Jun 22, 2017 - MYOPIA PROGRESSION THEORIES. Prepared by: Dr. Jacinto Santodomingo-âRubido, Clinical Affairs Manager & Senior. Research Scientist ...
MYOPIA PROGRESSION THEORIES Prepared by: Dr. Jacinto Santodomingo-‐Rubido, Clinical Affairs Manager & Senior Research Scientist, Menicon Co., Ltd. Date: 22nd June 2017 INTRODUCTION The prevalence of myopia has been increasing substantially in recent decades and continues to be on the rise. In fact, it has been estimated that by 2050 around 50% of the world’s population (~ 2 billion people) would be myopic.1 The mechanisms involved in myopia onset and progression remain unclear. Several theories, including (1) lag of accommodation; (2) mechanical tension; and (3) peripheral refraction have been proposed to explain the aetiology behind myopia progression,2 with the latter theory being the most popular currently (Figure 1). This document aims at providing a brief summary of the different and most commonly accepted theories used to explain how myopia progresses. Mechanical)Tension
Peripheral)Refrac2on
Lag)Accommoda2on
Figure 1. Myopia progression theories Lag of accommodation The lag of accommodation theory is based on the hypothesis that high lag of accommodation that occurs during near work in myopic eyes causes foveal hyperopic retinal blur that ultimately induces an abnormal axial growth of the eye leading to myopia (Figures 2 and 3).3 This theory is supported by observations that myopes have a reduced accommodative response compared with emmetropes and thus, an insufficient accommodative response to blur (Figure 2). Such hypothesis suggests that treating myopic children with plus lenses for near work (e.g. bifocals and progressive addition spectacles) to reduce accommodative lag during near
work decreases hyperopic foveal blur, axial elongation and ultimately myopia progression.
Figure 2. Reduced accommodative response during near work typically shown by myopic eyes. Image taken from Lopez-‐Gil et al., 2013.4
Figure 3. Foveal hyperopic retinal blur resulting from reduced accommodative response at near. Image taken from Yeo et al. 2016.5
Some previous studies have indicated that the wear of bifocals and progressive addition spectacles can reduce myopia progression, but the effect appears to be limited to the first year of treatment (~ 10 to 20% on average).6, 7 However, a meta-‐analysis of nine trials which compared the effect of bifocals and progressive addition spectacles in reducing myopia progression reported that the wear of such lenses with powers ranging from +1.50 to +2.00D were associated with a statistically significantly mean decrease in myopia progression of 0.25D in school-‐ aged children compared with single-‐vision spectacles, with the benefit being greater in children with a higher level of myopia at baseline and sustained for a minimum of 24 months.8 Mechanical tension The finding that the eye responds to transient changes in axial length following short periods of accommodation lead to the proposal of the mechanical tension theory.9 This theory suggests that the contraction of the ciliary muscle following accommodation results in forward and inward pulling of the choroid. Such ciliary-‐ choroidal tension restricts the equatorial growth of the eye thus decreasing the circumference of the sclera leading to a more prolate eye shape and ultimately to an elongation of the axial length of the eye that results in myopia (Figure 4).9, 10 In fact, recent research work using both optical biometers and OCT has confirmed the choroid thins in a reversible manner following short periods of accommodation.11-‐ 13
Figure 4. Schematic representation of the mechanical tension theory
Peripheral refraction Several previous studies have shown that chronic exposure to lens-‐induced hyperopic defocus accelerates the axial length growth of the eye in a predictable manner in various species ultimately leading to myopia, suggesting that foveal defocus influences eye growth.14, 15 However, later investigations on the effect of hyperopic defocus on ocular growth have highlighted the importance of peripheral image formation in the aetiology and progression of myopia. Specifically, the peripheral refraction theory indicates that peripheral hyperopic defocus has been suggested to play a significant role in the development of myopia (Figure 5).16, 17 In fact, several meta-‐analyses reported a significant reduction of myopia progression (~ 30 to 50% on average) with treatments that reduce peripheral hyperopic defocus (i.e. orthokeratology and centre-‐distance multifocal soft contact lenses).18-‐ 20 However, large studies in humans have failed to find peripheral refraction to be associated with myopia progression thus leaving uncertainties with regards to the validity of this theory in humans.21, 22
