Mark Wilcox, BSc, PhD is Chief Scientific Officer at the Brien Holden Vision Institute, in Australia and Professor of the School of Optometry and Vision Science at the University of New South Wales. He is a specialist in the field of eye health, particularly the areas of ocular inflammation and microbiology.
Contact lens disinfecting and cleaning systems are essential elements of successful and safe lens wear. Studies have shown that failure to maintain good hygiene in caring for contact lenses or contact lens storage cases may result in contact lens related complications1-3, such as microbial keratitis (MK), a potentially sight-threatening condition. Indeed, infrequent or inadequate hygiene of lens cases is a known risk factor for developing MK3. Lenses are often cleaned and disinfected in lens cases using multipurpose solutions (MPS), which now account for over 90% of disinfection solution types in United Kingdom and Australia4,5. However, studies have shown that despite use of MPS, contamination of lens storage cases remains common, occurring in 30-80% of cases6-10.
Lens cases have also been shown to be the most frequently6,11 and heavily12 contaminated of all the lens care accessories, including the bottle of disinfecting solution and the contact lens itself.
Bacteria and biofilm: Health hazards hiding in lens cases
Bacterial growth in a lens storage case is believed to be mostly biofilm; that is, a static adherent group of micro-organisms encased in polymeric material, composed of polysaccharides, proteins, nucleic acids and lipids produced simultaneously by the microorganisms in the biofilm and host cells or fluid. Biofilms are characterized by their resistance to stressful conditions, such as:
– pH variation,
– osmotic shock,
– biocidal substances13, such as disinfectants.
Microbial cells that grow in a biofilm environment often aggregate and are ten to one thousand times more resistant to biocides compared to single microbial cells exposed to a liquid environment 13-16. The most common micro-organisms found in lens storage cases are coagulase-negative staphylococci and Gram-positive bacilli 9,10. Of concern is that Gram-negative bacteria, which are much more likely to be potentially virulent ocular pathogens (such as Pseudomonas aeruginosa, Serratia marcescens), can also be isolated, and sometimes at very high levels10.
Despite being regularly bathed in disinfecting solutions that are designed to provide several log kills of bacteria suspended in solution, current (single-action) lens care systems are not entirely effective; they may not be strong enough to kill bacteria that grow as biofilms, and some MPS may select for a particular microbial ecology within a lens case.
Research has shown that cases used cleaned with certain MPS are more frequently colonised by strains of Delftia acidovorans, Serratia marcescens and Acromobacter sp. than when other MPS are used (10). Whilst the frequency of microbial keratitis caused by these microbes is still much lower than that caused by Pseudomonas aeruginosa, some have been isolated from contact lens-associated microbial keratitis and can lead to devastating outcomes17. However, producing a more lethal MPDS is not an easy task for manufacturers, given that these MPS would need up to an additional thousand-fold antimicrobial activity, yet maintain a safety profile that does not cause harm to the eye during lens wear.
New generation MPS: Dual disinfectants
The past 12 months has seen the release of new generation MPS from several manufacturers (Table 1), which will become more widely available worldwide over the next 12 months. All these MPS contain dual disinfectants (see Table 1) that provide, according to data released by the manufacturers, superior performance compared to single- disinfectant MPS when tested against microbes suspended in solution. These MPS are especially effective against pathogens that are more difficult to kill, such as fungi and Acanthamoeba.
The manufacturers of these solutions also report that the surfactants contained in these MPS (Table 1), which are not normally found in soft lens MPS, maintain the activity of lysozyme on lenses and improve lens wettability.
Meeting international disinfection standards
These MPS have had to pass the International Organization for Standardisation (ISO) standard 14729 disinfection test, which requires at a set reduction in the ability of certain bacterial and fungal strains to grow, including Pseudomonas aeruginosa ATCC 9027, Serratia marcescens ATCC 13880, and Staphylococcus aureus ATCC 6538 and Fusarium solani ATCC 36301, Candida albicans ATCC 10231, respectively. At present, there is no standard procedure for measuring the effectiveness of a solution against
Acanthamoeba is an omission that is being looked at by regulatory authorities and the ISO. It is likely a standard will be developed, and it might include the prevention of encystment of trophozoites or Acanthamoeba, which is thought to have been involved in the failure of Complete MoisturePlus MPS 18. It is likely that regulatory authorities have already begun asking manufacturers of the new MPS to test for encystment.
Uptake and release of disinfectants
Another test gaining favor with certain regulatory authorities is the so-called uptake and release test. This test is designed to determine whether the disinfectants in MPS are taken up by contact lenses and subsequently released. I believe the hypotheses behind this test are that uptake of disinfectants leads to diminishing disinfectant activity during lens disinfection in cases, and release of the disinfectants during lens wear might have a detrimental effect on the eye (solution-induced corneal staining) or on ocular comfort.
I have to say that there is very little evidence of any detrimental effect associated with lens uptake of disinfectants. Despite findings that PHMB can be taken up by lenses such as the widely available etafilcon A lenses (Acuvue 2; Vistakon, Johnson and Johnson Vision Care)19, there has never been any epidemiological evidence that these lenses in combination with PHMB containing single-disinfectant MPS (e.g. ReNu MultiPlus) have any increased risk for the development on MK. On the other hand, release of constituents of MPS (most likely the disinfectants, but this has not been conclusively proven) appears to lead directly to solution-induced corneal staining (SICS)20, which may also reduce ocular comfort21 although the effect of SICS on ocular comfort is controversial20,21.
Will dual disinfectant MPS improve contact lens performance?
The question remains whether these improvements in MPS formulations will translate into improvements in performance during use. At the Brien Holden Vision Institute, we have been fortunate to examine the effectiveness of RevitaLens OcuTec™ (Table 1) in a clinical trial, and found a significant reduction in the levels and frequency of Gram-negative bacteria in lens cases disinfected by this new MPS. It will be intriguing to see whether other dual MPS perform similarly well.
Another aspect of MPS performance is their ability to clean lenses. We have shown that single disinfectant MPS and the older generation dual disinfectant MPS (OPTI-FREE® RepleniSH®; OPTI-FREE® EXPRESS®) still allow the deposition of protein and cholesterol on the surface of silicone hydrogel lenses 22. Will the incorporation of the newer surfactants in MPS (Table 1) have any effect on this lens deposition during normal use by wearers? Indeed, given that we can find very little evidence that the level of protein or lipid deposition has much effect on lens performance 23, will the newer generation dual MPS really be an improvement even if they do show reductions in lens deposition and better cleaning ability?
The new dual disinfecting MPS may show improvements in several key properties of an MPS: disinfection (especially with respect to biofilms in lens cases) and cleaning/wetting of lenses. The “proof in the pudding” will be to see how well they perform in clinical trials and in use by consumers.
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