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04 May 2007

Overnight wear with silicone hydrogel contact lenses

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The innovation of combining silicone compounds with hydrogel materials has had many positive impacts on contact lens practice, one of which has been the resurgence of interest in wearing contact lenses overnight.


Practitioners should take time to learn about this modality, so they are able to counsel their patients on all aspects of this mode of wear.

Definitions

The definition of extended wear (EW) is widely accepted to be six nights followed by a night of no lens wear.

Continuous wear (CW) is widely accepted to be up to 30 nights followed by a night of no lens wear.

History of overnight wear

The overnight wear of soft contact lenses was spearheaded in the UK in the early 1970s by John de Carle, who first published his experiences with Permalens.1 Similar positive experiences and publications in the US led to the Food and Drug Administration approving the use of cosmetic soft contact lenses for continuous wear in 1981. In the following years much was learned about the physiological requirements of the cornea, including the hypoxic stress that the cornea undergoes beneath a contact lens and the relationship this stress has with infection risk. Indeed, by 1989 this association was so closely proven2,3 that the FDA changed their approval for cosmetic overnight lens wear from 30 consecutive nights of wear to just six.

In 1987 the Acuvue lens was the first 'true' disposable lens to be launched and it was initially marketed for weekly (six nights) overnight wear. Due to the reduced handling requirements and the fact the lens was initially designed for single use, it was widely expected that the introduction of these disposable contact lenses would reduce the infection risk associated with overnight wear. However, reports of infection linked with this modality continued. In the UK, practitioners remained nervous about prescribing overnight wear, and the Acuvue lens was subsequently marketed and used as a daily wear lens to be replaced every two weeks. About 10 years later, a large Dutch study confirmed these fears and supported the hypothesis that a disposable modality had done nothing to reduce the risk of corneal infection when soft contact lenses were worn overnight.4

Evolution of silicone hydrogel materials

By the late 1980s, it was clear that corneal health was closely linked to adequate oxygenation of the cornea. Soft lens materials had relied on their water content as the transport mechanism to provide the cornea with oxygen, with higher water content materials providing the greatest amount of oxygen.5

However, it was evident that in many cases, hydrogel lenses were not capable of containing sufficient water to provide the cornea with the level of oxygen required to maintain normal metabolic activity during daily wear, let alone overnight wear. The relatively high infection rates measured with extended wear disposable lenses spurred the industry into developing new products. The incorporation of silicone compounds into hydrogels seemed a natural step to increase oxygen transport, as it had proved successful in increasing permeability for gas permeable lens materials.6 However, adding silicones to act as 'super-transporters' of oxygen proved difficult. Perseverance paid off some 15 years later, when the problem of the inherent silicone property of hydrophobicity was overcome by paying careful attention to surface treating lenses to aid wettability.

The first silicone hydrogel lens to be released commercially was the PureVision lens by Bausch & Lomb, which was closely followed by the Focus Night&Day from CIBA Vision. Currently there are six silicone hydrogel lenses on the market: PureVision (Bausch & Lomb), Air Optix Night&Day (CIBA Vision), Air Optix (CIBA Vision), Acuvue Advance (Johnson & Johnson Vision Care), Acuvue Oasys (Johnson & Johnson Vision Care) and Biofinity (Cooper Vision).

Demands on Dk

The ease with which oxygen penetrates a material is termed its permeability (Dk). The term Dk/t provides a more clinically relevant number because it relates to the amount of oxygen that can penetrate through a lens of specific thickness (t). It has been reported that, for the daily wear of a contact lens to have no impact on corneal metabolism, its Dk/t must be between 24 and 35 x 10-9, with overnight wear raising this minimum Dk/t to between 87 and 125 x10-9.7,8

Less oxygen supply than this minimum has been shown to cause corneal swelling at greater levels than those found in non-lens wearing eyes. It is important to consider that these values are for an 'average' cornea, meaning that there is a significant proportion of the lens-wearing population with corneas with above-average oxygen requirements.

