Introduction
The term ‘extended wear’ (EW) has generally been applied to describe the wear of contact lenses for periods in excess of 24 hours between removal, including sleep with the lenses on the eye and regular, planned removal of the lenses. The term ‘continuous wear’ was originally used to describe prolonged wear of unspecified duration, but was discarded during the early 1980s because prolonged wear was considered to be an unachievable goal. In the late 1990s, ‘continuous wear’ was reintroduced as a term to describe intended lens wear of up to 30 days between removals. To a certain extent, the terms are used interchangeably, but for the purposes of this chapter, the term ‘extended wear’ will be used.
For many years, the ‘holy grail’ of contact lenses for patients, practitioners and researchers was a contact lens that could be worn safely on an extended basis. Various factors influenced this desire: convenience, a sense of vulnerability, particularly in patients with higher refractive errors, relief from tedious maintenance and handling procedures, and vocational and lifestyle issues – but it was mostly about convenience. Widespread interest in refractive surgery since the 1980s underscores the appeal of a longer-term solution to refractive error. The industry responded around the turn of the century through the commercialization of silicone-hydrogel (SiHy) materials, with the goal of providing adequate oxygen to the cornea for closed-eye wear. Practitioners, wary of the potential complications, have not embraced the EW concept despite the advantages of SiHy materials. Because of this trend, EW practice might nowadays be considered niche. With falling demand, research into EW has slowed to a very modest level. Nonetheless, clinicians should be versed in the EW modality, since (1) many contact lens wearers nap or occasionally sleep in their lenses and (2) increasing interest in myopia control means that the practice of orthokeratology, in which contact lenses are worn while sleeping, contributes to the overall number of patients in EW.
For a detailed history of EW, the reader is referred to . Analysis of the early ventures in EW provides important clues as to the pitfalls with this wearing style. The current status of EW and its current format can be traced to the prominent aspects of this history.
Experiences With Extended Wear
The earliest report of EW appears to be a 1957 description of a noncompliant patient wearing an oxygen-impermeable polymethyl methacrylate (PMMA) contact lens continuously for 3 months. Details of a planned clinical trial were published in 1965, reporting on 50 patients wearing PMMA on a continuous-wear basis over a 7-year period (that is, without planned removal or replacement). Although such lenses have long since been condemned as unsuitable for EW, the absence of complications in these reports is noteworthy and indicative of the resilience of the eye to sleeping in contact lenses.
Conventional Soft Extended Wear
The original soft lenses, designed for long-term repeated usage, are now referred to as ‘conventional’, ‘traditional’ or ‘unplanned-replacement’ hydrogel lenses to distinguish them from ‘frequent-replacement’ or ‘disposable’ lenses. Soft hydrophilic lenses were introduced around the world in the late 1960s and early 1970s as a result of the pioneering work of Wichterle. These lenses were very well received by the public, which was principally due to the comfort afforded. Contact lens sales promptly soared. But the inconvenience of caring for the lenses and the need to insert and remove them on a daily basis led to increased experimentation with extended periods of wear.
Early reports focused on the use of prolonged periods of wear for therapeutic use, particularly in aphakia. Perceived advantages included alleviation of the possible risks associated with removal and handling of lenses from diseased eyes and relief for patients with limited dexterity. The first material promoted for use in cosmetic EW was perfilcon A (Permalens, CooperVision, Rochester, NY). It was suggested at the time that this could be worn continuously, day and night, for many months or even years ( ).
Numerous studies of soft lens EW for cosmetic use soon appeared with encouraging results. The remainder of this chapter deals largely with EW lens usage for such a purpose. Following clearance by the US Food and Drug Administration (FDA) for cosmetic EW in 1981, a large proportion of contact lens prescriptions, particularly in the United States, were written for EW. An intense and exciting period of research followed during which many of the problems preventing complication-free EW were identified.
The reports of early clinical experiences with hydrogel EW lenses are too numerous to review here, but even a cursory review shows dramatic variation in quoted success rates. Such differences appear to depend upon patient selection, the type of lenses fitted, fitting approach (tight or loose), the diligence of follow-up, and criteria used by investigators in assessing the severity of adverse reactions and in the categorization of success. For example, the study of reports a success rate as high as 81% following 3 years’ of EW compared with 48% reported by . Variation in study design remains a common problem in the modern evaluation of EW success.
