Myopia and Myopic Progression Among Schoolchildren:A Three-Year Follow-Up Study

Olavi Pcirssinen* and Anna-Liisa Lyyraf

Purpose. To discuss the factors that might explain the rate of myopic progression and thedegree of myopia after a 3-year follow-up among schoolchildren with myopia.

Methods. Myopic progression among 238 schoolchildren was followed up in a randomizedclinical trial of myopia treatment. The associations between the explanatory factors and myo-pic progression and the final value of the spherical equivalent after the follow-up were studiedby analysis of variance and regression analysis.

Results. Myopia progressed faster among girls than boys. According to the regression modelsfor the boys, 25% of the variation of myopic progression and 57% of the final sphericalequivalent could be explained by initial spherical equivalent, age at receiving first spectacles,time spent on sports and outdoor activities, and on reading and close work. Among the girls,30% of myopic progression could be explained by age at receiving first spectacles, time spenton reading and close work, and reading distance. Similarly, 49% of the final spherical equiva-lent could be explained by age at receiving first spectacles, initial spherical equivalent, timespent on reading and close work, and reading distance. The rest of the variations could not beexplained by the variables measured in this study.

Conclusions. The factors with the most significant relationships to myopic progression weresex, age of onset, and degree of myopia at the beginning of the follow-up. Myopic progressionand final myopia were related to time spent on reading and close work and to reading distancebut not, however, to accommodation stimulus. Invest Ophthalmol Vis Sci 1993; 34:2794-2802.

JMyopia is a common refractive error in the literatecountries. It is clear that there is a hereditary influenceon the different components of the refractive ele-ments of the eye.1 However, the influence of variousexternal factors on myopia and on its progression isstill under discussion.23 That myopia is related to edu-cation and the higher.occupational groups would seemto be clearly established.45 Accommodation,6 conver-

Frtnn the * Department of Ophthalmology, Central Hospital of Central Finland, andthe ^Department of Statistics, University ofJyvaskyla, Jyviiskyla, Finland.This study was supported by a grant from the Academy of Finland. We thank alsothe instrnvientarium Corporation for sharing the costs of bifocal lenses.Submitted for publication October 23, 1992; accepted January 12, 1993.Propriety interest category: N.Reprint requests: Olavi Pdrssinen, Kannaksenkatu 5, SF-40600, Jyviiskyla,Finland.

gence,7 or both have most often been advanced as thereasons for these relationships.

Our previous report on a 3-year follow-up studyof myopia treatment showed that myopic progressionamong schoolchildren could not be prevented with theuse of bifocals or by avoiding spectacles in reading andclose work.8 The same report showed a relationshipbetween myopic progression and time spent on read-ing and close work, and also between myopia andreading distance. Progression was also faster amongthe girls than among the boys.

This article deals with the factors that might ex-plain the rate of myopic progression as well as thedegree of myopia after a 3-year follow-up among myo-pic schoolchildren by using multivariate statistical

2794Investigative Ophthalmology & Visual Science, August 1993, Vol. 34, No. 9Copyright © Association for Research in Vision and Ophthalmology

Myopic Progression 2795

methods. We also attempted to identify those factorsthat would discriminate the fastest and slowest rates ofmyopic progression. The possible relationships be-tween reading and myopia are also discussed.

MATERIALS AND METHODS

Myopic progression (final spherical equivalent [SFE]— initial SFE) among myopic schoolchildren was fol-lowed for a 3-year period as a part of a clinical trial ofmyopia treatment.8 Schoolchildren from the third andfifth grades of lower comprehensive school (mean age10.9 years), with no previously prescribed spectaclesfor myopia, were considered for inclusion in the study.Two hundred forty children were randomly allocatedto one of three treatment groups: full corrected spec-tacles to be used continuously, only for distant vision,or in the form of bifocals with +1.75 D add. Beforerandomization the children were invited to participatein a follow-up study of myopia and all agreed. Theresearch followed the tenets of the Declaration of Hel-sinki. Informed consent was obtained after the natureand the possible consequences of the study were ex-plained to the participants. The research was ap-proved by the institutional human experimentationcommittee. The 3-year follow-up proved possible for238 children. Cycloplegic refraction was performedannually. Reading distance was measured during eachannual examination using a Clement Clark accommo-dometer. The average value of the accommodationstimulus was calculated from annual measurements ofreading distances and the refraction values. Myopicprogression and the way of wearing spectacles wereaccounted for in these calculations. The following for-mulae were used to calculate accommodation stimulusbetween two control visits:

Continuous use: ACC = (100/Rj + 100/R2):2 +(SFD2 - SFD,):2

Distant use: ACC = (100/R, + 100/R2):2 + (SFD2

+ SFD,):2Bifocals: ACC = (100/R, + 100/R2):2 + (SFD2 -

SFD,):2-1.75,where ACC = accommodation stimulus, Rj and R2 =reading distances in two consecutive visits, SFDj andSFD2 = spherical equivalents of the right eye in twoconsecutive visits.

The average reading distance and accommodationstimulus for the whole follow-up period was calculatedfrom the consecutive values for annual reading dis-tance and accommodation stimulus.

Intraocular pressure was measured annually usingHaag Streit applanation tonometry. Keratometryreadings were measured at the beginning and at theend of the study using a Wesley Jessen System 2000

PEK camera.9 At the end of the follow-up period theaxial dimensions were measured by a Storz a-scan witha soft probe.

A questionnaire was used to determine the use ofspectacles by the children's parents. The children weredivided into two groups according to whether one orboth parents or neither parent had spectacles for dis-tant vision. The questionnaire at the end of the follow-up asked for the average, amount of time spent daily tothe nearest half hour separately on reading and onother types of close work outside school, separately forboth school days and weekends. The mean number ofhours spent daily on close work was calculated fromthose four estimates so that the values for school dayswere multiplied by five and those for weekends by two,and the different types of near work added togetherand divided by seven. The mean number of hoursspent daily on outdoor activities was calculated like-wise. In this report only the refraction values of theright eye are used. Because the differences betweenthe treatment groups in myopic progression and in thefinal SFE (spherical equivalent at the end of the fol-low-up) of the right eye were statistically non-signifi-cant, the treatment groups are mainly treated as awhole. Materials and methods have been published inearlier reports.810

Statistical MethodsThe comparisons of means for myopic progressionand final SFE between the sexes, between treatmentgroups and between parent groups (according towhether parents wore spectacles for distant vision)were made using analysis of variance. Analysis of vari-ance was also used to make the comparisons of themean values of myopic progression with regard to sexand to different groups concerning time spent onreading and close work, reading distance, and timespent outdoors. The degree of myopic progressionand final SFE were explained by age, initial SFE, timespent on reading and close work, time spent on out-door activities, and average reading distance. Theserelationships were examined by constructing regres-sion models separately for boys and girls. These mod-els were estimated and tested using the LISREL 7 soft-ware program (Scientific Software, Mooresville, IN).11

Comparisons of the refraction-related variables andselected background variables between the slowestand the fastest progressing groups were made by Stu-dent's tf-test. Comparisons of the relative proportionsof the sex, treatment, and parent groups between theslowest and the fastest progressing groups were madeusing x2-test. For the longitudinal examination of myo-pic progression at the four measurement points thegrowth curves were constructed for boys and girls sep-arately using Generalized Multivariate Analysis of Vari-

2796 Investigative Ophthalmology & Visual Science, August 1993, Vol. 34, No. 9

TABLE l. Comparisons of the Means for Myopic Progression and Final SphericalEquivalent (SFE)

ParentsSex Group

Boys One or both

Neither

Girls One or both

Neither

Source of Variation

Main EffectsSexParent groupTreatment group

TreatmentGroup

ContinuouslyOnly distantBifocalsContinuouslyOnly distantBifocalsContinuouslyOnly distantBifocalsContinuouslyOnly distantBifocals

