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1 From the Visual Development Unit, London and Cambridge, Department of Psychology, University College London, United Kingdom; the 2 School of Optometry, University of Waterloo, Canada; and the 3 Department of Ophthalmology, Addenbrooke’s Hospital, Cambridge, United Kingdom.
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Abstract |
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METHODS. Children identified as having significant hyperopia in a population screening program at age 8 to 9 months were assigned to treated (partial spectacle correction) or untreated groups. A control group of infants with no significant refractive errors at screening was also recruited. Measurements of retinoscopic refraction under cycloplegia were taken at 4- to 6-month intervals up to the age of 36 months, and changes in refraction of 148 subjects were analyzed longitudinally.
RESULTS. Refractive error decreased toward low hyperopic values between 9 and 36 months in both hyperopic groups. By 36 months, this reduction of hyperopia showed no overall difference between children who were treated with partial spectacle correction and those who were not. Despite the improvement, both hyperopic groups’ mean refractive error at 36 months remained higher than that of the control group. When infants in all three groups were considered together, the rate of reduction of refractive error was, on average, a linear function of the initial level of hyperopia.
CONCLUSIONS. The benefits of spectacle correction for infants with hyperopia can be achieved without impairing the normal developmental regulation of refraction.
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Introduction |
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The mechanisms that regulate human ocular development are poorly understood. Pioneering studies of other animals including primates5 6 7 8 9 have suggested that ocular development and refraction are partly regulated by visual feedback related to optical defocus. Making chicks artificially myopic using plus lenses produces compensatory ocular growth6 10 that can eliminate the refractive error. However there has been some controversy about the extent of these effects in mammals, for both hyperopic and myopic defocus, and their application to human development.11 12 13 14 15 16 17 Refraction in human infants is usually hyperopic, and generally develops gradually toward emmetropia during the first years of life.18 19 20 21 However, the extent to which defocus or accommodation induced by lenses may affect this process has not yet been resolved. In particular, there have been no studies to date comparing human emmetropization in matched groups of infants with corrected and uncorrected refraction who have hyperopic refractive errors. The purpose of the present study was first to examine refractive changes between the ages of 9 months and 3 years in human infants who had naturally occurring hyperopic refractions and compare them with infants with normal refractions, and second to compare changes in refraction in infants with hyperopia who were given correcting spectacles with changes in those who did not receive correction. We present results for three groups: infants who were significantly hyperopic at 9 months and were treated with partial spectacle correction (n = 44), infants who were significantly hyperopic at 9 months and were not treated (n = 37), and a control group with normal refraction at 9 months (n = 36). A further analysis examines the change in refraction of the subgroup of treated infants who consistently wore the prescribed correction.
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Methods |
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The research protocol adhered to the Declaration of Helsinki for research involving human subjects. All parents or guardians of the infants studied provided written consent to the screening and follow-up assessments.
Spectacle Correction and Compliance
Infants identified as having significant hyperopia but with no
meridian greater than +6 D, were alternately assigned to the treated or
untreated groups. For infants treated in the trial, spectacles
were prescribed according to the following protocol:
This protocol was adopted to ensure that astigmatic errors, which are known to reduce rapidly in infancy,24 did not become overcorrected during the period between follow-ups and that some accommodative demand remained, similar to that for an infant with average refraction of +1.0 D to +1.5 D. If at any follow-up visit refractive error had reduced below these criteria, the spectacle correction was discontinued, but the child was retained as part of the treated group for analysis.
A questionnaire at each follow-up asked parents what proportion of waking time the child had worn glasses in the current period. Every effort was made to encourage parents to provide honest answers and not to exaggerate the periods of spectacle wear. Infants with reported spectacle wear of 50% or more waking time were classified as compliant. Changes in refraction were initially analyzed according to intention-to-treat, but the data were also reanalyzed with those subjects who did not meet the criterion for compliance excluded from the treated group. The former analysis respected the original alternate assignment, whereas the latter served to evaluate more specifically the optical effect of correction.
Longitudinal Analysis
At screening, 208 of the 3166 infants (6.6%) met the criterion
for hyperopia. Of these, 199 (96%) who attended the follow-up
appointment and had cycloplegic retinoscopy, 177 (89%) were confirmed
hyperopic (at least one meridian of more than +3.5 D), but in the
present analysis, we considered only the 148 infants who had a meridian
of more than +3.5 D.