Despite the apparently low success rate, the retrospective study of provides good insight into the success of these trials. This article reviewed over 1000 patients entered into a number of FDA regulatory studies of EW lenses, with intended wearing times of 2–4 weeks. As noted above, nearly half (48.5%) of patients ‘survived’ EW with only minor problems for 3.5 years. Of the discontinuations, almost a third (31.3%) of the subjects did so prior to beginning EW, 16.2% were lost to follow-up, 14.5% found lens comfort inadequate, and 12% experienced ocular complications. The frequency of individual complications was as follows: injection 3.7% (of the discontinued group), abrasion 1.6%, conjunctivitis 1.4%, contact lens-related papillary conjunctivitis (CLPC) 1.4%, neovascularization 1.1%, dry eye, epithelial staining and oedema all less than 1%, and infections 0.2%.
The relative success observed in these trailblazing studies was quite astounding and provided considerable scope for optimism that relatively minor adjustments to the lens parameters would provide a safe, effective correction. By modern standards though, these studies used lenses and modalities that were grossly inappropriate for EW. The thick hydrogel lenses had oxygen transmissibility ( Dk / t ) levels that were later deemed to be inadequate. The period between lens removal was also up to 6 months in many of the early studies, and more recent research has arrived at the conclusion that this interval is too long for uncomplicated wear. But by far the greatest hazard with these early EW lenses was the emerging picture of severe complications due to infectious keratitis. A study conducted in the United States in 1987 and sponsored by the Contact Lens Institute (CLI) demonstrated that sleeping in contact lenses posed a significant risk over and above that of contact lens wear alone ( ). The adverse publicity subsequent to the publication of this study meant that EW usage decreased dramatically during the 1990s.
Nonhydrophilic Materials
The high oxygen permeability ( Dk ) of silicone elastomer led to an interest in using this material for extended contact lens wear. In the late 1970s, a number of fitting trials were undertaken. Unfortunately, the hydrophobic nature of the lens surface resulted in poor wetting and surface deposition ( Fig. 23.1 ). The highly elastic nature of the material and resistance of the material to water permeation also brought about lens binding. These factors prevented the success of this material except for limited use in the paediatric and aphakic fitting.
Towards the mid-1980s, rigid gas-permeable materials became a popular alternative to hydrogels for EW, the principal attraction being a higher Dk / t without the adverse properties of silicone elastomer. Other perceived advantages of rigid lenses over hydrogel lenses include a higher rate of tear exchange beneath these lenses, less corneal coverage and a more inert lens surface.
A considerable body of literature provided evidence for successful EW in patients using these lenses ( ). Certainly, preclinical evaluations confirmed that rigid lenses produced fewer oxygen-related physiological changes, such as corneal oedema, than hydrogels. However, there remained a range of limitations with rigid lenses for EW. Complications found with hydrogels, such as infectious keratitis, infiltrative keratitis and CLPC, were not eliminated. The elevated risk of infectious keratitis caused by sleeping in contact lenses observed with hydrogels was also evident with rigid lenses ( ). As well, rigid lenses induce a greater degree of initial discomfort than hydrogels and complications largely unseen with hydrogels, such as 3 and 9 o’clock staining, corneal distortion and lens binding.
Most practitioners have been unwilling to devote the additional time required to achieve a suitable fit and deal with patient complaints of discomfort, so rigid lenses are nowadays rarely fitted for traditional cosmetic EW. However, overnight wear of rigid lenses to achieve temporary reversal of refractive errors, known as orthokeratology, maintains a fervent, if specialized, following and has become a specialty area of contact lens practice ( Chapter 30 ).
Disposable Soft Lenses
Efficiencies of manufacture allowed Johnson & Johnson Vision Care (Jacksonville, FL) to bring low-cost disposable lenses made of etafilcon A material (Acuvue) to the US market in 1987. The original intention was that the lenses be worn continuously for 1 week at a time. At the end of each week, the lenses were to be discarded and replaced by a new set of lenses. In essence, this was a simultaneous launch of the concepts of weekly EW and lens disposability ( ).
Frequent replacement of lenses offers a range of potential advantages for increasing EW success. Practitioners perceived that the most significant problem with conventional hydrogel lenses was the build-up of surface deposits and associated complications, such as CLPC ( ). Frequent replacement restricts exposure of the eye to accumulated aged or denatured protein, which seems to act as an antigen in inducing inflammatory reactions such as CLPC and infiltrative keratitis. Control of deposit build-up by frequent replacement has the further advantage of reducing patient symptoms related to vision and comfort.
While reducing exposure of the eye to lens deposition is the outstanding advantage of disposable lenses, the regular provision of fresh, clean lenses also showed promise in other areas. There was hope that frequent lens replacement would lead to fewer bacteria being introduced to the eye, thereby reducing the risk of infectious keratitis during EW. In regard to lens maintenance, disposability offers simplification of cleaning procedures, easier lens fitting and replacement, and may serve to enhance patient compliance.
