Mark Wilcox, BSc, PhD is a professor of the School of Optometry and Vision Science at the University of New South Wales. He specialises in the field of eye health, particularly the areas of ocular inflammation and microbiology.
Epidemiological evidence suggests that poor hygiene of contact lens cases is significantly associated with increased risk of developing microbial keratitis1,2 or sterile keratitis.3 However, even in controlled clinical trials where lens case hygiene is emphasised to participants, the frequency of contamination of cases is high, as high as 92% of cases.4
It could be argued that contamination by all microbes is not a problem as microbial keratitis or corneal inflammation is only produced by certain microbial types, usually Pseudomonas aeruginosa, other Gram-negative bacteria, Staphylococcus aureus and certain other Gram positive bacteria, some fungi and Acanthamoeba sp. If lens case contamination by only these potentially more pathogenic bacteria is considered, then in studies with silicone hydrogel lenses and multipurpose disinfecting solutions show that 15-45% of cases are still contaminated in tightly controlled clinical trials.4
It is perhaps a combination of inadequate hygiene procedures, lack of understanding by users that case hygiene is important, and lack of activity of disinfecting agents against certain microbial types or microbes within biofilm communities in lens cases that lead to these high levels of contamination.
Recent experimental data has been published examining the effect of adding steps in the lens case hygiene procedure, or adding silver as an antimicrobial agent to lens cases, or heating lens cases as ways of reducing/minimising contamination of lens cases. Here I review these developments.
A new case-cleaning protocol
Data suggest that lens case hygiene should include the following combined steps:
- Rubbing the case with a finger
- Rinsing the lens case with the disinfecting solution by filling the case cup 80% full and then discarding the solution
- Wiping the interior side of the case well in a circular motion with a clean facial tissue
- Air-drying the lens case for six hours prior to re-use of the case
The results showed a significant (over 50%) reduction in the numbers of Gram-positive bacteria in lens cases in a clinical trial5. The median number of viable cells (live bacteria) per case was:
- New protocol: 5 viable cells
- Manufacturer’s recommended protocol: 13 viable cells
However, the effect on the level of the potentially more problematic Gram-negative bacteria could not be estimated, as this was low in cases using either the new or manufacturer’s recommended hygiene procedures, and the study was not powered sufficiently to detect differences.
Silver-infused lens cases
Several manufacturers have incorporated silver into lens cases. The effect of using silver cases (Proguard/Microblock, CIBA Vision, Atlanta, GA, USA) has been reported in two publications.6,7 In the first of these studies,6
- The silver lens cases were less frequently colonised by any bacteria than control non-silver cases (26-38% vs. 63-67%; p≤0.03), and
- 3% of silver cases were contaminated with Gram-negative bacteria (either P. aeruginosa or Serratia marcescens) compared to 8% of normal cases.
In the second trial,7
- The frequency of contamination of silver cases (71%) was similar to contamination in non-silver cases (82%), and
- There were significantly fewer numbers of bacteria colonising silver cases (7.6 x 102 viable cells per well) compared to non-silver cases (5.1 x 105 viable cells per well), and in particular fewer numbers of Gram-negative bacteria, Gram-positive bacilli and fungi (p≤0.04).
So, it does appear from all the data that the silver cases reduce the numbers of potentially pathogenic bacteria that contaminate cases, but their effect on the frequency of contamination is equivocal.
Use of a warming device
A warming device has recently been developed for use with contact lens cases.8 This device was originally developed to gently warm lenses prior to insertion to help with comfort of lenses. I have recently conducted a series of laboratory studies on the device, sponsored by Warm Contacts Pty Ltd., to determine whether the device could be used during drying of the lens case after removal and wear of lenses.
As there are no clinical data to evaluate the success of this device, I will compare it to laboratory data for silver cases and the “rub, rinse, wipe, dry” technique. For experiments using the warm contact device, biofilms containing 6.5 log10 viable cells of S. aureus or 7.2 log10 viable cells of P. aeruginosa were allowed to adhere to the insides of normal lens cases.
Incubating either bacterial biofilm at 60oC in the warm contact device in the presence of a residual small quantity of the multipurpose disinfecting solution OPTIFEE® RepleniSH® (Alcon laboratories, Fort Worth, TX, USA) resulted in total kill of the biofilm, i.e. no surviving bacterial cells could be cultivated (>6 log10 kill of bacteria). This was significantly better than drying in the presence of OPTIFEE® RepleniSH® at ambient temperature (14oC).
In data given in the publications of Amos and George7 and Dantam et al.9 there was only a maximum kill of 2.4 log10 of P. aeruginosa or 5.4 log10 of S. aureus when silver lens cases were challenged with bacteria. In data given in publications by Wu et al.10,11 using the “rub, rinse, wipe, dry” technique, there were reductions of between 3.53 and 6.7 log10 viable cells for S. aureus biofilms and 4.31-5.82 log10 viable cells for P. aeruginosa biofilms, depending on the MPDS and lens case used. These approached but did not exceed the 6-7 log10 reductions with the warm contact device.
So, there are new developments that are aimed at improving contact lens hygiene and/or reducing the ability of microbes to adhere to lens cases. Now, we need larger scale clinical trials of these devices to determine whether they have any effects on the rate of adverse events, especially microbially driven adverse events, during daily wear of contact lenses.
I value your views on these development, and have devised a series of questions in the clinical insights section of this web site for that purpose. Please take a couple of minutes to let me know what you think.
1. Stapleton F, Keay L, Edwards K, Naduvilath T, Dart JK, Brian G, Holden BA. The incidence of contact lens-related microbial keratitis in Australia. Ophthalmol 2008;115:1655–62.
2. >Radford CF, Minassian DC, Dart JK. Disposable contact lens use as a risk factor for microbial keratitis. Br J Ophthalmol 1998;82: 1272–5.
3. Bates AK, Morris RJ, Stapletion F, Minassian D, Dart JKG. ‘Sterile’ corneal infiltrates in contact lens wearers. Eye 1989;3:803-10.
4. Willcox MDP, Carnt N, Diec J, Naduvilath T, Evans V, Stapleton F, Iskandar S, Harmis N, Lazon de la Jara P, Holden BA. Contact lens case contamination during daily wear of silicone hydrogels. Optom Vis Sci 2010;87:456-64.
5. Wu YT, Teng YJ, Nicholas M, Harmis N, Zhu H, Willcox M, Stapleton F. Impact of lens case hygiene guidelines on contact lens case contamination. Optom Vis Sci 2011;88:E1180-E87.
6. Amos CF. George MD. Clinical and laboratory testing of a silver-impregnated lens case. Contact Lens Anterior Eye 2006;29:247-55.
7. Dantam J, Zhu H, Willcox M, Ozkan J, Naduvilath TJ, Varghese T, Stapleton F. Evaluation of silver impregnated anti-bacterial contact lens storage cases in vivo. Invest Ophthalmol Vis Sci 2012;In Press. doi: 10.1167/iovs.11-8197.
8. http://www.warmcontact.com – accessed on 17th February 2012.
9. Dantam J, Zhu H, Stapleton F. Biocidal efficacy of silver-impregnated contact lens storage cases in vitro. Invest Ophthalmol Vis Sci 2011;52:51-7.
10. Wu YT, Zhu H, Willcox M, Stapleton F. Removal of biofilm from contact lens storage cases. Invest Ophthalmol Vis Sci 2010;51:6329-33.
11. Wu YT, Zhu H, Willcox M, Stapleton F. The effectiveness of various cleaning regimens and current guidelines in contact lens case biofilm removal. Invest Ophthalmol Vis Sci 2011;52:5287-92.