Article

CURRENT AND FUTURE CONTROVERSIES IN CONTACT LENSES

Contact lens practitioners need to carefully consider the evidence when controversies emerge.

After hearing me advance a controversial argument to the audience at a conference, a contact lens industry mogul walked up to me and proclaimed: “Nathan, you always seem to be on the very edge of opinion.” I took that as a compliment, and it is largely true.

Throughout my career, I have often found myself playing the role of the anti-hero. This began early in my career, when I was often pitted against my academic nemesis, the late Professor Brien Holden, in debates on contentious topics. I usually found myself on the losing side, primarily because: 1) Professor Holden was a better debater than me; and 2) Professor Holden—through his influence—always managed to have me assigned to argue the intuitively implausible case.

But, I guess I also have myself to blame for being cast in the role of devil’s advocate because I have always taken great delight in writing controversial editorials and articles. I don’t do this because I am a nasty person, but rather, I like to get colleagues thinking about issues. So, in that context, I hope that I have done a service to the contact lens field, which ultimately advances as the arguments from all sides of these debates are digested, analyzed, and acted upon (or not) as appropriate.

It is with this background that I was delighted, but not surprised, to receive an invitation from Dr. Ed Bennett, Clinical Features Editor for Contact Lens Spectrum, to write this article. (I actually once found myself pitted against Dr. Bennett in a debate on rigid lenses; no surprise I lost that one!) In this article, I have decided to focus on 10 “controversial” contact lens topics—five of which are relevant to contact lens practice today, and five of which look more to the future. I invite you to mull over these controversies with me. (Note: Opinions expressed are those of the author and not of Contact Lens Spectrum).

CURRENT CONTROVERSIES

Contact lens wear is inflammatory. A question has haunted me throughout my career: “Is contact lens wear intrinsically inflammatory?”1,2 Contact lens practitioners typically associate the notion of ocular inflammation during contact lens wear with serious complications such as microbial keratitis. However, more subtle mechanisms may be at play. I have sought to answer this question by testing whether uncomplicated contact lens wear meets both the clinical definition of inflammation—rubor (redness), calor (heat), tumor (swelling), dolor (pain), and functio laesa (loss of function)—and the subclinical definition of inflammation—cytological changes and release of inflammatory mediators.

In reviewing the literature,3 I discovered that all of these criteria are met with hydrogel lens wear, and most are met with silicone hydrogel lens wear. Therefore, I was forced to conclude that contact lens wear is indeed intrinsically inflammatory (Table 1).3

TABLE 1 SUMMARY OF EVIDENCE OF INFLAMMATION DURING UNCOMPLICATED CONTACT LENS WEAR
CHARACTERISTIC MEANING EVIDENCE CONFIRMATION OF INFLAMMATION
Hy SiHy
CLINICAL MARKERS (CLASSICAL)
Rubor Redness Limbal and conjunctival hyperemia Yes No
Calor Heat Limbal and conjunctival warming Yes No
Tumor Swelling Corneal edema Yes No
Dolor Pain End-of-day discomfort Yes Yes
Functio laesa Loss of function Discontinuation from lens wear Yes Yes
SUBCLINICAL MARKERS (CONTEMPORARY)
Cellular reactions Increase in Langerhans cell density Yes Yes
Biochemical reactions Upregulation of inflammatory mediators Yes Yes
Hy: hydrogel contact lenses; SiHy: silicone hydrogel contact lenses

This conclusion may seem controversial, or at least unpalatable for the contact lens industry, which would consider such a proclamation as damaging from a marketing perspective. However, consideration of both classical and contemporary thinking about the role of inflammation in the human body leads to the perhaps surprising and counterintuitive conclusion that the chronic, subclinical inflammatory status of the anterior eye during contact lens wear is a positive phenomenon that reflects an upregulation of the immune system in a non-damaging way. Therefore, the eye is in a state of “heightened alert,” ready to ward off any extrinsic noxious challenge, such as a foreign body, infectious agent, or toxic solution.4 So, in reality, we should be thankful that contact lens wear is intrinsically inflammatory!

Orthokeratology is not worth the bother. The impetus behind the current interest in orthokeratology among a small number of enthusiasts worldwide appears to be that a “specialist” niche market can be created based upon often exaggerated claims of temporarily or permanently curing myopia. Another driving force is the natural academic curiosity of researchers.