In practice, the benefits with respect to ocular physiology, symptomatology and convenience are compelling, and replacement of lenses every day to every 2 weeks to 1 month now dominates contact lens prescribing around the world ( ). Clinical studies have certainly confirmed that frequent replacement reduces the occurrence of inflammatory events during EW. Both noninfectious corneal infiltrative events (CIEs) and CLPC show reduced incidence with more frequent replacement. However, these complications are not entirely eliminated, as will be discussed further below.
The early enthusiasm for prescribing EW with disposable hydrogel lenses was not maintained. Although the CLI-sponsored study was restricted to conventional replacement contact lenses, damage to the reputation of EW had been done ( ). A number of publications reported on corneal infections in association with frequent-replacement lenses. Academics and educators delivered widespread warnings of the potential complications of EW. By and large, practitioners around the world fell in line and EW prescribing dropped to very low levels (less than 1% of fits) globally by the end of the 1990s ( ).
Moreover, despite the apparent benefits and contrary to the hopes and expectations of the contact lens industry, frequent contact lens replacement failed to achieve the desired impact on the rate of infectious keratitis. Reports in the early 1990s suggested that the frequency of infections with disposable lenses was equal to or even greater than that with conventional replacement. In a nationwide survey of ophthalmologists in the Netherlands in 1996, demonstrated that rates of infection with disposable lenses, in both daily and EW, were essentially the same as those observed in the CLI study ( ). Despite the significant advantages offered by disposability, it was evident that the industry had to look elsewhere for answers to the challenge posed by infectious keratitis with EW.
Silicone-Hydrogel Contact Lenses
With the failure of frequent-replacement hydrogel lenses to solve the EW infection problem and persistent physiological compromise to the cornea during closed-eye wear, the industry turned its attention to the development of materials with high Dk . In the late 1990s, SiHy contact lenses were introduced to the market. The details of the advances incorporated in these materials are presented in Chapter 4 . In brief, the high Dk and good handling characteristics of silicone are combined with the flexibility and comfort of hydrophilic materials. Considerable advances in technology over previously manufactured lenses were required to combine these two ostensibly incompatible materials, not the least of which was new technology to produce a wettable, deposit-resistant surface. The early lenses used plasma surface coatings, but later generations of lenses managed to circumvent the need for this additional manufacturing step.
Clinical research demonstrated that EW of SiHy lenses produces fewer physiological changes, such as microcysts, corneal striae and endothelial blebs and less limbal and bulbar conjunctival injection, than did hydrogel lenses ( ). These benefits arise from the lowered resistance of the materials to oxygen flow. SiHy lenses may also, on occasion, provide the added benefits of less corneal staining, less subjective dryness and discomfort, and better handling, owing to properties such as hydration, modulus, surface and design characteristics.
These advantages of SiHy lenses were quickly recognized and some countries such as Australia, which had previously shunned EW, enthusiastically adopted these new materials. Other countries such as the United States and the United Kingdom, following the failure of the EW foray of the 1980s, were more cautious in approach. Nonetheless, by 2006 some 12% of all contact lens fits worldwide were for EW ( ).
Although the increased corneal oxygenation afforded by SiHy lenses virtually eliminated physiological or metabolic corneal disorders during EW, epidemiological studies revealed that, despite the hopes of the contact lens community, there was little impact on corneal infection rates ( ). Alongside this reality, daily disposability was increasingly seen as a safe, relatively affordable and convenient mode of wear.
As a result of the above factors, the fitting of soft lens EW in practice has been declining throughout the 21st century. Fig. 23.2 presents data for 17 nations between 2000 and 2020 ( ). The mean of all countries is shown by the black dotted line. A gradual decline in EW fitting, commencing around 2008, is apparent. The percentage of EW fits in relation to all soft lens fits, stratified by lens material type, is shown for the year 2020 in Fig. 23.3 ( ). This graph shows that all but 5 of the 23 countries surveyed in 2020 have a prescribing rate of ≤6% of all soft lens fits. As well, the vast majority of EW fits were with silicone-hydrogel lenses.
Despite the low rate of soft lens EW being prescribed, practitioners should bear in mind the considerable proportion of patients who sleep in their contact lenses and the accompanying implications for ocular health. In one survey of soft contact lens wearers, 16% said they sleep and 63% reported napping in their lenses ( ). Further, a small number of rigid gas-permeable fits (no more than 2% of total contact lens fitting) are for EW ( ). Orthokeratology constitutes around 3% of total worldwide fits and also should be considered in discussion with EW as these lenses are worn overnight, despite being removed during the daytime ( ).
