Wu, Yvonne T.*; Teng, Yuu Juan*; Nicholas, Mary*; Harmis, Najat†; Zhu, Hua‡; Willcox, Mark D. P.‡; Stapleton, Fiona§
Apart from daily disposable and extended wear contact lenses, other types of lens wear modalities require the use of a contact lens storage case in which contact lenses are disinfected and stored overnight. However, if the contact lens storage case itself is not cleaned properly, it can become contaminated with pathogenic microorganisms,1–4 which may predisposes the wearers to lens-related adverse events.5–8 In addition, once bacteria enter the lens case, they may adhere to the case and switch from a planktonic phenotype to a sessile biofilm phenotype. Bacterial biofilm contamination of contact lens storage cases has been well documented.3,7,9,10 Bacterial biofilm can be defined as a structured community of bacterial cells enclosed in a self-produced polymetric matrix and adherent to an inert or living surface.11 In response to a low nutrient environment, microorganisms in the sessile form are embedded in a glycocalyx, which is a polysaccharide-containing matrix12 produced by bacteria. The first colonizing bacteria adhere to a surface initially by weak, reversible bonds. These bacteria facilitate attachment of other pathogens by anchoring using cell adhesion protein molecules on their surfaces. They also begin to produce matrix that holds the biofilm together. Initially, biofilm can be easily removed because of the loose attachment of cells; however, the adhesion between microbial cells and the contact lens case surface becomes more persistent over time. It has been proposed that low metabolism and the complex architecture of biofilm community contribute to increased biofilm resistance to antibiotics.13,14 Indeed, current multipurpose disinfecting solutions (MPDS) may not be effective against bacteria in a biofilm,15 subsequently, dislodged “daughter” cells from biofilm may colonize new surfaces.16 Current recommended lens case hygiene practices and good hygiene compliance do not necessarily ensure a lens case free of contamination.3,8 This suggests that additional evidence-based cleaning methods should be incorporated into the current lens disinfection regimen to minimize microbial contamination.
Previous in vitro studies have demonstrated that cleaning measures such as rubbing and tissue wiping of contact lens cases are capable of reducing biofilm.17,18 These alternative cleaning methods have been shown to be more effective than the current manufacturers' guidelines in vitro.17 However, the efficacy of the alternative cleaning regimen in vivo is yet to be established.
Therefore, this study was conducted to evaluate and compare the effectiveness of an alternative contact lens case hygiene guideline, incorporating “rubbing” and “tissue-wipe” steps and specifying air-drying lens cases face down,19 to the current manufacturer's guidelines. We hypothesized that the alternative guidelines would be more effective in reducing contamination of the contact lens cases than that of the current manufacturers' guidelines.
METHODS AND MATERIALS
Forty contact lens wearers were recruited through the University of New South Wales, Sydney, Australia. The study procedures followed the tenets of the Declaration of Helsinki 2000 and were approved by the institutional ethics committee. The study was registered at Australian New Zealand Clinical Trials Registry (ANZCTR; registration number: ACTRN12610000662066). Each participant signed an informed consent form before enrolment in the study. Inclusion criteria were that participants were at least 16 years of age, wore soft contact lenses on a daily basis for at least 5 d/week, and were amenable to use a flat well style lens case for storing their contact lenses during the course of the study.
Contact Lens Care System
AMO COMPLETE EasyRub (Advanced Medical Optics, Dublin 4, Ireland), which consists of polyhexamethylene biguanide 0.0001% as preservative, poloxamer 237 0.05% as surfactant, edentate disodium, and a phosphate buffer system [sodium chloride, potassium chloride, sodium phosphate dibasic (heptahydrate), and sodium phosphate monobasic (monohydrate)], was used as the lens care solution. The lens case that accompanies AMO COMPLETE EasyRub solution was used for storing the lens. This case has a smooth non-ridged base and is made from acrylonitrile butadiene styrene, with lids made from polypropylene. Participants were advised to use the lenses of their choice on a daily wear modality and given AMO COMPLETE EasyRub and AMO lens storage case for 1 month after lens case hygiene instructions. Participants returned the cases and solutions to the clinic on their next visit. AMO COMPLETE EasyRub was chosen because it is commonly used20,21 and commercially available in practice.
Contact Lens Case Hygiene Guidelines
Participants were assigned to either use the alternative lens case cleaning guideline or the manufacturers' guideline according to a randomization table. A researcher explained and demonstrated the assigned contact lens case cleaning guidelines at the beginning of each phase. A contact lens case cleaning instruction sheet describing the lens case cleaning steps was given to participants at the beginning of each phase (Fig. 1). A sticker reinforcing the cleaning procedures was placed on the solution bottle to act as a daily reminder. The manufacturers' guideline was to rinse the lens case with supplied MPDS and air-dry face down; the alternative guideline was to rub lens cases in the presence of MPDS for 5 s, followed by wiping the lens cases with a clean tissue, and air-drying the lens case face down. No specific tissue-brand was nominated for this procedure. Participants were masked to whether they were following the alternative guidelines or the manufacturer's guidelines; rather, they were instructed to follow the guidelines as outlined on their instruction sheet.