However, despite a large amount of developmental activity from the small number of orthokeratologists around the world, overnight orthokeratology is still only capable of reducing myopia by about 2.00D, no matter what approach to fitting is adopted.5 Furthermore, the magnitude of the effect is unpredictable, and vision regresses at an uncertain rate back to the original state of myopia during the waking hours.5 As noted by Kwok et al,6 earlier findings of the limited efficacy of orthokeratology are simply being rediscovered today.

The great irony of orthokeratology is that it is not a procedure that will allow patients to avoid having to wear contact lenses; lenses still have to be worn overnight. Apparently, orthokeratology is for patients who do not want to have to rely upon wearing contact lenses. Presumably, this means that the primary justification for orthokeratology is its use as a psychological crutch for those who take this view.

The reality is that modern soft contact lenses are inherently safe, instantly comfortable, and provide sharp vision for patients who have all forms of ametropia. Orthokeratology lenses are expensive, difficult to fit, uncomfortable when applied, provide variable vision, and probably increase the risk of developing severe keratitis.5 The simple two-word question that I have repeatedly put to orthokeratology enthusiasts over the years—and to which I have never received a satisfactory answer—is “Why bother?”

A potential application of orthokeratology that might find limited utility in clinical practice is the accidental finding that orthokeratology lenses can partially arrest the progression of myopia.7 This is because of the unusual optics in the periphery of these lenses, which induce positive spherical aberration. But, if soft lenses can be just as effective in this regard, again I ask—“Why bother with orthokeratology?”

The extremely low international prescribing rates for orthokeratology8 indicate that this form of lens wear has failed to capture the attention of contact lens fitters. Those who have claimed that this approach to myopia correction or to retardation of myopia progression could be the savior of rigid lenses have been proven wrong.

Silicone hydrogels are superior to traditional hydrogels. The introduction of silicone hydrogel contact lenses onto the world market in 1999 heralded a revolution in eye care, and these products now represent the majority of lens sales worldwide. Clearly, silicone hydrogel lenses meet the oxygen needs of the cornea during open-eye wear.9 The oxygen permeability (Dk) values of these lenses are greater than 60 Dk, which is far above those of the “best” hydrogel extended wear lenses (35 Dk).10 Reports from numerous clinical trials have failed to reveal any problems relating to hypoxia with silicone hydrogel lenses.9 Thus, conditions such as epithelial microcysts, limbal redness, hypoxic staining, stromal neovascularization, edema, and endothelial polymegethism do not occur with these lenses.

There has recently been some discussion at conferences as to whether silicone hydrogel lenses really are superior to traditional hydrogel lenses. This question seems to have arisen from research demonstrating that the incidence of corneal infiltrative events is similar in wearers of hydrogel versus silicone hydrogel daily disposable lenses.11 Although equivalence is indeed demonstrated through this narrow prismatic view, this ignores the previous three decades of pain and suffering that practitioners had to endure in the pre-silicone hydrogel era.12

No one will deny that corneal infiltrative events (Figure 1) are a useful marker of the immune status of the eye. However, infiltrates are of no immediate clinical consequence. Unwanted hypoxia-induced changes that can occur with hydrogel lenses, such as limbal redness, corneal edema, and endothelial polymegethism, are of more immediate clinical relevance, and these adverse events have been obviated by silicone hydrogels.9 This is why these materials dominate the market now8 and will continue to do so.

Figure 1. Corneal infiltrative event in the central cornea (arrow) of a contact lens wearer. The infiltrates are penetrating to about one-third of the corneal depth.

Daily disposable lens wearers only require aftercare visits every two years. Despite the tremendous evolution of contact lens technology and clinical practice over the past three decades, our approach to the aftercare examination has remained conservative. In a clinical article titled “Aftercare of soft contact lenses” published in 1976, Kennedy13 advised that, following lens fitting, soft lens wearers should be examined “...at the end of one month, three months then six months...sooner if problems.” In the first textbook account of soft lenses, published in the United States in 1981, Mandell14 recommended an aftercare schedule after initial lens fitting of three days, one week, two weeks, one month, then every six months. Our general approach to aftercare does not appear to have fully evolved from those difficult, early years of fitting non-regular-replacement rigid and low-water-content hydrogel lenses.