Summary
As explained, EW prescribing ‘failed to break through the “glass ceiling” of 15%’ and it is therefore no surprise that it failed to become a mainstream lens-wearing modality. The continuing EW microbial keratitis rate of around 20 per 10,000 wearers per year remains a barrier to more widespread adoption and, given the industry’s reluctance to invest in further research, we are unlikely to see the substantial expansion of the EW market in the near term. A serendipitous breakthrough in an associated technology would seem to be necessary to revive interest.
Adverse Effects of Extended Wear
The presence of a contact lens on the eye brings with it a number of potential adverse responses. This is the case whether the lens is worn on a daily-wear or EW basis. A detailed discussion of adverse events with contact lens wear is provided in Chapter 38 . Some specific events are more prevalent during EW and these are presented below. The main feature of this discussion is to analyse the mechanisms of the ocular changes and to review risk factors and strategies to minimize these effects. Table 23.1 summarizes the major risk factors for complications with EW and these, as well as methods for mitigating the impact, are discussed in more detail below. Other issues common to daily-wear contact lenses, such as corneal staining, discomfort and the symptom of dryness, remain important in EW but are dealt with elsewhere in this book.
| Risk factor | Effect a | Remedy |
|---|---|---|
| Modifiable Risk Factors | ||
| Sleeping in lenses | 1, 2, 3 | Do not sleep in lenses |
| Long periods (>6 nights) between removal | 1, 2 | Reduce duration of EW |
| Long periods between lens replacement | 2 | Fit disposable lenses of greater replacement frequency |
| Hot climates | 1, 2 | Education and reinforcement; consider daily disposables if on vacation |
| No/poor handwashing | 1, 2 | Education and reinforcement of hygiene and care of lenses |
| Poor care and maintenance | 1, 2 | Education and reinforcement of hygiene and care of lenses |
| Poor storage case hygiene | 1, 2 | Frequent case replacement, education and reinforcement |
| Smoking | 1, 2 | Counsel on increased risks |
| Work environment | 1, 2 | Reduce exposure to ocular irritants, use eye protection, counsel on risks |
| Topical steroid therapy | 1 | Suspend EW until therapy completed |
| Internet purchase | 1, 2 | Education on importance of regular examinations, regular review, reinforcement of hygiene and care of lenses |
| Tightly fitting lenses | 2, 3 | Fit more mobile lenses |
| Low Dk / t | 3 | Fit SiHy lenses |
| SiHy lens wear | 2 | Fit hydrogel lenses |
| Nonmodifiable Risk Factors | ||
| Male gender | 1, 2 | Use discretion in prescribing; counsel on increased risks |
| Age (12–25 years) | 1, 2 | Use discretion in prescribing; counsel on increased risks |
| Diabetes | 1, 3 | Use discretion in prescribing; counsel on increased risks |
| Early period of EW | 1, 2 | Use discretion in prescribing; counsel on increased risks |
a 1 = risk of microbial keratitis; 2 = risk of corneal infiltrative events; and 3 = risk of physiological disturbance.
Infectious Corneal Ulceration
Infectious corneal ulceration ( Fig. 23.4 ) occurs with higher frequency in contact lens wearers than in nonlens wearers, and those who sleep in their lenses are at a higher risk than those who wear their lenses on a strictly daily-wear basis (that is, never sleeping with lenses in). The infection can lead to tissue necrosis and massive inflammation. Pseudomonas and Serratia species are the more commonly isolated bacteria from infectious corneal ulcers in EW patients. The incidence of infection by other microbial species such as fungi and Acanthamoeba may be higher in contact lens wearers but there is no specific evidence to suggest a heightened risk with EW. Although viral and chlamydial keratoconjunctivitis may also occur in contact lens wearers, there is no elevated risk apparent with the wearing of contact lenses.
Microbial keratitis can cause severe pain, photophobia and distress and, in some cases, vision loss. With early hydrogel EW, such problems were sensationalized by the lay press and numerous cases of legal action were initiated. The reported success of the early trials in the light of later reports of a raised incidence of infectious keratitis with EW is not surprising given the rarity of infection. Despite this low incidence, the large number of people adopting EW led to a disturbingly high frequency of hospital admissions with contact lens-related infections. This concern came to the attention of authorities, who described it as a ‘significant, preventable, public health problem’.
