Myopia Development Treatment Review

1. Literature Review

Introduction

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Refractive error (ametropia) prevalence varies with age, gender, ethnicity, and socioeconomic status. Overall ametropia prevalence was reported to be >2.3 billion worldwide in 2003 (Naidoo & Jaggernath, 2012), placing uncorrected refractive error as the primary cause of visual impairment (visual acuity <6/18) and the second leading cause of blindness (visual acuity <3/60) in the world (Resnikoff et al., 2004). In 2007, the global burden of uncorrected refractive error was conservatively estimated to be $268.8 billion (Smith et al., 2009). This is a global public health issue due to its effect on individual visual performance and quality of life, as well as economic productivity loss regarding subsequent care and disability. Ametropia is mainly described by the terms myopia (nearsightedness), hyperopia (farsightedness), astigmatism (typically occurs in conjunction with myopia and hyperopia as the cornea and crystalline lens are not perfect spheres), and presbyopia (age-related vision impairment at near distance, due to the natural loss of accommodation-the crystalline lens’ elastic ability to adjust light focus at various distances). Since levels of hyperopia are significantly low between ages 12-15 (IP et al., 2008), human clinical studies have not been implemented to investigate control strategies. In contrast, myopia is now considered an epidemic by the WHO and is the commonest refractive error among children and young adults (Morgan et al., 2010). In a recent publication, Holden et al. (2016) approximated global myopia prevalence at 1.4 billion people and 163 million with high myopia (≥ -6.00 D) in 2000, but predicted these values to reach nearly 5 and 1 billion respectively by 2050. Moreover, the same review showed that myopia distribution by 2050 will spread between ages 10-79, when previously compared to ages 10-39 in 2000. Besides children and young adults with progressive myopia, Holden et al. (2008) estimated 1.04 billion people were presbyopic and 517 million lacked adequate correction in 2005, while further predicted a prevalence of 1.8 billion by 2050. Presbyopia can also become a global burden due to increasingly ageing populations and longer life expectancies, while its correction is projected to be the largest growing segment of the contact lens industry (Bennett, 2008). Thus, is it possible to find an effective alternative to spectacles, daily contact lenses, and surgical lens implantation to correct presbyopia?

Uncorrected refractive error is preventable and easily treated via spectacles, contact lenses, or laser surgery. Orthokeratology (corneal reshaping therapy or reverse geometry rigid gas-permeable contact lenses) has been implemented since the 1960s. However, only in the last two decades it has generated significant attention worldwide. This is largely attributed to the global increased prevalence of myopia coupled with developments in lens materials and instrumentation. While myopia in the U.S. has increased from 25% to 42% in the past 30 years (Vitale et al., 2008), prevalence completely varies from 3% in Nepalese children (Garner et al., 1999) to 90% in Taiwanese university students (Want et al., 2009). Myopia progression refers to the uncontrolled growth of the eye via changes in axial length (or vitreous chamber). What drives myopia development? Although the risk factors associated with myopia are lower juvenile hyperopia, genetics, ethnicity, environmental factors (less time outdoors, prolonged near tasks), peripheral refraction, and binocular vision, the type of visual environment has been shown to specifically dictate its onset, progression, and cessation (Aller, 2014). Even at low and moderate levels of myopia, nearsighted people are more prone to cataracts, glaucoma, macular degeneration, retinal detachments, choroidal atrophy, and reduced cone photoreceptor density, while high myopia may ultimately lead to blindness. Thus, myopia carries a heavy socioeconomic burden. The cost of correcting myopia in Singapore, only for ages between 12-17 has been estimated to be $37.5 million (Lim et al., 2009), while the approximate individual cost for adults of age ≥ 40 was $709 annually (Zheng et al., 2013). In the United States, an approximate annual cost between $3.9-7.2 billion was reported (Vitale et al., 2006). Can myopia be permanently prevented? The global scientific community has been developing various interventions to control myopia, or reduce the eye’s axial length, such as topical pharmaceuticals (low-dose atropine of 0.01% or pirenzepine), contact lenses (soft multifocal, rigid gas-permeable, or orthokeratology), multifocal spectacles, and the general undercorrection of myopic refraction. How do these treatments work and are they safe? What happens to myopia progression once treatment is discontinued? Although orthokeratology is the most effective optical myopia control strategy, topical pharmaceuticals are considered to be the most effective treatment overall (Walline et al., 2011). In addition and with some variations, all of these strategies are off-label treatments (not approved by legislation for myopia control) and have limited commercial availability. Orthokeratology and soft multifocal contact lenses currently remain the most promising tools available to clinicians, but both require further longitudinal, large randomized controlled clinical trials, in order to become universally adopted.

Thus, this review will discuss (1) myopia development, (2) modes of treatment, and (3) new lens designs and materials.

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Methods

The electronic search was done through Web of Science. There were no database (Web of Science Core Collection, BIOSIS Citation Index, KCI-Korean Journal Database, MEDLINE, Russian Science Citation Index, SciELO Citation Index), date, or language restrictions. The search was selective for randomized controlled clinical trials in humans.

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The search terms included: myopia, myopia control, myopia progression, refractive error, ametropia, orthokeratology, corneal refractive therapy, myopia management, contact lenses, hybrid contact lenses, atropine, children, accommodation, hyperopia, presbyopia, lens design, blur, contrast sensitivity, glare, axial length, central corneal thickness, corneal topography, orthokeratology mechanism, peripheral refraction, orthokeratology efficacy, orthokeratology safety, orthokeratology regression, orthokeratology reversibility, orthokeratology complications, orthokeratology wearer profile, orthokeratology acceptability, orthokeratology satisfaction, corneal recovery, vision related quality of life.