Infants with any meridian more than +6 D (n = 18), anisometropia more than 1.5 D between parallel meridians (n = 5), or manifest strabismus (n = 1) were referred for immediate appropriate ophthalmic treatment and were not included in the trial of refractive correction. (One child’s condition fit two of these diagnostic categories.) Of the remaining 125, the intention-to-treat group comprised 62 infants, whereas the no-intention-to-treat group comprised 63. An outcome measure taken between ages 24 and 36 months was available for 46 infants in the intention-to-treat group (74% of those entering the group) and 43 in the no-intention-to-treat group (68%). Of those lost to the study between 9 and 36 months, 18 had development of a visual problem that met the criteria for referral for ophthalmic treatment, and they did not subsequently attend the follow-up visits reported in this study. The remaining 18 moved from the area, failed to attend, or attended but were uncooperative during attempted retinoscopy.
In the control group, 106 of the 162 infants (65%) recruited at screening had cycloplegic retinoscopy at 9 months; 105 of these (99%) were confirmed to have all meridians below +3.5 D. An outcome measure is available for 59 infants (56%). The higher rate of withdrawal in this group was mainly due to failure to attend. Presumably, their parents perceived less benefit of attendance than did those who had children with hyperopia.
To provide a more detailed picture of refractive development, our primary analysis considered the infants for whom, in addition to an initial and final retinoscopy measure, we had obtained an intermediate measure between 16 and 24 months. This reduced our groups to 44 intention-to-treat hyperopes (71%), 37 no intention-to-treat (59%), and 36 control subjects (34%), totalling 53 boys and 64 girls. A subsidiary longitudinal analysis considering only the initial and final measures was also performed.
If more than one refractive measure was available for an infant at the intermediate ages, the earliest available measure was used for the analysis; whereas for the outcome, the last available measure was used.
Statistics
Treated and untreated infants were compared on measures of
hyperopia and astigmatism, using repeated-measures analyses of variance
with age as a within-subjects factor, and treatment and gender as
between-subjects factors. The initial (9 month) values were used as
covariates to remove any effects due to small differences in the
distributions of initial refractions between the two groups. For
assessments of outcome, independent samples t-tests were
used to compare the final (36 month) means. In addition, linear
regression analyses were performed to determine the relation between
initial hyperopia or astigmatism and change over the course of the
study.
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Results |
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The analysis considered overall refractive outcome by taking whichever meridian was most hyperopic at each point. However, this is not necessarily a measure of developmental changes in any specific meridian or eye, because it may compare different meridians at different times. An alternative approach is to track whichever meridian is most hyperopic at 9 months for each child. This yielded an overall pattern of change very similar to that in the previous analysis, with a mean difference of 1.1 D between the treated and untreated groups at 18 months. The overall reduction of hyperopia was greater using this measure, however, with a final mean of +2.7 D for the untreated hyperopes and +3.0 D for the treated, a reduction of 1.6 D in both groups. The control group had a final mean of +1.4 D, a reduction of 0.5 D. Analysis of variance for the two hyperopic groups again found a significant age–treatment interaction (F = 9.35, P < 0.005), and an independent samples t-test found no difference at 36 months (power = 0.89 for a true difference of 1.0 D; 0.35 for 0.5 D at P < 0.05).
Of the 44 treated-group subjects in our main longitudinal analysis, 13 did not meet the criterion for compliance. We reanalyzed the data for the treated group, omitting these children. Figure 2 plots the mean level of hyperopia, calculated as the most hyperopic meridian at the time of measurement, distinguishing the treated hyperopes who were compliant from those who were not.
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Again, the data were reanalyzed in terms of the single greatest axis at 9 months. At 18 months the untreated group mean was +3.4 D, 1.1 D higher than the treated compliant group, but by 36 months the mean had declined to +2.9 D, leaving a difference of 0.2 D between the two, an overall mean reduction of 1.6 D in both groups. Analysis of variance again found that the age–treatment interaction was significant (F = 11.81, P < 0.02), but the final difference in means was not (power = 0.90 for a true difference of 1.0 D; 0.37 for 0.5 D at P < 0.05).
When we included in these four analyses the infants without a midpoint refraction (total n = 148), the results were essentially unchanged, although the control group showed slightly less reduction in hyperopia than in the analyses shown in Figures 1 and 2 . An example of these analyses is shown in Figure 3 (the larger samples increase power to 0.96 for a true difference of 1.0 D, 0.45 for 0.5 D at P < 0.05).
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Figure 4 plots change in hyperopia against refraction at 9 months, for the meridian initially most hyperopic in each case. The plot indicates that subjects across the whole range of initial measurements tended to converge toward emmetropia, and that a subject’s overall change was proportional to the initial degree of hyperopia. A regression analysis found this linear relationship to be highly significant (F = 56.35, P < 0.0001).