Lens options

Solving the initial manufacturing difficulties of incorporating silicone components was just the beginning, since these new lenses also had very different mechanical and surface properties from conventional HEMA-based lenses. They demonstrated a higher rigidity due to their higher modulus of elasticity, making the fit less forgiving, as the draping property of conventional hydrogel lenses is lost.9 To improve the wettability of the lens surface Air Optix Night&Day, Air Optix and PureVision all undergo surface plasma treatments. These surface changes are an integral part of the lens rather than a coating.10,11 Johnson & Johnson Vision Care chose to incorporate an internal wetting agent, polyvinylpyrolidone, (PVP), into the lens matrix which acts as a shield to cover the hydrophobic silicon on the lens surface.12 The Biofinity lens by CooperVision is unique in that it shows good wettability but has no surface treatments. Table 1 details the major properties of the currently available silicone hydrogel lenses.

Oxygen flow

Since there is no longer any doubt that the cornea relies on an adequate oxygen supply to maintain a healthy metabolism, then every patient deserves to be fitted with a lens that allows high levels of oxygen to reach the cornea. There has been much debate about 'how much oxygen is enough',13-15 and though this debate continues, practitioners should remember there is no evidence that providing more than enough oxygen to the cornea is a cause for concern. There is little reason, therefore, for not refitting successful hydrogel wearers into one of the silicone hydrogel lenses, and conversely, there are many reasons for refitting the unsuccessful hydrogel wearer into these new materials. The only caution is that with some of the current lenses available, the higher oxygen flow is accompanied by a higher modulus of elasticity, which may lead to mechanical side effects in some patients.16

Infection risks

The driving force for the development of silicone hydrogel lenses was the desire to reduce the rate of infection during overnight wear of contact lenses. So did this happen?

Previous work17,18 has shown that Pseudomonas aeruginosa - a microorganism commonly associated with microbial keratitis - shows a marked reduction in its ability to bind to the epithelium of corneas wearing lenses of higher permeability. Recent prospective studies19,20 have concluded that the risk of microbial keratitis is no lower with silicone hydrogel lenses than it is with hydrogels during overnight wear. This finding was disappointing to eye care professionals and the contact lens industry alike. However, the data did show that, despite wearing the lenses for longer periods than conventional lenses, the severity of infection was reduced and the recovery rate from infection was faster in patients who wore silicone hydrogel lenses.19,20

Other potential complications of overnight wear

Hypoxia and corneal acidosis

Hypoxic responses are inevitable with hydrogel overnight wear and even with hydrogel daily wear for some patients, because the Dk/t of these lenses barely meets the transmissibility required for daily wear, and falls far short of that required for overnight wear, as shown in Figure 1. Conversely, most silicone hydrogel lenses exceed the required transmissibility for even overnight wear, thus providing a much higher level of safety against hypoxic corneal responses, whichever mode of wear is practiced. As a direct result of their higher transmissibility, silicone hydrogel lenses are rarely associated with the hypoxic complications that have become familiar with hydrogel use, namely epithelial and stromal oedema, limbal hyperaemia, corneal vascularisation, endothelial polymegathism and myopic prescription shifts.21,22

Inflammatory responses

As with hydrogel lens wear, the incidence of inflammatory responses increases in overnight wear compared to daily wear, regardless of lens type. While the incidence of inflammatory responses in overnight wear has changed little with the introduction of silicone hydrogel lenses, their severity seems to be much less and generally a faster recovery rate is experienced by wearers of silicone hydrogels. 21-23 Despite this, complacency should be avoided and occurrences of complications such as infiltrative keratitis, contact lens acute red eye (CLARE) and contact lens peripheral ulcer (CLPU) should always be monitored closely until resolution.21,24

