Orthokeratology: The Evidence is Here

This is a special myopia issue so I’m going to cut to the chase and assume awareness of the growing prevalence of myopia,¹ the basics of orthokeratology (OK), and that OK has been shown to slow progression of myopia.^2–6 

Taking these three statements into account, I was surprised to read, in the latest International Contact Lens Prescribing survey report on OK lens fitting, that despite a doubling of lens fits from 2004 to 2017, OK still only accounts for 1.3% of all contact lens fits.⁷

How OK slows progression of myopia is not yet understood, though it is generally thought to be due to the way OK alters the retinal image profile

Let’s take a look at why this shouldn’t be the case.

THE BENEFITS OF OK FOR MYOPIA CONTROL

Do Children Like OK? 

The research suggests yes. Compared to wearing glasses, children rated OK better for distance vision, symptoms, appearance, satisfaction, effect on activities, academic performance, handling and peer perceptions.⁸ Higher vision-related quality of life compared to wearing glasses has also been reported, with 75% of participants preferring OK over glasses.⁹ 

OK is Safe to Fit in Children 

The predominant concern regarding OK contact lenses is microbial keratitis (MK) infection, which, due to the short history of OK use in children, is difficult to quantify. The currently most robust available measure estimates risk of MK in children wearing OK at 13.9 per 10,000 patient-years,¹⁰ which is comparable to silicone hydrogel soft reusable daily wear contact lenses in adults.¹¹ A vital consideration is to ensure good compliance12 and regularly reinforce that lenses, their case, or any devices used in lens handling, such as removal plungers, should never come into contact with tap water.¹³

OK is a Good Option for Correcting Low Astigmatism 

Current commercial soft contact lenses for myopia control are only available in spherical design, meaning they can only mask up to around 0.75D of astigmatism. In comparison, spherical OK is effective in correcting up to 1.50D of astigmatism.¹⁴ Toric OK designs are also available, offering the capability of correcting higher amounts of astigmatism.¹⁵ 

Young Children are No Harder to Fit with Contact Lenses 

Though admittedly for soft lenses, Walline et al reported it taking around 15 minutes more chair time to fit eight to 12 year old children compared to 13 to 17 year old teenagers, with most of this difference caused by insertion and removal training that can be easily delegated to support staff.¹⁶ I would argue that once accurate corneal topography is captured, fitting OK lenses follows a similar process to fitting soft lenses, and arguably could be considered simpler if lenses are ordered empirically from corneal topography.

RESEARCH HIGHLIGHTS

Mechanism for Slowing Progression of Myopia 

How OK slows progression of myopia is not yet understood, though it is generally thought to be due to the way OK alters the retinal image profile. When myopia is corrected with single vision glasses or contact lenses, light from an axis off-axis target tends to focus behind the retina to create a relative peripheral hyperopia profile (Figure 1).

Higher vision-related quality of life compared to wearing glasses has also been reported, with 75% of participants preferring OK over glasses

During overnight wear OK flattens the central cornea to create a negative shift in power, that provides the myopic refractive correction effect, surrounded by a ring of steeper plus power (Figure 1). This change to corneal profile causes the retinal image to take on a relative peripheral myopia profile (Figure 1),¹⁷ that has been shown in animal studies to slow axial eye growth, and with it myopia progression.¹⁸ It is the peripheral image being focussed anterior to the retina that is believed to promote slowing of eye growth.

Another potential mechanism is that the same change to the optics of the cornea also creates a negative shift in longitudinal spherical aberration.¹⁹ This has the effect of extending the depth of focus of the eye, which, through its influence on accommodation, can shift the whole retinal image profile in an anterior and therefore protective direction.²⁰ 

Customising OK for Myopia Control 

Evidence towards a relative peripheral refraction mechanism was published by Chen et al, who, when analysing results from their two-year longitudinal study, divided the measured data from their participant cohort either side of the mean measured pupil diameter.²¹ The authors reported greater myopia controlling effect in those that had larger than average pupils, leading them to suggest that larger pupils allowed more of the corneal plus power formed around the treatment zone to fall inside the pupil, thereby causing a larger area of the peripheral retina to experience myopia defocus. Consequently, if change to relative peripheral refraction is the predominant mechanism, this should lead to greater myopia control effect (Figure 1).

Following this argument, the logical next step to improving myopia control efficacy from OK is to bring more OK induced corneal plus power to fall inside the pupil (Figure 1). This approach, created by reducing the treatment zone (TZ) diameter, has been advocated by some OK lens fitting specialists.²² In a recent study with colleagues from the University of New South Wales, we tested this further by comparing a standard OK lens against a test OK lens that had been modified to reduce the TZ diameter.²³ After one week of wear, TZ diameter was reduced by 0.93±0.73mm from the test, compared to the standard OK lens design. However, while there was an observable difference to measured peripheral refraction profile, the difference did not reach statistical significance.