Results & Discussion

Myopia Development

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Myopia is caused by an increase in eye length or corneal curvature. Since light is focused in front of the retina distance vision is blurry, while close objects remain clear. Nearsightedness assumes onset around age 8 and continues to develop during adolescence between ages 15-16 (Thorn et al., 2005). Moreover, Walline et al. (2009) stated an average rate of progression per year of 0.50 D. Children at age 6 are of particular risk to develop myopia if they have hyperopia <0.75 D (Zadnik et al., 2015), indicating the need for early intervention. Myopia is a multifactorial problem with varying onsets. The literature is inconclusive whether males or females exhibit higher myopia prevalence.

The risk of becoming nearsighted is five to six times greater when both parents are nearsighted, especially for children between 6-14 years of age (Jones-Jordan et al., 2010). East Asian children between 11-15 years of age are eight times more susceptible to become nearsighted compared to Caucasian children in the same age group (Ip et al., 2008). Environmental factors such as reduced time outdoors and prolonged time engaging in near tasks (reading or using portable devices) are considered to be the primary risk factors for developing myopia regardless of ethnicity; particularly hours spent reading (Ip et al. 2008). However, Jones-Jordan et al. (2012) did not report a strong correlation between near work and myopia onset or progression. The variability of refractive error has been attributed to genetic factors (Hammond et al., 2001). Rose et al. (2008) further elaborated that the duration of near tasks has ethnic variations, where East Asian children spent 20% more time than their Caucasian counterparts. Additionally, the children of nearsighted parents spent less time outside, while engaged more in near work activities compared to children of parents without a refractive error. Increased outdoor time is especially important due to its preventative effect on myopia onset (Lin et al., 2014), but its effectiveness has not been shown in slowing down myopia progression (Jones-Jordan et al., 2012). French et al. (2013) have suggested that the preventative mechanism of increased outdoor activity is related to reduced accommodative demand, and higher levels of Vitamin D and retinal dopamine activity. Overall, Voo et al. (1998) had previously reported that myopia prevalence is highest among children of Asian ethnicities, followed by Hispanic, African-American, and Caucasian backgrounds.

Previous work has suggested that peripheral refraction is associated with myopia development and progression, but is not the cause. Mutti et al. (2007) demonstrated nearsighted children had higher peripheral hyperopia relative to the central refractive error, when compared to their emmetropic counterparts, but was maintained through 5 years regardless of the increasing eye elongation since the myopia onset. Thus, this association is mainly applied towards strategies to control myopia progression. A child’s binocular vision state is another possible association with myopia. Allen & O’Leary (2006) have reported higher levels of esophoria (inward eye deviation), as well as unstable and insufficient accommodative responses (higher accommodative lag or error) at near distances among nearsighted children and young adults compared to their emmetropic counterparts. These outcomes are thought to be involved in triggering eye growth by causing relative peripheral retinal blur. However, Rosenfield et al. (2002) have shown lower accommodative lags among both myopic and emmetropic subjects. This suggests that binocular vision also does not consistently indicate a causal relationship with myopia, but is important in regards to designing control strategies.

Modes of Treatment

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From the currently implemented myopia control strategies, only soft multifocal contact lenses, orthokeratology, and topical pharmaceuticals are considered to be clinically significant (the ability to reduce myopia progression by approximately 50%); Figure 1 (Smith & Walline, 2015). The mechanism of these treatments consists of peripheral refraction, positive spherical aberration, and accommodation. For instance, myopic undercorrection actually enhances its progression in nearsighted children (Chung et al., 2002). Additionally in their review, Smith & Walline (2015) noted the ability of multifocal spectacles to control myopia in comparison to single vision lenses, even in nearsighted children with binocular errors (esophoria or accommodative lag), but the effect was generally not clinically significant and did not last for more than a year. In regards to rigid gas-permeable contact lenses, the randomized clinical trial by Walline et al. (2004) reported only a temporary significantly reduced myopia progression due to changes in corneal curvature, but no effect on axial elongation. There are various designs all producing similar levels of myopia control, which are significantly more effective than conventional means (undercorrection, single vision spectacles and contact lenses, or multifocal spectacles), and although visual acuity may be compromised depending on the design, the primary goal is controlling myopia progression.

Figure 1. The reported efficacy (%) of atropine, soft multifocal contact lenses, and orthokeratology to reduce myopia by various controlled studies (Smith & Walline, 2015).

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Soft multifocal contact lenses

Soft multifocal contact lenses that are normally used to correct presbyopia are also promising for myopia control. The two soft multifocal lens designs are dual-focus (concentric) and aspheric. In 2011, Anstice & Phillips carried out a 20 month crossover study comparing a dual-focus center-distance concentric soft multifocal lens in one eye with a conventional single vision soft lens for distance correction in the other eye, which were exchanged after a 10 month interval. The researchers reported ≥30% reduction in myopia progression with the dual-focus lens. In a two year study, Cheng et al. (2013) showed that the myopia control mechanism with aspheric soft multifocal contact lenses is based on inducing the most positive spherical aberration, while Anstice & Phillips (2011) did not report significant difference in visual acuity and contrast sensitivity using a concentric design. However, Kollbaum et al. (2013) found reduced visual performance (decreased acuity by one line) and contrast sensitivity when compared a dual-focus center-distance multifocal and a center-near bifocal concentric soft designs to conventional spectacle-corrected performance, while no difference resulted when only the multifocal lenses were compared.

