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Risk factors for Myopia Development

The following article is presented to you by Pablo Sanz and Miguel García
Disclaimer: For all general public, some elements may require deeper knowledge.

As noted earlier, Myopia is one of the world leading causes for visual impairment, but what is the aetiology ?  What is the background facts involving its development?

What is Myopia?

Since the earliest studies on myopia, several theories about its etiology have been enunciated, but nowadays we can mainly  forge them into two flowlines: genes (1,2) and environment (3).

Scientific data and experience indicate that there is a very complex mixture of factors and both lines seems to be important.

However, there is no clear answer to why myopia is developed. Only in the last decades, there is an increased understanding about development and onset of myopia, that is, more detailed knowledge of what, how, when, where and why myopia develops.


Simplifying, the answers to these questions may be found in the following factors:

  • Genetic factors

There is a genetic predisposition to myopia in some individuals but it often requires of environmental help to being developed. Higher concordances in myopia prevalence have been found between monozygotic twins than in dizygotic ones, and even more linked than child-parent relationships.(4) Furthermore, it appears that Han ethnicity is more prone to develop Myopia. (5)

However, as genetic factors can take a large shot, we would explain them in deep in another article.

Genetic Myopia (Not available yet)


  • Environmental Factors
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Eskimo reading Saturday Evening Post in the Arctic region.
  • Educational level

The educational level have been correlated with myopia quite soon. Already in 1892, Hermann Cohn stated that the prevalence of myopia is related to the educational level (6).

Following this trend, we can find this study from Morgan et al, in Inuits, that reveals the same correlation.(7)

This educational level by itself, is not the original cause of myopia, and we should discern between the conglomerate factors that define it. Which seems to be related with more myopia development, as proposed, more indoor activity, near work activities…(8)

  • Near work and eye accommodation

It has been described that myopic children show higher degrees of accommodative lag (you are focusing on one object, but your eyes actually focus on a point behind it). This accommodative mismatch produces hyperopic retinal blur, which could provide a stimulus for myopic eye growth. (9)

There are still many unanswered questions and details to resolve about myopia-accommodation link. For this reason it is necessary to carry out more longitudinal and randomized research trials to confirm that near work is a real risk factor for myopia development. (10)

Accommodation and Myopia (Not available yet)

  • Peripheral refraction

Animal studies have shown that the peripheral retina plays an important role in determining eye growth, moreover there appears to be different theories (11) about how it works in humans, for more information, refer to the following article.

The peripheral refraction 

  • Outdoor exposures

Several studies have reported the association between outdoor time and lower likelihood of myopic refraction. The main idea behind this factor is the amelioration of myopia development due to the high levels of light (pupil construction, increased depth of focus, increased dopamine release).(12)

Outdoor and myopia 

Following this trend of time spend outdoors, several authors claimed a relationship between Urban vs Rural and population density, where rural lifestyle with more time outdoors can be related to less myopic prevalence. Similar effect has been described to physical activity, the most the better to avoid myopia, but before claiming them to be truly related factors, a deeper understanding is required of outdoors as can be not risk “per se” if not only incentives for more outdoor time.(13)

  • Others

The premature children are more predispose to develop  refractive errors, such as Myopia.(14), and seems to have shallower anterior chambers.


As you have seen, myopia is a complex trait: several variables, factors and small details are involved on its development.

  • Discussion

A small proportion of myopia are clearly inherited. These appear at an early age, and reach high values. However, the most common myopia occurs at school age, and does not reach these high values. In this type of nearsightedness, it seems that there may be a small genetic contribution, but environmental factors seem to be the most important, and this is what is contributing to the increase in myopia worldwide.

Based on the scientific findings, currently, myopia control is focused taking all these factors into account: genetics, outdoor exposure, new optical designs, pharmaceutical agents, etc.


Remember…

Stay up-to-date, Keep on reading and…

Myopia in Science!

  • References.
(1) Goldschmidt E, Jacobsen N. Genetic and environmental effects on myopia development and progression. Eye. 2014;28(2):126-133. doi:10.1038/eye.2013.254.

