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HomemieyecareA Far-Sighted Approach to Short Sightedness

A Far-Sighted Approach to Short Sightedness

Recent developments in the field of myopia research demonstrate the potential to change both the optometry profession and the lives of young myopic patients.

Myopia was first recognised as a visual condition by the Greek philosopher Aristotle more than two thousand years ago.1 Despite this early observation, it wasn’t until the development of concave spectacle lenses in the mid 16th century that the visual correction of myopia was first realised.2

In the 17th century, Johannes Kepler contributed to the first accurate description relating to the phenomenon of light being aberrantly focussed in front of the retina in the myopic eye.2 While the traditional management of myopia with single-vision lenses can accurately correct the refractive error, it is now well established that conventional lens designs do not slow the axial elongation that underlies progressive myopia.

Myopia is the most common human ocular disorder and represents a significant global public health concern.3,4 Epidemiological evidence indicates that the prevalence of myopia varies from approximately one-third of individuals in Western populations5,6 to more than 80 per cent in parts of Eastern Asia.7 In addition to cataract, macular degeneration, infectious disease and vitamin A deficiency, myopia is one of the most prevalent causes of visual impairment worldwide.5,8

Spherical soft and rigid contact lenses prescribed for distance correction are considered to neither enhance, nor significantly inhibit myopic progression

High myopic refractive error, commonly regarded as six dioptres or greater,9 poses a significant risk for developing sight-threatening eye pathology due to its association with comorbidities including retinal detachment, macular chorio-degeneration, premature cataract and glaucoma.8,10 As such, substantial research has focussed upon determining the factors that underlie myopic refractive error development, with the intention of providing a foundation for developing strategies to attenuate or even eliminate myopia.

The aetiology, pathogenesis and treatment of myopia has been intensively debated for decades.11 It is now accepted that myopia develops from a combination of both genetic and environmental factors,12 however the specific factors involved and their relative contributions remain unclear. Of particular interest is a growing body of evidence that certain clinical modalities may significantly alter paediatric myopic progression. This article provides a summary of the evolution of a number of the major strategies for retarding the progression of myopia.

Spherical Contact Lenses

As early as 1956, practitioners associated spherical rigid contact lenses with apparent reductions in childhood myopic progression.13 The first clinical studies were conducted in the 1980s, but were limited in their validity due to inadequate control of variables, hypoxic-related corneal changes due to low oxygen permeable materials, incomplete follow-up and/or poor selection of study participants. The most definitive study to date is the Contact Lens and Myopia Progression (CLAMP) study, funded by the US National Eye Institute, published in 2001. This randomised, controlled clinical investigation evaluated eight to 11 year olds (n = 100) over a three year period.14

It was reported that while patients wearing standard spherical rigid gas permeable lenses (RGPs) demonstrated less myopic progression compared with a control group of spherical soft lens wearers, the reduced progression rate was temporary (ie. only over the initial 12 months) and was not due to stabilisation of the vitreous chamber depth. The authors concluded that the growth in ocular axial length between rigid and soft lens wearers was similar, and hence rigid lenses should not be fitted with the intent of controlling myopic progression.15

Importantly, a related study, the Adolescent and Child Health Initiative to Encourage Vision Empowerment Study (ACHIEVE), in 2008, found that the wear of spherical soft contact lenses by children did not produce a significant difference in axial length or myopia relative to spectacle lens wear.16

Spherical soft and rigid contact lenses prescribed for distance correction are considered to neither enhance, nor significantly inhibit myopic progression.

Spectacle Under-correction

The under-correction of myopia with single vision lenses is a treatment option that has been advocated by some practitioners.17 At this stage, only one masked, randomised clinical trial (2002) has evaluated the efficacy of this method.18 In this investigation, myopic children (n = 106) aged between nine and 14 years underwent two years of full-time single-vision spectacle wear; patients were randomised either to full correction or to an under-corrected prescription of approximately 0.75D. Importantly, the under-corrected group demonstrated enhanced, rather than reduced, myopia progression compared with the fully-corrected group. Further research is necessary to explore these findings.

