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HomemistoryThe March of Myopia

The March of Myopia

The incidence of global myopia is escalating sharply. People with high myopia, generally classified as greater than -5 dioptres, are at increased risk of retinal detachment, cataracts, glaucoma and blindness. With almost 20 per cent of university-aged people in East Asia now in this category, half of these people are expected to develop irreversible vision loss. The economic consequences are enormous.

Worldwide, myopia is the most common refrractive error, affecting more than 1.44 billion people – over one-fifth of the world’s population. Its prevalence rose sharply late last century with continued unprecedented growth reported this century.

In Asia in particular, myopia affects up to 90 per cent of teenagers, up significantly from 60 years ago when it was between 10 and 20 per cent. In urban parts of Korea, an incredible 96.5 per cent of 19-year-old men are myopic. In Europe and the US, rates have doubled in half a century. Here in Australia, the incidence of myopia appears to be more stable, currently quoted at around 17 per cent.

With rates of myopia and high myopia increasing, the total burden of this disease is increasing. People with high myopia, generally classified as greater than -5 dioptres, have proportionally longer axial length and resultant thinning of the inner parts of the eye. As a result they are at increased risk of retinal detachment, cataracts, glaucoma and blindness. With almost 20 per cent of university-aged people in East Asia now in this category, half of these people are expected to develop irreversible vision loss. The economic consequences associated with this are enormous.

As myopia levels rise, so too have the levels of severe myopia, with the elevated risk of vision-threatening consequences

The alarming increase in this vision-threatening condition has prompted a steep rise in research to determine the causes and possible interventions that might reduce the ‘march of myopia’.

Understanding Myopia

Myopia is a highly heritable trait. Environmental factors are also known to play a key role with levels of education, cognitive ability, time spent outdoors, urbanisation, pre-natal factors and socioeconomic status
all potential risk factors.

Our refractive status is dynamic throughout our lifespan. Newborns are usually hyperopic and with age myopia increases as a result of changes to the eye. These changes include increased axial elongation, thinning of the lenses and flattening of the cornea. Growth in axial length is fastest in the year prior to onset of myopia. Some studies have shown that peripheral hyperopia stimulates axial elongation in patients with myopia. Myopia has been associated with low birth weight. Some studies showed an association between higher height and myopia but the links were weak.

Ethnicity

The incidence of myopia differs depending on ethnicity. One study of Singaporean males found myopia prevalence higher for Chinese (48.5 per cent) compared to Eurasians (34.7 per cent), Indians (30.4 per cent) and Malays (24.5 per cent).

Interestingly, closer to home, a study by a prominent Australian orthoptist Kathyrn Rose, found that children of Chinese origin living in Sydney had a much lower prevalence of myopia than Chinese children living in Singapore (3.3 per cent vs. 29.1 per cent). The difference in myopia incidence was attributed to the amount of time spent outdoors, with Sydney children spending 13.75 hours per week outside compared to 3.05 hours for their Singaporean counterparts.

Bookworms Need Glasses

Researchers have documented a strong association between levels of education and prevalence of myopia. More than 400 years ago Johannes Kepler noted that short-sightedness was more common in young people doing a lot of close work.1 In the 1860s a German pathologist, Rudolf Virchow, identified myopia as the single most harmful effect resulting from school attendance.2 Virchow’s conclusions were based on the work of ophthalmologist Dr. Hermann Cohn, who measured the eyesight of 10,060 school children, which showed that both the incidence and the degree of myopia increased with the number of years at school.

Many subsequent studies have shown that higher levels of education are associated with increased incidence of myopia. Late in the 19th century higher levels of outdoor activity were linked to lower levels of myopia.3 The nature vs. nurture debate has swung back and forth over the past century with school environment, including factors such as light levels, time spent outside and amount of close work, believed to be important factors in the development of myopia.

