repeat examiatio after 2 years was performed i the Shuyi District i Chia 13 i subjects betwee 5 ad 13 years of age. I that study, the mea chage i the power of the astigmatic cylider was foud to be 0. However, the progressio aalysis did ot cosider the astigmatic cyliders to be vector quatities, 14 therefore limitig the usefuless of the coclusios. We coducted a study to evaluate the icidece ad progressio rate of astigmatism i a cohort of school childre i Sigapore ad to ivestigate the possible epidemiologic associatios of these logitudial chages. METHODS Desig ad Patiets The Sigapore Cohort Study Of the Risk Factors for Myopia (SCORM) commeced i 1999. It was desiged to evaluate the risk factors for progressio of myopia i school childre. The details cocerig recruitmet of subjects ad the study procedures have bee published previously. 15,16 All school childre i grades 1 to 3, ages ragig from 7 to 9 years, attedig two Sigapore schools (located o the Easter ad Norther parts of Sigapore) were ivited to participate. Exclusio criteria icluded serious medical coditios such as leukemia or heart disorders ad allergies to eye drops. A total of 1019 childre, 507 (49.75%) boys ad 512 (50.25%) girls participated i the baselie examiatio visit. There were 528 7-year-olds, 325 8-year-olds, ad 166 9-year-olds with 737 Chiese, 229 Malay, ad 53 Asia Idia childre. Thereafter, yearly follow-up visits were scheduled for all participats. Amog the participats, 842 had three follow-up visits, whereas 38 were without ay follow-up. There were o statistically sigificat differeces i age, geder, ad baselie refractio i the childre who remaied i the study throughout the 3 years ad childre who were ot i the study at the ed of 3 years ( 177). However, the childre who were lost to follow-up were less likely to be Chiese (54.6%), compared with childre who remaied throughout the 3 years. Figure 1 shows the umber of subjects recruited ad the umber of childre with refractive data aalyzed at the ed of the 3-year study period. This study was approved by the ethics committee of the Sigapore Eye Research Istitute, ad all procedures adhered to the Declaratio of Helsiki. Iformed writte coset was obtaied from parets of the childre. Ivestigative Ophthalmology & Visual Sciece, November 2004, Vol. 45, No. 11 3914 Copyright Associatio for Research i Visio ad Ophthalmology Dowloaded From: https://iovs.arvojourals.org/pdfaccess.ashx?url=/data/jourals/iovs/932923/ o 11/08/2018
IOVS, November 2004, Vol. 45, No. 11 Astigmatism i Sigaporea Childre 3915 J45 (axis at 135 ) i the right eye ad positive J45 i the left eye (axis at 45 ) produces a A-patter symmetry. Compoud hyperopic astigmatism was defied as refractios with positive powers i each meridia, ad compoud myopic astigmatism was defied as those with egative powers i each meridia. Mixed astigmatism referred to cases with positive power i oe meridia ad egative power i the other. Statistical sigificace was set at the level of 0.05. As ot all childre had the same umber of follow-up visits, aalyses comparig subgroups were repeated with mea values weighted for the umber of visits. The umber of childre eligible for the icidece of astigmatism i each of the visits is show i Figure 1. All aalyses referred to the values for the right eyes, except for the symmetry of astigmatism aalyses, which aturally ivolve both eyes. Statistical aalysis was coducted usig the commercially available software (SAS, Cary, NC). RESULTS FIGURE 1. Study Procedures The same procedures were performed at aual school visits. The cycloplegic regime cosisted of 1 drop of 1% cyclopetolate repeated twice at 5-miute itervals. All measuremets were performed at least 30 miutes after the last eye drop istillatio. Cycloplegic autorefractio ad autokeratometry i two perpedicular meridias were performed (RK-5; Cao Ltd., Tochigike, Japa). Cycloplegic autorefractio was the most suitable method of logitudial measuremet i epidemiologic studies. 