MYOPIA BURDEN

Myopia is currently considered to be an environmentally driven condition which develops in genetically susceptible individuals. As discussed by speakers at the 3rd World Congress of Paediatric Ophthalmology and Strabismus in Barcelona, Spain, research designed to identify the contributing genetic and environmental factors and how they interact will hopefully lead to effective strategies for myopia prevention or control.

“Nowadays one of the main goals in genetics research is the identification of possibly modifiable factors and the translation ‘from bench to bedside’. This is especially important for myopia considering its prevalence and importance as a cause of low vision,” said Fabiana D’Esposito MD, PhD, University of Naples Federico II, Italy.

“The real answers in complex traits such as myopia, however, probably lie in epigenetic mechanisms. Research in this way aims at understanding the effects of external stimuli in eyes genetically predisposed to develop myopia,” she added.

Dr D’Esposito noted that findings from familial and twin studies provide strong evidence that isolated (non-syndromic) myopia can be inherited.

However, it is a genetically complex trait with more than 20 associated genetic loci identified so far. The genes in those loci are involved in many different biological processes, including neurotransmission, ion channel function, extracellular matrix formation and stabilisation, retinoic acid metabolism, and ocular development. As suggested by authors of a study demonstrating that an individual’s genetic risk of developing myopia was influenced by educational level, individual genes may be differentially affected by different environmental factors.

“Speculating that not all classes of myopia-related genes contribute to the gene-education interaction, the investigators of this study hypothesised that neurotransmission-related genes that are expressed in the outer retina may be particularly vulnerable to the effect of retinal defocus,” Dr D’Esposito said.

“To make the scenario even more complicated, we need to consider that education means a large use of vision from near, which is more correlated to lifestyle. There are also studies associating intelligence and myopia, but there too it is not easy to make an objective evaluation of intelligence levels, as those can be interconnected with educational levels.”

THE OUTDOORS CONNECTION

Discussing environmental factors, Kathryn Rose PhD, University of Technology Sydney, Australia, said that aside from education, time spent outdoors is the only other major variable that has been consistently reported to affect myopia risk.

“Other reported environmental factors associated with myopia include urban dwelling and higher levels of intelligence, socio-economic status, and hours spent in near work and/or reading. However, these factors are likely to be intertwined with education and time spent outdoors, and whether they have an independent role remains to be teased out,” Dr Rose said.

The first evidence that time spent outdoors may affect the development of myopia rather than time spent in near work came from the Sydney Myopia Study (SMS). Examining the relationship between near work, outdoor activities and myopia prevalence, the SMS found that children having a low level of outdoor activity and high level of near work had the highest odds ratio for myopia, whereas children doing a high level of near work who had a high level of outdoor activity were relatively protected. Subsequently, analyses of longitudinal data from the Sydney Adolescent Vascular and Eye Study (SAVES) generated similar findings.

Findings from the Orinda Longitudinal Study of Myopia suggested that heritable myopia is not necessarily inevitable. In that study, the risk of a child becoming myopic decreased with increasing time spent playing sports, irrespective of the number of myopic parents.

The evidence from epidemiological studies on time spent outdoors and myopia risk led to controlled interventional trials which demonstrated increased outdoor time reduced incident myopia. In the Guangzhou Outdoor Activity Longitudinal Study, schools were randomly allocated to add 45 minutes of outdoor activity class at the end of each day or to follow their normal activities. After three years, the cumulative incidence of myopia was significantly less in the intervention arm. In a study in Taiwan, increasing outdoor time by 80 minutes per school day through recess outside the classroom significantly reduced the rate of new onset myopia after just one year.

Dr Rose noted that physical activity alone does not appear to account for the protective effect of increased time spent outdoors. Rather, being outdoors is what seems to matter, and the mechanism may involve exposure to higher intensity light stimulating retinal release of dopamine that inhibits axial elongation. Findings from animal studies are consistent with this hypothesis.

Dr Rose concluded her presentation by discussing a number of other factors which have been speculated to affect myopia risk. She said that many of them, including full correction of myopia, time spent watching television, vitamin D, and diet, can be dismissed either due to a lack of temporal association with the rise in myopia prevalence or lack of supporting evidence.

“However, body posture for reading and viewing distance are two issues that warrant further investigation,” Dr Rose added.

Fabiana D’Esposito: fdesposito@alice.it

Kathryn Rose: kathryn.rose@uts.edu.au