The Myopia Boom

The myopia boom

Short-sightedness is reaching epidemic proportions. Some scientists think they have found a reason why.
Elie Dolgin
18 March 2015
Nature - International Weekly Journal of Science

This article discusses recent research on myopia.  It also brings up the possible benefit of the use of 10 000 lux lamps indoors, given the potential role of some aspect of natural sunlight in controlling myopia.  The role of bright light is unclear, but it seems to be related to myopia in animal studies.

From the article:

"Based on epidemiological studies, Ian Morgan, a myopia researcher at the Australian National University in Canberra, estimates that children need to spend around three hours per day under light levels of at least 10,000 lux to be protected against myopia. This is about the level experienced by someone under a shady tree, wearing sunglasses, on a bright summer day. (An overcast day can provide less than 10,000 lux and a well-lit office or classroom is usually no more than 500 lux.) Three or more hours of daily outdoor time is already the norm for children in Morgan's native Australia, where only around 30% of 17-year-olds are myopic. But in many parts of the world — including the United States, Europe and East Asia — children are often outside for only one or two hours."

Link:

http://www.nature.com/news/the-myopia-boom-1.17120

The complex interactions of retinal, optical and environmental factors in myopia aetiology

D.I. Flitcroft
Progress in Retinal and Eye Research
Volume 31, Issue 6, November 2014, pages 622-660

Visual experience and the physical environment seem to have an impact in the development of myopia. Experiments and observations in animals have shown that the eye can become myopic in response to visual stimuli from the environment. It is less clear in humans, though people who work in submarines tend to have higher levels of myopia than other military personnel, while rural populations are less likely to be nyopic than urban populations, and higher levels of education (more time spent doing near work and indoors) appears to also be related to myopia development.


Also, this article points out the need to study myopia carefully considering the three-dimensionality of the environment and the eye, and the interaction of these two.


Because near and distance visual experiences indoors and outdoors may affect the eye differently, different corrective powers might be needed for different tasks.

From the abstract:

"Animal studies, observational clinical studies and more recently randomized clinical trials have demonstrated that the retinal image can influence the eye's growth. To date human intervention trials in myopia progression using optical means have had limited success but have been designed on the basis of simple hypotheses regarding the amount of defocus at the fovea. Recent animal studies, backed by observational clinical studies, have revealed that the mechanisms of optically guided eye growth are influenced by the retinal image across a wide area of the retina and not solely the fovea. Such results necessitate a fundamental shift in how refractive errors are defined. In the context of understanding eye growth a single sphero-cylindrical definition of foveal refraction is insufficient. Instead refractive error must be considered across the curved surface of the retina. This carries the consequence that local retinal image defocus can only be determined once the 3D structure of the viewed scene, off axis performance of the eye and eye shape has been accurately defined. This, in turn, introduces an under-appreciated level of complexity and interaction between the environment, ocular optics and eye shape that needs to be considered when planning and interpreting the results of clinical trials on myopia prevention."

From the text:
“Optical structure of the environment
Several early studies that predate the current interest in off axis refraction have pointed to a possible influence of the structure of the environment on eye growth in the form of lower field myopia, a phenomenon first described in pigeons by Catania (1964). In a more detailed analysis it has been shown that the refraction of pigeon eyes is essentially uniform off-axis in the superior visual field but shows progressive myopia up to 5 Dioptres which closely followed the geometric distance from the pigeon’s eye to the ground suggesting it is an adaptive phenomenon (Fitzke et al., 1985). Lower field myopia is a phenomenon that has now been described in a range of ground feeding bird species (Hodos and Erichsen, 1990) and amphibians (Schaeffel et al., 1994b) but not in raptor bird species that spend little time on the ground (Murphy et al., 1995). Young, 1961 and Young, 1963 demonstrated that rearing monkeys in highly restrictive visual environments led to myopia, a finding that was an extension of a much earlier study in German by Levinsohn published in 1919. Studies in chicks have also demonstrated that rearing in a low ceiling environment creates the inverse of lower field myopia with development of myopia in the superior field associated with local expansion of the inferior sclera (Miles and Wallman, 1990).”

