Percy Lazon de la Jara is Head of Clinical Research at the Brien Holden Vision Institute and Visiting Fellow at the School of Optometry and Vision Science, University of New South Wales, Australia.Myopia has become a significant public health issue. A recent study1 shows that the incidence of myopia in the United States has increased by approximately 40%, and other studies report that the incidence has reached 84% in school children in some East Asian countries such as China, Singapore and Taiwan2-4. This rise in the prevalence of myopia is of concern to health authorities because of potential pathological ocular conditions that may increase the risk of blindness. It has been shown that the risk of cataract, glaucoma and chorioretinal abnormalities increases with higher levels of myopia5-9. Thus, there is a pressing need to improve our understanding of the mechanisms behind myopia development and progression.
Previous attempts to control myopia
The prescription of spectacles and/or contact lenses for vision correction is unable to prevent axial elongation of the eye, and thus myopia progression, particularly relevant during childhood, is not halted with traditional vision correction. Previous attempts at controlling myopia have included optical means such as bifocal and multifocal (or progressive addition) ophthalmic lenses and contact lenses, which have shown little or no beneficial effect in slowing myopia progression10-12. Phamacological agents such as atropine and pirenzepine achieve reduction in myopia progression in the short term (1- to 2-year studies)13-14; however, lack of compliance in using eye drops affects efficacy, and rebound following cessation of the treatment has been reported15. Importantly, the potential side effects of long-term use of these agents are unknown.
There is strong evidence that lens-induced defocus can modulate eye growth in several animal models16-19. In addition, findings from the COMET Study11 showed that progressive addition lenses were effective for a subgroup of children presenting larger lags of accommodation, shorter reading distances or lower baseline myopia. These results indicate that retinal defocus may play a role in myopia progression.
However, studies in which children were under-corrected have reported mixed results. Chung et al.20 under-corrected subjects by +0.75D bilaterally, causing an acceleration in their myopia correction compared to fully corrected subjects. On the other hand, Phillips21 showed that use of monovision in a group of children slowed axial elongation and significantly reduced myopia progression in the near-corrected eye.
The role of the peripheral retina
More recently, interest has emerged in the role of the peripheral retina in emmetropization and/or refractive error development, based on experimental work in animal models22-24. Smith et al.24 showed that a functioning fovea is not critical for emmetropisation or refractive error development in infant monkeys. Furthermore, in form-deprivation experiments, the peripheral retina was found to play a significant role23.
Interestingly, several studies have reported that the human myopic eye experiences relative peripheral hyperopia25-27. These findings suggest that visual feedback through defocus, especially peripheral retinal defocus, modulates eye growth. It has been speculated that the presence of peripheral defocus (light focussing behind the peripheral retina) may act as an optical signal, triggering eye elongation and subsequent myopia. It has been further postulated that changing the retinal image shell by shifting the peripheral defocus in front of the retina may result in slowed axial elongation.
Support for this hypothesis comes from reports that orthokeratology contact lenses can be effective in myopia control28,29. Orthokeratology changes the cornea from a prolate to an oblate shape, which impacts the image shell of the myopic eye, shifting the original peripheral hyperopic defocus into peripheral myopic defocus. Pilot studies in children wearing orthokeratology lenses found that the change in axial length was significantly smaller than that measured in children wearing regular vision correction strategies, such as single-vision spectacles and soft contact lenses28,29.
A case for inducing retinal defocus
A study aiming to manipulate peripheral retinal defocus through the application of specially designed contact lenses has shown some promising results after 12 months of wear30. Spherical equivalent/refractive error and axial elongation were reduced significantly in eyes wearing the novel design compared to that measured in a group of children wearing single-vision spectacles. Conversely, results from a 10-month study using concentric dual focus contact lenses that induced simultaneous myopic defocus reported a significant reduction in myopia progression and axial length in the test eyes compared to the control eyes31.
Alternative theories
Some research groups32-35 have reported that the prevalence of myopia in children who are more active outdoors is lower than in children with less outdoor exposure, suggesting that outdoor activity may have a protective effect.
Conclusion
Controlling myopia progression by optical means is an appealing concept and offers a fantastic opportunity for our profession to make a significant contribution in the public health arena. The results reviewed in this editorial are encouraging and hold promise that perhaps, through optical devices, myopia progression may be halted or significantly retarded. The need for rigorous randomised clinical trials is key to determine the effect of retinal defocus in myopia progression. Collaboration between industry, our profession and the scientific community is imperative to tackle this public health problem.
To succeed, the future of controlling myopia progression lies in holistic management, through implementation of complementary strategies, such as optical intervention and environmental behaviour.
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