F

4.5137.1476.3931.925

df

4112

Myopic Progression

Mean

-1.62-1.69-1.49-1.30-1.55-1.33-1.77-2.23-2.12-1.28-1.47-1.91

SD

0.841.070.780.850.880.370.961.031.110.940.841.03

P

0.0020.0080.0120.148

(n)

(19)(21)(22)(18)(16)(15)(25)(23)(20)(14)(13)(16)

3470

F

.572

.408

.442

.938

Mean

-3.11-3.04-3.11-2.80-2.88-2.53-3.16-3.64-3.55-2.70-2.68-3.40

Final SFE

df

4112

SD

1.291.481.071.120.950.641.111.171.250.950.721.49

(n)

(19)(21)(22)(18)(16)(15)(25)(23)(20)(14)(13)(16)

P

0.0080.0370.0070.393

Comparisons were performed by three-way analysis of variance among boys and girls in two groups of parents regarding their wearing ofspectacles for distant vision and in the three treatment groups.

ance (GMANOVA) model.12 The estimation and test-ing of hypotheses for the growth curves and the calcu-lations of confidence intervals were also computedusing the GMANOVA program.13

RESULTS

The differences in myopic progression and final SFEbetween the boys and girls, between the treatment

groups, and between the two groups according to par-ents' use of spectacles were studied by analysis of vari-ance (Table 1).

Myopic progression was significantly sloweramong boys, and among those whose parents did nothave spectacles for distant vision. The differences be-tween the treatment groups (in the right eye) were notsignificant. No interaction among these three explana-tory factors was found.

TABLE 2. Regression Models of Myopic Progression and Final SFE for Boys and Girls

ExplanatoryVariable

AgeInitial SFEClose workOutdoorsReading distanceNR2

Boys

0

0.32*0.25f

-0.160.23f

—

1080.25

Myopic Progression

(SE)

(.09)(.09)(.09)(.09)

Girls

P0.40*

—-0.18J

—0.18J

1130.30

(SE)

(-09)

(.08)

(.09)

Boys

0.24*0.67*

-0.120.17J

—108

0.57

Final SFE

(SE)

(.07)(.07)(.07)(.07)

Girls

0.34*0.51*

-0.16J—

0.15f113

0.49

(SE)

(0.07)(0.07)(0.07)

(0.07)

The explanatory variables used were: age at commencement of study (Age); initial spherical equivalent (SFE); daily time spent on readingand close work (Close work); time spent on outdoor activities (Outdoors), and average reading distance during the follow up (Readingdistance). Only effective variables were included in the models. Estimates for regression coefficients /3 and their standard errors are in pa-rentheses. N, number of observations. R2, determination coefficients.* P< 0.001.f P < 0 . 0 1 .%P < 0.05.

Myopic Progression

MYOPIC PROGRESSION (D)

3 -

2 -

1 -

>

1

1

f

. '

n n nrni

GIRLSp = 0.004BOYS

s2.0 2.1-3.0 3.1-4.0 • 4.0

TIME SPENT ON READING AND CLOSE WORK (h)

3

2

1

n

MYOPIC PROGRESSION

-

_

•i a

J (D)

1 "

n n nn?i

GIRLS\ p= 0.003

BOYS

B20-24.9 25-29.9

READING DISTANCE (cm)

> 30

MYOPIC PROGRESSION (D)

3 -

2 -

1 -

s2.0 2.1-3.0 3.1-4.0 >4.0

TIME SPENT ON SPORTS AND OUTDOOR ACTIVITIES (h)

FIGURE l. Comparisons of the mean values of myopic pro-gression with regard to sex and to (A) time spent on readingand close work; (B) reading distance and (C) time spent out-doors. Results of two-way analysis of variance: P values fordifferences among four groups on x-axis and between sex.Bars represent standard deviations.

2797

The correlations among the different explanatoryfactors about certain living habits and myopic progres-sion and the final SFE were analyzed by constructingregression models for boys and girls separately, in-cluding age at the start of the study (age of receivingthe first spectacles for myopia) and the initial SFE ascontrol variables (Table 2).

