Category Archives: Summaries

Eye Growth and Sign of Defocus

The following article is presented to you by Najnin Sharmin
Disclaimer: The following text may content specific terms, requiring more in deep knowledge in the field.
  • Human Eye Growth

During birth to adulthood the human eye grows very little. The eye of a newborn is around 70% of the size of an adult and the growth is approximately 7.6 mm from birth to adulthood. The “Eye Socket” also grows with the eyeball. Different kind of variations can occur during the eyeball growth and this can cause optical errors shifting the location of the best focus within the eye. If the eye is too short in length, it will focus images behind the retina. This case is known as “Hyperopia” or far-sightedness. Difficultly in reading, headaches, eye strain, fatigue are some consequences of Hyperopia. On the other hand, If the eyeball grows too long, it will focus images in front of the retina. This case is known as “Myopia” or near-sightedness. Myopia also causes headaches, eye strain and squinting if not treated.

What is Myopia?

  • Accommodation

The human eye changes the optical power by altering the shape of its lens to focus objects at various distances, this mechanism is known as “Accommodation”. Young people can change the optical power by up to 15 dioptres by changing the ciliary body. Their eye can change focus from infinite distance to just 6.5 cm from the eye. But accommodation cannot shift images back in focus on the retina in myopic eyes. For a relaxed eye, the accommodation level is zero, when the power of the eye is 60 D. Accommodation and eye growth are intricately linked but not the same.

Risk Factors 

  • What is Sign of defocus

Like any other optical system, human eyes also suffer from aberrations.There are different kind of optical aberrations e.g. defocus, tilt, spherical aberration, astigmatism, coma, distortion etc. In optics, defocus is one kind of aberration in which an image is simply out of focus. High levels of axial aberration (spherical aberration) is responsible for night myopia. Moreover, low-order aberrations cause Myopia (positive defocus) and Hyperopia (negative defocus). One of many common techniques to measure eye aberrations is the Hartmann-Shack wavefront sensor (HS-WFS). It is comprised of a camera with an array of microlenses called “lenslets” mounted in or near to the camera.

The sign of defocus is very important for the rapid control of accommodation and also for regulating the slower long-term growth of the eye (1). Human eyes typically have a positive Spherical aberration (SA) when accommodation is relaxed. The amount of positive SA falls when the eye accommodates, vanishes with about 2 or 3 diopters (D) of accommodation, and grows steadily more negative with further accommodation of eye (2,3,4-8) because of the changes in eyeball shape (2,9) and refractive index distribution of the crystalline lens (10).

  • Retina alone detect the sign of defocus?

A fundamental question in emmetropisation (ideal vision) is – whether the retina by itself can perform the image processing necessary to derive the sign of defocus without any help from the brain?

An experiment on chicks shows that eye growth can be locally stimulated by local degradation of the retinal image, even after the optic nerve was cut. So, it was clear that the retina has at least the complete machinery to convert image features into growth signals (11).

When we talk about light absorption in the retina, we tend to consider the retina as a single surface. In general, the retina is a multilayered surface. According to the antenna model of outer-segment pigments, those retinal layers can be considered as layered circular discs (12). Each disc has around 4,000 (1 µm) to 4,000,000 (5 µm) pigment molecules that can absorb light. Each outer segment has approximately 1000 lamellae and the interspacing between the lamellae is approximately 20 nm. (12).

Fig.1 Photoreceptor outer-segment model.

In this retinal model, defocus symmetry is broken by the light acceptance of layered membrane infoldings –

Fig 2. Role of defocus seen in a cross section through the middle of the outer segments consisting an array of 19 hexagonally packed outer segments, when each segment has 1000 layers each containing 12 dipoles. The figure shows the light incidence near the (a) upper entrance and (b) far-end exit of a single outer segment (13).

The role of defocus for accommodation can be noticed from Fig. 2 suggesting that the best focus is obtained once the amount of light within the outer segment is maximized (13). Moreover, those stacked pigments may determine the sign of defocus.

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Myopia in Science!

  • References.
1) Thibos LN, Bradley A, Liu T, and Lo´pez-Gil N. Spherical Aberration and the Sign of Defocus. Optom Vis Sci 2013; 90:1284 –1291.

2) Young T. The Bakerian Lecture: on the mechanism of the eye. PhilTrans R Soc Lond 1801;91:23 – 88.

3) Tscherning MH. Physiologic Optics, 3rd ed. Philidelphia, PA:Keystone Publishing; 1920.

4) Ivanoff A. On the influence of accommodation on spherical aberration in the human eye, an attempt to interpret night myopia. J Opt Soc Am 1947;37:730.

