The contemporary definition of myopia is a disease based on the latest scientific research. Researchers indicate that myopia is an independent risk factor for glaucoma, cataracts, choroidal neovascular membranes and retinal detachment. In pathological myopia, there is three times greater risk for early onset cataracts, eight times greater risk for myopic macular degeneration, retinal holes and tears, and an eighteen times greater risk for glaucoma [1].
Significantly, the higher the myopia, the higher the risk. Myopia affects 1.6 billion people worldwide [2]. Greater than 80% of the population in Hong Kong, Taiwan, and Singapore are myopic [2]. In the US, 42% of the total population and 25% of children are myopic [2]. The incidence of myopia is rapidly increasing and it is estimated that 50% of the population will be myopic by 2030 [2].
Researchers have affirmed that there is no safe level of myopia. Risk factors for myopia include race, genetics, parents with myopia, the environment, time performing near work and time spent outdoors. Additionally, through genome-wide linkage analysis, a mutation in exon 10 of the gene LEPREL1 has been shown to cause myopia [3].
In the Avon Longitudinal Study of Parents and Children (ALSPAC) study of 2833 participants age 2 to 15 years old, greater time outdoors was associated with a reduced risk of incident myopia from age 3 onward. Additional time outdoors from age 3 – 9 years of age was associated with a reduced incidence of myopia between ages 10 -15 years old [4].
There are several theories why time outdoors reduces the incidence of myopia. According to multiple human and animal model studies of myopia, it is actual light exposure, not just a decrease in reading time that decreases the incidence of myopia [5]. One theory believes that exercise is protective and when children are outdoors, they exercise more. Another theory is that direct light exposure release vitamin D which may inhibit eye growth and the onset of myopia [5]. A different theory is that light is brighter outdoors and slows abnormal myopic eye growth [5].
Yet another postulation is that pupil constriction in brighter light may result in a greater depth of focus. The prevailing theory is that light outdoors which is bright stimulates a release of dopamine from specialized cells in the retina. Dopamine then initiates a molecular signaling cascade that concludes with normal eye growth, without the prevalence of myopia [5].
Which theory do you believe is correct?
References:
- Tano Y. Pathologic myopia: where are we now?. Am J Ophthalmol. 2002 Nov;134(5):645-60.
- Holden B, Fricke T, Wilson D. Global Prevalence of Myopia and High Myopia and Temporal Trends from 2000 through 2050. Ophthalmology 2016.
- Mordechai S, Gradstein L, Pasanen A. Am J Hum Genet. 2011 Sep 9;89(3):438-45. doi: 10.1016/j.ajhg.2011.08.003. Epub 2011 Sep 1.
- Shah RL, Huang Y, Guggenheim JA. Time Outdoors at Specific Ages During Early Childhood and the Risk of Incident Myopia. Invest Ophthalmol Vis Sci. 2017 Feb 1;58(2):1158-1166. doi: 10.1167/iovs.16-20894.
- Zadnik, K, Mutti D. Why your kids might be able to see better if they play outdoors more often. The Conversation. September 2017.