In a two year aspheric lens design study on myopic children wearing center-distance soft multifocal lenses with +2.00 D power add, Walline et al. (2013) reported a maintained 50% and 29% reduction in myopic refraction and eye elongation respectively, when compared to a conventional single vision soft lens. Bickle & Walline (2013) noted that although myopia control is the primary concern, children must be able to tolerate lens wear. After comparing the visual performance of center-distance multifocal soft lenses having +3.00 D and +4.00 D power adds to conventional single vision lenses, the researchers concluded that add powers greater than +2.50 D may not be visually acceptable. A study by Sankaridurg et al. (2011), where Chinese children were fitted with a peripheral plus-powered aspheric soft contact lens for one year, resulted in 34% and 33% reduction in myopic refraction and axial elongation respectively, when compared to the nearly two-fold reduced myopia control effectivity of the flat optical profile in spectacle lens wearers among children of the same age. This lens design corrected the distance refractive error within its central optical diameter, while controlled myopia progression by reducing the relative peripheral hyperopia. Although new developments are constantly made, the literature suggests that the current majority of contact lens designs for myopia control follow this methodology (Gifford & Gifford, 2016). The study by Sankaridurg et al. (2011) contradicts the use of conventional soft single vision contact lenses for myopia control treatment by clinicians, since their optical profile only exerts relative peripheral hyperopia. Additionally, multifocal spectacles are also a contradicted treatment option, as Hasebe et al. (2005) have reported that nearsighted children misuse the near zone within the lens profile and any control effect is limited to one year. Studies have been inconclusive regarding inducing a relative peripheral myopia refraction profile with aspheric lens designs, while the impact of accommodation requires further research.

Orthokeratology

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Overnight orthokeratology is considered the most effective optical treatment in slowing myopia progression. Although the complete mechanism is not yet understood, changes to the anterior and posterior surface of the cornea occur. Anterior surface reshaping is the primary change consisting of flattening the central two mm. cornea (epithelial thinning) and steepening the five-six mm. mid-periphery (stromal thickening); while changes to the posterior surface (corneal bending and edema) have very little contribution (Alharbi & Swarbrick, 2003). Moreover, hyperopic defocus in the paracentral two-five mm. cornea was shown to be the driving force of axial elongation (Mutti et al., 2007). Cheah et al. (2008) have noted the mechanism to be based on redistribution of corneal tissue: epithelial cell compression by driving positive tear forces, instead of migration or cell loss. Hyperopia can also be corrected with orthokeratology in the opposite manner by redistributing tissue to and steepening the central cornea with paracentral flattening. Orthokeratology is particularly superior to other optical myopia control strategies due to its ability to treat moderate and high nearsightedness (3.00 D to 6.00 D), as well as being less susceptible to eye movement and blinking (Smith, 2013). In addition to myopia control, orthokeratology provides the same convenience as laser surgery by providing quality unaided daytime vision, with the further advantages of being reversible, non-invasive, and allowing patients to take a break from lens wear. Laser refractive surgery is also not recommended for patients under age 18. Orthokeratology is also used to correct refractive error regression and restore corneal regularity due to complications arising from refractive surgeries. Another advantage unique to overnight orthokeratology is that the lenses never leave one’s home and parents have the opportunity to be involved with every aspect of their use in cases of children wearers. Orthokeratology is perhaps the most promising tool for clinicians in the nearest future, as new lens designs and materials have been shown to produce increasingly consistent and predictable outcomes.

The main orthokeratology properties consist of induced peripheral myopia shift (Kang & Swarbrick, 2011), increased positive spherical aberrations (Gifford et al., 2013), and higher levels of insufficient accommodation (Zhu et al., 2014). Similarly to peripheral plus-powered aspheric soft contact lens designs, orthokeratology lenses also correct ametropia within the central optical diameter and promote peripheral myopia (Kakita et al., 2011). Although induced hyperopic defocus in the peripheral retina by other optical treatments corrects myopic ametropia, this mechanism does not control axial elongation. The literature has associated orthokeratology’s myopia control efficacy with several factors such as: patient age, gender, anterior chamber depth, baseline myopia, induced relative peripheral myopia, pupil size, corneal power changes, contact lens oxygen transmissibility (Dk/t), aberrations, and accommodation. However, Cho & Cheung (2012) found no correlation between gender or initial myopia, refractive cylinder, and corneal toricity with orthokeratology myopia retardation. Chen et al. (2012) found that subjects with larger pupil sizes achieved greater orthokeratology efficacy, as larger retinal space was available for myopic defocus, which is also found in laser refractive surgery patients. Although changing orthokeratology lens parameters, such as decreasing the central treatment zone size to increase mid-peripheral corneal steepening has not resulted in greater myopia control (Kang et al., 2013). Lum & Swarbrick (2011) found that lenses made of higher Dk/t material produce greater myopia reduction via faster epithelial thinning and less overnight edema. In conjunction with higher positive spherical aberrations, orthokeratology is capable of manipulating the mid-peripheral and anterior retina to cause both a myopic shift and reduce myopic progression (Charman et al., 2007). The induced aberrations lead to the loss of contrast and reduced image quality in people without accommodative error, but due to their peripheral nature and rapid visual adaptation the side effects are limited (Mathur & Atchison, 2009). Also, these and additional light distortions such as haloes and glare are only short-term (Stillitano et al., 2008). Thus, orthokeratology can also be effectively applied for presbyopia treatment; since the increase in positive spherical aberrations post-orthokeratology reduces accommodative demand for high-spatial frequency tasks at near and enlarges the depth of field (Faria-Ribeiro et al., 2016). In a longitudinal overnight orthokeratology study on young adults, Felipe-Marquez et al. (2015) found no accommodative function changes post-orthokeratology lens wear for either short-term (three months) or long-term (three years) duration.