Light Levels

In the 19th century Cohn also examined light levels in classrooms. He found that children in classrooms in narrower streets and on lower floors of buildings had the highest levels of myopia. The rates of myopia were found to be four to five times higher in towns than in village schools. Cohn concluded that, providing heat and glare were controlled, light levels, ideally boosted by glass roofs above, could never be too high. Good day lighting became a priority in schools in the 1950s, with classroom designs incorporating large windows and verandas to facilitate lessons being conducted outside. A lack of concern for building design and the thermal effects of large poorly orientated windows, which resulted in overheating and glare, lead to a move away from daylight in schools toward artificial lighting.

Current research – including the use of glass cubes for classrooms spaces, conducting classes outside and encouraging children to go outside during break times – is examining more closely the effect of light levels on children’s eyes. Animal studies have supported the hypotheses that low light levels indoors may be a risk factor for the development of myopia and that bright light can slow myopia progression. However, light may not be the only factor in what is a very complex process of myopia development. Some researchers have postulated that the protective effect of being outdoors may result from the greater viewing distances when children are outside.

Current hypotheses associate light stimulation with the release of dopamine in the retina, which blocks excessive elongation of the eye. Dopamine production by the retina is part of the normal diurnal cycle; researchers now suspect that disrupting this cycle makes the eye grow in an irregular fashion. Ian Morgan, a leading myopia researcher from Canberra, has estimated that children need to spend around three hours per day at light levels of at least 10,000 lux.

As we move toward recommending children spend more time outdoors, it is important that we consider appropriate levels of protection from environmental hazards. Research currently being conducted by internationally recognised genetic ophthalmologist, David Mackey at the Lion’s Eye Institute in Perth, aims to determine the optimal balance of safe sun exposure while still having a protective effect against myopia.

Diet and Environment

One study showed an association between low vitamin D serum levels and high myopia. Low vitamin D levels are associated with reduced exposure to ultraviolet (UV) light because UV light is required to synthesise vitamin D. It is therefore most likely that vitamin D levels are a proxy for other consequences of sun exposure rather than a risk factor for myopia. This would seem to indicate that it is light rather than UV light specifically that prevents myopia. Similar studies on the exact spectral composition required for humans now need to be conducted.

Studies of our diet and its relationship with refractive status showed no clear link between diet and myopia development or myopia progression. Notably, children with vegetarian diets were found to be more likely to have refractive errors compared with their meat-eating peers.

Genetic Influence

Not until 1962 did the nature vs. nurture debate begin to turn more strongly toward genetic influence. Twin studies showed parental history of myopia as a significant risk factor for both onset and progression of myopia. Though influential at the time, the report by the UK Medical Research Council4 did not adequately account for the limited environmental variation with twins, underestimating the impact of environment on myopia. Since this time, more than 100 regions of the genome have been linked to short-sightedness. The advent of genome-wide association studies (GWAS) has contributed to an incredible unravelling of the complex genetic code associated with myopia. More than 20 susceptibility loci have been mapped with several gene variants also identified. Genetic changes, however, occur slowly, so the recent steep rise in the rates of myopia internationally cannot be attributed to genes alone – environment is playing a significant influence.

The gene/environment interaction cannot be overlooked because several genes are likely to influence and control eye growth, and the expression of these genes may be modified by the environment. The sharp rise in myopia in Asia in particular indicates strong environmental influence associated with increased educational attainment in conjunction with less time spent outdoors.

Outdoor Activity

The argument has now almost swung full circle with many researchers now believing the sharp rise in myopia to be associated with environmental influences. Put simply, gene pools do not swing that rapidly, so a child’s environment must be a major factor. Studies in the US early this century found an association between the development of myopia and the amount of time spent outside.5 Important work conducted in Sydney reinforced this with the finding that children in Australia who spent less time outside, were more likely to develop myopia, even when correcting for their levels of activity or near work.6

Time spent outdoors appears to have a protective effect, reducing a child’s risk of becoming myopic. Playing sports is inversely associated with myopia, but importantly not for indoor sports. This has added weight to the argument for outdoor activity being the factor, rather than physical activity.

