Interpretation of the statistical findings of this survey needs to be placed in the context of the very large data set analyzed. Even small numeric differences in the percentages presented are likely to be “statistically significant,” because this outcome is directly related to sample size. Thus, although a given difference or change might be statistically significant, we need to consider the broad implications of our findings. For example, the global contact lens market is worth an estimated U.S. $6 billion at retail values.20 Therefore, a global difference in prescribing EW of, say, 1% would have a potential fiscal impact of $60 million, which is a meaningful shift. However, from a clinical perspective, such a small change may not necessarily influence practitioner perceptions or behavior.
There are many possible reasons for the difference between nations with respect to the extent of EW contact lens fitting. Differences may exist between nations with respect to the training, attitudes, and collective confidence of the predominant practitioner groupsa—opticians, optometrists, and ophthalmologists—in relation to EW fitting. Differences in the extent and nature of commercial and professional promotion of EW may also influence the volume of EW contact lens prescribing in different regions. Access to therapeutics has little apparent impact on EW prescribing; for example, Japan, where virtually all contact lenses are prescribed by ophthalmologists, has the third lowest rate of EW prescribing (1%), whereas the United States, in which most contact lens practitioners are able to prescribe therapeutics, has a much higher EW prescribing rate (10%). Differences between nations with respect to the likelihood of litigation arising from clinical problems because of EW may also impact on the uptake of this modality.
There will also be variations in EW prescribing in different regions of the world, and even within some large countries, relating to socioeconomic factors and levels of public awareness of eye care. For example, in China, the provision of contact lenses and associated eye care in large urban centers, with ready access to large hospitals and other forms of sophisticated urban infrastructure, is likely to be very different from that in remote, rural areas. In some parts of East Asia, standards of contact lens care possibly lag behind those in the western world.
We speculate that the relatively high rate of EW fitting in Australia, which is largely with silicone hydrogel lenses, is attributed to the fact that much of the initial trial work for silicone hydrogel EW lenses was conducted in that country. This activity generated considerable interest among local practitioners, who were apparently keen to try out these new products as soon as they became available. However, the low rate of EW fitting in Japan may be linked to the belated release of silicone hydrogel lenses in that country, primarily as a result of stringent regulatory constraints and resultant delays in obtaining approvals to launch these products. Additionally, Japanese contact lens fitters—who are primarily ophthalmologists—may harbor more conservative attitudes and concerns relating to the ocular health risks of overnight lens wear.8,9,21,22
The higher age at which people are fitted with EW lenses could be attributed to a variety of factors. The strategy of fitting patients with DW lenses before moving on to EW has the advantage of familiarizing them with lens handling and lens maintenance procedures, resulting in a higher age when EW is commenced. In view of the known increased ocular health risks associated with EW generally,8,9,21,22 practitioners may prefer to fit such lenses to more mature (and therefore older) patients who would perhaps better understand the risks and responsibilities involved in overnight wear and be more likely to adopt a more sensible approach to solving any problems that arise during wear. A higher age might also be associated with a higher disposable income and less resistance to purchase the more expensive EW option.b
The reason for the over-representation of males being fitted with EW lenses is unclear but could relate to gender differences in lifestyle. More males are involved in sport23,24 and may prefer the convenience of EW when engaging in such activities.
The greater tendency for EW lenses to be dispensed as a refit could be explained by a preference of practitioners for lens wearers to first become accustomed to the lens handling and maintenance rituals of DW before progressing to EW. Practitioners also may be interested in gauging the ocular response to DW, which can serve as a baseline reference against which any adverse reactions to subsequent EW can be measured. The higher proportion of EW lenses prescribed as refits is commensurate with the necessarily higher age of those fitted with EW (as discussed above).
The majority of lenses prescribed for EW are made from silicone hydrogel materials. There is disagreement in the literature as to whether lenses made from these materials reduce the risk of severe microbial keratitis with EW compared conventional hydrogel lenses.8,9,21,22 Nevertheless, the superior oxygen transmissibility of silicone hydrogel lenses5 is known to alleviate hypoxic ocular complications—such as corneal edema, stromal neovascularization, and endothelial polymegethism25—which can be more pronounced with extended lens wear because of reduced oxygen availability beneath the closed eyelid during sleep.26 Therefore, the health benefits of silicone hydrogel lenses for closed-eye lens wear demand that EW patients are only fitted with this lens type.
It is somewhat disappointing to observe that 28% of EW fits were with conventional hydrogel lenses. A possible reason for the continued use of conventional hydrogel lenses for EW is their lower cost. If this were to be the case, greater use of conventional hydrogel lenses may be observed in poorer countries. Our demonstration of a lower rate of prescribing silicone hydrogel lenses for EW in nations with a lower gross domestic product at purchasing parity per capita tends to support this hypothesis.
The finding that a higher proportion of presbyopic designs are fitted for EW compared with DW is commensurate with the observation that those wearing EW lenses are slightly older. The lower rate of prescribing toric lenses for EW (14%) vs. DW (19%) may relate to clinical considerations of the oxygen performance of these lenses. Toric lenses have regions of increased thickness to mediate in-eye orientation. The oxygen transmissibility in these thicker regions is reduced,27 which may be problematic in EW in view of already reduced corneal oxygen availability during closed eye lens wear.26 For this reason, practitioners may prefer to prescribe thinner spherical designs, thus avoiding fitting toric lenses for EW at the expense of vision decrement resulting from a degree of uncorrected astigmatism. Throughout our survey period (2006–2010), there has been a progressive increase in the number of silicone hydrogel toric lens designs introduced into the market, which could also explain why the total proportion of toric lenses fitted is lower for EW (which are predominantly made from silicone hydrogel materials) than for DW.