The major epidemiological study sponsored by the CLI, as mentioned above, determined that the incidence of clinically diagnosed infectious keratitis for daily-wear and EW hydrogel lenses was approximately 4 and 20, respectively, per 10,000 wearers per year ( ). Subsequent studies have found similar estimates for the incidence of microbial keratitis, confirming that EW entails a higher relative risk than does daily wear of hydrogel materials, regardless of whether lenses are worn in disposable or conventional mode, and whether they are hydrogel or SiHy lenses ( ). These epidemiological studies and other case reports have highlighted a number of risk factors for keratitis with extended contact lens wear (see Table 23.1 ).
The precise pathogenesis of contact lens-related microbial keratitis remains elusive. The large-scale field experiments of frequent replacement of lenses and use of SiHy materials tested whether corneal lens spoliation and hypoxia, respectively, were important mechanisms of corneal infection. Neither advance ‘moved the needle’ with respect to the apparent rate of EW infection. Ongoing research implicates changes in virulence factors of P. aeruginosa on prolonged exposure to the ocular environment and decay in the antimicrobial activity of the postlens tear fluid during lens wear as important aetiological factors ( ).
Noninfectious Inflammatory Events
While infectious keratitis is most often cited as the reason for the failure of EW in the 1980s, the high incidence of ‘noninfectious’ corneal inflammatory events probably contributed strongly to practitioner aversion to this mode of wear. Corneal inflammation is characterized by the migration and accumulation of inflammatory cells into the epithelial and anterior stromal space. Such events are collectively termed CIEs. These events may or may not be symptomatic. Although noninfectious CIEs are not sight threatening, they constitute a sufficiently acute response to cause interruption or even termination of EW. Indeed, the FDA of the United States has used CIEs as a surrogate measure of the safety of contact lenses in premarket approval studies for 30-day continuous wear, as the low incidence of infectious keratitis makes measurement of the rate impractical in trials of new lens materials.
CIEs pose a considerable challenge for clinicians and researchers alike with respect to diagnosis and classification and these are discussed in more detail below. The principal concern is to separate those incidents that are truly infectious, caused by virulent organisms, likely to progress and in need of urgent intensive treatment from those that are apparently sterile or caused by less virulent organisms, self-limiting and in need of no or minimal treatment.
Although a cornea may not suffer frank infection during a so-called ‘sterile’ inflammatory keratitis, there remains considerable circumstantial evidence that at least some of these events are due to the presence of bacteria and their exo- and endotoxins at the corneal surface. Strategies that minimize the bacterial load at the corneal surface may therefore help reduce the risk of CIEs.
During EW, the incidence of symptomatic CIEs has been found to be between about 2.5%–6% per year while the inclusion of asymptomatic events raises this range to 6–26% per year. EW of contact lenses causes some two to four times increased risk of CIEs compared with daily wear. The wide range of values for rates of CIEs probably arises from differences in experimental designs. Prospective studies with more frequent subject visits certainly show higher rates of asymptomatic CIEs. A series of studies following the introduction of SiHy lenses with large sample sizes has shown a consistent doubling of the rate of CIEs with these lenses compared to hydrogel lenses ( ). This finding seems to be consistent across wearing modalities. We hypothesize that this may relate to the ability of hydrogel lenses to attract and retain nondenatured protein to the lens surface, which may provide both antibacterial and immunogenic advantages.
Another inflammatory condition, CLPC ( Fig. 23.5 ) was also a principal hindrance to successful EW with early hydrogel lenses. As noted above, the use of frequent-replacement contact lenses helped to lessen the incidence of the condition with hydrogels. However, the introduction of first-generation high-modulus SiHy lenses that were intended to be worn continuously for 30-day periods saw the re-emergence of CLPC as a significant complication of contact lens wear. This chronic inflammatory reaction of the palpebral conjunctiva resembles vernal conjunctivitis but is directly related to contact lens wear (see Chapter 38 for more details). Both mechanical and antigenic stimuli are implicated as causative agents. Thus CLPC can be controlled in part by using lenses with a more frequent replacement schedule, a lower modulus and a lower surface coefficient of friction.
Acute Physiological Effects
When worn during eye closure, hydrogel lenses induce corneal oedema, which is observable clinically on awakening as striae and folds ( Fig. 23.6 ). The effects persist for some time following eye opening, and in some cases throughout the day. In some circumstances, visual disturbance, in the form of haze and blur, is noted. Although the acute effects on corneal oedema resolve rapidly, wearing a hydrogel contact lens in EW produces a recurring pattern of oedema during the night followed by partial resolution during the day ( ). Chronic exposure to this unfavourable environment is considered by some to be detrimental to long-term corneal health.
