The reported rates of myopia progression (-0.70 dioptres (D) per year in Singapore children and -0.63 D per year in Hong Kong children) among myopic children in east Asian countries are among the highest in the world. The knowledge of changes in biometric patterns in myopic children will enable a better understanding of stimuli influencing growth patterns and continued shifts in refraction. Most previous studies have evaluated biometry parameters at one point of time in cross sectional studies. Changes in biometry parameters were documented in the Correction of Myopia Evaluation Trial (COMET) of 469 children aged 6–11 years with myopia between -1.25 and -4.5 D: the increase in axial length was 0.75 mm in the single vision lens group (n = 234) over a 3 year period. There are few reports describing the risk factors for changes in biometric parameters, although from cross sectional analysis of 2583 school aged children in the Collaborative Longitudinal Evaluation of Ethnicity and Refractive Error (CLEERE) study, boys and older children had longer axial lengths.
There are few community based or school based longitudinal data describing risk factors influencing changes in biometry parameters in myopic children. We aimed to evaluate the pattern of biometry parameter changes and associated factors in young myopic children aged 7–9 years followed prospectively for 3 years in Singapore.
DISCUSSION
In our study, the 3 year increases in axial length (mean = 0.89 mm) and vitreous chamber depth (0.92 mm) in Singapore children are high. In contrast, the 3 year changes in anterior chamber depth, lens thickness, and corneal curvature were minimal. Children who were young, female, and who had myopic parents were more likely to have greater axial length increases. No associations between near work activity and biometry parameter changes were found.
The 3 year increase in axial length is high (0.89 mm), but similar to the single vision lens group of a Hong Kong study of myopic children aged 7–10.5 years (n = 133) enrolled in a trial evaluating progressive lenses, who showed similar increases in axial length (0.32 mm per year = 0.96 mm over 3 years). In the COMET trial, the rate of increase in axial length was slightly lower in myopic children aged 6–11 years in the single vision lens group: 0.75 mm over 3 years. In contrast, in a university study of 149 Norwegian engineering students (mean age of 20.6 years) across the refractive error range with a mean 3 year refractive change of -0.52 D, the 3 year change in axial length was 0.34 mm. Differences in the refractive error range, subject age limits, interethnic composition, and overall dropout rates, however, limit the conclusions from comparing the results of these studies.
Greater changes in axial length in younger children and females parallel the greater changes in refraction in the same age and sex patterns in this Singapore cohort. In our study, children who had one or two parents with myopia had greater increases in axial length change compared with children without any myopic parents. In a 24 year longitudinal study of 122 US children, the odds ratio of incident myopia for two myopic parents versus none was 5.09 (95% confidence interval (CI) 1.69 to 14.49). Previously, most epidemiological investigations have been cross sectional in nature. A cross sectional study of adults aged 17–45 years showed that if there were one or more highly myopic parent, the odds ratios of mild (SE -1.25 to -2.5 D), moderate (SE -3.75 to -4.75 D), and high myopia (SE at least -5.0 D) were 2.5, 3.7, and more than 5.5, respectively. In the Orinda Longitudinal study of 716 children aged 6–14 years, children with two myopic parents had more myopic refractive error compared with children with one or no myopic parents. Few studies, however, have evaluated the effect of parental history, a possible marker of shared common family environment or genetic susceptibility, on biometric parameter changes.
In the Finnish children’s study, reading was significantly associated with the progression of myopia. To our knowledge, there are few studies that have evaluated the effects of reading on the change in axial length. Reading in books per week was not associated with greater increases in axial length in our study. The initial cross sectional analysis of SCORM data showed that children who read more and who had myopic parents had larger axial lengths. The lack of any significant effects of lifestyle and familial risk factors on longitudinal changes on axial length change could perhaps be attributable to the narrow age range of the population, the rather young cohort, relatively short follow up, misclassification of near work activity, or to the possibility that the role of near work differs between myopia onset and myopia progression.
The rapid progression of myopia in our study is accompanied by elongation of the eye in the axial dimension. The posterior segment changes in severely myopic eyes include the stretching of the retina and the development of optic disc crescents and other pathological changes. In Singapore children, corneal power appears to have a minor role in the progression of myopia, consistent with the pattern found in other longitudinal studies. In a 2 year follow up study of 142 Hong Kong children aged 6–17 years with refractive error from -7.5 to +3.5 D, the myopic shifts in refractive error were driven by increases in axial length and vitreous chamber depth. Contrary to our findings, the flattening of the cornea (but only in the horizontal meridian) was associated with the progression of myopia in Hong Kong children. Lens thinning does not explain the shift of refractions towards the myopic direction, and among adult clinical microscopists, this effect is also minimal.
The advantages of our study include the relatively large sample size, the low dropout rate, and the availability of longitudinal biometry parameters in a school based cohort in a myopia endemic country. The 3 year dropout rate was 18.4% and children who withdrew from the study were not significantly different from children who did not withdraw.
In summary, the 3 year increase in axial length is high (0.89 mm). Axial length changes are greatest in the first 2 years of the study and highest in younger children, females, and children with a parental history of myopia. Children who read more were not more likely to have greater changes in biometry parameters. The rate of progression of myopia was significantly associated with changes in vitreous chamber depth.
Conclusions:
The 3 year change in axial length of Singapore children aged 7–9 years at baseline was high and greater in younger children, females, and children with a parental history of myopia. Myopia progression was driven largely by vitreous chamber depth increase.
No comments:
Post a Comment