Children engaging in more than 14 hours of outdoor activity per week significantly reduce their risk of becoming myopic.5 Trials have been conducted in classrooms across Taiwan and China to determine the exact mechanism or group of mechanisms. Light levels are currently hypothesised to be the strongest environmental risk factor in myopia.

Prevention of Myopia

Many of the methods adopted clinically to prevent or reduce myopia are based on the view that accommodation of the eye is involved to some degree. Methods that have been explored have included:

  • Visual training and biofeedback
  • Under-correcting myopia
  • Use of bifocal or progressive lenses
  • Peripheral lens designs
  • Pharmacotherapy
  • Contact lenses and orthokeratology

Visual Training

Visual training, which was largely based on the assumed connection between accommodation and myopia, was first promoted early last century by Skeffington. Studies conducted in the 1940s showed a positive improvement from the training but methods were not detailed. Later work at the Wilmer Institute at Johns Hopkins showed no refractive change with the conclusion that “myopia was not reduced”. Further improvements to the model that incorporated auditory feedback were later introduced. Still, training showed no significant reduction in myopia.

Under-correction

Under-correction for individuals with myopia – by as much as one dioptre – has long been used by clinicians to avoid excessive accommodation and try to slow progression of myopia. Texts dating back to at least 1880, including those by Landolt, instruct clinicians to record the least minus correction for maximum visual acuity and minimum accommodation. Most recently studies have confirmed that under-correction of 0.75D leads to a more rapid progression of myopia. It has also been demonstrated that over-correction by 0.75D has no effect on myopic progression.

Bifocal and Progressive Lenses

The premise applied to the use of bifocal and progressive lens designs is that excessive accommodation can affect the eye structurally and mechanically or that disrupting the relationship between accommodation and convergence can affect refraction. The largest and longest prospective study investigating the use of bifocals, the Houston Study,7 showed no reduction in the progression of myopia with bifocal wear. Though initial success and improved compliance was shown in the initial trials relating to progressive lenses, later studies showed no difference between children with single vision and progressive lens wearers. More recently the COMET study8 showed a small (0.20D) reduction in progressive lens wearers’ myopia. This was not considered significant enough for a change in
clinical practice.

Peripheral Lens Designs

Early studies showed that partial occlusion of chick retina resulted in asymmetry in eye shape. Similar studies were shown the same result in monkeys. These results lead researchers to believe that peripheral image quality may be used as a treatment strategy for myopia control. A large study of children conducted by Mutti9 showed that relative peripheral hyperopia can be a predictor of myopia. Spectacle lens designs with a concentric zone with peripheral defocus were found to influence refractive development in chicks. Subsequent studies with spectacle lens designs offering peripheral defocus have shown some reduction in myopic progression. Contact lenses adopting a similar design have shown similar results for reducing myopic progression.

Pharmacotherapy

The protective effect of outdoor activity is thought to be due to the constriction of the pupil that occurs outdoors in bright light. Constricting the pupil increases the depth of focus and decreases blur, which could slow eye growth. Sub-optimal accommodation or accommodative lag has a major influence on eye growth. Atropine and pirenzepine drops have been shown to slow myopia progression, though the mechanisms remain unclear and rigorous scientific analysis about the long-term adverse effects have not been undertaken.

Contact Lenses and Orthokeratology

The use of contact lenses, not just for correction of refractive error but also for reducing myopia, originated in the 1950s. Flatter base curves were incorporated into fitting strategies, resulting in a temporary reduction in myopia. The key question with respect to orthokeratology was and still is: are the lenses purely flattening the eye or do they have an effect on slowing eye growth. A large retrospective study conducted in Hong Kong using orthokeratology lenses overnight showed they were effective in reducing low to moderate levels of myopia. Other studies demonstrated a slowing of axial elongation of the myopic eye.

Where to From Here

Myopia is as common as it is complex. Rapid changes in the incidence of myopia have been associated with increased urbanisation and increased pressure to achieve academically. As myopia levels rise, so too have the levels of severe myopia, with the elevated risk of vision-threatening consequences. It is interesting to see the shift in thinking over time and that it appears we are now promoting earlier thinking, for the need to increase light and spend time outdoors to reduce onset and slow progression of myopia.