Bifocal and Progressive Addition Spectacle Lenses

The effect of bifocal and progressive addition spectacle lenses (PALs) for altering the accommodative demand in young myopes has been well studied. These modalities are based upon the premise that a reduced accommodative response to near objects in myopic children may be associated with retinal blur and aberrant ocular growth.19 The benefit of a near addition is postulated to reduce the accommodative demand during near work and thereby reduce myopic progression.20 On the whole, the reported treatment effects for slowing myopic progression with these lens modalities have been small, ranging from 0.15 to 0.50 dioptres over 1.5 to three years.21-25

The largest treatment trial to be undertaken thus far is the Correction of Myopia Evaluation Trial (COMET) study (2003), which enrolled 469 myopic children aged six to 11 years and randomised them to PALs or single vision lenses.21 This multi-centre, randomised, double-masked study found that after three years, there was no clinically meaningful difference in myopic progression between the two groups overall. Subsequent analyses demonstrated that a clinically significant myopia control effect was observed in children with large accommodative lags and near esophoria, suggesting that this sub-population of children could benefit from wearing PALs.22 The results of a follow-up study, the COMET-II, evaluating PALs versus single-vision lenses for slowing myopic progression in children with these accommodative-convergence characteristics are expected to be published this year.

Pharmacologic Intervention

Studies examining the effect of pharmacologic agents on myopic progression date back to the 1970s.26 A number of recent, well-designed studies have centred on the use of the topical muscarinic- receptor antagonists atropine and pirenzipine. Although the mechanism of action of these agents in reducing myopic progression is uncertain, it may involve one or more of their effects relating to reducing accommodation, altering neurotransmitter release and/or changing patterns of systemic growth hormone release.27-29

Recent clinical data supporting the efficacy of atropine is derived from the Atropine in the Treatment of Myopia (ATOM) study (2006), which examined 400 children between the ages of six and 12 over a two year period.30 On average, eyes treated with 1 per cent topical atropine demonstrated a 77 per cent reduction in childhood myopic progression compared with placebo-treated eyes. Similar effects have been reported in other studies in myopic Taiwanese children.31,32

Despite the reported efficacy of atropine, its use as a standard treatment for myopia control is not generally accepted due to its significant adverse ocular effects. The well-established local side effects, particularly at higher doses (1 per cent), include photophobia, due to mydriasis and blurred vision, secondary to cycloplegia; effects that have been recognised to potentially impair a child’s ability to perform at school and to undertake sporting activities.33

Furthermore, the long-term side effects of atropine eye drops in children are also relatively unknown, but may include a risk of long-term ultraviolet-related retinal damage and cataract formation as a result of chronic papillary dilation.34 Pirenzepine is an atropine-like agent that is a more selective M1 muscarinic receptor antagonist with higher ocular tolerability than atropine and has been shown to be effective in attenuating axial elongation in experimental models of myopia.35,36 Although some short-term clinically trials in children have demonstrated significant reductions in myopic progression with topical pirenzepine gel,37,38 long-term follow up data is not yet available.

The Next Generation of Myopia Control

A heightened understanding of the genesis and temporal development of myopia has been derived from laboratory based form-deprivation experiments. Whilst it was previously thought that the central retina governed emmetropization, recent animal studies highlight the importance of the peripheral retina in regulating, at least in part, the development of myopia.

Using foveal ablation in primates, the paracentral retina, rather than the central macular region, has been identified as key to the process of axial elongation in response to hyperopic defocus .39 The concept of “relative hyperopia in myopes” (Figure 1) was first coined by the Dutch army ophthalmologist, Hoogerheide, who described the effect of traditional concave lenses for correcting myopia, in which the central image is focussed on the retina but a hyperopic focal plane is created in the peripheral retina (i.e. light is focussed behind the peripheral retina).40 Hyperopic defocus now has an established role in driving axial elongation in both experimental models of myopia41-43 and human myopic eyes.44,45 The scientific rationale for many recent treatments designed to impart a myopia control effect is based upon this improved understanding of the proposed role of the peripheral retina in modulating axial elongation.