A randomized cross-over clinical trial (n = 40) was performed (Fig. 2) to assess the effectiveness of the contact lens case hygiene guidelines. The total duration of the study was 2 months, with each phase lasting for 1 month. Participants were randomly allocated to two parallel groups, and each group was assigned to use either the manufacturer's guideline or the alternative guideline for the first month, and then switched to the other guideline for the second month. At the end of each month, participants were asked to return their lens cases and lens care solution. The used lens cases were sent to a microbiological laboratory within 2 h of collection for microbial investigation. The solution usage was recorded by measuring the amount in millilitres of the remaining solution in the bottle. The habitual contact lens cases and solution of participants were also collected at baseline for microbial investigation.
To assess whether participants followed the instructions assigned to them, a self-administered contact lens hygiene practice questionnaire was filled out by participants on three occasions (initial visit as a baseline, phase 1 and phase 2) to monitor compliance. The questionnaire included questions on whether participants rubbed and rinsed their case, and the position of the lens cases while air drying. Compliance was also monitored by recording whether the lens case was dry on collection. Participants' solution bottles were also collected at the end of each phase to calculate the volume of solution used.
Microbial Analysis of Contact Lens Cases
Microbial sampling of lens cases was performed within 2 h of lens case collection. The lens cases were opened under aseptic conditions. Only the right compartment was sampled, as previous studies had shown that both wells of the lens cases usually yield identical organisms.2,22 Before sampling, the status of the lens case well (wet or dry) was recorded. Any solution in the case was discarded, and the entire internal well of the right compartment was rubbed with a sterile calcium alginate swab premoistened with phosphate buffered saline. The calcium alginate swabs were then vortexed in phosphate-buffered saline containing 1% hexametaphosphate for ∼5 s. Aliquots (0.4 mL) were plated onto “chocolate” agar (Oxoid Australia, Australia) and Sabouraud Dextrose agar plates containing chloramphenicol and gentamicin (Oxoid Australia). Chocolate agar plates were incubated in 5% CO2 at 37°C for 3 days for the recovery of bacteria. Sabouraud plates were incubated at room temperature for 7 days for the recovery of fungi.
After the incubation period, the morphologically unique colony forming units (CFU) were counted. Representative bacterial colonies of each morphological type were streaked on purity plates (chocolate agar), incubated for 24 h, and Gram stained. Bacteria were identified as either Gram positive or Gram negative and not further identified. Positive cultures on Sabouraud plates were identified as either yeasts or mold based on morphology. All contact lens cases collected were given a unique code and researchers were masked for microbial sampling and identification.
Based on a pilot study, a sample size of 33 participants was required to detect 1 log CFU difference in microbial recovery between the manufacturers' and the alternative guidelines (power of 80% and statistical significance level of 0.05). Analysis of the categorical data was performed using non-parametric comparisons by either Pearson chi-square or Fisher exact test. Binary outcome comparisons and the analysis of compliance level between related samples were compared using the McNemar test. The level of contamination between related samples was analyzed using the Wilcoxon signed rank test. All statistical analyses were performed using SPSS v14.0 statistical software.
A total of 40 participants were recruited in the study, with 78% (31/40) female. The ages of the participants ranged from 17 to 32 years with an average age of 24 ± 4 years. The majority of the participants were university students. The study population consisted of experienced contact lens wearers, 49% (17/35) of the participants had been wearing contact lenses for 5 to 10 years. Five participants (5/40) failed to provide baseline data but completed both phases. At the end of the study, there was one lost to follow-up in each phase. Both participants were unable to return to the clinic to hand in their lens cases and solutions as scheduled. Thirty-eight participants completed both phases of the study.
Baseline Case Contamination
The majority of the participants used multipurpose solutions for their lens care before the study, with only one participant using a hydrogen peroxide-based disinfecting solution. Twenty-nine percent (10/35) of the contact lens cases were more than 3 month old (Fig. 3).
The overall rate of contamination of the baseline contact lens cases was 77% (27/35). The median CFU was found to be 20 (range, 0 to 106). The most commonly recovered bacterial types were Gram-positive bacteria (70%), followed by Gram-negative bacteria (22%), and fungi (8%). The level of microbial contamination of cases at baseline was not significantly different between the two groups of participants who were randomly allocated to either phase of the study (p = 0.69).
Participants' hygiene behavior at baseline is shown in Table 1. Ninety-one percent of lens wearers reported washing hands before lens handling; however, only 57% reported using soap during hand washing. Less than one-half participants rinsed or air-dried lens cases after use.
Alternative Guidelines vs. Manufacturer's Guidelines
The levels of contamination recovered from lens cases using each of the two cleaning guidelines are given in Table 2. Overall, the level of contamination was significantly lower with use of the alternative guidelines (median CFU: 12, range of 0 to 104 CFU) compared with the manufacturers' guidelines (median CFU: 28, range of 0 to 105 CFU; p = 0.004). The rate of contamination for the alternative guidelines was found to be 72% compared with the manufacturer's guidelines 82%, but this difference did not reach statistical significance (p = 0.21). Frequency of different types of microorganisms recovered in both guidelines is shown in Table 2. For those participants who had been assigned first to the alternative guideline phase, when they were crossed over to the manufacturer's guidelines, their lens cases had significantly lower level of lens case contamination using the manufacturers guidelines than those who were assigned to the manufacturer's guideline phase initially (p = 0.03).