17 Data o demographic variables ad computer use were obtaied from a detailed eight-page questioaire admiistered maily to assess the risk factors of myopia. The questioaire was completed by the parets of the childre. Oly the data from the baselie questioaire were used i the study. Data Aalysis The follow-up status of the study populatio. Astigmatism was defied as a cylider power of at least 0.5 or 1.0 D. Myopia was defied as a spherical equivalet of at least 0.5 D. J0 ad J45 were Fourier-trasformed vectors 18 derived from the average of five refractios, as described i a previous report. 14 Briefly, if C is the cylidrical power, ad A the axis of the cylider i the spherical/ cylider otatio, J0 (Ccos2A)/2 ad J45 (Csi2A)/2. The cumulative icidece rate of astigmatism was defied as the proportio of participats with astigmatism preset at the 3-year follow-up examiatio i subjects without astigmatism at the baselie visit. The age-, geder-, ad race-specific cumulative icidece rate of astigmatism ad 95% cofidece itervals (CI) were calculated. The multivariate adjusted hazard ratios ad 95% CI were derived with the Cox proportioal hazards model. Chage i J0 or J45 was defied as the J0 or J45 at the baselie examiatio subtracted from the fial examiatio divided by the duratio of follow-up i years. Multiple liear regressio models were costructed with chages i refractio as the depedet variable ad the relevat demographic factors as covariates. For progressio studies, oly subjects with at least two visits were icluded. For studies ivestigatig the sig of J0, the J0 may have bee exactly 0 i the baselie visit, i which case the sig of J0 i the secod visit was used i the aalysis. The A ad V patters of astigmatism symmetry betwee right ad left eyes were defied as i a previously published report. 19 Briefly, a positive J45 (axis at 45 ) i the right eye ad egative axis i the left eye (axis at 135 ) refer to a V-patter symmetry, whereas a egative The childre ( 1019) were examied at the baselie visit. Amog the iitial 1019 childre, 197 (19.3%) had cylider powers of 1.0 D or worse. Amog the remaiig 822 childre, 680 childre atteded a 3-year follow-up visit (Fig. 1). For studets who were examied i all follow-up visits ( 680), the mea cylider at baselie was 0.44 D (SD: 0.25), ad media cylider was 0.43 D (rage, 0.00 1.00). At the last follow-up visit, the mea cylider was 0.53 D (SD: 0.41) ad media cylider was 0.45 D (rage, 0.00 2.80). The 3-year cumulative icidece rate of astigmatism (cylider at least 1.0 D) was 11.5% (95% CI: 9.1 13.9; Table 1). Myopic childre at baselie had a higher icidece rate of astigmatism tha omyopes (P 0.001). The hazard ratio of the developmet of astigmatism i myopes compared with omyopes was 5.40 (95% CI: 3.59 8.10), whe cotrollig for age, geder, ad ethicity. I the childre without myopia, the 3-year icidece rate of astigmatism (at least 1.0 D) was oly 4.5% (95% CI: 2.7 6.4), compared with 29.2% (95% CI: 22.7 35.6) i childre with myopia. Whe astigmatism was defied as a cylider magitude of 0.5 D or worse, the 3-year cumulative icidece rate was 33.6% (95% CI: 29.1 38.1). The mea J0 at baselie was 0.28 D (95% CI: 0.25 0.30), whereas the mea J45 at baselie was 0.01 D (95% CI: 0.00 0.02). The