5) Atchison DA, Collins MJ, Wildsoet CF, Christensen J, Waterworth MD. Measurement of monochromatic ocular aberrations of human eyes as a function of accommodation by the Howland aberroscope technique. Vision Res 1995;35:313–23.

6) Plainis S, Ginis HS, Pallikaris A. The effect of ocular aberrations on steady-state errors of accommodative response. J Vis 2005;5:466–77.

7) Lopez-Gil N, Fernandez-Sanchez V, Legras R, Montes-Mico R,Lara F, Nguyen-Khoa JL. Accommodation-related changes in mono-chromatic aberrations of the human eye as a function of age. Invest Ophthalmol Vis Sci 2008;49:1736–43.

8) Cheng H, Barnett JK, Vilupuru AS, Marsack JD, Kasthurirangan S, Applegate RA, Roorda A. A population study on changes in wave aberrations with accommodation. J Vis 2004;4:272–80.

9) Lopez-Gil N, Fernandez-Sanchez V. The change of spherical aberration during accommodation and its effect on the accommodation response. J Vis 2010;10:12.

10) Navarro R, Palos F, Gonza´lez LM. Adaptive model of the gradient index of the human lens. II. Optics of the accommodating aging lens. J Opt Soc Am (A) 2007;24:2911–20.

11) Schaeffel F. Can the retina alone detect the sign of defocus? Ophthalmic Physiol Opt 2013;33,362–367.

12) J. J. Wolken. Light detectors, photoreceptors, and imaging systems in nature. (New York, Oxford University Press, 1995).

13) Vohnsen B. Directional sensitivity of the retina: A layered scattering model of outer-segment photoreceptor pigments. BOE 2014;5:1569–1587.

Peripheral vision and myopia

The following article is presented to you by Petros Papadogiannis
Disclaimer: The following text may content specific terms, requiring more in deep knowledge in the field.
  • What is peripheral vision

Peripheral vision is the part of our vision that is outside the center of our gaze, and it is the largest portion of our visual field. For both eyes the combined visual field is 130°–135° vertical and 200°–220° horizontal with 180-200 degrees comprising the peripheral vision. It is weaker in humans than in many other species, and this disparity is even greater where it concerns our ability to distinguish color and shape. This is due to the density of the receptor cells on the retina and the enlargement of optical errors in the periphery. As a result, reduced visual acuity and contrast sensitivity occurs.

  •  Retinal shape and myopia

Myopic eyes have multiple variations on their retinal shape. This phenomenon is related to the potential models of retinal stretching that occurs during axial elongation. The picture below represents the 4 models of retinal stretching that can occur in myopia. The solid circles represent the shape of the retina of an emmetropic eye, the dashed shapes represent the myopic retinas, and the arrows indicate the regions of stretching. (1,2)
It was found that despite the existence of myopia in both the central and peripheral retina, myopic error in the periphery is smaller. (1)
Also, in 2009 Tabernero and Schaeffel found that myopes (even those with medium refractive error) appear to have more irregular shape than emmetropes, on the peripheral retina. (8)

Eye expansion.
Source: Eye shape and retinal shape, and their relation to peripheral refraction, OPO 2012
  •  What do animal studies show?

Animal studies have shown that the peripheral retina can trigger or stop the growth of the eye depending on the location of the peripheral image relative to the retina. When an image is focused on the central retina and for the peripheral retina, the image is focused behind, this results in a relatively hypermetropic periphery and a defocused image. This defocused image sends a growing signal to the eye and makes the eye myopic.
By their experiments in laboratory animals, Smith et all found that visual signals from the peripheral retina can dominate against the visual signals from the central retina in terms of regulation of eye’s refractive status. (3)
The concept that dominates is that cones are more involved than rods(they are located in the peripheral retina) in the detection of visual signals that contribute to eye growth. But a study of 2010 in mice, shows that rods are important for the detection of the signals that are involved in the procedure of emmetropization and the development of myopia.(4)

  •  Does peripheral refractive status affect the onset and progression of myopia?

A number of studies in humans, have shown that peripheral refractive errors are ante-dated to the onset of central myopia and can, therefore, be a risk factor for the onset and progression of myopia.
In a 1971 study in young trainee pilots, Hoogerheide found that emmetropes with peripheral hypermetropic refraction had greater possibilities to develop myopia, compared to emmetropes that appeared to have myopic astigmatism in the periphery. (5)
More recently, Schmid (2011) verified an important association between the greater steepness of the retina (more prolate eye shape) and the central myopic shift in children.(6)
On the other hand, Mutti in 2011 didn’t manage to verify the influence of peripheral hypermetropia in the onset of myopia. Particularly, despite the fact that he found a correlation between the magnitude of the peripheral hypermetropia and myopia progression, the total influence of peripheral hypermetropic state in central refraction was limited. (7)
To conclude with, although the hypothesis that a relatively hypermetropic periphery can drive the development of human myopia remains unproven, the existing research support the possibility of an interaction between the states of focus on axis and in the periphery.