The age of receiving first spectacles for myopiawas significantly related to myopic progression andfinal myopia among both boys and girls. Similarly SFEon receiving the first spectacles had a significant rela-tionship with final SFE and with myopic progressionamong the boys.

Time spent on reading and close work was asso-ciated with a faster rate of myopic progression and ahigher degree of myopia at the end of the study, al-though the association was slightly weaker among theboys. The amount of time spent outdoors was clearlyconnected with myopic progression and final SFEamong the boys, so that the greater the amount of timespent outdoors, the lower was the rate of myopic pro-gression and the degree of myopia at the end of thefollow-up. A shorter reading distance was clearly con-nected with a faster rate of myopic progression andhigher myopia at the end of the follow-up among thegirls.

According to the regression models these factorsexplained 25% of the variation in myopic progressionamong the boys and 30% among the girls, and 57%and 49% of the variation in final SFE, among the boysand the girls, respectively.

We also compared the mean values of myopic pro-gression with the factors characterizing habits of read-ing and outdoor activities classified in four groups (Fig-ure 1).

Figure la shows that when more time was spent onreading and close work, myopia progressed faster (P =0.003). Progression was significantly faster among thegirls than among the boys (P = 0.004). The figures

MYOPIC PROGRESSION (D)

-1

- 2

-3

BOYSGIRLS

BEGINNING FIRST yr SECOND yr THIRD yr

FIGURE 2. Curves showing final spherical equivalent changesfor boys and girls.

2798 Investigative Ophthalmology & Visual Science, August 1993, Vol. 34, No. 9

TABLE 3. Comparisons of Annual Myopic Progression in Dioptes (D) Between Boys and GirlsUsing Two-Tailed t-test

Variable

First yearSecond yearThird year

Mean (D)

-0.56-0.46-0.43

Boys

SD(n)

0.41 (119)0.36(119)0.44(119)

Mean (D)

-0.79-0.57-0.46

Girls

SD(n)

0.51 (120)0.47(118)0.42(119)

t-test(P Value)

<0.0010.0380.618

were virtually the same when reading time only wastaken into account.

Myopic progression was also faster among chil-dren with a shorter reading distance (P = 0.002; Figlb). The difference between boys and girls was alsosignificant (P = 0.003). Myopic progression wasslightly slower when more time was spent on outdooractivities, but the relationship was nonsignificant (Figlc). Nevertheless, the difference between the boys andthe girls was significant.

The differences in the data in Figure 1 were testedusing the two-way analysis of variance. There were nointeractions between sex and these three explanatoryvariables.

The longitudinal investigation of myopic progres-sion during the 3-year period was carried out by fittingthe growth curve model for the SFE of the right eyethrough the four time points separately for both sexesusing the GMANOVA model. The second-order poly-nomials were obtained as the growth curve models forthe SFE; for the boys

SFEt = -1.44 - 0.58t + 0.03t2 , t = 0,1,2,3,and for the girlsSFEt = -1.44 - 0.86t + 0.08t2 , t = 0,1,2,3,

where SFE is the expectation of the spherical equiva-lent of the right eye, t = 0 at the beginning of the studyand t = 3 at the final examination 3 years later. Thehypotheses suggesting identical and parallel curves forboth sexes were both rejected (P < 0.001), and themyopic progression among the girls was found to beclearly faster.

The graphs of the growth curves of SFE are shownin Figure 2.

There was a divergent trend between the boys andthe girls as well as a slight decrease in the rate of myo-pic progression. The annual progression among thegirls was faster than that among the boys during thefirst 2 follow-up years but not thereafter (Table 3).

The amount of time spent on reading and closework was somewhat greater among the girls and theamount of time spent outdoors was correspondinglyshorter. However, the differences in these variablescould not fully explain the divergence in myopic pro-gression between the sexes.