mea absolute J45 at baselie was 0.11 D (95% CI: 0.10 0.12). The J0 chage per year was 0.012 D (95% CI: 0.007 0.018; Table 2). Chiese childre had a worseig of J0 that averaged 0.022 D, whereas o-chiese childre had a reductio i J0 by 0.014 D. The differece i the progressio rate of J0 betwee ethic groups was sigificat (P 0.001). Girls also had sigificatly greater progressio of J0 tha did boys (P 0.001). Similarly, myopia at baselie, ad the use of computers were also associated with a greater progressio rate of J0 (P 0.049). The absolute J45 did ot appear to chage over the period of the study: 0.001 D per year (95% CI: 0.001 to 0.004; Table 3). However, the Chiese childre showed a fluctuatio of J45 at 0.004 D per year (95% CI: 0.000 0.007), whereas the o-chiese childre showed 0.005 D per year (95% CI: 0.010 0.000; P 0.007). The myopes also showed a sigificatly icreasig slope compared with omyopes (P 0.003). Ethicity (P 0.02) ad myopia status (P 0.03) were still sigificat whe the aalysis was adjusted for age ad geder. Whe the J0 ad J45 progressio rates were weighted accordig to the umber of follow-up visits, the results were essetially the same (ot show). The rates of progressio of J0 were separately aalyzed i childre with differet subtypes of astigmatism at baselie (i.e., Dowloaded From: https://iovs.arvojourals.org/pdfaccess.ashx?url=/data/jourals/iovs/932923/ o 11/08/2018
3916 Tog et al. IOVS, November 2004, Vol. 45, No. 11 TABLE 1. Icidece Rates of Astigmatism Nomyopes Myopes (Baselie) 3-Year Cumulative Icidece of Astigmatism Multivariate Hazard Ratios of Astigmatism* (Baselie) 3-Year Cumulative Icidece of Astigmatism* Multivariate Hazard Ratios of Astigmatism* All 680 11.5 (9.1 13.9) 268 3.4 (1.2 5.5) Age at baselie (y) 7 342 9.6 (6.5 12.8) 1.00 130 3.9 (0.5 7.2) 1.00 8 232 14.7 (10.1 19.2) 1.24 (0.82 1.85) 97 2.1 (0.0 4.9) 1.64 (0.50 5.37) 9 106 10.4 (4.6 16.2) 0.69 (0.38 1.26) 41 4.9 (0.0 11.5) 1.27 (0.24 6.59) P 0.41 0.23 0.99 0.78 Chiese 502 13.9 (10.9 17.0) 1.00 165 3.6 (0.8 6.5) 1.00 No-Chiese 178 4.5 (1.5 7.5) 0.81 (0.48 1.37) 103 2.9 (0.0 6.2) 1.02 (0.33 3.14) P 0.001 0.42 0.75 0.97 Geder Male 336 10.7 (7.4 14.0) 1.00 142 3.5 (0.5 6.6) 1.00 Female 344 12.2 (8.8 15.6) 1.10 (0.75 1.60) 126 3.2 (0.1 6.2) 0.98 (0.33 2.93) P 0.54 0.62 88 0.015 0.97 Myopia at baselie Nomyopia 488 4.5 (2.7 6.4) 1.00 Myopia 192 29.2 (22.7 35.6) 5.40 (3.59 8.10) Cumulative icidece is expressed as the percetage of childre with astigmatism, with the 95% CI i paretheses. Nomyopes had cylider power of at least 1.0 D. * Adjusted for all other factors i the table. SE at least 0.5 D ad cylider power greater tha 1.0 D. compoud myopic astigmatism, mixed astigmatism, ad compoud hyperopic astigmatism). I those with compoud myopic astigmatism ( 320), the progressio rate was 0.057 D per year (95% CI: 0.047 0.066). I those with mixed astigmatism ( 86), there was o sigificat progressio of J0 (0.000 D/y; 95% CI: 0.019 to 0.020). I those with compoud hyperopic astigmatism ( 552), the J0 showed a reducig tred at 0.012 D/y (95% CI: 0.017 to 0.007). The aalysis for progressio of J0 ad J45 was repeated i those childre with iitial cylider power of 0.5 D or worse ad repeated agai for those with iitial cylider power of 1.0 D, ad essetially the same results were foud (ot show). Most childre with positive J0 (with-the-rule astigmatism) remaied with-the-rule at the ed of the follow-up, with oly 8.3% covertig to agaist-the-rule astigmatism.