The majority of EW lenses are prescribed for monthly replacement. This perhaps reflects lens availability at the time of our survey, whereby monthly replacement lenses have dominated the market in respect of lenses that are suitable for EW.28 It is unclear why 2.5% of EW fits were designated as daily replacement. This could be due to the occasional use of daily disposable lenses for 24 h (e.g., one day and one night) as a social convenience, a practice that would be contrary to the recommendations of manufacturers and most practitioners.29
Eighty one percent of those wearing lenses on an EW basis use some form of care solution. This indicates a need among most EW patients for occasional lens removal and subsequent maintenance and storage between scheduled lens disposal time points. The majority of patients use multipurpose solutions. The rate of use of hydrogen peroxide with EW is greater than for DW, possibly because of the perceived superior antimicrobial efficacy of hydrogen peroxide systems.30 Those who do not use lens care solutions (a) are generally able to wear lenses continuously for the full-designated lens wear replacement frequency interval, (b) replace lenses whenever a problem occurs (e.g., sudden discomfort), which may be sooner than the scheduled lens replacement time point, or (c) are non-compliant, e.g., using tap water to rinse lenses that need to be temporarily removed.
An Historical Perspective
The contact lens industry launched silicone hydrogel contact lenses onto the market around the turn of the century with the recommendation that they be worn exclusively on an EW basis. Market penetration of EW increased dramatically between 1999 and 2003, before levelling off from 2003 to 2007, and declining slightly thereafter. The slowing down of EW prescribing in the mid-2000s is possibly because, at least in part, of the introduction in 2004 of silicone hydrogel lenses specifically designed and promoted for DW.
A key factor that is likely to influence the prescribing of EW lenses is the perceived safety of this modality. Numerous epidemiological studies have addressed this issue, as outlined above. The muted world-wide prescribing of silicone hydrogel lenses for EW suggests that practitioners and lens wearers generally judge the health risks of this modality to outweigh the potential benefits.
Despite the obvious benefits of lifestyle convenience offered by the EW modality, and the enhanced biocompatibility of silicone hydrogel lenses that are available for this purpose, it appears that practitioners still prefer to prescribe, and patients prefer to wear, contact lenses for DW. Throughout the 40-year history of soft lenses, repeated efforts at promoting EW by the contact lens industry have failed to result in this modality exceeding a global average “glass ceiling” prescribing rate of about 15%. EW is unlikely to become the mainstream lens wearing modality until the already low risks of severe keratitis with this modality (around 20 cases per 10,000 wearers/yr9,21,22) become equivalent to that for DW (around four cases per 10,000 wearers/yr9,21,22). In the mean time, if contact lenses are to be fitted to a patient for EW, then silicone hydrogel lenses are the preferred option in view of their superior oxygen performance compared with conventional hydrogel lenses.
The International Contact Lens Prescribing Survey Consortium
Nathan Efron, Australia; Philip B. Morgan, United Kingdom; Craig A. Woods, Canada; Suresh Awasthi, Nepal; Joseph T. Barr, U.S.; Vadim Belousov, Russia; Jolanta Bendoriene, Lithuania; Aris Chandrinos, Greece; Byoung Sun Chu, South Korea; Nir Erdinest, Israel; Philip Fine, Israel; Martha Y Gonzalez, U.S.; José Manuel González-Méijome, Portugal; Hans-Jürgen Grein, Germany; Christina N. Grupcheva, Bulgaria; Jorgen Gustafsson, Sweden; Magne Helland, Norway; Hreinn Ingi Hreinsson, Iceland; John C. J. Hsiao, South Korea; Lee Kai Hung, Singapore; Motozumi Itoi, Japan; Deborah Jones, Canada; Razmig Knajian, Lebanon; Carla J. Mack, U.S.; Florence Malet, France; Edoardo Marani, Italy; Sebastian Marx, Germany; Giancarlo Montani, Italy; Jason J. Nichols, U.S.; Alex Ong, Singapore; Alice Pesinova, The Czech Republic; Geraint Phillips, New Zealand; Simona Radu, Romania; Ole Ravn, Denmark; Svend-Erik Runberg, Denmark; Jacinto Santodomingo, Spain; Mirna Stabuc Silih, Slovenia; Kah-Ooi Tan, Singapore; Inga-Lill Thunholm-Henriksson, Sweden; Ioannis G. Tranoudis, Greece; Eef van der Worp, The Netherlands; and Edit Vodnyanszky, Hungary.
School of Optometry
Queensland University of Technology
Kelvin Grove, Queensland 4059
The appendix is available online at http://links.lww.com/OPX/A72.
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a Unregistered merchants are regrettably also involved in contact lens fitting in certain unregulated parts of the world, but this category of lens supplier was not surveyed in this work.
b Some practitioners charge a higher professional fee for fitting EW lenses in view of the additional costs relating to (a) greater chair time required for the higher frequency of follow-up visits, (b) logistical considerations (a requirement for 24-hour access), and (c) additional administration (e.g., gaining informed consent and more frequent reminders).
contact lenses; fitting; survey; extended wear; silicone hydrogel lenses
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