It is roughly a decade since silicone hydrogel lenses were first launched onto the contact lens marketplace. Since then there has been an expansion of both the availability of lens types and the prescribing rate of these materials. Victoria Evans reviews the long-term studies on contact lenses for extended wear.
It was evident just a few years after their launch that silicone hydrogel lenses would become a dominant contact lens type due to their positive physiological benefits.1
Indeed, between the period of 2000 to 2008, Australian practitioners led this worldwide trend, with silicone hydrogel lenses peaking at 63 per cent of all contact lens fits in 2008.2 In 2009, the 10th annual survey of contact lens prescribing in Australia showed silicone hydrogel lens materials were approximately 48 per cent of new contact lens fits and 54 per cent of refits.3
A list of the silicone hydrogel contact lens materials currently approved for sale as disposable lenses in Australia is shown in Table 1.4 There are now three materials approved for up to 29 or 30 nights’ wear, three materials approved for up to six nights’ wear and an additional three materials approved for daily wear. In addition to the lenses listed below, many of the brands are available in toric and bifocal or multifocal options and custom made silicone hydrogel lenses are now also available from a variety of manufacturers.
The superior oxygen permeability of these materials has led to a virtual elimination of hypoxia related complications of contact lens wear, even when worn on a continuous wear basis for up to three years.5 At the launch of the first silicone hydrogels for continuous wear in 1999 there was great hope that the high oxygen permeability would significantly reduce the risk of microbial keratitis (M.K.) with overnight wear and prescribing rates of extended and continuous wear were expected to increase.6 In 2009, 7 per cent of all soft lens fittings in Australia were for extended wear and, while these fits were nearly all in silicone hydrogel materials, it illustrates that the penetration of extended wear modality remains relatively low, despite the massive uptake of silicone hydrogels for daily wear.3
With a decade of fitting experience behind us, the clinical performance of the first silicone hydrogel lens materials for overnight wear is well documented; indeed they represent some of the most studied contact lens materials in the scientific literature. The past five years in particular have seen the publication of results from long term clinical trials of extended wear, as well as population-based surveys of adverse event rates and risk factors for all lens wear materials and modalities. This information allows us to examine the track record of these materials and determine whether they met early expectations of their performance for overnight wear.
The three largest and most recent surveys of the general population designed to ascertain the incidence and risk factors associated with M.K. with contact lens wear were conducted in the United Kingdom, Australia/New Zealand and the United States, respectively.
The UK study included 367 contact lens (C.L.) wearers who reported to Moorfields Eye Hospital; 1,069 C.L. wearing controls from the hospital population and 639 C.L. wearing controls from the local population over a two year period.7 They found the relative risk of M.K. was increased with daily disposables, reduced for rigid gas permeable lenses and was no different for silicone hydrogel or other types of soft lenses compared to planned replacement soft lenses.
The Australia/New Zealand study was a prospective study of all M.K. cases reported over a 12 month period.8 297 M.K. cases were matched with 1,798 controls, identified by a telephone survey of 35,914 individuals. The U.S. study differed in that it was specifically designed to assess the incidence of M.K. in lotrafilcon A extended wear, and recruited 6,245 participants from 131 practices over a one-year period.9 In these two studies, the risk rates of M.K. were again roughly consistent with those first published for hydrogel lenses10 and a selection of the data is summarised in Table 2. Most importantly the incidence of loss of visual acuity following microbial keratitis was very low, 3.6/10,000 in the US study and 0.6/10,000 in the Australian study.
Low Risk of MK
The above studies have confirmed that while silicone hydrogel lenses have not eliminated M.K., the risk remains extremely low and the risk of vision loss even lower. Putting these risks in context of other ophthalmic risks, the risk of M.K. with silicone hydrogel lenses is roughly equivalent to the risk of retinal detachment after cataract extraction and 30 times less than the risk of experiencing a corneal inflammatory event during low Dk extended wear.11 In the context of other life risks, the risk of M.K. with extended wear is less than the risk of casualty in a car accident. In July 2010 in NSW there were 327 casualties per 100,000 population (or approximately 32.7/10,000).12
As well as identifying incidence, the above studies were able to identify risk factors for Microbial Keratitis. In addition to overnight wear, the Australian group found the risk factors for M.K. were poor lens storage case hygiene, smoking, internet purchase of C.L.s, <6 months wear experience and higher socio-economic class.8 Thyroid disease and self-reported poor health were more common in cases of microbial keratitis than in the control groups.13 The U.S. group found the rate of M.K. was lower for users reporting wear of three or more weeks continuously than those wearing the lenses for shorter periods.9
With the wide availability of silicone hydrogel lenses, overnight wear remains an option for individual patients who seek extra flexibility from their contact lenses. These patients may prefer daily wear but wish to nap while wearing lenses; they may need occasional overnight wear or want up to 30 nights extended wear. Contact lens industry surveys of wearers reveal that 50 per cent of patients nap in their lenses or desire to sleep in them overnight.14 Of the 18 per cent who currently sleep in their lenses, 36 per cent sleep in them for more than seven nights and 21 per cent sleep in them for more than 29 nights. Critically, 76 percent of these patients are wearing lenses not approved for overnight wear.14 Careful history taking is required to identify these wearers, otherwise an opportunity to refit them with more breathable lens materials may be missed.
Good patient selection, an understanding of the risks of infection in the context of other life risks, combined with education on the modifiable risk factors such as smoking, hygiene and internet purchase of lenses, allows the practitioner and patient to ensure the best possible lens wearing experience every time.
Victoria Evans PhD BOptom (Hons) is a freelance writer and consultant to the ophthalmic industry. Her prior role was Head of Clinical Research for the Institute for Eye Research. She has worked in clinical practice in Sydney, in contact lens research at the CCLRU, and in tear film and dry eye research at CRCERT and Oxford University. Vicki was sponsored by Ciba Vision to research this study.
Table 1. Disposable silicone hydrogel lenses approved for overnight wear in Australia
Table 2. Annualised incidence/ persons of microbial keratitis with lens wear for soft lenses and silicone hydrogel lenses