A review by Gonzalez-Meijome et al. (2016) stated that six overnight orthokeratology clinical trials in children of ages 8-12 and varied ethnicities successfully controlled myopia between 30-50%, irrespective of lens design or material; adopted from the same review, Table 1 summarizes orthokeratology studies conducted in 2004-2012. Kang & Swarbrick (2016) also compared the induced peripheral refraction between three orthokeratology lens designs and reported minimal differences in myopia control efficacy. In a two-year randomized study on children, Cho & Cheung (2012) showed myopia control efficacy with orthokeratology of 43-50% with consistently reduced axial elongation annually (-0.14 mm / year), in comparison to an average growth rate seen in children wearing spectacles (0.63 mm / year). Chan et al. (2014) have reported successful myopia control of 32-42% for duration of two to five years, while Walline et al. (2009) had previously shown a 55% reduced axial length after two years of lens wear.

Table 1. Summary of orthokeratology studies conducted in 2004-2012 and their myopia control efficacy specifically based on axial length measurement (Gonzalez-Meijome et al., 2016).

Studies on orthokeratology reversibility have found varying lengths of recovery. Soni et al. (2004) found recovery of central corneal thinning after one night, corneal curvature after one week, and refractive correction after two weeks. Hiraoka et al. (2009) had previously discovered completely recovery of ocular biometry and refraction to baseline, post- one week lens discontinuation in subjects of age 20-37, after one year orthokeratology treatment. Santodomingo-Rubido et al. (2014) confirmed that short-term refractive changes after one week discontinuation of two year orthokeratology lens wear, among subjects of age 6-12, were due to corneal shape recovery and not myopia progression. In 2015, Swarbrick et al. conducted a one year contralateral study design comparing overnight orthokeratology lens wear and the use of conventional daytime rigid lenses, which were swapped after a six month period. Although orthokeratology lens wear leads to lower axial length, Swarbrick et al. stated that stable myopia control cannot be attained after only six months of wear due to the reported rebound effect, where the eye switched to a rigid lens grew nearly two-fold in the second six month interval. In a two-year study on nearsighted children, Cho & Cheung (2016) found increased axial elongation when orthokeratology lens wear was ceased for seven months and children wore spectacles, while the treatment effect in slowing myopia progression was restored with resumed lens wear for additional seven months, but maintained for the full two year duration. The authors also reported a six month rebound effect and that ceasing orthokeratology lens wear at an age ≤14 leads to faster myopia progression. Downie & Lowe (2013) compared the myopia refractive stability in near-sighted children between overnight orthokeratology and spectacles over an eight year period. While the myopic refractive error increased in all spectacle wearers, 64% of the orthokeratology group maintained total stability of their baseline myopia. The authors noted that myopic refractive instability resulting from overnight orthokeratology lens wear is due to a higher asymmetry in the baseline corneal power along the vertical meridian, while factors such as gender, ethnicity, treatment commencement age, treatment period, baseline spherical and cylindrical refraction, or pupil size had no influence. Furthermore, in an 11-year study, the COMET group (2013) reported that myopia stability among nearsighted Asian children occurred at age 16 and 60% of the group demonstrated stabilization. Although the majority of central corneal thinning consists of central epithelial thinning (Alharbi & Swarbrick, 2003), this effect occurs within the first week of wear and has been found to stabilize for a full month (Li et al., 2016). Previously, Mountford (1997) reported the regression of refractive error among Caucasians eight hours post-lens removal to be between approximately 0.50-0.75 D. However, Chan et al. (2008) found regression to be insignificant and unpredictable in Chinese subjects. Thus, further research is warranted, in order to investigate possible correlations between the above results with ethnic differences in lid morphology and corneal asphericity.

ACCEPTABILITY & SAFETY

In interviews with 94 orthokeratology patients, over 90% ranked their post-lens wear vision as either ‘good’ or ‘very good’ (Chan et al., 2008). Also, patients who wore their lenses every night reported higher satisfaction (Santolaria et al., 2013), while children treated with orthokeratology reported greater satisfaction regarding visual quality, cosmetics, academic performance, and peer perception in comparison to spectacles wearers (Santodomingo-Rubido et al., 2013). In comparison to daily soft contact lenses, the results from a Refractive Quality of Life Survey in a randomized clinical trial revealed 67.7% of participants (age 18-40) preferred wearing overnight orthokeratology (Lipson et al., 2005).

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Through technological advancements and new materials, orthokeratology is deemed a safe myopia control strategy. However, this is a contact-lens treatment, which if applied in a lacking regulated clinical environment and thorough patient education can result in poor compliance, as any other type of contact lens. Watt & Swarbrick (2007) reviewed 123 cases of microbial keratitis in children fitted with overnight orthokeratology; 52% occurred in the year 2001, 69% involved East Asian children, and wrongful contact lens care regimes via tap water application. Due to the myopia epidemic, the field of orthokeratology has significantly expanded and the rate of associated complications has decreased in the last decade. Moreover, the risk of contact lens-related infiltrative complications was shown to be the lowest for ages 8-15, which also matches the typical commencement range for myopia control implementation (Chalmers et al., 2011). Adverse events related to overnight orthokeratology are also significantly lower than those associated with overnight soft contact lenses for continuous or extended wear (Keay et al., 2007). After systematically reviewing 170 publications on the safety of orthokeratology, Liu & Xie (2016) noted the most frequent complication was mild corneal staining, while the risk of microbial keratitis was similar to other overnight treatments. In a two year study, Santodomingo-Rubido et al. (2012) found that the majority of complications resulting from overnight orthokeratology occurred between 6-12 months of wear and none were visually compromising. A review of seven orthokeratology studies by Sun et al. (2015) did not report severe adverse events or any keratitis cases, while the commonest complications only consisted of mild corneal stains and pigmented arcs (associated with initial refraction). In a more recent review, Li et al. (2016) similarly showed that all adverse events associated with orthokeratology lens wear were not clinically significant and had fast recovery. Table 2 summarizes the adverse events reviewed by Wen et al. (2015) from eight randomized studies during 2005-2013.