Mechanical

Common complications attributed to mechanical effects include contact lens papillary conjunctivitis (CLPC), superior epithelial arcuate lesions (SEALs) and mucin ball formation.16,21 The first two examples are arguably more prevalent with silicone hydrogel than hydrogel lenses and both are associated with overly mobile lenses. Though hypoxia may play a role in their development with hydrogel wear, material rigidity is more likely the cause with silicone hydrogels. Refitting with a different lens design is frequently required to avoid future repetition. Mucin balls are primarily a phenomenon associated with silicone hydrogel lenses, more so with the higher modulus and exaggerated when the lens fit is loose.25 It is thought that the lens movement shears the tear film to create balls of mucin under the lens which can create transient depressions in the epithelial surface.25

While unlikely to cause symptoms in daily wear, these mucin balls can, in some patients, increase to significant numbers during extended or continuous wear. In such cases using lubricating drops prior to sleep or removing the lens to rub and rinse is sufficient to manage this problem.25

Overnight wear or daily wear?

Overnight wear lenses should not be offered as a 'safer' option to a patient who is non-compliant with their lens care regime. A patient who ignores the rules of lens care will probably ignore the rules of lens replacement and will cease lens wear when symptoms are present, both of which may lead to more serious complications in a continuous wear modality. There are situations, however, when an absence of good hygiene can make extended wear a favourable option. Consider the patient who has a pastime of hiking, or who is engaged in the armed forces. Clean water supplies for hand washing may be scarce and in such situations the risks associated with overnight wear may be less than the risks associated with lens insertion with dirty hands. Similar risks may apply to specific occupations which involve hand contamination.

Good reasons for considering an overnight wear modality:

●Poor handling ability: this could be due to physical or mental disabilities, high refractive errors or poor visual acuity

●Lifestyle: outside activities such as hiking, mountain climbing, camping etc

●Work habits: shift workers, armed forces, emergency workers, long and/or irregular work hours

●New parents: babies and young children with irregular sleep patterns demand a functional parent within seconds of waking

●Therapeutic/bandage lens requirements

Good reasons for avoiding an overnight wear modality

●Non-compliant patients: the consequences of being non compliant in extended wear mode are potentially higher than in daily wear mode

●Smoking: ulcers and infiltrates are more prevalent in the smoking population compared to the non-smoking population26

●Poor general health: patients with diabetes (particularly those who are insulin dependent) or health problems which lead to an increase in the risk of inflammation should be advised against continuous wear due to their higher risk of developing inflammatory and/or infective adverse events27

● Patients with chronic blepharitis or meibomian gland dysfunction: these patients typically have a higher bacterial load (especially gram positive organisms) on the ocular surface28,29 which may increase the risk of developing corneal infection or inflammation. Extra care even in daily wear contact lenses is advised5

●Chronic desiccation staining: this chronic break in the epithelial barrier can lead to corneal infection at a higher rate in overnight wear than in daily wear.

●History of inflammatory response: Once there has been one corneal inflammatory response event there is a much higher risk of the patient developing a repeated inflammatory event24

Patients who fall into the 'avoid overnight wear' groups listed above should not be left to a fate of problematic conventional hydrogel lens wear. Silicone hydrogel lenses are not solely prescribed for overnight wear and just because a lens is licensed for overnight wear does not mean that it can't be worn as a daily wear lens, indeed Acuvue Advance does not even have approval for overnight wear.

Silicone hydrogel lenses can offer huge benefits when prescribed in daily wear modality and are fast becoming the first choice of material for new fits and refits.30,31

The advantages of switching from hydrogel to silicone hydrogel regardless of mode of wear have been well documented and include:

●Increased oxygen supply to the cornea

●Reduced hypoxic events15,32

●Whiter eyes33,34

●Reduction of neovascularisation35

●Improved comfort36

●Reduced dryness36,37

●Facilitation of a longer wearing time38

●Potential increase in 'comfortable' wear time

●Facilitation of short naps or flexible wear.