(table 4). O the cotrary, of the childre with egative J0 (agaist-the-rule TABLE 2. J0 Progressio for All Studets with at Least Oe Follow-up Visit Over 3 Years (Baselie) Astigmatism ( J0) Adjusted Astigmatism ( J0)* All 981 0.012 (0.007 0.018) 0.018 (0.012 0.024) Age at baselie (y) 7 510 0.010 (0.003 0.017) 0.019 (0.012 0.027) 8 315 0.015 (0.006 0.024) 0.019 (0.011 0.028) 9 156 0.013 (0.005 0.026) 0.015 (0.003 0.026) P 0.71 0.47 Chiese 714 0.022 (0.017 0.028) 0.030 (0.024 0.037) No-Chiese 267 0.014 ( 0.024 0.005) 0.005 ( 0.004 0.015) Geder Male 488 0.003 ( 0.004 0.011) 0.007 ( 0.000 0.015) Female 493 0.021 (0.014 0.028) 0.028 (0.021 0.036) Myopia at baselie Nomyopia 660 0.008 ( 0.014 0.003) 0.014 ( 0.020 0.007) Myopia 321 0.055 (0.047 0.063) 0.049 (0.040 0.058) Computer (h/d) 0.5 549 0.009 (0.002 0.015) 0.013 (0.006 0.020) 0.5 416 0.019 (0.011 0.027) 0.022 (0.015 0.030) P 0.04 0.049 Data are mea chage per year, with 95% cofidece iterval i paretheses. * Adjusted for all other factors i the table. Dowloaded From: https://iovs.arvojourals.org/pdfaccess.ashx?url=/data/jourals/iovs/932923/ o 11/08/2018
IOVS, November 2004, Vol. 45, No. 11 Astigmatism i Sigaporea Childre 3917 TABLE 3. Progressio of J45 for All Childre with at Least Oe Follow-up Visit Over 3 Years (Baselie) Absolute Astigmatism ( J45) Adjusted Absolute Astigmatism ( J45)* All 981 0.001 ( 0.001 0.004) 0.002 ( 0.001 0.006) Age at baselie (y) 7 510 0.003 ( 0.006 0.001) 0.003 ( 0.007 0.001) 8 315 0.006 (0.001 0.011) 0.005 ( 0.000 0.010) 9 156 0.005 ( 0.002 0.012) 0.004 ( 0.002 0.011) P 0.06 0.06 Chiese 714 0.004 (0.000 0.007) 0.006 (0.002 0.010) No-Chiese 267 0.005 ( 0.010 0.000) 0.002 ( 0.007 0.004) P 0.007 0.02 Geder Male 488 0.003 ( 0.001 0.007) 0.003 ( 0.001 0.008) Female 493 0.000 ( 0.004 0.004) 0.001 ( 0.003 0.005) P 0.27 0.40 Myopia at baselie Nomyopia 660 0.001 ( 0.005 0.002) 0.001 ( 0.005 0.003) Myopia 321 0.007 (0.003 0.012) 0.006 (0.000 0.011) P 0.003 0.03 Data expressed as i Table 3. * Adjusted for all other factors i the table. astigmatism) at baselie, 41.2% coverted to with-the-rule astigmatism. At the baselie visit, the most commo type of oblique astigmatism was V-patter symmetry, followed by (axes at 135 i both eyes), A-patter symmetry, ad (axes at 45 i both eyes; Table 5). This distributio of symmetry persisted at the last visit of the follow-up, with most childre retaiig the origial patter of astigmatism symmetry. DISCUSSION Because of a icrease i the mea cylider power of oly 0.09 D over 3 years (0.44 0.53 D) i this cohort of school childre, the 3-year cumulative icidece rate of astigmatism of 33.6% (cylider power of 0.5 D or worse) or 11.5% (cylider power of 1.0 or worse) was ot expected. This discrepacy could be related to measuremet fluctuatio ad differig treds of astigmatism i some childre. Cases of decreasig astigmatic severity may reduce the public health sigificace of the icidece rate. We foud that the icidece of astigmatism was five times greater i myopes tha i omyopes. With-the-rule astigmatism teded to icrease slightly i this logitudial study, with myopes, girls, ad Chiese childre showig sigificatly icreasig treds. A greater umber of hours of computer use were also associated with this worseig. The magitude of oblique astigmatism (J45) did ot chage sigificatly i the etire cohort, but the Chiese ad myopes TABLE 4. Chages i the Directio of J0 (with-the-rule Astigmatism) i Childre with Oe Follow-up Visit Over 3 Years Baselie J0 (Baselie) J0 Positive at Fial Visit* J0 Negative at Fial Visit All 975 819 (84.0) 156 (16.0) J0 positive 827 758 (91.7) 69 (8.3) J0 positive 148 61 (41.2) 87 (58.8) Data are the umber of childre, with the percetage of the etire group i paretheses. * With-the-rule