Public health campaigns currently by the Health Promotion Board operating in Singapore encourage children to “Keep myopia at bay” and “Go outdoors and play”. In addition to public health campaigns, vision screening is conducted annually with the aim of preventing and reducing myopia as well as delaying the onset.

There is a plethora of online articles dedicated to preventing myopia. Parents are known to consult Dr. Google and be provided with information that they will happily present in the consulting room as gospel. The International Prevent Myopia Association website petitions children to wear reading glasses to relax their eyes. Clinicians need to be fair-armed and forewarned to inform patients appropriately. Optometrists and ophthalmologists need to continue to critically examine the literature to keep ahead of the ‘latest’ success or otherwise in this very complex and heterogeneous ocular disorder.

Annette Hoskin is a Research Fellow at the Lions Eye Institute Centre for Ophthalmology and Visual Science at The University of Western Australia, researching the causes and promoting prevention of eye injuries. At Shamir OHS Eyres Eyewear she is a technical consultant, developing product and quality systems for the manufacture of high performance sport and safety eyewear. A consulting optometrist, Ms. Hoskin is also a committee member for Australian Standards and International Organization for Standardization Committees for sunglasses, eye protection and spectacle lenses.

Get Involved

Want to learn more about the research being conducted at LEI?
Go to www.mackeylab.org/eps. If you live in Perth, you can enrol to take part in the study.t

Take Home Messages

1. Encourage children to spend more time outdoors safely (with appropriate
sun protection)
2. Ensure myopic children have regular eye examinations
3. Inform parents and carers about the consequences of not doing point one and two
4. Don’t under-correct myopes
5. Maintain awareness of the significant impact of this growing health care issue.


To earn your points from this article answer the assessment available at mivisionclean2.flywheelsites.com/the-march-of-myopia

References:

1. Mark H. Johannes Kepler on the eye and vision. American Journal of Ophthalmology. 1971;72:869-78.

2. Virchow R. On certain influences of schools injurious to health. Boston Medical and Surgical Journal. 1869;81:256–9.

3. Fuchs E. Textbook of Ophthalmology: Appleton and Company; 1899.

4. Sorsby A. Refraction and its components in twins. .
Privy Council. Medical Research Council Special report
no. 303. London: HMSO, 1962.

5. Jones LA, Sinnott LT, Mutti DO, Mitchell GL, Moeschberger ML, Zadnik K. Parental history of myopia, sports and outdoor activities, and future myopia. Investigative ophthalmology and visual science. 2007 Aug;48(8):3524-32. PubMed PMID: 17652719. Pubmed Central PMCID: 2871403.

6. Rose KA, Morgan IG, Ip J, Kifley A, Huynh S, Smith W, et al. Outdoor activity reduces the prevalence of myopia in children. Ophthalmology. 2008 Aug;115(8):1279-85. PubMed PMID: 18294691.

7. Grosvenor T, Perrigin DM, Perrigin J, Maslovitz B. Houston Myopia Control Study: a randomized clinical trial. Part II. Final report by the patient care team. Am J Optom Physiol Opt 1987;64(7):482-98.

8. Gwiazda J, Hyman L, Hussein M, Everett D, Norton TT, Kurtz D, et al. A randomized clinical trial of progressive addition lenses versus single vision lenses on the progression of myopia in children. Investigative ophthalmology & visual science. 2003 Apr;44(4):1492-500. PubMed PMID: 12657584.

9. Mutti DO, Hayes JR, Mitchell GL, Jones LA, Moeschberger ML, Cotter SA, et al. Refractive error, axial length, and relative peripheral refractive error before and after the onset of myopia. Investigative ophthalmology
& visual science. 2007 Jun;48(6): 2510-9. PubMed PMID: 17525178. Pubmed Central PMCID: 2657719.

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