A lot of peer reviewed papers about adverse events for first generation extended wear silicone hydrogels are available, but what about silicone hydrogel daily wear, a modality that we saw in the last editorial is rapidly increasing? There are numerous articles in the optical press detailing daily wear silicone hydrogel studies, but these typically discuss subjective responses or favourable clinical outcomes of short term studies. Since it is evident that the interaction of different lens care solutions with silicone hydrogel lens materials produce varying effects on the ocular surface in terms of solution induced corneal staining (SICS), focus is starting to shift to other adverse events in longer term well controlled studies.
Contact Lens Papillary Conjunctivitis (CLPC) is a major cause of lens discontinuation and Superior Epithelial Arcuate Lesions (SEALs) a source of frustration. In an 18 month study of new contact lens wearers using the two first generation silicone hydrogels , CLPC and SEALs were reduced in daily compared to continuous (30 night) wear. For CLPC, the surface cleaning and lower exposure of lens to upper lid is likely to be of benefit in daily wear, while for SEALs, the more stable daily tear film may play a part (see this month’s case report by Judy Kwan for further information). Corneal shape changes are minimal  and while still of unknown significance, conjunctival epithelial flaps that appear at the lens edge are also reduced .
Infection And Inflammation
It is very apparent from the post-marketing FDA studies in the US  and the Australian  and UK  Microbial Keratitis studies that silicone hydrogel lenses do not protect against corneal infection. A recent analysis of the non-infective complications in the UK study , described in this month’s article review by Nikki Peng, indicates silicone hydrogels have not reduced the relative risk of these less severe but more common adverse events either. Risk factors for sterile keratitis were similar to what we have seen previously, including overnight wear, increased days per week, poor hygiene, smoking and less CL experience. Overnight wear and less experience were also risk factors for mechanical disorders.
In the 18 month study of neophytes , fewer inflammatory adverse events also occurred in daily wearers compared to those on continuous wear. The continued effects of the lens on epithelial cells and the increased stagnation of tears and trapped debris are more likely to cause mechanical disruption and/or toxic response, especially if the lens has become contaminated with bacteria, fungus or amoeba. We are aware of the intense response of a contaminated lens in Contact Lens Acute Red Eye (CLARE) after overnight wear . Similar pathogens are isolated from CLARE and Infiltrative Keratitis (IK); however, it is thought the IK response seen in DW is less intense than CLARE due to flushing of the tears  and anecdotally we see more milder infiltrative responses in daily wear compared to overnight wear as well as lower incidence.
However, CLARE can happen after one day of wear and indeed in an analysis of the sterile infiltrates in the US FDA post-market study , an increased incidence was found for those who could not maintain a 30-night extended wear schedule, compared to those that could. Clearly there is a person factor operating—either there is lower tolerance to lens wear, and/or other external factors such as microbes; there is more chance of mechanical and/or microbial challenge due to handling lenses; or a combination of these factors.
A group from Leiden University in the Netherlands  recently showed small mutations on a gene associated with corneal inflammation increases the susceptibility to microbial keratitis and modulates the severity of the response. This confirmed work on animal models by Linda Hazlett and her group  linking IL-10 with more severe pseudomonas aeruginosa keratitis.
We repeatedly see smoking as a risk factor for eye disease. It is probable that smoking is an indicator of the propensity to take risks rather than some physiological or physical factor as has previously been suggested . At BCLA this year, some preliminary data was presented which showed risk taking personality was associated with occasional overnight wear and poor lens replacement compliance . Morgan and Efron have done some excellent work on the area of compliance and further understanding of the drivers of this behaviour is important, particularly with the links with outbreaks of disease we have seen in recent times.
In our studies we have seen higher incidence of daily wear adverse events with MPS solution compared to hydrogen peroxide . Clearly for part time wearers, peroxide could be problematic as there is not preservative when stored long term. Daily wear silicone hydrogels provide a welcome addition to the array of silicone hydrogel lens types available.
With an aging population, that are better able to handle silicone hydrogel lenses than hydrogels , the increased oxygen benefit of this modality and the expanded market of lens materials and designs, there are many options available. Lens solution compatibility and wearer compliance will almost certainly continue to be challenges for practitioners, but active research in these areas is likely to produce new technology and guidelines to deliver the more favourable outcomes.
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