(2) Mohamed Dirani, Matthew Chamberlain, Sri N. Shekar, Amirul F. M. Islam, Pam Garoufalis, Christine Y. Chen, Robyn H. Guymer, Paul N. Baird; Heritability of Refractive Error and Ocular Biometrics: The Genes in Myopia (GEM) Twin Study. Invest. Ophthalmol. Vis. Sci. 2006;47(11):4756-4761. doi: 10.1167/iovs.06-0270.

(3) Ramessur R, Williams KM, Hammond CJ. Risk factors for myopia in a discordant monozygotic twin study. Ophthalmic & Physiological Optics. 2015;35(6):643-651. doi:10.1111/opo.12246.

(4) Rong SS, Chen LJ, Pang CP. Myopia Genetics—The Asia-Pacific Perspective. Asia-Pacific J Ophthalmol. 2016;5(4):236-244. doi:10.1097/APO.0000000000000224.

(5) Chin MP, Siong KH, Chan KH, Do CW, Chan HHL & Cheong AMY. Prevalence of visual impairment and refractive errors among different ethnic groups in schoolchildren in Turpan, China. Ophthalmic Physiol Opt 2015; 35: 263270. doi: 10.1111/opo.12193

(6) Schaeffel F. Myopia V What is Old and What is New ? 2016;93(9):1022-1030. 

(7) Morgan RW, Speakman JS, Grimshaw SE. Inuit myopia: an environmentally induced “epidemic”? Canadian Medical Association Journal. 1975;112(5):575-577. 

(8) Morgan IG, Rose KA. Myopia and international educational performance. Ophthalmic Physiol Opt 2013; 33: 329338. doi: 10.1111/opo.12040 

(9) Gwiazda J, Thorn F, Bauer J, Held R. Myopic children show insufficient accommodative response to blur. Invest Ophthalmol Vis Sci. Mar 1993;34(3):690-694.

(10) Huang HM, Chang DST, Wu PC (2015) The Association between Near Work Activities and Myopia in Children—A Systematic Review and Meta-Analysis. PLOS ONE 10(10): e0140419. doi: 10.1371/journal.pone.0140419

(11) Atchison DA, Rose R. The Possible Role of Peripheral Refraction in Development of Myopia. 2016;93(9):1042-1044. doi:10.1097/OPX.0000000000000979.

(12) Ngo C, Saw SM, Dharani R, Flitcroft I. Does sunlight (bright lights) explain the protective effects of outdoor activity against myopia? Ophthalmic Physiol Opt. 2013;33(3):368-372. doi:10.1111/opo.12051.

(13) Guggenheim JA, Northstone K, McMahon G, et al. Time Outdoors and Physical Activity as Predictors of Incident Myopia in Childhood: A Prospective Cohort Study. Investigative Ophthalmology & Visual Science. 2012;53(6):2856-2865. doi:10.1167/iovs.11-9091.

(14) Quinn GE Dobson V Kivlin J . Prevalence of myopia between 3 months and 5 1/2 years in preterm infants with and without retinopathy of prematurity: Cryotherapy for Retinopathy of Prematurity Cooperative Group. Ophthalmology. 1998;105(7):1292–1300

What is Myopia?

The following article is presented to you by Pablo Sanz and Miguel García
Disclaimer: For all the general public, not technical knowledge is required.

Originally described by the ancient Greeks as “myopos”, this condition makes reference to how the myopic people squint their eyelids to create a sharp and clear image.

But what´s myopia? And why they do this characteristic grimace? Let us begin with how the image is formed into the eye.

In a normal eye, the ray-lights coming from far distance pass through the different mediums inside the eye creating a sharp point on the retina.

On the other hand, if we are dealing with any error condition, we have a mismatch between this focused image and the retina, where our light receptors reside. Both ray schemes are shown below:

Raylight image formation

Note that this is an easy approximation, while even in the best ideal case, a point refracted by crossing a circle(pupil) is not a point. Far from this, due to the eye aberrations, this merged point is always a stain/blur.

So, myopia also known as short-sightedness or nearsightedness , constitutes a failure of matching image formation and receptors location, being the image formed in front of them as shown in the right scheme.