Different profiles of myopic progression emerged.Among six boys and six girls myopic progression dur-ing the follow-up was 0.25 D or less. Conversely, sevenboys and seventeen girls had a myopic progression of

TABLE 4. Comparisons of Variables Connected With Refraction Between the Slowest andFastest Progressing Groups Using Two-Tailed t-test

Variable

Slow Fast

Mean SD(n) Mean SD (n)t-test(P Value)

Age at enteringthe study (yr)

Initial sphericalequivalent (D)

Myopic progression (D)Initial corneal

refraction (D)Final corneal

refraction (D)Final anterior

chamber depth (mm)Final lens

thickness (mm)Final axial

length (mm)

11.3

-1.4-0.5

44.1

44.2

3.67

3.36

24.11

0.9 (60)

0.6 (60)0.3 (60)

1.3(55)

1.3(35)

0.38 (33)

0.14(32)

0.74 (38)

10.3

-1.5-2.9

44.1

44.1

3.78

3.36

24.99

1.0(60)

0.7 (60)0.6 (60)

1.4(55)

1.3(39)

0.35 (44)

0.20 (44)

0.77 (46)

<0.001

0.214<0.001

0.965

0.751

0.166

0.843

<0.001

Myopic Progression 2799

TABLE 5. Comparisons of SelectedBackground Variables Between the Slowestand Fastest Progressing Groups Using x2-test

Variable

SexBoysGirls

Treatment groupWearing spectacles

continuouslyWearing spectacles

only for distantvision

BifocalsParents' spectacles

for distant visionOne or bothNeither

Slow(%)

5545

42

3127

4951

Fast(%)

3268

32

3830

6931

n

5268

44

4134

6745

X2-test(P Value)

0.010

0.463

0.028

more than 3 D. There was considerable variation inSFE at beginning of the follow-up in both the stable(-0.38 D to -2.68 D) and the fastest progressing cases(-0.38 D to -3.00 D). On the basis of the initial SFEwe could not discriminate the stable from the fastestprogressing cases.

We then tried to ascertain which of the differentvariables would discriminate between the fastest andslowest progressing quartiles in this myopic child pop-ulation. The comparison groups consisted of the 60slowest and the 60 fastest progressing cases (Table 4).

The fastest progressing group received their firstspectacles approximately 1 year earlier than those withthe slowest progression. At the beginning of the fol-low-up there were no significant differences betweenthe groups in SFE although myopic progression wasabout 2.4 D more in the fastest progressing quartile.

There were no significant differences between thegroups in corneal refraction at the beginning or at theend of the follow-up. Of the axial dimensions at theend of the follow-up only a greater total axial lengthwas observed in the faster progressing group. Table 5shows the other comparisons between the slowest andthe fastest progressing groups.

The faster progressing group contained moregirls, and the parents of the children in this groupmore often had spectacles for distant vision. The com-parison did not show any significant differences be-tween the treatment groups.

The faster progressing group had a shorter read-ing distance, but despite this their accommodationstimulus in reading was less than in the slowest pro-gressing group (Table 6).

Intraocular pressure seemed to be slightly higherin the faster progressing quartile, although this differ-ence was not statistically significant. Furthermore, thetime spent on reading or on both reading and closework was longer, and the time spent outdoors shorterin the faster progressing group.

DISCUSSION

The subjects selected for this study consisted of arather visually homogenous group of schoolchildrenreceiving their first spectacles for myopia at beginningof the study. The age of the children on entering thestudy was also within narrow limits (mean 10.9 years,range 8.8-12.8 years), thus imposing certain limita-tions on interpretation of the results. Had the studyincluded hyperopic and emmetropic cases, the varia-tion in the different explanatory factors used herewould perhaps have been wider. However, even in thedesign of this study of myopic schoolchildren manyvariables were found to be related to myopia and to itsprogression.

