August 2019

REFRACTIVE

Presentation spotlight
Hard-wired for myopia?


by Stefanie Petrou Binder, MD EyeWorld Contributing Writer

Myopia is thought to be passed on by a combination of genetic and environmental factors, although which one is more influential for the condition isn’t decided. Speaking on epidemiology and the genetics of myopia at the 36th Congress of the European Society of Cataract and Refractive Surgeons, Caroline Klaver, MD, shed some light on this debate of nature versus nurture.
The estimated prevalence of myopia among 20-year-olds in Asian populations is 80–90%, and 20% have high myopia with risk of ocular pathology.1 In Europe, there is a 50% prevalence of myopia among the same age group.2 There are indications these numbers are rising.
“Myopia develops in childhood and teenage years,” Dr. Klaver said. “Onset before 13 years of age means that the individual is much more likely to become highly myopic (at least –6 D). This is concerning because one out of three high myopes will develop a severe visual impairment in their lifetime.”
High axial length has been associated with a lifelong risk of bilateral visual impairment and blindness. “Studies show that eyes with an axial length over 26 mm are very prone to develop visual loss in both eyes, while axial lengths over 30 mm incur a 95% lifetime risk of developing visual impairment or blindness,” Dr. Klaver said.

Genetics of refractive error and myopia

Genetics play a strong role in myopic development. Myopia is not determined by any single gene, rather a combination of many different genes affect both refractive error and myopia.
Dr. Klaver was involved in a genome-wide association meta-analysis that implicated light-induced signaling as a driver for refractive error. The meta-analysis in 160,420 participants and replication in 95,505 participants increased the number of established independent signals from 37 to 161 genetic loci. The study showed a high genetic correlation between Europeans and Asians (p>.78).3
The meta-analysis’ expression experiments and comprehensive in silico analyses identified retinal cell physiology and light processing as prominent mechanisms. It also identified functional contributions to refractive error development in all cell types of the neurosensory retina, retinal pigment epithelium, vascular endothelium, and extracellular matrix. Newly identified genes implicated novel mechanisms such as rod-and-cone bipolar synaptic neurotransmission, anterior segment morphology, and angiogenesis. The study results support the notion that refractive errors are caused by a light-dependent retina-to-sclera signaling cascade and delineate potential pathobiological molecular drivers.
“When you combine all of this genetic data into a genetic risk cohort, you see that persons with a high genetic risk score are 40-times more likely to develop high myopia than those who have a low genetic risk score. However, the genetics don’t do very well in terms of prediction,” Dr. Klaver said.
Analyses have demonstrated a large genetic overlap between Europeans and Asians; an 80% genetic effect correlation was observed in a genome-wide association study of corneal astigmatism,4 with an 88% genetic impact correlation. Another analysis showed a 79% genetic effect correlation and an 87% genetic impact correlation between European and Asian populations.3
“This genetic data shows that although Asians are more prone for myopia, their myopia is not genetically determined. Actually, they have a somewhat lower genetic risk than European populations do, and overall the overlap in the genes is about 90%,” Dr. Klaver said.

Lifestyle factors

How you deal with a genetic predisposition to myopia may make a big difference. Studies have shown that lifestyle in children is an important risk factor.
One study says that in economically developed societies, myopia tends to appear earlier rather than later, during the school years of childhood, and notes a rapid, environmentally induced change in the prevalence of myopia associated with increased education and urbanization.5
“Education is the strongest classical risk factor. We know that when you go to university, you have about a 60% risk of developing myopia,” she said.
According to the outcomes of a recent birth cohort study on the socioeconomic influence on myopia in 5,711 children aged between 6 and 9 years that clocked activities like playing outside, biking to school, walking to school, watching TV, computer use, outdoor/indoor activities, and playing sports, myopes spent more time indoors, had lower vitamin D levels, and spent more hours on near work.6
“Overall, studies have shown that the more time spent outdoors per week or per day, the lower the risk of myopia becomes. The best advice would be for children to spend 2 hours or more outside per day. I like the 20–20–2 rule of thumb: 20 minutes of near work, 20-second break to focus on something far in the distance, 2 hours of outdoor exposure per day,” Dr. Klaver said.
Healthy smartphone use is being studied, with inconclusive data thus far. One current study monitors smartphones with an app that measures data such as light conditions, how long kids are on their phones, and how much continuous use.

About the doctor

Caroline Klaver, MD
Erasmus University
Medical Center
Department of Epidemiology
Rotterdam, Netherlands

References

1. Morgan IG, Rose KA. Myopia: is the nature-nurture debate finally over? Clin Exp Optom. 2019;102:3–17.
2. Cooper J, Tkatchenko AV. A review of current concepts of the etiology and treatment of myopia. Eye Contact Lens. 2018;44:231–247.
3. Tedja M, et al. Genome-wide association meta-analysis highlights light-induced signaling as a driver for refractive error. Nat Genet. 2018;50:834–848.
4. Shah RL, et al. A genome-wide association study of corneal astigmatism: The CREAM Consortium. Mol Vis. 2018;24:127–142.
5. Morgan I, Rose K. How genetic is school myopia? Prog Retin Eye Res. 2005;24:1–38.
6. Tideman JWL, et al. Environmental factors explain socioeconomic prevalence differences in myopia in 6-year-old children. Br J Ophthalmol. 2018;102: 243–247.

Financial interests

Klaver: None

Contact information

Klaver
: c.c.w.klaver@erasmusmc.nl

Hard-wired for myopia? Hard-wired for myopia?
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