In the first article in the series we considered the factors which could influence the development of myopia. In this second article we will consider the contact lens interventions which have been postulated to reduce the progression of myopia. Firstly we need to consider the potential risk of uncontrolled myopia development.

Risks of pathological myopia

In the previous article, pathological myopia was defined as that level of myopia at which the economic burden on an individual or society should be considered. This burden is not only in the provision of appropriate refractive correction but in the socio-economic consequences of the ocular diseases associated with significant myopia. The predominant feature in higher levels of myopia is the increase in vitreous chamber depth and its effect on axial length. This increase in size leads to stretching of the ocular layers. Known consequences of this stretching are retinal thinning, retinal breaks and lattice degeneration (figures 1 to 3). The result of these retinal changes is an increased risk of retinal detachment.

Figure 1: Retinal atrophy in pathological myopia

Stretching is not confined to the retina both the choroid and sclera are similarly stretched. This leads to conditions such as lacquer cracks, Fuchs spot (or Forster-Fuchs spot), choroidal neovascularisation (CNV) and macular holes (figure 4).

CNV was found in 10% of highly myopic eyes (> -8.00DS) with those eyes showing lacquer cracks being the most prone to its development.1-3 Lacquer cracks are thought to indicate breaks in the retinal pigment epithelium (RPE) and Bruch’s membrane due to elongation of the eye. The second most common cause of CNV in high myopia was found to be patchy atrophy (figure 1). In this case the RPE and choriocapillaris have undergone complete atrophy. Thirty percent of patients with CNV in one eye developed a similar change in the other eye after, on average, a period of eight years. The advent of anti-VEGF treatments has allowed patients to receive treatment for myopic CNV which may reduce the ongoing impact of the condition.

Figure 2: Retinal tear

Thinning of the sclera may lead to staphyloma, in which there is a backward bulging of the posterior wall of the eye. The degree of staphyloma correlates with the thinning of the sclera. The thinning may occur in an irregular pattern giving rise to quite abnormal eye shapes.6-13 In their study of 424 Asian subjects aged over 40 and with myopia of -6.00DS or more, Chang et al found 23% had some form of staphyloma.4 It has been suggested that there may be a link between posterior staphyloma and the development of splits within the macular tissue which may lead to macular hole formation.5 The optic disc is also more likely to show abnormality in high myopes, with an increased presence of tilted disc or significant areas of peripapillary atrophy. Both of these features can lead to difficulty in disc evaluation for changes associated with glaucoma. Stretching of the tissues around the optic disc and a subsequent reduction in support may be part of the explanation why pathological myopes are more susceptible to glaucoma.3,6,7

Figure 3: Lattice degeneration

Cataract is another condition which is often seen alongside myopia. The Blue Mountain Eye Study found in their 10 year follow up of 3,644 subjects aged ≥49 years that 10% of myopes had some degree of nuclear cataract compared with 33% of hypermetropes.8 For myopic individuals, the incidence increased with increase in their degree of myopia. The myopic shift seen in the ageing eye as a result of nuclear sclerosis creates index myopia which is unrelated to any stretching of the ocular tissues. For posterior subcapsular cataracts, the group found a higher incidence in myopes than in hypermetropes. As with nuclear cataracts, the incidence increased with the magnitude of the refractive error. Cortical cataract was found to have no association with myopia among the individuals in the study programme. The actual mechanism involved in cataract formation in high myopes is unclear.7

Figure 4: Full thickness macular hole

Pathological limits

So what level of myopia is safe? The Blue Mountain Eye Study recorded myopic retinopathy changes in eyes with myopia of as little as -3.00DS (1%).2,9 Cataract surgery is now one of the most commonly completed surgeries in the NHS and, while it is not without risk, it is considered a routine procedure, even in patients with severe myopia. Currently the consequences of tissue thinning are the most likely cause of visual impairment in the developed world. All of the changes associated with pathological myopia will increase significantly over the next decades as myopic children currently being identified reach adulthood. Measures to look at the control of myopia progression will become critical.

Research to identify those factors in children at risk of high myopia as early as possible will be vital in order to allow the timely delivery of appropriate myopia control interventions. Members of the Collaborative Longitudinal Evaluation of Ethnicity and Refractive Error (CLEERE) group have suggested that children with hyperopia which is lower than expected for their age should be identified as potential future myopes.10 They suggest the cut off points shown in table 1 for cycloplegic spherical equivalent refractive errors.

Table 1: Degree of hyperopia as a predictor of future myopia 10

Much of the work around our understanding of the development and progression of myopia has been based on animal studies. Species involved in this have been birds, such as the chick, mammals such as the tree shrew and primates such as the rhesus monkey. This research topic is outside the scope of this article.