Table 2. Adverse events reviewed from eight randomized controlled studies spanning 2005-2013 (Wen et al., 2015). The most common complication was mild corneal staining, but no severe events were reported.

Pharmaceuticals

Topical pharmaceutical therapy (low-dose atropine of 0.01% or pirenzepine) is considered to be the most effective myopia control strategy overall. However, atropine is not often clinically prescribed due to its side effects (temporary photophobia or light- sensitivity, and blurry vision at close distance) via pupil dilation and accommodative reduction, while pirenzepine is not commercially available (Smith & Walline, 2015). Chia et al., (2014) conducted a two year randomized study on nearsighted children and concluded that a possible mechanism of action is based on reduced accommodation and changes of the crystalline lens curvatures. Similarly to orthokeratology, there is a rebound effect after atropine cessation (Tong et al., 2009).

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The problem with treatment

Although clinicians are shown to be aware of the available myopia control strategies, as well as the research evidence behind their efficacy and safety, the lack of global standardization in treatment protocol persists. In a review by Wolffsohn et al. (2016), important trends among current clinical practice regarding myopia control were noted: undercorrection continues to be employed as a control strategy particularly in India, Spain, Portugal, and South America, pharmaceutical efficacy was reported to be underestimated, while the efficacy of increased outdoor activity was overestimated, and >68% of nearsighted children were still prescribed with single vision spectacles or contact lenses. In addition to clinical standardization, further longitudinal randomized clinical trials are still necessary to confirm the mechanism, efficacy, safety, predictability, and economic feasibility of myopic control strategies, in order to gain clinical acceptance worldwide. These steps would be expected to additionally result in increased practitioner training and patient education that can potentially end the myopia crisis.

New Lens Designs & Materials

Despite its consistent levels of myopia retardation by approximately 50%, convenience, reversibility, and no significant side effects, overnight orthokeratology rigid lenses only constitute 1% of new contact lens fits worldwide (Wolffsohn et al., 2016). In an international survey, Efron et al. (2013) found that rigid lens fitting is at a steady decline and consisted 10.8% of all contact lens fits worldwide, while orthokeratology represented 11.5% of the rigid contact lens fits. Soft materials dominate the contact lens industry, as rigid lenses are primarily used today for specialized vision correction in conditions including keratoconus, keratoglobus, corneal ectasia, pellucid marginal degeneration, irregular corneas, and orthokeratology. In addition to their higher durability, superior optical quality, and good tear exchange (higher oxygen permeability), only rigid lens materials can reshape the cornea and induce the necessary refractive shift in specialty conditions. However, rigid lenses require an adaptation period, present significant initial discomfort, and lack positional stability on the eye (most likely to decenter).

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Thus, the current patent and future outlook for myopia control based on new lens designs and materials is hybrid orthokeratology. In a hybrid contact lens, the center is made of a rigid, highly gas-permeable material surrounded by bonded soft silicon ‘skirt’. This innovation is meant to combine the superior optics and stable vision provided by a rigid material, with the comfort and centration of soft materials. The expectation is for the ability to exert maximum myopia control across all levels of myopia, as well as ease clinician prescribing and increase patient compliance. Hybrid contact lenses are already used for managing many of the above mentioned specialty conditions. In keratoconic patients, hybrid lenses have been shown to provide an optical improvement (visual acuity, contrast sensitivity) among rigid gas-permeable intolerant patients (Abdalla et al., 2010), as well as higher satisfaction and vision-related quality of life scores (Hashemi et al., 2014). Additionally, hybrid contact lenses are capable of comparable visual quality for presbyopic patients to soft multifocal contact lenses (Pinero et al., 2015). The possible risks associated with hybrid lenses include giant papillary conjunctivitis, breakage of the soft skirt (Abdalla et al., 2010), infection (Lee & Gotay, 2010), hypoxia (Pilskalns et al., 2007), or edema (Fernandez‑Velazquez, 2011).

Despite their promise, hybrid contact lenses have not yet been applied to orthokeratology for myopia control. There is currently no peer-reviewed literature and only limited information comparing hybrid contact lenses to traditional forms of treating advanced eye diseases. Their overall mechanism, efficacy, safety, and acceptability are unclear. This presents a tremendous opportunity for future research, which can explore the possibility of further improvements.

Conclusions

Although uncorrected refractive error (myopia, hyperopia, astigmatism, or presbyopia) is preventable and easily treated, it remains a global public health issue. Moreover, the myopia epidemic is characterized by increasingly early onset and high progression rates. There are various control strategies, but a standardized clinical protocol still has not been established. Orthokeratology is currently the leading myopia control strategy available, due to its combined convenience, efficacy, safety, and acceptability. However, other options continue to be implemented despite their reduced effectiveness. Management challenges regarding pharmaceutical prescribing continue unsolved, while the remaining optical strategies, including laser surgery, are only capable of restoring visual acuity and do not prevent myopic eye growth. Due to the lack of accepted widespread use and supply availability of effective treatments, myopia often continues to be wrongfully treated with conventional remedies (undercorrection, single vision spectacles and contact lenses, or multifocal spectacles).