TABLE 6. Comparisons of Selected Background Variables Between the Slowest and FastestProgressing Groups Using Two-tailed t-test

Variable

Reading distance (cm)Accommodation stimulus (D)Intraocular pressure (mm Hg)

At commencement tothe study

At the end of the studyTime spent on

reading (hours/day)Time spent on reading

and close work (hours/day)Time spent outdoors (hours/day)

Mean

24.11.5

17.415.6

1.2

2.93.2

Slow

SD(n)

4.3 (56)0.9 (55)

2.8 (58)2.2 (59)

0.3 (58)

0.8 (58)1.4(60)

Mean

22.00.3

17.516.4

1.4

3.52.5

Fast

SD (n)

3.8 (58)1.2 (58)

2.6 (60)2.5 (58)

0.4 (60)

0.9 (60)1.1 (60)

t-test(P Value)

0.005<0.001

0.8360.085

0.006

0.0010.003

2800 Investigative Ophthalmology & Visual Science, August 1993, Vol. 34, No. 9

Heredity

That heredity has an influence on refraction is obvi-ous. In a recent study of 109 pairs of twins by Teikariet al, the heritability of myopia was 0.58 when myopiawas considered a dichotomous variable.14 It can beassumed that the effect of heredity on refraction issimilar to that of many other physiologic phenomena,both in terms of direct influence and influence on sus-ceptibility to external factors.

In a previous Finnish study the percentage ofmyopes among those adults who wore spectacles forpoor distant vision was 66%.15 Thus a higher fre-quency of myopia can be assumed in the case of thoseparents of the children in the current study who hadspectacles for poor distant vision compared to thosewho did not have spectacles for this purpose. Al-though this variable was apparently a rather weak vari-able in the heredity of myopia, it nevertheless had arelationship both with myopic progression and finalmyopia in this study.

Boys-Girls

Myopic progression was significantly faster among thegirls than the boys. Interestingly, the difference in therate of progression was significant only during the first2 years of the follow-up and remained approximatelythe same thereafter. In the data studied by Goss, sexdid not have a great effect on the rate of progressionof childhood myopia.16 Perhaps this difference be-tween the sexes is evident only at the beginning ofmyopia and when the onset of myopia is early. Thevariables used in this study could not fully explain thedifference in myopic progression between boys andgirls. It has been proposed that the progression ofmyopia in children is related to the rate of their rate ofgrowth.17 Some of the difference in myopic progres-sion between the sexes might be explained by thefaster maturation of girls at the ages studied.

Age of Onset and Degree of Myopia

Earlier studies have shown that myopic progression isfaster in the younger age groups. For example, in thestudy by Mantyjarvi myopic progression was 0.93 D/yrin 8-year-olds and 0.52 D/yr in 13-year-olds.18 In theHouston myopia control study there was also a morerapid rate of progression among those who enteredthe study at an early age with a large amount of myo-pia.19 In the current study the rate of myopic progres-sion was also related to the age of onset of myopia andalso independently to the degree of myopia at the be-ginning of the study. Thus the greater amount of myo-pia in adulthood among those whose myopia beginsearlier is not at least solely dependent on more years of

progression but also on faster progression whenyounger.

Although the degree of myopia at the beginningwas, on average, related to the rate of myopic progres-sion, this did not hold true in individual cases. In thisstudy there were stable cases of myopia varying from-0.38 D to -2.68 D. The distant vision of these chil-dren had been tested by school nurses at 1- or 2-yearintervals at least twice before the commencement ofthe study. Thus it can be supposed that their myopiahad developed during a few years and then stabilized.What halted their myopic progression remains to beanswered.

The relationship between faster myopic progres-sion and earlier onset of myopia may depend on astronger hereditary influence and/or a stronger sus-ceptibility to external influences.

Living Habits

It is known that myopia is common among school chil-dren and educated people,15 and less common amongnonliterate populations.20 Myopia has also been shownto be rare among occupational groups such as farmersand fishermen.4 Further, restricted spaces have beenshown to increases myopia in animal experiments.21 Inour previous study of men aged between 33 and 37years, 77% of the subjects were correctly assignable asmyopic by means of discriminant analysis taking intoaccount interest in reading, length of education, occu-pational status, and body mass index.22 Richler andBear, respectively, using multiple regression coeffi-cients, found an age-related relationship between myo-pia and near work.23

In this study the amount of time spent on readingand on reading and close work and reading distancewere related to myopic progression and final SFE, al-though there were clear differences between the boysand the girls.