Fitting silicone hydrogel lenses

Due to the higher rigidity of some silicone hydrogel materials, the fit may be less forgiving than a hydrogel lens. Lens movement is required for flushing toxins and debris from underneath all lenses but excessive movement in these materials may increase lens awareness and lead to tarsal plate changes.21,39,40 A flat lens is likely to exhibit lens edge fluting, which will reduce comfort, as typically the lens will irritate the lower lid. A lens which is centrally steep may lead to fluctuating vision which can be solved with a flatter base curve (Figures 4-6). High molecular weight sodium fluorescein is an extremely useful agent for assessing the fit of these stiffer lenses.

It is common for patients to undergo a period of adaptation before fully acceptable comfort is achieved.

This adaptation may take as long as four weeks but in most cases lasts for only three to seven days. Typical complaints may include the lens feeling 'heavy', having an awareness of the lens edge, dryness and sporadic blurring of vision. These symptoms can be due to the very different surface properties of the lens, its rigidity or even the rapid oxygenation of the cornea. Such initial adaptation problems are less of an issue when fitting a neophyte with silicone hydrogel lenses because they have no previous experience to compare with and will not experience re-oxygenation symptoms.

To facilitate an adaptation period the first review should not be scheduled too soon. The review should be planned to coincide with two to four hours of lens wear, so that vision and lens fit are stable and any potential solution incompatibility staining will be evident.41,42

It is highly recommended to counsel the patients being refitted on the potential comfort problems they may encounter during the first few weeks. This will help them to feel more confident during the adaptation period. When fitting lenses for overnight wear it is important to discuss the schedule of review appointments and related costs as well as providing counsel on how to deal with adverse events.

Switching patients to overnight wear

For those patients who are experiencing difficulties with their current contact lenses, it is not a difficult task to persuade them to try something new, particularly as trial lenses are easily accessible. It is recommended that the symptomatic daily hydrogel lens wearer should be switched to a silicone hydrogel yet remain in daily wear mode to begin with, so the only change is the lens. This way any new benefits or symptoms can be attributed specifically to the change in lens rather than creating uncertainty between the lens and the modality. It is when the lens parameters have been finaliSed that the options of flexible, extended or continuous wear can be broached. On the other hand, persuading the happy, symptom-free hydrogel wearer to switch lens type can be quite challenging. In these situations the key is to educate the patient on the benefits of a more highly permeable lens.

When discussing the possibility of overnight wear to patients it is important to provide a carefully balanced argument. Patients will appreciate the honesty of being told about the potential downsides of switching to extended or continuous wear, as well as being informed of the positive aspects. In an age of ever-increasing awareness of the legalities of 'informed consent', a document that provides information on all aspects of contact lens overnight wear is a useful tool.

Patients commonly believe that if a lens type or wear modality has not been suggested or offered by their eye care practitioner, they must not be suitable. It is helpful here to use the marketing power that the manufacturers of overnight wear have generated. Patients who already wear contact lenses are very aware of the many media advertisements for new contact lenses. Most patients are aware that there have been dramatic advances in the fields of technology and medicine over the past 20 years. Patients often welcome an opportunity to wear the latest contact lens designs and materials. Useful reminders to patients include discussions of providing an increased oxygen supply to the cornea throughout long wearing periods, the increased convenience of overnight wear, whiter eyes and faster healing rates should an adverse event occur.

Tips for success with overnight wear in silicone hydrogels

Patients are quick to adopt revolutions in technology and will welcome a new product or modality. Ensuring that the practice maintains appropriate literature on overnight wear products is a good way of promoting this modality as is inserting a feature in the practice newsletter or including information with reminder letters. This modality should be mentioned to all patients, and a discussion of the potential problems and benefits should take place.

Advantages of overnight wear

●Convenience

●Immediate clear vision on awaking

●Less handling

●Less maintenance

● Reduced or no care products- reduced chemical sensitivities

●Possibly a reduction in costs (depending on current regime).

Disadvantages of overnight wear

●Increased risk of corneal infection

●Potential mechanical responses, for example tarsal plate changes

●Potential inflammatory responses

●Possibly an increase in costs (depending on current regime).

By following these steps a practice's overnight wear base should grow.