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Also, as with general overnight contact lens wear, infective microbial keratitis and increased visual light disturbances are possible risks of orthokeratology, but research has thoroughly demonstrated the overall safety of the procedure. Likewise, patient satisfaction and acceptability are widely reported. The combination of diligent clinical practice with patient education is essential to ensure appropriate lens fitting and complication management, as well as lens care regime compliance and disciplined follow-up attendance respectively. The precise mechanism of both aspects of myopia development risk factors (ametropia, environmental factors, gender, ethnicity, genetics, peripheral refraction, or binocular vision) and associated modes of treatment (topical pharmaceuticals or contact lenses) remain poorly understood. The literature has suggested that implementing multiple interventions simultaneously (increased time outdoors, orthokeratology, and pharmaceuticals) may be the ultimate treatment method. Since orthokeratology has been proven effective for correcting any refractive error, new lens designs and materials such as hybrid orthokeratology must be investigated. Further longitudinal, large randomized controlled trials are required in all of the discussed points regarding myopia and its treatment, particularly into the occurring corneal changes and physiological control of eye growth.

2. Research Studies

The proposed studies are based on orthokeratology contact lens treatment (corneal reshaping overnight), which is considered to be the most effective optical myopia control strategy available, without significant side effects, and is reversible. However, it currently involves rigid lenses that are no longer widely fitted due to the need for adaptation. To overcome this in people requiring rigid lenses for eye pathology, a soft ‘skirt’ can be attached to make the lens more comfortable (a hybrid lens), but this has not yet been applied to corneal reshaping.

The studies will focus on custom design and contact lens materials (hybrid orthokeratology), as well as the mechanism of orthokeratology for myopia control and presbyopia correction. The overall aims are to develop a new orthokeratology lens in collaboration with industrial partners, test the lens and its efficacy in comparison to traditional orthokeratology lenses, and evaluate other potential benefits such as increasing the depth of focus in presbyopia. The studies will follow the principles of Declaration of Helsinki and are pending approval by the Aston University Ethics Committee (Sections 4 & 6).

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Study 1 – Hybrid Orthokeratology

Purpose

This study will compare the clinical performance of two contact lens designs for myopia control (effectiveness and mechanism of action). Industrial partners include Contamac (Saffron Waldon, UK) and No7 Contact Lenses (Hastings, UK).

Research Questions

• Does adding a soft skirt to a rigid orthokeratology (hybrid) lens achieve same topography profile as conventional orthokeratology lens design, with perhaps better centration and comfort?
• Could soft stabilisation allow further peripheral optical manipulation to alter the peripheral image shell?
•    What is the mechanism of short-term orthokeratology myopia retardation effects?

Design

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• Bilateral contact lens wear in a nonrandomized cross-over design (an innovative hybrid lens by No7 first in some subjects and after traditional orthokeratology in others with a 1-week washout period in between) will be efficient and would work especially well regarding aspects of eye comfort.
• Difficult to mask a hybrid study as will be obvious from lens size and insertion technique, but will use objective measures of eye shape and corneal biomechanics.
• Sample size target: 30; by accounting for potential participant drop out, in order to achieve 80% statistical power for a significance level of α = 0.05 with a confidence level of 95%, the suggested sample size group based on an effect size of 0.8 for a two-tailed t test of the difference between two independent means via a priori power analysis is 26 (G*Power 3.1, University of Dusseldorf). Previous research has shown relevant standard deviations (SDs) of 0.15 mm for axial length and 0.50 D for refractive error.
• Participant eligibility: 18 years or older Aston University students (no vulnerable individuals), as well as staff and associated family and friends, first time orthokeratology lens wearers (interested in having a contact lens neophyte and existing soft contact lens wearing group; difficult to find enough rigid lens wearers), regular sleep patterns, healthy eyes (no active disease/systemic medications-diabetes, dry eyes, corneal irregularities-keratoconus, scarring, previous refractive surgery), no previous contact lens wear history required, Rx or prescription (initial myopia between -1.00 D to -5.00 D, as higher myopia has increased risk of decentration and staining; with-the-rule astigmatism up to -2.50 DC and against-the-rule astigmatism up to -1.00 DC).
• Fitting, handling (insertion, removal), sterilization (peroxide and multipurpose care solutions), and patient guidelines via No7 manufacturer’s guide.
• Measurements: computerized ETDRS (Early Treatment Diabetic Retinopathy Study) logMAR VA (Visual Acuity) for distance and near, refractive error (autorefractor), ocular health (slit-lamp and Efron Grading Scale), NITBUT (Non-invasive Tear Brea-Up Time), topography (Medmont E300), iris and pupil diameters, aberrometry, OCT (Optical Coherence Tomography), ORA (Ocular Response Analyzer), electronic VCTS (Vision Contrast Test System) for contrast sensitivity, halometer for glare, visual analog questionnaires (National Eye Institute Refractive Error Quality of Life Instrument or NEI-RQL-42, and Ocular Surface Disease Index or OSDI).
• Data collection (screening, baseline, and follow-ups): monocular and binocular examination with corrected refractive error at distance and near, without/with lenses to evaluate patient impression/comfort, fit, VA, Rx, anterior eye health, topography, contrast sensitivity, glare, axial length and choroidal thickness, corneal curvature/thickness, response factor, and hysteresis, defocus curves, questionnaires, and monitor wear time via compliance report. Compare results between the baseline (before lens insertion, before sleep) and after each wearing session (within 2 hours after lens removal the next morning).
• Statistical analysis: Microsoft Excel software to represent objective clinical data and subjective patient responses, where p < 0.05 is considered statistically significant; descriptive statistics to analyze baseline variables (age, gender, ethnicity, refractive error, medical and contact lens history responses); parametric statistics (repeated-measures one-way ANOVA or t test for independent samples) to analyze outcome differences between the two orthokeratology lens types at selected visits and to evaluate changes over time from baseline for individual groups.  