astigmatism. Agaist-the-rule astigmatism. showed a icreasig tred. The effect of ethicity remaied after adjustmet for myopic status ad other demographic factors. Agaist-the-rule astigmatism teded to become withthe-rule, rather tha vice versa. J0 progressio was foud i astigmatic childre with compoud myopia, J0 reductio was foud for those with compoud hyperopia, ad o detectable chage i J0 was foud i those with mixed astigmatism. I the Shuyi study, 13 it was foud that the astigmatic error showed little chage over the 28.5-moth period. A decrease of 0.004 cylidrical diopter (SD 0.301) was foud. Almost all eyes had cylidrical powers of less tha 0.75 D at baselie as well as the repeat measuremet. I the eyes with 0.75 D or more (8.4%) at the iitial measuremet, a geeral reductio i the magitude of the astigmatism was foud. Icreasig astigmatic errors of above 0.50 D or more were associated with older age, ad icreases of 1.0 D or more were associated with greater cylider powers at baselie ad female geder. However, the Shuyi study foud that the reductio of the cylider was ot associated with age ad sex o multiple regressio. These results of astigmatic chage, however, did ot treat cyliders as vectors, ad therefore could ot be directly compared with fidigs i the preset study. It has bee metioed that vectorial aalysis is ecessary for the algebraic maipulatio of astigmatic cyliders, 18 because the directio as well as the magitude of cyliders could chage with time. The stregths of this study icluded a reasoable sample size ad homogeeity of the method of refractio ad biometry, performed i a age group ad ethic backgroud that had ot bee adequately studied i the past. I the preset study, the schools were ot radomly selected, ad this may limit the ability to geeralize the fidigs to all school childre. Because of the differet subjects, the results of this study may ot be applicable to childre Wester coutries. The 3-year lost-tofollow-up rate was low (Fig. 1). The preset fidigs suggests that severity of astigmatism remaied stable i mea cylider power i school age childre. Beig myopic was the major risk factor for developmet of astigmatism. I geeral, if omyopic school childre had o astigmatism (usig the same defiitio), the chaces of havig cylider powers of above 1.0 D over the ext 3 years was fairly low (o the order of 3.4%). Gwiazda et al. 11 reported that cylider power may icrease slightly after 10 years of age, Dowloaded From: https://iovs.arvojourals.org/pdfaccess.ashx?url=/data/jourals/iovs/932923/ o 11/08/2018
3918 Tog et al. IOVS, November 2004, Vol. 45, No. 11 TABLE 5. Chages i Right Left Eye Symmetry of J45 (Oblique Astigmatism) i Childre with at Least Oe Follow-up Visit over 3 Years J45 Symmetry of Right ad Left Eyes at Baselie J45 Symmetry of Right ad Left Eyes at Fial Visit All 976 118 (12.1) 433 (44.4) 177 (18.1) 248 (25.4) 104 40 (38.5) 31 (29.8) 12 (11.5) 21 (20.2) 407 32 (7.9) 283 (69.5) 24 (5.9) 68 (16.7) 204 28 (13.7) 32 (15.7) 91 (44.6) 53 (26.0) 261 18 (6.9) 87 (33.3) 50 (19.2) 106 (40.6) Data are as expressed i Table 4. although this tred was ot statistically sigificat. More studies should be performed i older childre to determie whether astigmatism progresses beyod the age rage i the preset study. The associatio of computer use with progressio of J0 remaied eve after adjustmet for myopia ad had ot bee reported i previous studies o astigmatism. Computer use was foud to be associated with prevalece of astigmatism i a previous report based o childre i this same cohort study. 