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Myopia in Science!

  • References.
1) Pavan K Verkicharla,Ankit Mathur,Edward AH Mallen,James M Pope,David A Atchison. Eye shape and retinal shape, and their relation to peripheral refraction. OPO 2012; 32: 184–199
2) Strang NC, Winn B & Bradley A. The role of neural and optical factors in limiting visual resolution in myopia. Vision Res 1998; 38: 1713–1721.
3) Earl L. Smith. The Charles F. Prentice Award Lecture 2010: A Case for Peripheral Optical Treatment Strategies for Myopia Optom Vis Sci. 2011 September ; 88(9): 1029–1044
4) S. B. Jabbar; A. E. Faulkner; G. F. Schmid; F. Schaeffel; J. Abey; P. M. Iuvone; M. T. Pardue. Rod Photoreceptor Contributions to Refractive Development and Form Deprivation Myopia in Mice. Investigative Ophthalmology & Visual Science 2010; 51: 1726
5) Hoogerheide J. · Rempt F. · Hoogenboom W.P.H. Acquired Myopia in Young Pilots. Ophthalmologica 1971;163:209–215
6) Schmid GF. Association between retinal steepness and central myopic shift in children. Optom Vis Sci. 2011 Jun;88(6):684-90.
7) Donald O. Mutti; Loraine T. Sinnott; G. Lynn Mitchell; Lisa A. Jones-Jordan; Melvin L. Moeschberger; Susan A. Cotter; Robert N. Kleinstein; Ruth E. Manny; J. Daniel Twelker; Karla Zadnik Relative Peripheral Refractive Error and the Risk of Onset and Progression of Myopia in Children Investigative Ophthalmology & Visual Science.2011;52:199-205
8) Juan Tabernero; Frank Schaeffel. More Irregular Eye Shape in Low Myopia Than in Emmetropia. Investigative Ophthalmology & Visual Science.2009;50:4516-4522.

The effects of outdoors on Myopia

The following article is presented to you by Pablo Sanz and Miguel García
Disclaimer: For all the general public and specialists, some technical knowledge might be required.

Let en-light our blog, pick our sunglasses and let´s talk about the influence of outdoor time on the onset, development as well as progression of myopia. Besides, as far as 100 years ago (1), some studies started to conjecture about ambient light and its impact on the development of the eye. Starting to be considered as plausible public action to stop myopia prevalence increase, especially in those areas with high risk of development such as East Asia, the topic triggered interest again.

For more in-depth treatment of the issue of outdoors effect we should keep in mind different terms such as time exposure and light intensity, because many factors could contribute to this “shielding effect“.

During the last years a large number of research studies investigated the hypothesis that time spent outdoors protects against the development and progression of myopia.

Since the beginning of this hypothesis, all researches pointed to this direction. Earlier, it was shown in chickens (2) and children that ambient light plays an important role at compensation of myopic defocus and onset of myopia. While at early stages in humans, it was though that physical activity could have a major input, Rose et al (3) showed that light conditions where the key.

To get a better overview on this matter we should introduce the sentence scientific evidence.

  • But what´s evidence?

In a scientific environment, there is no place for believes, and the evidence relies in the studies published and their repeatability. If we want to grade the evidence they give, we do so according to the type of article, as following pyramid illustrates.

Evidence piramyd
Fig 1. Pyramid of evidence

As pointed out by the pyramid, meta-analysis are the highest source of evidence in science. And a recent meta-analysis from Xiong et al, 2017 (4), analyzed over 25 studies and they concluded that time outdoors prevent the development, but has no effect on slowing progression of eyes that are already myopic.

Other studies that looked into the possible use of longer outdoor hours to prevent myopia (5) as public policies, concluded that an extra hour could have greater impact on the onset and development of myopia in children between 5 to 8 years. Similar recommendation were given by He et al 2015,(6) where they claimed that 45 min of outdoor activities for schools in China could prevented myopia onset.

“Although research about understanding the exact mechanism is still underway, based on current results approximately 3 hours of outdoor activity during a day may be considered protective against myopia.”

– Verkicharla, 2016 (7)

Continue reading The effects of outdoors on Myopia

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
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.


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)

‘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.