The conclusion that I draw from the research to date is that modifying TZ diameter ‘may’ provide an improved myopia control, however further research is needed to prove whether this is in fact the case. Most notably, longitudinal studies are needed to assess whether reducing TZ diameter causes greater slowing of axial eye elongation compared to standard OK lens designs. Until this is shown to be the case, fitting standard OK lens designs, which have been shown to slow myopia progression across numerous longitudinal studies,^24,25 should be used when considering fitting OK for myopia control.

Partial Correction 

OK is effective and has USA Food and Drug Administration (FDA) approval in correcting up to -6.00D. Higher amounts of myopia correction are possible, and indeed some manufacturers provide lens designs specifically aimed at correcting high myopia. However, there is currently no reported evidence-based research to verify the reliability of correcting high myopia with OK. Although the FDA holds no jurisdiction over Australia, they are still a highly accredited body and we need to be extra cautious when fitting children, so my suggestion is that until shown to be safe to do otherwise, the suggested -6.00D limit should be respected. The good news is that OK has been shown to be just as effective in slowing axial eye growth when myopia is only partially corrected with the residual myopia corrected by glasses.²⁶ When faced with cases of myopia beyond -6.00D or astigmatism beyond -1.50D this means that instead of discounting OK as not being suitable, you can fit up to these limits and, once fit is stable, refract and provide glasses to correct the shortfall.

Results from the first six months of a well-controlled study comparing combined OK and atropine 0.01% against OK-only showed slower axial eye elongation in the combined therapy cohort, and the combined therapy to be well tolerated

Rebound of Effect 

Cho and Cheung investigated rebound of effect after completing two different myopia control studies. Participants were given the option to discontinue wear and monitored over seven months.²⁷ Those that chose to continue wear exhibited slower axial eye growth, indicating loss of myopia control effect, though axial elongation was similar to spectacle wearing children in earlier studies, leading the authors to suggest that this was not a true rebound effect. OK wear was then reinstated to find that axial eye growth slowed to the same rate as the participants who had continued OK wear throughout. The message to keep front of mind is that children who discontinue OK should be monitored for at least six months and the treatment resumed if myopia progression is observed.

OK and Atropine 

Both OK^24,25 and atropine²⁸ have been shown to slow the progression of myopia, naturally leading to the question of what may happen if both treatments were to be combined. To date there is limited research on this topic. The two longitudinal studies that have been conducted either suffer poor methodology²⁹ (no baseline results reported) or raise concern about subject matching³⁰ (the OK-only control cohort contained a higher proportion of younger subjects, which may have resulted in a faster natural progression of myopia in this group). Taking these faults into consideration however, they still hint that combining atropine with OK ‘may’ offer additional benefit for slowing progression of myopia. Fortunately, more thorough research is currently underway. Results from the first six months of a well-controlled study comparing combined OK and atropine 0.01% against OK-only showed slower axial eye elongation in the combined therapy cohort, and the combined therapy to be well tolerated.³¹ 

SUMMARY

In this article I’ve shown that, as long as you own a corneal topographer, OK is arguably no different to fit in children; takes only 15 minutes longer in tasks that can be delegated to support staff; is preferred by children in comparison to glasses; and is safe as long as patients remain compliant and avoid contact with tap water. The research shows good efficacy for slowing progression of myopia without the need to modify lens design. If full correction cannot be achieved, then over-correcting the residual with glasses is as efficacious for slowing progression of myopia and there is promising, albeit only early research findings, to suggest that combining with atropine may further improve efficacy. Given the increasing prevalence of myopia in children it’s hard to see why fitting rates for OK should not be increasing, and I encourage you to do your part in moving this goal post.

Dr Paul Gifford is a research scientist and industry innovator who graduated as an optometrist from City University, London in 1995, then worked in clinical practice for a decade before being awarded his PhD in hyperopic orthokeratology and contact lens optics in 2009 from the University of New South Wales (UNSW), Sydney. Dr Gifford’s experience includes every facet of the optometry profession, from clinical practice to academia, research and industry. He holds over 40 peer reviewed and professional publications, and has presented more than 40 conference lectures in Australia and internationally. Dr Gifford holds three professional fellowships; and has been conferred two prestigious research awards from the British Contact Lens Association, along with seven other research awards during his PhD, and two post-doctorate research grants. He holds an adjunct academic position at UNSW and consults to the contact lens industry on projects relating to product and systems design and software solutions including machine learning , and is a co-founder and director of the education platform Myopia Profile. 

References 

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