Orthokeratology (Ortho-K)

Anecdotal reports of apparent myopia stabilisation in association with overnight orthokeratology, also known as corneal reshaping, have existed for decades. The validity of this clinical perception has been verified by three recent major international studies.

The first, conducted in Hong Kong by Cho et al. (2005) demonstrated a 46 per cent reduction in ocular growth rate in orthokeratology-treated eyes compared with a control group of single vision distance spectacle wearers over a two year period.46 Similar findings were reported in the United States by Walline and colleagues in 2009.47 Results from the first randomised, prospective clinical trial in orthokeratology were presented at ARVO in 2010.

This study, conducted by the Research in Orthokeratology (ROK) group led by Professor Swarbrick at The University of New South Wales, showed that over six months, orthokeratology slowed eye growth for eyes wearing orthokeratology lenses compared to contralateral eyes wearing alignment fit conventional RGPs.48 Although long-term prospective data is not yet available, a retrospective clinical study (the Corneal Reshaping Inhibits Myopia Progression, CRIMP study) conducted in Melbourne has provided preliminary evidence that orthokeratology treatment can reduce, and in some cases, may even fully arrest the rate of progression of childhood myopia in the longer term.49

The leading theory to explain how orthokeratology may regulate myopic progression relates to its effect on manipulating the mid-peripheral visual experience. Orthokeratology induces a redistribution of the corneal epithelium, involving central epithelial displacement (i.e. central corneal “flattening”) and a relative thickening of the epithelium in the mid-peripheral cornea.50 The typical fluorescein pattern of a well-fitting orthokeratology lens and a post-treatment corneal topography map are shown in Figure 2.

The resultant change in corneal contour increases the degree of positive spherical aberration; the effect is such that mid-peripheral light is focussed anterior to the retina, bending the image plane anteriorly within the globe, to create a myopic retinal image shell. Orthokeratology treatment therefore has the advantage of maintaining a myopic peripheral refraction whilst accurately correcting the axial error. Further work is required to determine the precise role of the peripheral optic profile in regulating ocular growth. In particular, a number of studies are currently underway to determine the optimum peripheral myopic defocus that may enable such treatment to be even more predictable and consistent.

Other New Modalities

The technique of manipulating the peripheral image shell to induce relative positive peripheral power has been applied in the development of other potential modalities for myopia control, including bifocal soft contact lenses and aberration-controlled spectacle lenses. In the DIMENZ (Dual focus inhibition of myopia evaluation in New Zealand) study (2011), a dual-focus soft contact lens, consisting of a distance centre and concentric near zone, imparted a 37 per cent reduction in myopic progression over ten months, compared with standard single-vision contact lenses.51 Researchers at the Vision Cooperative Research Centre and Brien Holden Institute have also published data on a myopia-control spectacle lens, the Zeiss “MyoVision” spectacle lens, which was reported to reduce the rate of myopic progression by approximately one third, in children aged six to 12 years with a history of parental myopia.52

The increasing prevalence of myopia, its negative impact upon quality of life and the associated economic burden, together underscore the value of eliminating myopic refractive error. Despite extensive research in the field of myopia control, many questions remain to be answered.

Most of the described treatments still lack the large-scale, high-quality masked randomised clinical trials that are necessary to fully test the efficacy of any treatment. Furthermore, as several myopia control studies have exhibited significant treatment effects in the first year that do not continue thereafter, the longevity of any reported treatment effects need careful evaluation. Further study into the true mechanism underlying the reported treatment effects is indicated.

Undoubtedly, myopia control will continue to be an area of particular interest to optometrists, as further evidence evolves regarding the benefits of technological innovations in lens designs for young myopic patients. Accumulating evidence certainly indicates that the regulation of myopia is changing from an aspiration to a reality. This is an exciting era for optometrists; the potential to not only manage, but treat or even eliminate myopia, is likely to be closer than expected.

Dr. Laura Downie, BOptom, PhD(Melb), PGCertOcTher, DipMus(Prac), AMusA is an optometrist who specialises in contact lenses. She has been published in scientific journals, is a regular contributor to mivision and is a clinical instructor to undergraduate optometry students.

References

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