●Promote silicone hydrogel lenses and overnight wear within the practice with literature. This will help to 'normalise' the products and the overnight wear modality

● Be proactive. Discuss the benefits of silicone hydrogel contact lenses with all current and potential contact lens wearers

● Ensure you provide information about the expected benefits of the new lenses as well as the potential problems that may arise

● Assess the lens fit carefully especially if there are initial visual or comfort issues, use of high molecular sodium fluorescein is preferred

● Be aware of alternative lens designs and also lens care incompatibilities so that you are better prepared to deal with any difficulties which may arise

● Make all costs and review schedules very clear. ●

References

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2 Schein OD, Glynn RJ, et al. The relative risk of ulcerative keratitis among users of daily-wear and extended-wear soft contact lenses. A case-control study. Microbial Keratitis Study Group. N Engl J Med, 1989 32112: 773-778.

3 Poggio EC, Glynn RJ, et al. The incidence of ulcerative keratitis among users of daily-wear and extended-wear soft contact lenses. N Engl J Med, 1989 32112: 779-783.

4 Cheng KH, Leung SL, et al. Incidence of contact-lens-associated microbial keratitis and its related morbidity. Lancet, 1999 3549174: 181-185.

5 Efron N, Brennan N. How much oxygen? In search of the critical oxygen requirement of the cornea. Contax, 1987July: 5-18.

6 Benjamin W. RGP material selection strategy and the Gel/RGP oxygen "gap". Int Contact Lens Clin, 1993 209/10: 200-202.

7 Holden BA, Sweeney DF, et al. The minimum precorneal oxygen tension to avoid corneal edema. Invest Ophthalmol Vis Sci, 1984 254: 476-480.

8 Harvitt DM, Bonanno JA. Re-evaluation of the oxygen diffusion model for predicting minimum contact lens Dk/t values needed to avoid corneal anoxia. Optom Vis Sci, 1999 7610: 712 - 719.

9 Dumbleton KA, Chalmers RL, et al. Effect of lens base curve on subjective comfort and assessment of fit with silicone hydrogel continuous wear contact lenses. Optom Vis Sci, 20027910: 633 - 637.

10 Jones L, Subbaraman LN, et al. Surface treatment, wetting and modulus of silicone hydrogels.Optician, 2006 2326067: 28-34.

11 Tighe B. Silicone hydrogels: Structure, properties and behaviour. in Silicone Hydrogels:Continuous Wear Contact Lenses, D. Sweeney, Editor. Oxford, Butterworth-Heinemann,2004, p 1 - 27.

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13 Morgan P, Brennan N. The decay of Dk ? Optician, 2004 2275937: 27-33.

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16 Dumbleton K. Noninflammatory silicone hydrogel contact lens complications. Eye Contact Lens, 2003 291 Suppl: S186-189 discussion S190-181, S192-184.

17 Ren DH, Yamamoto K, et al. Adaptive effects of 30-night wear of hyper-O2 transmissible contact lenses on bacterial binding and corneal epithelium : A 1-year clinical trial.Ophthalmology, 2002 1091: 27-39.

18 Cavanagh HD, Ladage P, et al. Effects of daily and overnight wear of hyper-oxygen transmissible rigid and silicone hydrogel lenses on bacterial binding to the corneal epithelium: 13-month clinical trials. Eye Contact Lens, 2003 291 Suppl: S14-16 discussion S26-29, S192-194.

19 Schein OD, McNally JJ, et al. The incidence of microbial keratitis among wearers of a 30-day silicone hydrogel extended-wear contact lens. Ophthalmology, 2005 11212: 2172-2179.

20 Stapleton F, Edwards K, et al. The incidence of contact lens related microbial keratitis in Australia. Invest Ophthalmol Vis Sci, 2005 ARVO abstract #5025.

21 Sweeney D, du Toit R, et al. Clinical performance of silicone hydrogel lenses. in Silicone hydrogels: Continuous wear contact lenses, 2nd. D Sweeney, Editor. Oxford, Butterworth-Heinemann, 2004, p 164-216.