Peripheral refraction

A number of research papers have reported a possible role for the peripheral refraction in the development and progression of myopia. In their review article, Charman and Radhakrishnan discuss these numerous studies and evaluate the interaction between peripheral refraction and the development of refractive error.11 Both animal and human studies have found that an eye which shows a relative hyperopic peripheral refraction is more likely to develop myopia.11,12 Children with a relative myopic peripheral refraction show an emmetropic or hypermetropic foveal refractive error.13

In essence, the presence of the image shell behind the peripheral retina appears to drive axial length growth. This is clearly an oversimplification of the complex interactions which occur within the developing eye. Research in this field however has led to the suggestion that manipulation of the peripheral refraction to create relative peripheral myopia may slow or even prevent myopia progression. As with all research into progressive conditions, longitudinal studies are required to confirm this hypothesis. Two such studies have reported their findings over the past three years.14-15

They concluded that both baseline and changes in peripheral refraction did not support the hypothesis that the presence of relative peripheral hypermetropia provided a driver for myopia development and progression. In a recent paper presented at the International Myopia Conference in China, Atchison and Rosén presented a different explanation for the role of the peripheral refraction in myopia development.16 They suggest the ocular growth may stop when the periphery is rendered emmetropic.

These findings contrast with a number of papers which have reported on the use of contact lenses which have been designed to manipulate peripheral refraction. These contact lenses fall into three categories;

  • orthokeratology 17
  • soft multifocal lenses (used off label)
  • custom-made soft lenses designed to manipulate peripheral refraction or peripheral aberrations.

In these studies, a reduction in the increase in axial length over time has been used as an indication of a reduction in the progression of myopia. The remainder of this article will look at each of these three.

Orthokeratology lenses (OK)

In 2004 the Contact Lens and Myopia Progression (CLAMP) study reported their findings on the comparison of the influence of conventional rigid gas permeable lenses with soft contact lenses on the progression of myopia.18 They found no clinically significant difference in progression between the two modalities. The longitudinal orthokeratology research in children (LORIC)19 study, the corneal reshaping and myopia progression (CRAYON)20 study and Kakita et al21 reported a reduction in the rate of increase in axial length over a two-year period. The LORIC study and Kakita et al compared the difference between orthokeratology lenses and single vision spectacles and reported a reduction in axial length change of 0.25mm and 0.22mm respectively. In the CRAYON study a comparison was made between orthokeratology and single vision soft lenses with a corresponding reduction in axial length increase of 0.32mm.

In both the LORIC and CRAYON studies, conclusions regarding the axial length changes were made against a control group of children recruited to a previous myopia control study. This retrospective check is not an ideal situation. More recently trials have been conducted where myopic children have been recruited to a prospective trial and randomly assigned to either OK or single vision spectacles. Santodomingo-Rubido et al22 looked at the effect of OK lenses on a group of Spanish children and concluded that over a period of two years, while axial length increased in both OK lens and spectacle lens wearers, the OK group showed a similar reduction to that of the Kakita et al study.21

Hiroaka et al in their five-year follow up of children fitted with either OK lenses or spectacles found that OK lenses did have an effect on the rate of increase in axial length.23 This effect was most noticeable in the first three years of OK lens wear. Cho and Cheung in the ROMIO study found that over a two-year period children in OK lenses showed a 43% slower rate of increase in their axial length when compared with the spectacle wearing group.24

The children in the Hiroaka et al study were 10 years old at the commencement of the study. This may explain why they recorded a reduction in effect after three years as the children were approaching adolescence when myopia progression generally tends to slow. The ROMIO group divided their subjects into younger (under nine years) and older (nine to 10 years) and also by their rate of myopia progression;

  • Slow – -0.50DS or ≤0.18mm change in axial length per year
  • Fast – over -1.00DS or >0.36mm change in axial length per year
  • Moderate – those children who fell between these two categories.

They concluded there were more fast progressors in the younger spectacle wearing subjects (65%) than the OK lens group (20%) and suggest that OK lens wear should be commenced in younger myopic children.

Swarbrick et al in their novel study design compared the effect of OK and rigid gas permeable (RGP) lens wear.25 In this study the children wore an OK lens in one eye and an RGP lens in the other for a period of six months. After a two week recovery period, in which no lenses were worn, the lenses were reversed and worn for a further six months. In both phases they recorded a reduction in the axial length of the eye wearing the OK lens. They did note that on reversal of the lenses the RGP wearing eye showed a rapid change in axial length which they feel is a rebound effect. They also point out that this raises an issue around the length of time children will be required to wear OK lenses if we are to control their myopia progression. In all of the trials mentioned here subjects were fitted with commercially available orthokeratology lenses.