Using a more accurate definition, myopia has been defined as a common optical aberration of the eye in which the conjugate focus of the retina is at some finite point in front of the eye, when the eye is not accommodating.

Some signs related to myopic subjects are primarily: blurred distance vision or out of focus image and deteriorated vision in low lighting conditions while better vision at short distance than hyperopes.


*Why they squint their eyes? With this characteristic gesture they are simulating a pinhole, reducing the amount of rays arriving to the eye and generating an artificial pupil of smaller size. This pupil reduces the size of the blur on the retina but also the amount of light and the area you can see.


As this post is supposed to be an easy explanation on myopia, further comprehension about aberrations of a myopic eye can be found in other entries:

Myopia and eye aberrations (Not available yet)

This refractive error is caused by many factors, but basically due to elongation of the eye.


  • Which is the prevalence of myopia?

Now, knowing what this refractive error is, you might be curious to know the predominance of this eye condition around the world.

Nearsightedness has been estimated that affects 1.6 billion people worldwide and during the year 2020, 2.5 billion people would be affected by myopia. (1)(2)

1-s2-0-s0161642015002808-gr5
‘Myopia Prevalence, based in birth and educational level in Europe, Meta-Analysis’ Click in the image for more information.

Furthermore, the prevalence of myopia varies with age and other factors: genetic, ethnicity, geographic location, environmental factors, lifestyle, etc. As you can found in the following post:

Risk Factors 

Current recommendations for Myopia (Not available yet)

Typically appears between 6 and 12 years of age, and the mean rate of progression is approximately 0.50 Diopters per year, based on studies of mostly Caucasian children. (3, 4)

The prevalence of myopia is approximately 25% in the western population and much higher (70% to 90%) in different regions of Asia (5, 6).

Among persons the prevalence is about 35% to 40% in their 20s to 40s and decreases to about 15% to 20% among those in their 60s, 70s, and 80s. (7, 8)

All these values led us to think that this optical condition is emerging as a major public health concern, generating an economic burden for each individual with myopia that does not have to go unnoticed. (9) For this reason, the research myopia field is focused on understand and reduce all the factors that may produce the increase and development of myopia.


Stay up-to-date, Keep on reading and

Myopia in Science!

  • References.
(1) Kempen JH, et al. The prevalence of refractive errors among adults in the United States, Western Europe, and Australia. Arch Ophthalmol. Apr 2004;122(4):495-505.

(2) Holden BA, et al. Global Prevalence of Myopia and High Myopia and Temporal Trends from 2000 through 2050. Ophthalmology. 2016 May;123(5):1036-42.

(3) Jensen H. Myopia progression in young school children and intraocular pressure. Doc Ophthalmol 1992;82:249-55.

(4) Parssinen O, Hemminki E, Klemetti A. Effect of spectacle use and accommodation on myopic progression: final results of a three-year randomised clinical trial among schoolchildren. Br J Ophthalmol 1989; 73:547-51.

(5) Shortt AJ, Allan BDS. Photorefractive keratectomy (PRK) versus laser-assisted in-situ keratomileusis (LASIK) formyopia. Cochrane Database Syst Rev 2006; (2):CD005135 3. Dirani M, Islam FMA, Baird PN. The role of birth weight in myopia – the Genes in Myopia twin study. Ophthalmic Res 2009; 41:154–159

(6) Saw SM, Tong L, Chua WH, Chia KS, Koh D, Tan DT et al. Incidence and progression of myopia in Singaporean school children. Invest Ophthalmol.Vis.Sci. 2005; 46:51-7.

(7) Vitale S, Ellwein L, Cotch MF, et al. Prevalence of refractive error in the United States, 1999-2004. Arch Ophthalmol 2008; 126:1111-9.

(8) Katz J, Tielsch JM, Sommer A. Prevalence and risk factors for refractive errors in an adult inner city population. Invest Ophthalmol Vis Sci 1997; 38:334-40.

(9) Zheng YF et al. The economic cost of myopia in adults aged over 40 years in Singapore. Invest Ophthalmol Vis Sci. 2013 Nov 13;54(12):7532-7.