“In humans, there is some limited evidence that the local environment influences refractive development. Time spent in restrictive environments such as submarines or underground ballistic missile installations has been found to be associated with increased rates of myopia as compared to military personnel in more usual working environments (Greene, 1970 and Kinney et al., 1980). Rural populations have been observed in epidemiological studies to have very low levels of myopia as compared to urban populations. For example school children in rural Melanesia have a reported myopia prevalence of only 2.9% (Garner et al., 1988) as compared to a myopia rate in Taiwanese cities of 12% at the age of 6 increasing to 84% by age 16–18 (Lin et al., 1999). Myopic prevalence in children has been correlated with increasing urbanization in both the far east (Yang et al., 2007 and Zhan et al., 2000), Greece (Paritsis et al., 1983) and Australia (Ip et al., 2008).
Intriguing recent results have demonstrated that time spent outdoors (Rose et al., 2008) is associated with refractive status. Children who spent little time outdoors and large amount of time on near-work activities were more likely to be myopic than the control group (odds ratio = 2.6; 95% CI, 1.2–6.0). Yet the group who were in the highest third for near work and highest third for outdoor activity had no significant increased myopia risk suggestive of a protective effect of time outdoors. The authors proposed light exposure as the most likely factor in preventing myopia progression. An alternative possibility that merits consideration… is that the three dimensional structure of the environment is an important factor due to the impact this has on the patterns of defocus across the retina.”

“An alternative or additional possibility for the impact of being outdoors on myopic progression may be the profound differences in the pattern of retinal defocus generated indoors and outdoors. On the basis of animal studies, sustained hyperopic defocus should promote local eye growth and myopia. Contrasting indoor scenes with outdoors reveals a marked increase in the level of hyperopic defocus for both near and distant fixation while indoors. Being outdoors may therefore be protective on the basis that it provides minimal amounts of peripheral defocus and hence may provide a so-called STOP signal for eye growth. Even if the human eye were responding only to the amount of blur and not its sign, the amount of blur across the retina and indeed its variation with eye movements is far less outdoors than indoors. This optical effect may indeed be further enhanced by the impact of light levels on pupil size which would be expected to be much smaller outdoors so light might interact with peripheral defocus via the depth of focus changes induced by pupil constriction.”

“The impact of the environment and its three dimensional structure have still to be taken into account in such designs. A specific lens design will have different impact on off-axis retinal blur when used for reading than for outdoors for example. This suggests that lenses designed for specific tasks may be more appropriate than a general-purpose lens. This may be a practical option for glasses but is less practical for contact lenses. A combination of contact lenses for general use with additional glasses that correct detrimental patterns of retinal defocus for specific tasks may prove to be a workable solution. “

“Incorporation of the full range of interactions detailed in this paper into optical strategies should provide a method of designing appropriate corrections for different tasks, viewing conditions and eye shapes.”

Link:
http://www.ncbi.nlm.nih.gov/pubmed/22772022

Relationship between changes in crystalline lens shape and axial elongation in young children.

Relationship between changes in crystalline lens shape and axial elongation in young chilren.

Ishii K, Yamanari M, Iwata H, Yasuno Y, Oshika T.

Department of Ophthalmology, Faculty of Medicine, University of Tsukuba, Ibaraki, Japan.

Invest Ophthalmol Vis Sci. 2013 Jan 28;54(1):771-7.

This study of 25 children highlights the interaction between the lens and the length of the eyeball in the development of myopia.  It's not just that the eyeball is longer; the lens also works differently in myopic children, affecting the length of the eyeball, and playing a role in the development and progression of myopia.

From the text:
"CONCLUSIONS:
Axial elongation is related to the entire contour of the crystalline lens. This result shows that axial elongation progresses in parallel to change in the crystalline lens shape."

Link to abstrract:
http://www.ncbi.nlm.nih.gov/pubmed/23307966

Other articles that highlight the role of the lens in the development of myopia:

How does nearsightedness develop in children?." ScienceDaily. ScienceDaily, 1 March 2012. http://www.sciencedaily.com/releases/2012/03/120301113258.htm

Myopia: It's Not Just the Length, but the Lens
Myopia is commonly described as having an eyeball that is “too long.” Not exactly.

By Colleen Mullarkey, Senior Editorhttp://www.revoptom.com/content/c/33495/x

Crystalline Lens Power in Myopia
GARNER, L F O.D., Ph.D.; YAP, M O.D., Ph.D.; SCOTT, R O.D., M.Sc.http://journals.lww.com/optvissci/Abstract/1992/11000/Crystalline_Lens_Power_in_Myopia.5.aspx