In January 2016 Parker and Leach published results from a survey of optometry schools in North America and Puerto Rico into the education provided in the use of orthokeratology as a mechanism for myopia control.26 For the purposes of the consideration of myopia control, optometry schools were asked to indicate the age when they would recommend OK lenses. While 30% of schools surveyed suggested that they would treat from as young as five or six years of age, most qualified this decision with a comment that the child’s maturity and parental participation in the process would influence the situation.

Wolffsohn et al, in their report into global trends in myopia management attitudes, reported that orthokeratology was perceived to be the most effective method of myopia control.27 The survey found that, of the practitioners who responded, the Asian and Australian practitioners were more confident in the efficacy of OK lenses for myopia control than those in North America or Europe. Practitioners from Asia (particularly those in China and Hong Kong) perceived themselves to be more active in employing myopia control. This may well reflect the endemic nature of myopia in these countries. Despite this, Asian practitioners commenced OK intervention at a higher level of myopia than those in Australia and North America. Practitioners responded with concerns regarding the increased instrumentation required for the fitting of OK lenses. Corneal topography is an essential requirement for fitting these lenses. Respondents also raised concerns regarding the safety of orthokeratology.

Liu and Xie published their systematic review into the safety of orthokeratology in January 2016.28 Microbial keratitis (MK) is clearly the condition which is of greatest concern to the contact lens practitioner when considering any modality of contact lens wear (figure 5). The overnight wear of OK lenses is the risk factor here, since closed eye wear of contact lenses has been associated with an increased risk of MK. Bullimore et al reported their results from a large multicentre retrospective that found the risk of MK in orthokeratology to be similar to that of other overnight wear lenses.29 In most cases, the causation of the MK has been put down to poor compliance with lens care instructions or poor training of patients and/or practitioners.

Appropriate regulation of the practice of orthokeratology (by stating minimum requirements for instrumentation and training) has addressed many of the issues around the occurrence of MK. Evaluation of the organisms involved in the reported infections show that Pseudomonas aeruginosa and Acanthamoeba spp are the most common. In both of these organisms prompt treatment is critical to ensure a favourable outcome. The use of the ‘look good, feel good, see good’ adage should be stressed particularly to orthokeratology patients along with the advice to seek urgent clinical advice if they have any concerns. The other side effects of orthokeratology such as lens binding, corneal staining and other corneal related changes are beyond the scope of this article. Clearly orthokeratology has a place within the control; of myopia progression but the technical difficulties involved in this modality of lens wear have probably contributed to the limited uptake in some centres.

In Wolffsohn et al’s survey, 4.1% (±11.3) of progressive myopes were being offered multifocal contact lenses, while only 2.1% (±7.9) were being offered novel soft contact lenses as a mechanism of myopia control.27

Soft multifocal lenses (off label)

Walline et al published their findings on the use of a commercially available soft multifocal lens in 2013.30 Children in this study aged between eight and 11 years old were fitted with Proclear multifocal ‘D’ lenses (centre distance design) and followed for a period of two years. Refractive errors were restricted to -1.00 DS to -6.00DS and <1.00 D of astigmatism. All children were fitted with a +2.00DS add.

Over a two-year period, the group found there was a statistically significant reduction in the rate of axial length increase (29%) in the children wearing the multifocal lens when compared with an age matched group of single vision soft lens wearers who had participated in a previous study (ACHIEVE). The multifocal group also showed a 50% reduction in the progression of their refractive error. Aller et al recently published their results for a 12-month trial of children and young people fitted with either Acuvue Bifocal (centre distance design) or Acuvue 2 single vision soft lenses.31 The inclusion criteria for the study were similar to that of the Walline group, with an additional limit of anisometropia ≤2.00DS. Evidence of at least 0.50DS of myopia progression in the previous 12 months was required. All candidates had a near associated phoria.

In contrast to the Walline study, reading additions for the Aller et al study were selected to eliminate or produce maximum reduction in the near associated phoria. Decisions on the reading add were tempered by the need to provide maximum distance acuity. This use of adds to eliminate near esophoria correlates with that of previous studies into the provision of multifocal spectacles in the provision of myopia control.32,33 Over the period of the study both the bifocal and single vision lens groups showed a change in refractive error and axial length. The group report a 72% reduction in myopia progression in the bifocal group and an 80% reduction in axial length extension.

Custom designed soft lenses

In 2011, Anstice and Phillips reported on the effect of a custom designed soft contact lens. The lens had a central zone to correct the distance refractive error.34 This was surrounded by concentric zones which created 2.00DS of simultaneous myopic defocus while the wearer was looking in the distance and at near. The diameter of the zones was designed to allow action to occur even when the pupil was constricted for near. The children were assigned to two groups, one having the custom lens in the dominant eye initially and the other having the lens in the non-dominant. A single vision soft lens was worn in the other eye. After a period of 10 months wear the lenses were reversed.