Protocol

• 45-60 min assessments: 6-8h daily wear during sleep; Initial study (1-2 nights)-Day 1, Day 2; If promising results (3 month extension)-1 week, 2 weeks, 4 weeks, and 12 weeks. This is because orthokeratology effects are normally exhibited and retained in 24-48 hours. Measurements will be taken again 72 hours post-lens wear to assess any regression changes.
• Examination will include:
• Explanation and signing informed consent document
• Taking medical and contact lens history
• Uncorrected VA for distance (at 6 m) and near (40 cm based on reading speed and print size) on ETDRS logMAR chart at 40-48 lux luminance (medium to low illumination similar to driving at dusk or mesopic vision conditions), as well as peripheral refraction using an aberrometer
• Subjective refraction
• Overefraction (ETDRS logMAR-starting from the best distance correction and adding 0.25 D increments to achieve best corrected vision
• Anterior eye health (slit-lamp assessment), including ocular diagnostic dye (sodium fluorescein)
• Topography: 10 measurements generally two seconds after blinking to ensure a stable tear film, if marginal dry eye is found, a saline solution can be administered prior the measurements. Obtain corneal profile (power and shape), aberrometry (data for a 4.50 mm pupil diameter measured up to the fourth Zernike order), and measure horizontal visual iris diameter (HVID) and pupil diameter (under photopic and scotopic environment to ensure the treatment zone is not smaller than the pupil size). Obtain ocular biometric parameters in the anterior segment
• OCT for corneal epithelium analysis, axial length, and choroidal thickness
• ORA for corneal curvature/thickness, response factor, and hysteresis (biomechanics)
• Contrast sensitivity/contrast threshold 3 m under mesopic and photopic conditions to measure the subject’s sensitivity to a particular object size
• Glare via halometer
• Defocus curves (Rx profile or range of clear vision between +1.50 and -5.00 D in 0.50 D increments) with letter randomization
• Contact lens fitting (No7 manufacturer’s guidelines)
• Questionnaires: NEI-RQL-42 & OSDI on baseline and follow-up visit within 8 weeks
• Safety: frequent aftercare (1st night-early morning visit within 2hrs after awakening to ensure no corneal oedema; then 1 week, 2 weeks, 1 month, and every 3 months) to evaluate fit, Rx, wear time and to ensure ocular health, lens integrity, and review compliance (contamination: lens cases (39%) and suction holders (34%); clean daily-brushing with saline and multipurpose solution, disinfect weekly-boiled water in 10min and protein tablets, and replace monthly both solutions and cases; replace lenses every 6 months; cool and dry place storage).
• Adverse response expectation: mild corneal staining (<Grade 2 on Efron), lens binding (46%-most common non-visual problem), multipurpose care solution sensitivity/toxicity (staining and discomfort-Polyquad most damaging; rinse with saline before insertion and use rewetting drops); also, possible drop by 2 D/1 or 2 lines in distance acuity, haloes/reflections or monocular diplopia, and slight contrast sensitivity reduction. Overall low risk.
• Good results expectation: Px comfortably wears lenses overnight, good vision (at least expected to be the same level as the pre-treatment), no/minimal staining, topography shows well-centered treatment zone of central corneal flattening and paracentral steepening.

Study 2 – Presbyopic Orthokeratology

Purpose

This study will assess the clinical performance of presbyopic orthokeratology contact lenses. Partners include Teknon Hospital (Barcelona, Spain) and Precilens (Paris, France).

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Research Questions

• What are the efficacy, safety, and acceptability (dropout rates) of short-term presbyopic orthokeratology?
• What are the associated refractive and corneal topographic changes?
• What are the mechanism, as well as the optical effect on the peripheral retina of presbyopic orthokeratology?