I have recently reviewed current aftercare practice and, in particular, the preferred frequency that lens wearers should return for routine visits.15 The current norm seems to be to advise annual aftercare visits. Four key clinical reasons for conducting a routine aftercare visit were identified: preserving ocular health, maintaining good vision, optimizing comfort, and ensuring satisfactory lens fitting performance. In reviewing the literature in the context of these clinical considerations, overwhelming evidence supports the intrinsic safety of daily disposable hydrogel lenses, especially those made from low-modulus silicone hydrogel materials.11

Of course, a sensible balance must be sought between seeing our patients frequently enough to ensure ongoing ocular health and safety versus not unduly inconveniencing our patients with unnecessary aftercare visits. In view of this, and in consideration of all of the available evidence, it is proposed that an aftercare visit every two years may be justifiable for uncomplicated daily disposable lens wearers.15 Clearly, however, such an approach has implications for practice-associated lens supply logistics, health care insurance, professional regulation, and medico-legal considerations. Let the debate begin!

Subtypes of corneal infiltrative events cannot be clinically differentiated. Around the turn of this century, Sweeney et al16 described a schema for clinically differentiating between four symptomatic subtypes of contact lens-associated corneal infiltrative events (CIEs): microbial keratitis (MK), contact lens-induced peripheral ulcer (CLPU), contact lens-induced acute red eye (CLARE), and infiltrative keratitis (IK). This schema was initially very popular and adopted by many research groups and clinicians.

However, the clinical utility of this schema has subsequently been called into question. Baum and Donshik17 found it confusing and ambiguous on the basis that there is considerable overlap between the signs and symptoms that describe the six subtypes of CIE. These authors concluded that “Although...[Sweeney at al]...have defined six subsets of infiltrates, we believe that they have failed to distinguish any.” Other authors have expressed similar concerns. Barr18 queried the use of the various terms proposed by Sweeney at al,16 and Nilsson19 has questioned whether CLPU can be said to exist as a discrete condition.

I set out to determine whether it is possible to clinically differentiate between these conditions. Criteria for MK, CLPU, CLARE, and IK were applied to a dataset of 111 contact lens-associated CIEs, spanning a wide range of clinical severities, presenting consecutively to a hospital clinic. A Venn diagram analysis was used to determine the extent to which these conditions could be clinically differentiated.20

Of the 111 CIEs, 20% could be classified unambiguously as either MK, CLPU, CLARE, or IK, 56% could be classified as one of two conditions, 13% could be classified as one of three conditions, and 0% could be classified as one of four conditions. Furthermore, 11% of CIEs could not be classified as any of the four conditions (Figure 2).20

Figure 2. Venn diagram showing how the 111 corneal infiltrative events observed in the Manchester Keratitis Study were classified according to the criteria of Sweeney et al.16 Each dot represents a single corneal infiltrative event. Thirteen corneal infiltrative events are not shown because they did not fit into any category.
Reprinted from Efron and Morgan20 with permission of Lippincott Williams & Wilkins.

The study authors concluded that the considerable overlap between the clinical presentation of MK, CLPU, CLARE, and IK is such that it is not possible to clinically differentiate between them with any degree of certainty.20 A preferred approach might be to consider CIEs as part of a disease continuum whereby these events can manifest in various degrees of severity, also depending on the point at which the condition is observed in the course of the natural history of the disease.

FUTURE CONTROVERSIES

Semi-scleral designs will not reinvigorate the rigid lens industry. I respect the passion, skill, and dedication of practitioners around the world capable of successfully fitting rigid contact lenses. But sadly, rigid contact lens fitting is becoming a lost art. I made this point with artistic panache at a conference in The Netherlands in 2010, where I play-acted the Grim Reaper chanting an obituary for rigid lenses (Figure 3).

Figure 3. The author play-acting the Grim Reaper at the grave of rigid contact lenses, presented at a contact lens conference in the Netherlands in 2010.

There are numerous reasons for the decline in rigid lens prescribing, which I have discussed in detail elsewhere.21 The reality is that rigid lenses are uncomfortable, and modern consumers desire instant gratification, which, in the domain of contact lenses, means instant comfort. Of course, we are able to convince some of our patients who have unusual conditions, such as keratoconus, post-corneal trauma, high astigmatism, and cosmetic disfiguration, that the optical and cosmetic benefits that can be achieved with rigid lenses far outweigh the rigid lens discomfort.

A significant advance in rigid technology over the past decade has been the development of semi-scleral and mini-scleral designs. Such lenses offer greater stability compared to smaller-diameter lenses and, consequently, better and more stable optics and reputedly enhanced comfort. Such lenses may be especially suited for the correction of refractive astigmatism.22 There are disadvantages, however, in that such lenses are more difficult to fit; lens application and removal is more challenging; the lenses are subject to fogging; and such lenses come at greater cost.

