Myopia Control Using Orthokeratology - Vision Magazine Online

THE IMPACT OF THE MYOPIA “EPIDEMIC”

Myopia is a leading cause of irreversible blindness and uncorrected refractive errors also remain a significant cause of correctable blindness. High myopia presents a greater risk for eye health issues such as retinal detachment, glaucoma and myopic maculopathy. The prevalence of myopia has been increasing over the past decades. In 2000, approximately a quarter of the world’s population was myopic, and researchers are predicting that half of the world’s population will be myopic by 2050 (Fig. 1). Higher rates of myopia, nearing 90%, occur in some Asian populations (Fig. 2). The increase in myopia prevalence suggests that environmental factors play a role in its development, though there is certainly a genetic component as well. Myopia is significantly impacting global health care costs, as a direct consequence of its management with spectacles, contact lenses and refractive surgery and there are significant costs associated with the treatment of myopia-related complications. So, the myopia “epidemic” is indeed a public health concern.

Figure 1. Line graph demonstrating the prevalence and degree of myopia is increasing rapidly over time. Holden BA, Fricke TR, Wilson D, Resnikoff S, 2016.

Figure 2. Line graph showing estimated prevalence of myopes in Asian countries. I. Morgan, 2015.

WHAT CAUSES MYOPIA PROGRESSION?

The exact mechanism of myopia development is still unclear. However, it is accepted that myopia progression is caused in large part by the elongation of the eye. Research suggests that peripheral hyperopic defocus causes elongation of the eye and plays a dominant role in the development of myopia progression. The periphery can be blurred even if the centre is clear, and this peripheral blur is not really something patients notice. At near, the periphery is more out of focus than it is when looking in the distance, so near work may be implicated in the progression of myopia. Some more recent studies have shown a greater association between myopia development and the time spent outdoors than the time spent doing near work. Increasing myopia is more closely related to how little time people spend outdoors than how much time people spend reading. So minimal time spent outdoors could be a risk factor for myopia development.

Figure 3. Schematic showing light rays from a distant object falling on the fovea of an emmetropic eye (left picture) and light rays from a distant object falling in front of the fovea in a myopic eye because of an elongated axial length (right picture). J. Terhost, 2018.

Fig 5. Sedentary, significant indoor lifestyle with extensive near work leads to myopia progression. P.Doyon, 2017

With the discovery of the phenomenon of form deprivation myopia (FDM) in primates, research has demonstrated that ocular growth and refractive development are regulated by visual feedback associated with the eye’s refractive status.

When susceptible visual systems are corrected with myopic lenses, it creates an image shell described as a positive curvature of field. These visual systems, while being corrected centrally, suffer from a relative hyperopia peripherally that causes conflicting visual stimulus. The result is elongation of the eye to correct the peripheral hyperopia. What this means in simple terms, is that even though a child gets spectacles, he would see clearly mainly because the image is focused on the retina but peripherally it is focused past (that is, posterior to the peripheral retina).

Figure 6. Form deprivation studies confirm increased vitreous chamber length in myopia progression. Form deprivation myopia (FDM) occurs in a wide variety of animals, including humans which suggests that the mechanisms responsible for FDM are probably fundamental to ocular development.

Figure 7. Schematic showing conventional myopic spectacles focusing the central light ray on the fovea, while the peripheral light rays are focused past the retina (peripheral hyperopia).

HOW DOES ORTHOKERATOLOGY SOLVE MYOPIA PROGRESSION?

Extensive research in orthokeratology has shown us that this method of vision correction is a safe and effective modality and has the benefit of retarding myopia progression. Unlike conventional methods of correcting myopia such as glasses or regular contact lenses, myopia-correcting orthokeratology lenses create a negative image shell on the retina of a treated patient. If, however, the peripheral retina is experiencing a relative myopia effect (negative curvature of field), then elongation may slow.

Figure 8. Schematic showing reverse geometry orthokeratology lens allowing the central light ray to reach the fovea and the peripheral rays to fall in front of the retina creating a peripheral myopic defocus. The peripheral myopic defocus negates the peripheral hyperopic defocus that is linked to eyeball growth.

A popular explanation for myopic refractive changes is that they result from movement of the corneal epithelium away from the centre of the cornea and towards the periphery. It appears as though the paracentral region thickens thus flattening the central cornea undergoing corneal reshaping. The paracentral thickening appears to be essentially stromal hypertrophy.

Figure 9. The effect of orthokeratology of the cornea showing central thinning and peripheral stromal hypertrophy known as the “epithelial shift theory”.

ORTHOK LENS DESIGN FOR MYOPIA CONTROL

The key design feature is that we utilise an aspheric back optic zone (BOZ) along with a facilitation curve to get a vivid mid-peripheral steepening which we need to create for myopia control. In addition, a Jessen factor of +1.50D to +2.50D is used based on practitioner preference. This means, for example, a patient who has a flat or spherical K of 43.D and is a -4.00D myope, will require a base curve of 37.50D and assuming a Jessen factor of 2.50D for myopia control. The alignment curve will be that curve that produces a bull’s eye fluorescein pattern. The power of the lens will be +2.50D and the rest of the curves will be determined by the corneal eccentricity and tear film profile using an orthokeratology software. The relief curve assists in creating a peripheral myopic defocus that is required to halt myopia progression. Mathematically, orthokeratology gives the practitioner wide control in designing lenses for myopia control using specialised lens design programs and controlled tear clearance. Essentially, the tentative reverse geometry lens parameters are entered in an orthokeratology design program and the eye care practitioner looks at simulations and arrives at the best lens design that corrects the refractive error and caters for myopia control. Other methods of myopia control include: 0.01% atropine therapy, progressive addition lenses, specifically-designed single vision lenses, soft multifocal contact lenses, vitamin D supplement, ambient lighting and outdoor recreation. However, in my opinion orthokeratology gives the practitioner greater control on the myopia control treatment plan.

Figure 10. Bull’s eye NaF pattern is the end-point of the ideal fitting orthoK lens.

image021-1 — Figure 11. An orthokeratology program is used to design the reverse geometry lens and also to evaluate simulations to yield the optimal tear film profile to achieve emmetropia and myopia control.

CONCLUSION

For decades, practitioners actively fitting orthokeratology lenses reported anecdotal evidence that myopia appears to develop at a much slower rate in children wearing an overnight orthokeratology modality than in children wearing soft lenses or glasses. Now, research has shown that indeed a significant slowing of axial elongation in children wearing overnight orthokeratology versus other correction methods. Optical treatment strategies that take into account peripheral refractive errors are more likely to be successful than those that only manipulate central vision.

After the eye care practitioner has just completed the comprehensive exam of a new myopic patient, when making recommendations, the practitioner owes the patient the benefit of knowing all of his or her options including orthokeratology.

With world population booming, increased urbanisation along with sedentary, indoor lifestyle will accelerate the myopia epidemic. Eye care practitioners should consider halting myopia progression.