19 The exact explaatio for this associatio was uclear ad could be explored i further studies. To summarize, i Sigaporea school childre, the chage i mea cylider power was slight over 3 years. The 3-year icidece rate of astigmatism was 11.5% (cylider power of 1.0 D or worse). The presece of myopia at baselie was the most importat risk factor for icidet cases of astigmatism. Progressio of astigmatism was geerally ot severe, but was show to be affected by ethicity, presece of myopia, the axis, ad subtype of the astigmatism. I additio, the progressio of with-the-rule astigmatism may be affected by the umber of hours of computer use. Refereces 1. Miller JM, Dobso V, Harvey EM, Sherrill DL. Compariso of preschool visio screeig methods i a populatio with a high prevalece of astigmatism. Ivest Ophthalmol Vis Sci. 2001;42: 917 924. 2. Igram RM, Walker C, Wilso JM, Arold PE, Dally S. Predictio of amblyopia ad squit by meas of refractio at age 1 year. Br J Ophthalmol. 1986;70:12 15. 3. Abrahamsso M, Fabia G, Adersso AK, Sjostrad J. A logitudial study of a populatio based sample of astigmatic childre. I. Refractio ad amblyopia. Acta Ophthalmol (Copeh). 1990;68: 428 434. 4. Garber JM. High coreal astigmatism i Navajo school childre ad its effect o classroom performace. J Am Optom Assoc. 1981;52: 583 586. 5. Thor F, Held R, Fag LL. Orthogoal astigmatic axes i Chiese ad Caucasia ifats. Ivest Ophthalmol Vis Sci. 1987;28:191 194. 6. Hammod CJ, Sieder H, Gilbert CE, Spector TD. Gees ad eviromet i refractive error: the twi eye study. Ivest Ophthalmol Vis Sci. 2001;42:1232 1236. 7. Mash AJ, Hegma JP, Spivey BE. Geetic aalysis of idices of coreal power ad coreal astigmatism i huma populatios with varyig icideces of strabismus. Ivest Ophthalmol. 1975;14: 826 832. 8. Igram RM, Barr A. Chages i refractio betwee the ages of 1 ad 3 1/2 years. Br J Ophthalmol. 1979;63:339 342. 9. Gwiazda J, Scheima M, Mohidra I, Held R. Astigmatism i childre: chages i axis ad amout from birth to six years. Ivest Ophthalmol Vis Sci. 1984;25:88 92. 10. Abrahamsso M, Fabia G, Sjostrad J. Chages i astigmatism betwee the ages of 1 ad 4 years: a logitudial study. Br J Ophthalmol. 1988;72:145 149. 11. Gwiazda J, Grice K, Held R, McLella J, Thor F. Astigmatism ad the developmet of myopia i childre. Visio Res. 2000;40: 1019 1026. 12. Maples WC, Atchley J, Ashby W, Fickli T. A epidemiological study of the ocular ad visual profiles of Oklahoma Cherokees ad Miesota Chippewas. J Am Optom Assoc. 1990;61:784 788. 13. Zhao J, Mao J, Luo R, et al. The progressio of refractive error i school-age childre: Shuyi district, Chia. Am J Ophthalmol. 2002;134:735 743. 14. Tog L, Carkeet A, Saw SM, Ta D. Coreal ad refractive error astigmatism i Sigaporea school childre: a vector based Javal s rule. Optom Vis Sci. 2001;78:881 887. 15. Saw SM, Chua WH, Hog CY, et al. Nearwork i early oset myopia. Ivest Ophthalmol Vis Sci. 2002;43:332 339. 16. Saw SM, Chua WH, Hog CY, et al. Height ad its relatioship to refractio ad biometry parameters i Sigapore Chiese childre. Ivest Ophthalmol Vis Sci. 2002;43:1408 1413. 17. Zadik K, Mutti DO, Adams AJ. The repeatability of measuremet of the ocular compoets. Ivest Ophthalmol Vis Sci. 1992;33: 2325 2333. 18. Thibos LN, Wheeler W, Horer D. Power vectors: a applicatio of Fourier aalysis to the descriptio ad statistical aalysis of refractive error. Optom Vis Sci. 1997;74:367 375. 19. Tog L, Saw SM, Carkeet A, et al. Prevalece rates ad epidemiological risk factors for astigmatism i Sigapore school childre. Optom Vis Sci. 2002;79:606 613. Dowloaded From: https://iovs.arvojourals.org/pdfaccess.ashx?url=/data/jourals/iovs/932923/ o 11/08/2018