So, will semi-scleral designs reinvigorate the rigid lens industry? I think not. Semi-scleral designs may replace conventional designs to some extent, but rigid lenses will remain as a specialist option for especially challenging cases. Additionally, the overall declining rate of rigid lens prescribing8 is likely to continue.

All contact lenses will be single-use only within five years. I have been espousing the virtues of daily disposable lenses for many years now.23 At conferences, I often pose the following question to the audience: “Putting aside the issue of cost, and assuming that a lens wearer falls within the available parameter range of current products, can you think of any reason why you might prescribe anything other than daily disposable lenses?” This question is generally met with stony silence, although one issue of concern that occasionally has been mentioned is “environmental impact.” I quickly point out that I have looked at this and discovered that the environmental impact of waste generated by daily disposable lenses is minimal.24

Contact lens practitioners and contact lens wearers alike derive far more benefit from daily disposable lenses than from reusable lenses. The overall cost of daily disposable lenses to lens wearers is not that much more, and in many instances is considerably less, compared to the cost of reusable lenses.25 I believe that within the next five years, the major global contact lens companies will gear up their production to exclusively produce daily disposable lenses. Indeed, there are signs that this is already happening. I also believe that such a move would be warmly embraced by contact lens practitioners, thus relegating reusable lenses to the annals of history.

A fourth incarnation of extended wear will fail. No-one disagrees that extended contact lens wear is the ultimate modality of lens wear in terms of convenience. This has undoubtedly been the impetus for repeated attempts by the contact lens industry over the past 40 years to introduce extended wear contact lenses into the world market. There have been three major attempts at this, each with a different rationale, and all have failed (Figure 4).

Figure 4. Three failed attempts at introducing extended wear contact lenses into the market.

The first attempt was by CooperVision in the late 1970s with the launch of Permalens. The rationale was that it was a high-water-content lens with a high oxygen transmissibility, which would alleviate hypoxic problems. By today’s standards, the oxygen permeability of these lenses was low, and unacceptably high rates of microbial keratitis were encountered.26

The second attempt at launching an extended wear lens was the Acuvue lens by Johnson & Johnson.27 This lens was designed to be disposed of weekly. The rationale was that a fresh lens every week would reduce the rate of infections. This did not happen. However, the Acuvue lens used for daily wear went on to become one of the most successful contact lenses ever launched onto the market.

The third attempt at extended wear came with the near-simultaneous launch onto the market of the PureVision lens (Bausch & Lomb) and Focus Night & Day (Ciba Vision).28 The rationale for these lenses was that, being made from silicone hydrogel materials, hypoxia would be obviated and cases of keratitis would be minimized. There was some benefit; although the incidence of keratitis was not diminished, the severity of keratitis in those affected was reduced (Figure 5).29

Figure 5. Data from the Manchester Keratitis Study,29 showing the distribution of clinical severity scores for corneal infiltrative events with respect to wearing modality and lens type. The severity distribution is greater for hydrogel extended wear (larger red/orange bands) versus for silicone hydrogel extended wear.

Nevertheless, negative press relating to cases of keratitis among silicone hydrogel extended wear lens wearers, albeit of lesser severity, dampened enthusiasm for wearing this type of lens. Until truly efficacious anti-infective lenses are developed—and it is still unclear what that might entail—a fourth incarnation of extended wear is bound to fail.

Digital/electronic contact lenses will dominate in the future. The digital revolution, coupled with advances in nanotechnology and battery power, introduces the prospect of incorporating intelligent microsystems into contact lenses to create a variety of novel applications.30 In the domain of biosensing, the challenge is to develop a contact lens into which a sensor is incorporated that can respond in a coherent manner to whatever biological stimulus is of interest. Second, there must be a means of communicating the information gathered to the users or their clinicians via an external display or analytical device. A third element may be needed to provide energy to the biosensing lens. One such device that has entered the market works via a circumferential strain gauge mounted in a silicone hydrogel contact lens.31 These lenses respond to changes in corneal curvature caused by fluctuations in intraocular pressure.

Other biosensing possibilities include lenses that analyze tear fluid to determine the level of metabolites such as glucose levels,32 lipids, and specific proteins. Remote monitoring of retinal vasculature from a contact lens-mounted device offers other advantages in terms of monitoring ophthalmic or systemic disease.