In this study there was also a slight negative corre-lation among the boys between myopic progressionand the amount of time spent on sports and outdooractivities. It cannot be said whether the relationshipbetween slower progression rate and longer amountsof time spent outdoors was attributable to some effectof distant gazing or simply to being away from readingand close work.

Accommodation, Convergence, and EyeMovements

What then might link myopia and reading and closework? Accommodation6 and convergence7 have beensuggested as the reasons sons for the development ofmyopia. If accommodation played a significant role inmyopic progression the feedback mechanism would

Myopic Progression 2801

probably halt the process when reading with under-corrected glasses or without glasses. The results ob-tained in the earlier report8 and from this study didnot support the hypothesis of accommodation as a sig-nificant cause of myopia. As the matter of fact, in thisstudy the accommodation stimulus was smaller amongthe fastest progressing quartile than among the slow-est progressing quartile. The overall low accommoda-tion stimulus values found here can be explained bythe study design as one third of the children were rec-ommended to do close work without spectacles and asecond one third were prescribed bifocals. However,when a person with myopia reads without spectaclessmall changes in reading distance cause a greater per-centage of changes in the amplitude of accommoda-tion than when reading with spectacles, and we do notexactly know the effects of these changes.

The relationships between short reading distanceand myopic progression and final myopia found in thisstudy could support the hypothesis that convergenceis one factor inducing myopia. Greene has supposedconvergence, and specially the stress between theoblique muscles, to be an important factor in myopicprogression.7 However, in a previous Finnish study of26- and 46-year-olds it was shown that education in-creased the prevalence of myopia more than doingother kinds of precision close work.15 It can be sug-gested that the use of eyes in close work like sewing,watch-repairing, and assembling electronic equipmentis different from that in reading. A fact that has beenlargely neglected when discussing the possible rela-tionships between reading and myopia is the physio-logic nature of the reading process. Skilled readersmove their eyes, on average, every quarter of a secondwhen reading.24 Thus, there are constant saccadicback-and-forth movements of the eye during reading.These eye movements clearly cause repeated pressureand stretch pulses on the eye during reading. It is obvi-ous that there are fewer eye movements, although theworking distance is the same, when doing many otherkinds of close work.

It seems feasible to assume that the structure aswell as the thickness of collagen is influenced by hered-ity. The width of the eye muscle attachments and theirstrength can also vary because of heredity. The same isobviously true of axial length prior to any significantexternal influences. Hence, there are many variablesinfluenced by heredity, and in a multiplicity of combi-nations, all of which can cause different affinities ofthe eye in response to various external factors.

CONCLUSION

Several variables in this study were related to myopiaand myopic progression among schoolchildren, and

also discriminated the fastest and slowest progressingmyopes. The factors that had the most significant rela-tionships to myopic progression were sex, age of on-set, and degree of myopia at the beginning of the fol-low-up. In addition to having parents who had specta-cles for poor distant vision, more time spent onreading and close work, and short reading distance butnot high accommodation stimulus were all related tomyopic progression. Of the refractive elements of theeye, greater axial length was related to higher myopia.It remains to be answered what, in addition to heredi-tary factors, could be the reason for greater axiallength and myopia, but we suggest that studies of myo-pia should be addressed in part to determine the me-chanical forces affecting the eye and sclera duringclose work and especially when reading.

Key Words

myopia, progression, reading distance, accommodation,convergence

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3. Curtin B. The Myopias. Philadelphia: Harper & Row,Publishers; 1985:61-152.

4. Tscherning M. Studien iiber die Aetiologie der Myo-pie. Albrecht von Graefe's Arch Ophthalmol. 1883;29:201-272.

5. Parssinen TO. Relation between refraction, educa-tion, occupation, and age among 26- and 46-year-oldfinns. Am) Optom Physiol Opt. 1987;64:136-143.

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