Figure 5. Schematic representation of the peripheral theory
CLOSING REMARKS Over the last two decades, three main theories have been proposed to explain the mechanisms behind myopia onset and development, although none of them have been able to fully explain the rationale behind myopia progression, suggesting the condition involves complex physiological and biological processes which are affected by environmental and genetic factors. There is some overlapping between the theories as the three share assumptions in common indicating that perhaps these theories might not be exclusive of each other to explain how myopia progresses. Research efforts by laboratories throughout the world are underway into the biological, neurophysiological and environmental bases for myopia onset and development that will help in mapping pathways to effective therapy. REFERCENCES 1. Holden BA, Fricke TR, Wilson DA, et al. Global prevalence of myopia and high myopia and temporal trends from 2000 through 2050. Ophthalmology 2016;123:1036–1042. 2. Berntsen DA, Mutti DO, Zadnik K. Study of Theories about Myopia Progression (STAMP) design and baseline data. Optom Vis Sci. 2010 Nov;87(11):823-‐32. 3. Gwiazda J1, Thorn F, Bauer J, Held R. Myopic children show insufficient accommodative response to blur. Invest Ophthalmol Vis Sci 1993;34:690-‐4. 4. López-‐Gil N1, Martin J, Liu T, et al. Retinal image quality during accommodation. Ophthalmic Physiol Opt 2013;33:497-‐507. 5. Yeo A, Paillé D, Drobe B, Koh P. Myopia and effective management solutions. Available at: http://www.pointsdevue.com/article/myopia-‐and-‐effective-‐ management-‐solutions. [Accessed on 20th June 2017]. 6. Gwiazda J, Hyman L, Hussein M, et al. A randomized clinical trial of progressive addition lenses versus single vision lenses on the progression of myopia in children. Invest Ophthalmol Vis Sci 2003;44:1492-‐500. 7. Berntsen DA, Sinnott LT, Mutti DO, Zadnik K. A randomized trial using progressive addition lenses to evaluate theories of myopia progression in children with a high lag of accommodation. Invest Ophthalmol Vis Sci 2012;53:640-‐9. 8. Li SM1, Ji YZ, Wu SS, et al. Multifocal versus single vision lenses Intervention to slow progression of myopia in school-‐age children: a meta-‐analysis. Surv Ophthalmol 2011;56:451-‐460.
9. Drexler W1, Findl O, Schmetterer L, et al. Eye elongation during accommodation in humans: differences between emmetropes and myopes. Invest Ophthalmol Vis Sci 1998;39:2140-‐7. 10. Mutti DO1, Sholtz RI, Friedman NE, Zadnik K. Peripheral refraction and ocular shape in children. Invest Ophthalmol Vis Sci 2000;41:1022-‐30.
11. Woodman EC1, Read SA, Collins MJ. Axial length and choroidal thickness changes accompanying prolonged accommodation in myopes and emmetropes. Vision Res 2012;72:34-‐41. 12. Ghosh A, Collins MJ, Read SA, et al. Axial elongation associated with biomechanical factors during near work. Optom Vis Sci 2014;91:322-‐9. 13. Woodman-‐Pieterse EC, Read SA, Collins MJ, Alonso-‐Caneiro D. Regional Changes in Choroidal Thickness Associated With Accommodation. Invest Ophthalmol Vis Sci. 2015;56:6414-‐22.
14. Schaeffel F, Glasser A, Howland HC. Accommodation, refractive error and eye growth in chickens. Vision Res 1988;28:639–57. 15. Hung LF, Crawford ML, Smith EL. Spectacle lenses alter eye growth and the refractive status of young monkeys. Nat Med 1995;1:761– 5. 16. Smith EL, Kee CS, Ramamirtham R, et al. Peripheral vision can influence eye growth and refractive development in infant monkeys. Invest Ophthalmol Vis Sci 2005;46:3965–3972.
17. Smith EL 3rd, Ramamirtham R, Qiao-‐Grider Y et al. Effects of foveal ablation on emmetropization and form-‐deprivation myopia. Invest Ophthalmol Vis Sci 2007;48:3914-‐3922. 18. Huang J, Wen D, Wang Q, et al. Efficacy comparison of 16 Interventions for myopia control in children: a network meta-‐analysis. Ophthalmology 2016;123:697-‐708.
19. Li SM, Kang MT, Wu SS, et al. Efficacy, safety and scceptability of orthokeratology on slowing axial elongation in myopic children by meta-‐ analysis. Curr Eye Res 2016;41:600-‐8. 20. Li SM, Kang MT, Wu SS, et al. Studies using concentric ring bifocal and peripheral add multifocal contact lenses to slow myopia progression in school-‐aged children: a meta-‐analysis. Ophthalmic Physiol Opt 2017;37:51-‐ 59. 21. Mutti DO, Sinnott LT, Mitchell GL, et al. Relative peripheral refractive error and the risk of onset and progression of myopia in children. InvestbvOphthalmol Vis Sci 2011;52:199–205.
22. Atchison DA, Li SM, Li H, et al. Relative peripheral hyperopia does not predict development and progression of myopia in children. Invest Ophthalmol Vis Sci 2015;56:6162–6170.