Visual augmentation such as the creation of virtual reality are currently being achieved via the use of large, cumbersome headsets, although interesting applications have been demonstrated with spectacle-mounted devices. In addition, some patents have been issued for visual augmentation using contact lenses.30 Although it is difficult to predict the success of these approaches, the contact lens world must be prepared to embrace these developments, perhaps by modifying traditional modes of contact lens practice and enhancing eyecare training as appropriate.

Myopia control contact lenses are over-hyped and doomed to fail. While extended wear was the holy grail of the 20th century, myopia control is the holy grail of the 21st century. The promise of a lens that will arrest the progression of myopia is hugely attractive, especially to parents of young children showing early signs of developing myopia. This is especially relevant in view of the worldwide myopia epidemic, which is especially evident in Asia.33

Aside from the inconvenience and cost of correcting myopia with spectacles or contact lenses, there are adverse consequences in terms of ocular health. It has been shown that each diopter less of myopia is associated with a 42% reduced likelihood of severe retinal change.34

Clinical trials on the efficacy of myopia control lenses are in their infancy.35 Preliminary results suggest that, under optimal circumstances, the “myopia control effect” of soft contact lenses may be around 40% (range 25% to 72%).35 Thus, as an example, by wearing myopia control contact lenses, a young child destined to become a –10.00D myope might end up becoming only a –6.00D myope. There are potential health benefits of this reduction, as discussed above; however, there are also considerable challenges with this approach.

Central vision is compromised with myopia control lenses because the deliberate optical defocus in the lens periphery overlaps onto the lens center.36 The question remains as to whether the disbenefit of this moderate degradation of vision outweighs the longer-term benefit of arresting myopia progression.

Also, the concept of myopia control is a “hard sell” to parents of myopic children because a procedure of uncertain efficacy is being advocated for a potential benefit that might be a decade away. This contrasts sharply with the instant gratification of sharp vision encountered immediately when a young myope receives traditional contact lenses for the first time. So, the jury is still out on the question of the long-term commercial success of myopia control contact lenses.

CONCLUSION

These days, contact lens practitioners need to be alert to exaggerated and unsubstantiated claims with respect to contact lens products and procedures. By all means, read the claims and participate in the debates. But, in the end, we must always seek the truth; in this regard, we are fortunate that contact lens practice is an evidence-based scientific discipline and that the ultimate truth will eventually emerge in the refereed scientific literature. CLS