22 Stapleton F, Stretton S, et al. Silicone hydrogel contact lenses and the ocular surface. Ocul Surf, 2006 41: 24-43.

23 Willcox M, Sankaridurg P, et al. Inflammation and infection and the effects of the closed eye. in Silicone hydrogels: Continuous wear contact lenses, 2nd. D. Sweeney, Editor. Oxford, Butterworth-Heinemann, 2004, p 90-125.

24 Dumbleton K: Adverse events with silicone hydrogel continuous wear. Contact Lens & Ant Eye 2002 25 137 - 146.

25 Dumbleton K, Jones L, et al. Clinical characterization of spherical post-lens debris associated with lotrafilcon high-Dk silicone lenses. CLAO J, 2000 264: 186-192.

26 McNally JJ, Chalmers RL, et al. Risk factors for corneal infiltrative events with 30-night continuous wear of silicone hydrogel lenses. Eye Contact Lens, 2003 291 Suppl: S153-156discussion S166, S192-154.

27 O'Donnell C, Efron N, et al. A prospective study of contact lens wear in diabetes mellitus.Ophthalmic Physiol Opt, 2001 212: 127-138.

28 Dougherty JM, McCulley JP. Comparative bacteriology of chronic blepharitis. Br J Ophthalmol, 1984 688: 524-528.

29 Seal DV, McGill JI, et al. Microbial and immunological investigations of chronic nonulcerative blepharitis and meibomianitis. Br J Ophthalmol, 1985 698: 604-611.

30 Morgan PB, Efron N. A decade of contact lens prescribing trends in the United Kingdom (1996-2005). Cont Lens Anterior Eye, 2006 292: 59-68.

31 Morgan PB, Woods C, et al. International contact lens prescribing in 2006. Contact Lens Spectrum, 2007 221: 34-38.

32 Keay L, Sweeney DF, et al. Microcyst response to high Dk/t silicone hydrogel contact lenses. Optom Vis Sci, 2000 7711: 582-585.

33 Papas EB, Vajdic CM, et al. High-oxygen-transmissibility soft contact lenses do not induce limbal hyperaemia. Curr Eye Res, 1997 169: 942-948.

34 Papas E. On the relationship between soft contact lens oxygen transmissibility and induced limbal hyperaemia. Exp Eye Res, 1998 672: 125-131.

35 Dumbleton KA, Chalmers RL, et al. Vascular response to extended wear of hydrogel lenses with high and low oxygen permeability. Optom Vis Sci, 2001 783: 147-151.

36 Dumbleton K, Keir N, et al. Objective and subjective responses in patients refitted to dailywear silicone hydrogel contact lenses. Optom Vis Sci, 2006 8310: 758-768.

37 Schafer J, Barr JT, et al. A characterisation of dryness symptoms with silicone hydrogel contact lenses. Optom Vis Sci, 2003 8012s:

38 Dumbleton K, Woods CA, et al. Comfort and adaptation to silicone hydrogel lenses for daily wear. Optom Vis Sci, 2006 83E-abstract 060066.

39 Skotnitsky C, Sankaridurg PR, et al. General and local contact lens induced papillary conjunctivitis (CLPC). Clin Exp Optom, 2002 853: 193-197.

40 Skotnitsky CC, Naduvilath TJ, et al. Two presentations of contact lens-induced papillary conjunctivitis (CLPC) in hydrogel lens wear: local and general. Optom Vis Sci, 2006 831: 27-36.

41 Jones L, MacDougall N, et al. Asymptomatic corneal staining associated with the use of balafilcon silicone-hydrogel contact lenses disinfected with a polyaminopropyl biguanide preserved care regimen. Optom Vis Sci, 2002 7912: 753-761.

42 Garofalo RJ, Dassanayake N, et al. Corneal staining and subjective symptoms with multipurpose solutions as a function of time. Eye Contact Lens, 2005 314: 166 - 174

● Jill Woods is research associate, Centre for Contact Lens Research, University of Waterloo




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