Origins of Myopia. Sotiris Plainis, MSc, PhD, FBCLA. Laboratory of Vision and Optics, University of Crete

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1 Origins of Myopia Sotiris Plainis, MSc, PhD, FBCLA Laboratory of Vision and Optics, University of Crete Financial disclosure: Co-owner of contact-lenses.gr

Growth must be coordinated between components (corneal, lens, axial

2 Emmetropization and eye growth Emmetropization is an active process. The adult eye is not simply a larger version of the newborn eye. Growth must be coordinated between components (corneal, lens, axial length), e.g. corneal power changes may balance axial elongation Newborn Adult Newborn x 1.5

3 Corneal power (D) Corneal power (D) Ισχύς κερατοειδή (D) Εµµετρωποίηση Emmetropia Ο µηχανισµός που συντονίζει (συνήθως σε ηλικίες < 8 ετών), µε τη συµµετοχή γενετικών και περιβαλλοντικών παραγόντων, την Emmetropia describes ανάπτυξη του the οφθαλµού ocular µε condition σκοπό την in επίτευξη which της optical εµµετρωπίας balance is achieved, since this occurs for a wide range of axial lengths (flatter / less powerful Eίναι corneas πιθανόν for να longer «επέµβει» emmetropic ώστε να αντισταθµίσει eyes)! σε κάποιον βαθµό (Sorsby 1950: τα 21.0 διαθλαστικά to 26.0 σφάλµατα mm, brit (π.χ army) ο κερατοειδής να αναπτυχθεί µε Axial length: 21.4 λιγότερη to 26.2 ισχύ για mmνα διασφαλίσει ευκρινή εικόνα σε έναν οφθαλµό µε µεγαλύτερο του κανονικού αξονικό Axial µήκος ή length: το αντίστροφο) to 25.7 mm ADULTS CHILDRENΕμμέτρωπες (9 με 14 χρονών) 237 αγόρια 44 Αξονικό µήκος: 21.4 µε 26.2 mm 392 emmetropic eyes Axial length (mm) Αξονικό μήκος (mm) Axial length (mm)

$50D) is the result of the increased axial length of the eye Low myopia (< 3D) is mainly due to a mismatch between axial length and corneal refractive$

4 Myopia («μύειν» + «ωψ») Myopia (spherical equivalent < -0.50D) is the result of the increased axial length of the eye Low myopia (< 3D) is mainly due to a mismatch between axial length and corneal refractive power, while myopia > 3D is axial. Atchison, 2006 Replotted data Rozema Gullstrand study group

5 The march of myopia (WHO report) Holder, Fricke, Wilson, Jong, Naldoo, Sonkoridurg, Fikk, Resnlkoff 2015;in WHO report in myopia (2016)

6 Distribution of refractive error in the population Although the prevalence of myopia varies with age, geography etc Striking evidence exists for rapid increases in the prevalence of myopia Young adults (Stenstrom, 20-35, Sweden, 1946) (Sorsby, army, UK, 1960) Mean: +0.95D +0.85D Adults (20-80 years, Europe) (Rozema, The Gullstrand project, 2013) Mean: -0.90D Refractive error (D)

7 Distribution of refractive error at fixed ages (Taiwan) Prevalence increased from 70% (1983) to over 80% (2000) Peak of refractive error distribution moves towards higher myopia levels (from 1983 to 2000) Plainis & Charman, OPO 2016 [replotted from Morgan & Rose, 2005]

8 Refractive error (D) Myopia progression Myopia progression depends on age of myopia Onset Early onset myopia involvesmore years of progression and results to higher myopia levels Development of Myopia Thorn et al. Optom Vis Sci (2005) Thorn et al. Optom Vis Sci 2005;82:E28

9 Education is a key causal factor The link between education and myopia is strong and consistent, and shows up in the association between myopia and years of schooling in adults. In populations of a range of ethnic backgrounds, more educated people are found to be more myopic. Time outdoors Morgan et al., 2018

compared to Bulgaria Myopia prevalence increases in

10 Myopia and educational pressure (Heraklion vs Stara Zagora) A lot more children with myopia in Greece compared to Bulgaria Myopia prevalence increases in secondary compared to primary schools Plainis et al. OPO (2009)

