Silicone Hydrogels for Daily Wear: Comparison of Complications with Extended Wear

Notwithstanding the improved oxygen transmissibility of silicone hydrogel lenses, a number of mechanical, inflammatory and infectious complications have still been reported to occur with these lenses when they are worn on a continuous or extended wear (EW) basis. The frequency of complications associated with the DW of these lenses is less clear. This editorial will endeavor to address this topic.

Many of the mechanical complications reported with overnight wear of silicone hydrogel lenses would still be expected to occur when these relatively stiff materials are worn on a DW basis. In the early published clinical trials, superior epithelial arcuate lesions (SEAL), contact lens papillary conjunctivitis (CLPC) and mucin balls were reported to occur more frequently with silicone hydrogel lenses than with conventional hydrogel materials. Newer designs and the availability of wider parameters seem to have resulted in fewer reports of these complications. A daily wear modality, with fewer hours of physical contact between the lens surfaces and edges, may result in an even lower incidence of mechanical complications, particularly mucin balls, which are thought to result from stiff lens materials shearing the tear film and rolling up small balls of tear film mucin with lipid1. This process is further facilitated with overnight lens wear when the tear film collapses. Anecdotal reports and clinical trials with DW of silicone hydrogel lenses at the CCLR support this supposition. In clinical trials conducted at the CCLR with Focus Night & Day and PureVision lenses, 8 to 18% of eyes showed mucin balls at follow up visits in DW studies compared to 40 to 50% of eyes in CW studies².

In these short-term DW trials with Focus Night & Day and PureVision lenses² we have observed a 6 to 8% prevalence of SEAL, similar to that previously reported in CW and EW studies³. The prevalence is anticipated to be lower with the newest silicone hydrogel material, Acuvue Advance, since it has a modulus that is much closer to conventional materials, being only 1.5 times more rigid than the etafilcon material of regular Acuvue lenses. Erosions have also been reported to occur occasionally with silicone hydrogel lens wear⁴. These complications are likely to also occur sporadically in the DW modality.

CLPC in silicone hydrogel wearers is considered to be both mechanically and immunologically mediated^{5, 6}. For this reason, the reduced exposure with DW of silicone hydrogel lenses may result in significantly less CLPC when compared with overnight lens wear modalities. We will not know whether this hypothesis is substantiated until results from longer term DW studies are reported.

A number of inflammatory complications have been reported to occur with silicone hydrogel materials. These include contact lens peripheral ulcer (CLPU), infiltrative keratitis (IK) and contact lens acute red eye (CLARE). The majority of these events transpire as a result of colonization of the ocular adnexa and /or the contact lenses by micro-organisms, predominately bacteria. There appears to be a patient predisposition to these events, and those patients harbouring high levels of bacteria on their eyelids may be more prone to inflammation, particularly when lenses are worn overnight.

Even in a DW modality, CLPU may occasionally be observed with silicone hydrogel lens wear. The prevalence of this complication, however, is likely to be significantly lower than reported when the same lenses are worn overnight⁷. Approximately 5% of patients wearing silicone hydrogel lenses on a CW basis are likely to experience IK and it is expected that this complication will also be observed less frequently when lenses are worn on a DW basis.

CLARE is a unilateral acute inflammatory response to gram-negative organisms which colonise the lens and release endotoxins. Patients with CLARE are woken in the night while wearing their lenses and therefore this complication should not occur if lenses are worn strictly on a DW basis. CLARE can occur with DW, however, if lenses are inadvertently worn overnight or during naps, particularly with patients who have respiratory tract infections⁸.

The incidence of asymptomatic infiltrative keratitis (AIK) with DW of conventional lens materials and CW of silicone hydrogels is similar (2-3%⁷); therefore, it is unlikely that the incidence of AIK with DW silicone hydrogels will be different. Asymptomatic infiltrates (AI) with no associated hyperemia or staining are more common, and will likely be seen in all wearing modalities.

Microbial keratitis (MK) is a much more serious adverse event which may occur in silicone hydrogel lens wearers. Fortunately, the prevalence of MK within the general population is extremely low, due in part to the exceptional defence mechanisms that protect the ocular surface; however, MK remains the most serious complication of contact lens wear. The increased oxygen permeability afforded by silicone hydrogel lenses results in a healthier epithelium and less bacterial binding^{9, 10} which should provide better protection against infection. Nonetheless, several cases of MK with silicone hydrogel lenses have been reported^11-15. The actual rate of infection will not be available until the conclusion of ongoing prospective clinical trials¹⁶. In Australia, rates for MK with silicone hydrogel lenses of 1 in 4,000 for CW and 1 in 13,000 for DW have been reported¹⁷. These rates are still significantly lower than the currently accepted figure for low Dk soft lens EW (1 in 500 or 0.2%)^{18, 19}.

DW with silicone hydrogel lenses may also be associated with complications which do not occur with these materials when they are worn on a CW basis. The care regimens used for daily cleaning and disinfection of contact lenses were developed for conventional materials and a number of recent reports have indicated that, for some patients, certain components of these care regimens may result in asymptomatic hypersensitivity staining when used in conjunction with silicone hydrogel materials²⁰. Hydrogen peroxide systems, alternative disinfection components and new formulations of multipurpose solutions readily address these issues.

The type of complications observed with silicone hydrogel lenses worn on a DW basis are likely to be similar to those seen during EW. As the cornea is not in a chronic anoxic state during DW with silicone hydrogels, it is better able to recover, and we anticipate that complications will be less severe than those seen with low Dk lenses worn on a DW basis.

References

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2. CCLR Data on file.
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