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Refining the aetiology of myopia through negative results
  1. C A McCarty
  1. Correspondence to: C A McCarty Marshfield Clinic Research Foundation, 1000 North Oak Avenue (ML1), Marshfield, WI 54449, USA; mccarty.catherinemcrf.mfldclin.edu

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Negative data help researchers to further refine the directions to identify the pathogenesis of myopia

Uncorrected or undercorrected refractive error is the leading cause of preventable vision impairment in the world today, with an estimated 75 million cases of low vision due to refractive problems.1 Because refractive error is a major cause of avoidable blindness, the World Health Organization has named refraction as one of the five priorities for Vision 2020—the Right to Sight.2 Not all blindness associated with refractive error is avoidable as high myopia can lead to choroidal atrophy and subsequent myopic retinal degneration.3

People with myopia are more likely than people with hypermetropia to have undercorrected refractive errors.4 Although debate continues about the relative contribution of genetics and environment in the development of myopia, there is general consensus that myopia has become more common3 and indeed meets the definition of an epidemic: “the occurrence in a community or region of ... health-related events clearly in excess of normal expectancy.”5 Some therapeutic interventions for myopia have been tested, but have been largely unsuccessful.3 A better understanding of the causal mechanisms in the development of myopia is necessary to identify strategies for primary prevention.

In this issue of the BJO (p 5), Lee and colleagues question the postulated role of intraocular pressure (IOP) in the pathogenesis of myopia, based on their results from a cohort study of children in Singapore. Given conflicting data in the medical literature, how does a busy ophthalmologist respond to questions related to these research results from patients and what is the public health significance of these data?

Causal criteria can assist scientists and clinicians in the evaluation of epidemiological associations for non-communicable diseases.6 Although not perfect, these causal criteria are useful for considering the importance and clinical relevance of findings from a single study in the context of previous research. The causal criteria include: (1) strength of association, (2) consistency of findings, (3) specificity, (4) temporality, (5) plausibility, (6) biological gradient (dose response), (7) coherence, (8) experimental evidence, and (9) analogy. One issue that makes it difficult to accurately synthesise research findings is publication bias, the tendency for negative study results not to be published.

The epidemiological criteria for causality can be used to consider Lee et al’s research findings and potential implications for education of patients with dissemination of these research results via the internet and/or media releases. Earlier studies have yielded conflicting data, and none has been able to adequately address the issue of temporality. Strengths of the current study include the standardised examination procedures, high response rate, and control of the major risk factor for myopia (genetics) by selection of an ethnically homogeneous population. The data displayed graphically in the figures show quite convincingly that not only is there no statistically significant relation between IOP and refraction or axial length, but also there is no suggestion of a non-significant trend or dose-response relation. These data tip the balance of consistency towards acceptance of the null hypothesis, at least for Chinese children. A meta-analysis may be in order to confirm this suggestion by statistically combining the results from all previous studies and would have been a useful addition to the current paper.

In summary, within the context of the causal criteria, the data from Lee et al do not support the use of ocular hypotensives to delay myopia progression in Chinese children. This same research group published negative results in 2001 resulting from their evaluation of the role of night lighting in the development of myopia in this same group of schoolchildren.7 We applaud the authors for reporting their negative data because negative data help researchers to further refine the research directions to identify the pathogenesis of myopia, a very important research area given the global epidemic of myopia and associated low vision.

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Negative data help researchers to further refine the directions to identify the pathogenesis of myopia

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