Intensive near work and asymmetric head postures Many children adopt asymmetric head postures and short working distances, depending on the task:

11 Intensive near work and asymmetric head postures Many children adopt asymmetric head postures and short working distances, depending on the task: shorter distance for video games, then reading, and third writing Reading distance decreases (and head tilt increases) as luminance and contrast decrease. Accommodation lag observed in nearwork was thought to be a key factor - this view was undermined by evidence that accommodation is not important in animal studies of experimental myopia. However, interacting defocus signals, associated with accommodative lag, seem to play an important role. Charman, OPO, 2010

Time outdoors as a protective factor It has been found that close work (intensive education) multiplies the risk of developing myopia, but in all cases the risk is reduced as long as the children

12 Time outdoors as a protective factor It has been found that close work (intensive education) multiplies the risk of developing myopia, but in all cases the risk is reduced as long as the children spent more time outdoors / in the countryside This may be due to the fact that outdoor environment is dioptrically homogeneous - also to the intensity and wavelength composition of natural daylight that seems to play a role in the appearance of myopia Low Time outdoors Moderate High French et al., 2013

13 Animal research: Effect of lens compensation Compensation in axial length elongation following lens treatments (from +9.0D to -6.0D) Final axial length is reduced Final axial length is increased Earl Smith, OVS, 2011

14 Animal research: Eye-growth following form deprivation The vision-dependent mechanisms that regulate refractive development operate in a regionally selective manner* Nasal-field form deprivation Full-field form deprivation Earl Smith, OVS, 2011 *the process of lens compensation is largely independent in the two eyes.

15 Animal research: Effects of peripheral form deprivation When conflicting signals exist between the central and peripheral retina, peripheral visual signals can dominate central refractive development (experiments with monkeys using peripheral form deprivation or annular lenses to produce relative hyperopia). Monkeys were wearing binocular diffuser spectacles that had either 4 or 8 mm central apertures Earl Smith, OVS, 2011

16 How Peripheral retina controls refractive development? Animal models show that visual experience plays an important role in refractive development (emmetropization). Foveal and retinal image both play a role. Peripheral hyperopia tends to develop myopia. In order to work, peripheral mechanism requires: The existence of some kind of detectors that provide directional information (eg peripheral oblique astigmatism?) the existence of a uniform distant visual world (of near-zero dioptric vergence) - This condition is satisfied by typical outdoor environments (but not by indoor conditions)

$Peripheral defocus is important Refraction (sphere) changes with$ fovea Outdoors Periphery is a potential signal for emmetropisation 50

17 Peripheral defocus is important Refraction (sphere) changes with eccentricity: in myopes the periphery is hypermetropic compared to the fovea Outdoors Periphery is a potential signal for emmetropisation 50 degree radius field Indoors Accommodation stimuli (D)

18 Conclusions Intensive near work and education are of importance in the development of myopia. Emmetropization appears to be guided by the state of focus on retina - axial, as well as peripheral focus could play an important role in refractive development (but as yet the weighting of contributions is uncertain) Outdoor environments seem to have an important inhibitory effect on myopization, probably due to the whole visual field being at around zero vergence, so that the peripheral mechanism can work at maximum efficiency.

19 Visual psychophysics and electrophysiology lab S. Plainis, Institute of Vision & Optics

FEATURE REVIEW ON LINE. Prentice Award Lecture 2010: A Case for Peripheral Optical Treatment Strategies for Myopia. Earl L.

FEATURE REVIEW ON LINE. Prentice Award Lecture 2010: A Case for Peripheral Optical Treatment Strategies for Myopia. Earl L. 1040-5488/11/8809-1029/0 VOL. 88, NO. 9, PP. 1029 1044 OPTOMETRY AND VISION SCIENCE Copyright 2011 American Academy of Optometry FEATURE REVIEW ON LINE Prentice Award Lecture 2010: A Case for Peripheral