Results for the lenses were that both refractive error and axial length increases were reduced during the dual focus contact lens period. The custom designed soft lens of Sankaridurg et al had a central distance correction zone (1.5mm semi chord).35 The power increased to reach a relative power of +1.00DS at the 2mm zone and +2.00DS at the edge of the treatment zone. Results for this lens were that, over 12 months of wear, there was a 34% reduction in refractive error progression when compared with a matched group of spectacle wearers. Axial length changes were reduced by 33%. The Defocus Incorporated Soft Contact (DISC) is a custom soft lens which has a central correction zone surrounded by concentric zones of correction and 2.5DS of negative defocus.36 Children in this study wore either the DISC lens or a standard design soft lens. Over the two years of the study myopia progression was reduced by 25% and axial elongation by 31% in the DISC group. All of these dual focus lenses have produced a reduction in myopia progression and axial length extension. Lam et al point out that the optimum amount of retinal defocus to slow myopia progression is yet to be confirmed.36

In the Cambridge anti-myopia study, a soft lens was designed to alter the ocular spherical aberration while correcting the distance refractive error.37 This change in spherical aberration was used to reduce the lag of accommodation. This lag has been shown to contribute to myopia progression. Younger subjects (under 16.9 years) showed a reduction in myopia progression for the first 12 months. Over the two-year period of the study there was no statistically significant effect of the control of aberration.

In 2016, Cheng et al reported on their lens designed to create positive spherical aberration.38 Generally, soft lenses create negative spherical aberration in the periphery and as a result hyperopic blur. Participants in this study wore either the daily disposable spherical aberration lens or a standard daily disposable lens for a period of up to two years. This was followed by a wash out phase when the participants wore Acuvue moist daily disposable lenses. Over the first six months a 63% reduction in axial elongation was seen when compared to the control group. This change did not translate into a clinically significant effect on the refractive error with a reduction of < -0.25D in the first year. No rebound effect was noted over the withdrawal phase in the participants.

Paune et al compared the effect of orthokeratology (OK) lenses against a custom soft radial refractive gradient (SRRG) lens and a control group of spectacle wearers.39 In this study, the SRRG lens had a distance refractive zone and a progressive design which reached +2.00DS add at a point which corresponded to a retinal eccentricity of 35º. This eccentricity corresponds with the findings of relative peripheral hyperopia mentioned in the earlier section.11-13 Both the OK and SRRG lenses produced a reduction in myopia progression and axial length increase. The OK lens was more efficient with a 67% reduction in myopia and a 38% reduction in axial length change. The SRRG produced a 43% and 27% reduction respectively.

One concern with multifocal lenses has always been their effect on vision at low levels of illumination or low contrast. Kolbaum et al compared the visual performance of the MiSight contact lens (CooperVision) with the Proclear multifocal.40 MiSight is not yet commercially available in the UK. It is a daily disposable lens which has a series of concentric treatment zones which deliver 2.00DS of myopic defocus in the periphery with clear distance vision in the centre.

The results from this study showed that both the Proclear and MiSight lenses resulted in a reduction in measured logMAR acuity in low light and low contrast when compared with the patient’s spectacle correction. There was no significant difference between the two lens types. The effect of these lenses on children will need to be considered if they are to become widely used for control of myopia. Loss of low contrast elements of vision may have an adverse effect on children who are involved in sport for example.

Conclusion

Recent papers which have reviewed the use of contact lenses in the control of myopia have concluded that both orthokeratology and multifocal soft lenses have a role to play in the control of myopia progression.41-44 Troilo suggests that the use of multifocal soft lenses is probably the method of choice since orthokeratology is more expensive and the lenses are more complicated to fit.43 Gifford and Gifford suggest that the ideal myopia-controlling contact lens requires the following properties.42

  • Induce peripheral myopia without compromising vision
  • Reduce lag of accommodation
  • Reduce near esophoria
  • Provide controlled release of antimuscarinic agents (these will be discussed in the next article)
  • Provide a beneficial shift in positive spherical aberration to improve near point depth of focus without compromising image quality
  • Measure ocular biometrics and analyse surroundings to provide real-time advice and training in avoiding visual situations or environments that increase the risk of myopia progression.

Other questions which also need to be addressed are;

  • At what age should myopia control commence?
  • What is the optimal duration of treatment?
  • What is the optimal level of retinal defocus?

It is clear that, if we are to address the myopia epidemic, specialist myopia control contact lenses will become one of the tools in the arsenal. The choice of lens type used will also depend upon a practitioners experience and practice equipment. The commercial availability of the custom designed lenses is a current limitation on the work of myopia control. Given the risks of pathological myopia outlined at the beginning of this article it is incumbent on us to offer the latest myopia control mechanisms to our at risk patients.

Dr Annette Parkinson is a senior lecturer at the University of Bradford responsible for the teaching of investigative techniques and visual impairment and rehabilitation

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