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Myopia: Is the nature-nurture debate finally over?

Summary


Increased time outdoors may help protect children from developing myopia

Rapid increases in the prevalence of myopia (nearsightedness) have generated considerable concern and research over the past several decades. This increase has been especially noted in East and Southeast Asia, where the prevalence of myopia in young adults has increased from around 20-30% of the population to 80-90% over one to two generations. The concern of increasing myopia, however, isn’t restricted to Asia. Some projections suggest that as much as 50% of the world population will be myopic by the year 2050, with around 10% being highly myopic. Academic discussions about what causes myopia generally include debates over the role of nature (genetic influences) and nurture (environmental factors). The rapid increase in the prevalence of myopia indicates that more than genetics is involved, as gene pools cannot change that fast. Today, myopia geneticists generally acknowledge that environmental factors play a major role in the rapid increase in the prevalence of myopia.  This understanding has important implications: If “most myopia in a population depends on environmental exposures, then varying those environmental exposures provides an obvious approach to prevention.”

The aim of this review was to “trace the history of the nature-nurture debate on myopia, outline the basis of the idea that myopia is under tight genetic control, with little place for environmental factors, and explain how exaggerated interpretations of the genetic evidence led to incorrect conclusions.” More wide-spread schooling, at one point, was considered to be a factor in the increase of myopia. This thinking was based on observations of people with more education being more likely to be myopic. Intervention initiatives focusing on adjustments to print size and style and to differing light exposures proved to be ineffective in reducing the likelihood of being myopic. Yet, the belief that increased exposure to print can lead to myopia lives on.

Debate continues, however, on the role of light intensity in the development of myopia. Recent research adds support to the idea that exposure to bright light may be a protective factor in the development of myopia. This idea is supported by a large body of research indicating that more time outdoors may help protect children from the development of myopia through regulation of dopamine, which affects the development of the shape of the eye.

Understanding the causes of increased prevalence of myopia is complicated by the fact that myopia is “aetiologically heterogeneous” – meaning that there are diverse causes and forms of the condition.  One form of myopia is referred to as “school myopia”. This form “is strongly influenced by environmental factors such as exposure to schooling and the amount of time spent outdoors.” Other forms of myopia are strongly influenced by genetics.  Evidence suggests that the form referred to as “school myopia” is the type that has increased dramatically in certain populations, including children in East and Southeast Asia. Research provides little evidence of gene–environment interactions making major contributions to differences in myopia between ethnic or social groups within populations.

“The authors of this review believe that the current evidence, now soundly based on modern epidemiological and genetic analysis, largely puts the long argument over nature and nurture, or genes and environment, to rest. Environmental factors can have a major impact on the development of myopia.” These findings indicate that preventive measures would do well to target environmental rather than the genetic factors. Increasing time outdoors may be one way to protect children from developing myopia.

Citation

Morgan, I.C., Rose, K.A., (2019). Myopia: Is the nature-nurture debate finally over?. Clinical and Experimental Optometry, 102(1), 3-17.

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