Design

• Bilateral contact lens wear in a nonrandomized design (presbyopic lenses by Precilens, CE marked) will be efficient and would work especially well regarding aspects of eye comfort. The lenses used are rigid Double Reservoir Lenses (DRL) made of Boston XO2 (hexafocon B) material with oxygen permeability (Dk) of 141.
• Sample size target: 30; by accounting for potential participant drop out, in order to achieve 80% statistical power for a significance level of α = 0.05 with a confidence level of 95%, the suggested sample size group based on an effect size of 0.8 for a two-tailed t test of the difference between two independent means via a priori power analysis is 26 (G*Power 3.1, University of Dusseldorf). Previous research has shown relevant standard deviations (SDs) of 0.15 mm for axial length and 0.50 D for refractive error.
• Participant eligibility: adults (no vulnerable individuals) of age 50-65 recruited from Aston University Health Clinic, as well as staff and associated family and friends, must have trouble with near vision tasks, first time orthokeratology lens wearers (interested in having a contact lens neophyte and existing presbyopic contact lens wearing group; difficult to find enough rigid lens wearers), regular sleep patterns, healthy eyes (no active disease/systemic medications-diabetes, dry eyes, corneal irregularities-keratoconus, scarring, previous refractive surgery), no previous contact lens wear history required, Rx or prescription (-0.25 D to +1.00 D with astigmatism up to -1.00 DC for distance vision; +1.25 D to +2.50 D at near).
• Fitting, handling (insertion, removal), sterilization (peroxide and multipurpose care solutions), and patient guidelines via Precilens manufacturer’s guide.
• Measurements: computerized ETDRS (Early Treatment Diabetic Retinopathy Study) logMAR VA (Visual Acuity) for distance and near, refractive error (autorefractor), ocular health (slit-lamp and Efron Grading Scale), NITBUT (Non-invasive Tear Brea-Up Time), topography (Medmont E300), iris and pupil diameters, aberrometry, OCT (Optical Coherence Tomography), ORA (Ocular Response Analyzer), electronic VCTS (Vision Contrast Test System) for contrast sensitivity, halometer for glare, visual analog questionnaires (National Eye Institute Refractive Error Quality of Life Instrument or NEI-RQL-42, and Ocular Surface Disease Index or OSDI).
• Data collection (screening, baseline, and follow-ups): monocular and binocular examination with corrected refractive error at distance and near, without/with lenses to evaluate patient impression/comfort, fit, VA, Rx, anterior eye health, topography, contrast sensitivity, glare, axial length and choroidal thickness, corneal curvature/thickness, response factor, and hysteresis, defocus curves, questionnaires, and monitor wear time via compliance report. Compare results between the baseline (before lens insertion, before sleep) and after each wearing session (within 2 hours after lens removal the next morning).
• Statistical analysis: Microsoft Excel software to represent objective clinical data and subjective patient responses, where p < 0.05 is considered statistically significant; descriptive statistics to analyze baseline variables (age, gender, ethnicity, refractive error, medical and contact lens history responses); parametric statistics (repeated-measures one-way ANOVA or t test for independent samples) to analyze outcome differences between the two orthokeratology lens types at selected visits and to evaluate changes over time from baseline for individual groups.  

Protocol

• 45-60 min assessments: 6-8h daily wear during sleep; Initial study (1-2 nights)-Day 1, Day 2; If promising results (3 month extension)-1 week, 2 weeks, 4 weeks, and 12 weeks. This is because orthokeratology effects are normally exhibited and retained in 24-48 hours. Measurements will be taken again 72 hours post-lens wear to assess any regression changes.
• Examination will include:
• Explanation and signing informed consent document
• Taking medical and contact lens history
• Uncorrected VA for distance (at 6 m) and near (method of directly measuring the habitual near VA is by using functional presbyopia with a cutoff at 0.4 logMAR or N8 at 40 cm based on reading speed and print size) on ETDRS logMAR chart at 40-48 lux luminance (medium to low illumination similar to driving at dusk or mesopic vision conditions), as well as peripheral refraction using an aberrometer
• Subjective refraction
• Overefraction (ETDRS logMAR-starting from the best distance correction and adding 0.25 D increments to achieve best corrected vision; ADD-minimum amount of near correction to read; functional vision-expectation of the same VA as in the best refraction add)
• Anterior eye health (slit-lamp assessment), including ocular diagnostic dye (sodium fluorescein)
• Topography: 10 measurements generally two seconds after blinking to ensure a stable tear film, if marginal dry eye is found, a saline solution can be administered prior the measurements. Obtain corneal profile (power and shape), aberrometry (data for a 4.50 mm pupil diameter measured up to the fourth Zernike order), and measure horizontal visual iris diameter (HVID) and pupil diameter (under photopic and scotopic environment to ensure the treatment zone is not smaller than the pupil size). Obtain ocular biometric parameters in the anterior segment
• OCT for corneal epithelium analysis, axial length, and choroidal thickness
• ORA for corneal curvature/thickness, response factor, and hysteresis (biomechanics)
• Contrast sensitivity/contrast threshold 3 m under mesopic and photopic conditions to measure the subject’s sensitivity to a particular object size
• Glare via halometer
• Defocus curves (Rx profile or range of clear vision between +1.50 and -5.00 D in 0.50 D increments) with letter randomization
• Contact lens fitting (Precilens manufacturer’s guidelines)
• Questionnaires: NEI-RQL-42 & OSDI on baseline and follow-up visit within 8 weeks
• Safety: frequent aftercare (1st night-early morning visit within 2hrs after awakening to ensure no corneal oedema; then 1 week, 2 weeks, 1 month, and every 3 months) to evaluate fit, Rx, wear time and to ensure ocular health, lens integrity, and review compliance (contamination: lens cases (39%) and suction holders (34%); clean daily-brushing with saline and multipurpose solution, disinfect weekly-boiled water in 10min and protein tablets, and replace monthly both solutions and cases; replace lenses every 6 months; cool and dry place storage).
• Adverse response expectation: mild discomfort, vision adaptation requirement, mild corneal staining (<Grade 2 on Efron), lens binding (46%-most common non-visual problem), multipurpose care solution sensitivity/toxicity (staining and discomfort-Polyquad most damaging; rinse with saline before insertion and use rewetting drops); also, possible drop by 2 D/1 or 2 lines in distance acuity, haloes/reflections or monocular diplopia, and slight contrast sensitivity reduction. Overall low risk.
• Good results expectation: Px comfortably wears lenses overnight, good vision (at least expected to be the same level as the pre-treatment), no/minimal staining, topography shows well-centered treatment zone of central corneal steepening and paracentral flattening.