REFERENCES

  1. Efron N. Review: Is contact lens-induced corneal œdema inflammatory? Aust J Optom. 1985 Sept;68:167-172.
  2. Efron N. Is contact lens wear inflammatory? Br J Ophthalmol. 2012 Dec;96:1447-1448.
  3. Efron N. Contact lens wear is intrinsically inflammatory. Clin Exp Optom. 2017 Jan;100:3-19.
  4. Medzhitov R. Origin and physiological roles of inflammation. Nature. 2008 Jul 24;454:428-435.
  5. Carney LG. Orthokeratology. In Contact Lens Practice. 2nd ed., Efron N, ed. Oxford: Butterworth Heinemann Elsevier, 2010:332-338.
  6. Kwok LS, Pierscionek BK, Bullimore M, Swarbrick HA, Mountford J, Sutton G. Orthokeratology for myopic children: wolf in sheep’s clothing? Clin Exp Ophthalmol. 2005 Aug;33:343-347.
  7. Lee YC, Wang JH, Chiu CJ. Effect of orthokeratology on myopia progression: twelve-year results of a retrospective cohort study. BMC Ophthalmol. 2017 Dec 8;17:243.
  8. Morgan PB, Woods CA, Tranoudis IG, et al. International contact lens prescribing in 2017. Contact Lens Spectrum. 2018 Jan;33:28-33.
  9. Covey M, Sweeney DF, Terry R, Sankaridurg PR, Holden BA. Hypoxic effects on the anterior eye of high-Dk soft contact lens wearers are negligible. Optom Vis Sci. 2001 Feb;78:95-99.
  10. Efron N, Morgan PB, Cameron ID, Brennan NA, Goodwin M. Oxygen permeability and water content of silicone hydrogel contact lens materials. Optom Vis Sci. 2007 Apr;84:328-337.
  11. Chalmers RL, Hickson-Curran SB, Keay L, Gleason WJ, Albright R. Rates of adverse events with hydrogel and silicone hydrogel daily disposable lenses in a large postmarket surveillance registry: the TEMPO Registry. Invest Ophthalmol Vis Sci. 2015 Jan 8;56:654-663.
  12. Efron N. Contact Lens Complications. Oxford: Butterworth-Heinemann-Optician. 1999 Oct 26.
  13. Kennedy JR. Aftercare of soft contact lenses. J Am Optom Assoc. 1976 Mar;47:369.
  14. Mandell RB: Contact Lens Practice. Springfield: Charles C. Thomas, 1981:579.
  15. Efron N, Morgan PB. Rethinking contact lens aftercare. Clin Exp Optom. 2017 Sep;100:411-431.
  16. Sweeney DF, Jalbert I, Covey M, et al. Clinical characterization of corneal infiltrative events observed with soft contact lens wear. Cornea. 2003 Jul;22:435-442.
  17. Baum J, Donshik PC. Corneal infiltrates associated with soft contact lens wear. Cornea. 2004 May;23:421-422; author reply 422-423.
  18. Barr JT. Corneal infiltrates: What’s the difference? Contact Lens Spectrum. 2004 Aug;19:11.
  19. Nilsson SE. Seven-day extended wear and 30-day continuous wear of high oxygen transmissibility soft silicone hydrogel contact lenses: a randomized 1-year study of 504 patients. CLAO J. 2001 Jul;27:125-136.
  20. Efron N, Morgan PB. Can subtypes of contact lens-associated corneal infiltrative events be clinically differentiated? Cornea. 2006 Jun;25:540-544.
  21. Efron N. Obituary – rigid contact lenses. Cont Lens Anterior Eye. 2010 Oct;33:245-252.
  22. Michaud L, Bennett ES, Woo SL, et al. Clinical evaluation of large diameter rigid-gas permeable versus soft toric contact lenses for the correction of refractive astigmatism. A multicenter study. Eye Contact Lens. 2018 May;44:164-169.
  23. Efron N. Why are we still fitting reusable soft contact lenses? Clin Exp Optom. 2014 Sep;97:386-388.
  24. Morgan SL, Morgan PB, Efron N. Environmental impact of three replacement modalities of soft contact lens wear. Cont Lens Anterior Eye. 2003 Mar;26:43-46.
  25. Efron N, Efron SE, Morgan PB, Morgan SL. A ‘cost-per-wear’ model based on contact lens replacement frequency. Clin Exp Optom. 2010 Jul;93:253-260.
  26. Stark WJ, Martin NF. Extended-wear contact lenses for myopic correction. Arch Ophthalmol. 1981 Nov;99:1963-1966.
  27. Donshik P, Weinstock FJ, Wechsler S, et al. Disposable hydrogel contact lenses for extended wear. CLAO J. 1988 Oct-Dec;14:191-194.
  28. Morgan PB, Efron N. Comparative clinical performance of two silicone hydrogel contact lenses for continuous wear. Clin Exp Optom. 2002 May;85:183-192.
  29. Efron N, Morgan PB. Rethinking contact lens associated keratitis. Clin Exp Optom. 2006 Sep;89:280-298.
  30. Legerton JA. Technology in your practice. Contact Lens Spectrum. 2017 Aug;32:28-34.
  31. Sunaric-Megevand G, Leuenberger P, Preussner PR. Assessment of the Triggerfish contact lens sensor for measurement of intraocular pressure variations. Acta Ophthalmol. 2014 Aug;92:e414-e415.
  32. Chu MX, Miyajima K, Takahashi D, et al. Soft contact lens biosensor for in situ monitoring of tear glucose as non-invasive blood sugar assessment. Talanta. 2011 Jan 15;83:960-965.
  33. Morgan IG, He M, Rose KA. Epidemic of pathologic myopia: What can laboratory studies and epidemiology tell us? Retina. 2017 May;37:989-997.
  34. Vongphanit J, Mitchell P, Wang JJ. Prevalence and progression of myopic retinopathy in an older population. Ophthalmology. 2002 Apr;109:704-711.
  35. Sankaridurg P. Contact lenses to slow progression of myopia. Clin Exp Optom. 2017 Sep;100:432-437.
  36. Kollbaum PS, Jansen ME, Tan J, Meyer DM, Rickert ME. Vision performance with a contact lens designed to slow myopia progression. Optom Vis Sci. 2013 Mar;90:205-214.