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To determine whether a relationship exists between initial astigmatism and its reduction by 36 months, a regression analysis was performed (Fig. 6) for the same group of infants used in our regression analysis of emmetropization (n = 159). In each infant, we tracked the eye that was more astigmatic at 9 months. Comparing astigmatism (expressed as a signed value obtained by subtracting the vertical meridian from the horizontal), with change (expressed as the difference between astigmatism values at 36 months and 9 months), we again found a highly significant linear relationship (F = 315.86, P < 0.0001).
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Possible Biases
To evaluate differential withdrawal from the study as a possible
source of bias, we compared the initial (9 month) retinoscopy measures
of greatest axis and astigmatism of subjects who remained in the study
with those who were withdrawn. Independent samples t-tests
found no significant differences, in either hyperopic group or in the
control subjects. There was also no statistically significant
difference in numbers of boys and girls in any of the three groups.
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Discussion |
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Emmetropization
Our data show that, for the group as a whole, a substantial
reduction of hyperopia, both spherical and cylindrical, occurs during
the second and third years of life. The average reduction of refractive
error is a linear function of the initial level. The regression lines
imply that a child with an initial refraction of +0.86 D would on
average show zero change in this meridian, and so the process of
emmetropization can be considered as a convergence of refractions
toward a low hyperopic value. The linear relationship directed to a low
hyperopic end point is consistent with our data in other groups of both
hyperopic and myopic infants.21
25
The change in
astigmatism is also proportional to initial refractive error, implying
convergence toward a near emmetropic value in the case of the
cylindrical as well as the spherical component of refraction,
consistent with earlier data on the reduction of infant
astigmatism.26
27
24
It must be appreciated, however, that there is substantial variability in the extent of emmetropization within the hyperopic group. It is apparent from Figure 4 that some infants show marked reductions in hyperopia over the first 3 years of life, whereas others show little change. The basis of this variability is not yet known.
The occurrence of withdrawal between 9 and 36 months raises the question of whether those who remained in the study were a biased sample. However, we found no evidence for any differences in initial refraction between children who were withdrawn and those who remained in the study.
Effect of Spectacle Correction
The comparison of corrected and uncorrected groups suggests
a small, transient effect of refractive correction between 9 and 18
months of age. However, by 36 months this effect had disappeared, and
the infants with initial hyperopia had reached a common refraction
irrespective of treatment. This conclusion remained the same whether we
analyzed in terms of the original assignment to treated or untreated
groups, or whether we considered as treated only those who consistently
wore their spectacles. Thus, we find no evidence that partial spectacle
correction for infantile hyperopia interfered in any persistent way
with the developmental trend toward emmetropia.
The analysis that included the largest number of hyperopes—all those with a 9 month and 36 month measurement (total n = 89)—had a power of 0.96 in detecting a true difference of 1 D at P < 0.05. Thus, although there is wide variability in initial refraction and refractive change, with groups of this size we can show with some confidence that spectacle correction does not substantially interfere with emmetropization.
The general reduction of hyperopia meant that many infants (n = 21) in the treated group did not fulfill the criteria for prescription before the age of 36 months.
The finding that spectacle correction did not impede emmetropization applied to the refractive population we have described. We cannot be sure how refractive correction might affect the development of very large hyperopic errors, which show very variable degrees of emmetropization (see the squares in Fig. 4 ), or how it would affect children who have strabismus before receiving a correction. We did not gather systematic data on ethnic origin or socioeconomic status. However, the study group and the population from which it is drawn had a very strong predominance (>90%) of white origin, and, because the screening was based on high attendance within a socially mixed geographic area, covers the range of socioeconomic groups. There was no indication of differential withdrawal between different districts within the overall area.
Our results are also specific to the practice of partial spectacle correction as described in the Methods section. A full correction of refractive error would ensure that the accommodative demand for an infant with hyperopia was reduced to a lower level than for control infants (who in general had a small, uncorrected hyperopia). It is possible that such a reduction in accommodation would influence the emmetropization process. However, our partial corrections did not produce such an effect.
We have found that the partial refractive correction of infants with hyperopia according to the protocol described in the present study has beneficial effects of reducing the incidences of strabismus and poor acuity by two thirds in children who comply in wearing the prescribed correction.3 4 The present results indicate that these benefits can be achieved without the optical treatment’s impairing the normal developmental regulation of eye growth and refraction.
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Acknowledgements |
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Footnotes |
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Submitted for publication November 16, 1999; revised March 7 and May 22, 2000; accepted June 23, 2000.
Commercial relationships policy: N.
Corresponding author: Janette Atkinson, Department of Psychology, University College London, Gower Street, London WC1E 6BT, UK. j.atkinson{at}ucl.ac.uk
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) Untreated group (n = 43);
(
) compliant treated group (n = 31); (
)
noncompliant treated group (n = 15); (•) =
control group (n = 59); (
) high hyperopes excluded
from trial (n = 11).