Sixteen percent of lens cases were dry on receiving in the manufacturers' guidelines compared with 60% received dry in the alternative guidelines. Lens cases received wet in the manufacture's guideline had significant higher level of bacteria than those lens cases received dry in both guidelines (p < 0.04; Fig. 4). The levels of bacterial recovery between wet and dry conditions were not significantly different in the alternative guideline (p > 0.05).
Use of Disinfecting Solution
During the two phases, the solution usage in the alternative guideline was on average 200 ± 112 ml compared with 178 ± 99 ml when the manufacturer's guideline was used and no statistically significant difference was found (p = 0.098). In both guidelines, participants who used more than 180 ml (half a bottle) per month had consistently lower levels of lens case contamination than those who used less than half a bottle, although this was not statistically significant (p = 0.092). Using the alternative guidelines, a greater proportion of sterile lens cases were found for participants who used more than 180 mL/month (p = 0.043).
The hygiene behaviors of participants in each guideline are listed in Table 3. Apart from specified cleaning methods, the other hygiene behaviors were relatively similar between the alternative guidelines and the manufacturer's guidelines (p > 0.05). Two participants rubbed the lens cases in the manufacturer's guideline despite this not being instructed.
This study evaluated and compared the effectiveness of an alternative and current manufactures' lens case cleaning guidelines in asymptomatic contact lens wearers. The alternative guidelines resulted in a lower level of microbial contamination compared with the manufacturer's guidelines, which is consistent with previous in vitro findings.17 The baseline contamination rate of the contact lens cases collected in this study was 77%, which falls within the range of 19 to 85% in the reported literature.1,2,23–26
As this study specifically aimed to compare the effectiveness of two lens case cleaning guidelines, critical cleaning of contact lens case was reinforced using several strategies, such as lens case cleaning demonstrations by a researcher, take-home instructions sheets, and instruction stickers on solution bottles. Additionally, both subjective and objective approaches were adopted to evaluate the level of compliance, such as self-reported questionnaires, lens case status (wet or dry on receipt), and solution usage. It was anticipated that participants' compliance to the alternative guidelines would be lower owing to the additional cleaning steps (rubbing and tissue-wiping) involved in the procedure. In fact, no significant difference in the level of compliance was found between the alternative guidelines and manufacturer's guidelines based on the hygiene questionnaire. Other non-reinforced hygiene aspects such as rinsing and rubbing of contact lenses remained relatively consistent throughout the study and their compliance levels were comparable to those in published literature21,27,28 as no attempt was made to modify participants' habitual hygiene behaviors not directly related to lens cases. These steps were taken to increase the chance that any changes in the level of lens case contamination could be attributed to differences in two guidelines.
Despite participants being requested to air-dry their lens cases, more than one-half the lens cases were wet on receipt when participants were following the manufacturer's guideline. This could be due to the majority of lens cases being collected shortly after contact lenses insertion to the eyes, thus, not allowing enough time for the lens case to air dry. Our result showed that wet lens cases collected from participants following the manufacturers' guidelines were significantly more frequently contaminated than cases received dry. This is in alignment with epidemiological data showing that not air-drying lens cases is a risk factor for microbial keratitis.29 Of note, “dry or wet” state of a lens case did not make any significant difference in the level of microorganisms recovered in the alternative guidelines. It is possible that in the alternative guideline rubbing and wiping lens cases may have dislodged a significant amount of contaminants, therefore the remaining level of microorganisms between wet or completely dry state was not statistically significant.
Minor differences in the composition of different brands of tissue should not undermine the proof of principle of this work; therefore, participants were advised to use a clean tissue paper to wipe lens cases without brand specification, as this would be more practical for lens wearers. It was speculated that tissue-wiping lens cases as part of the alternative guideline regimen might shed fibers in the lens storage case and result in potential adverse responses in lens wearers. However, on study completion, no adverse reactions were reported.
Solution volume data demonstrated that a greater number of sterile lens cases were found in participants who used more than half a 360 mL bottle per month in the alternative guideline. We would speculate that the reduction of recoverable microbes was due to a longer rinse of the lens case while rubbing or a higher filling level of the lens case during the soaking cycle of contact lenses as previously suggested.20 At the end of each phase, none of the participants reported running out of the solution (360 mL) and the alternative guidelines did not result in greater consumption of the disinfecting solution than the manufacturer's guideline over this 1-month period.
There was no significant difference in the level of microbial contamination of cases at the baseline between the two groups, hence the initial randomization appeared effective. Of note, the group following the manufacturers' guidelines in the second phase of the study had significantly lower levels of contamination compared with the group of subjects who undertook this regimen first. This could be a carry-over effect as a wash out period was not included. However, compliance levels of lens case cleaning behaviors were similar between the two groups.
For those contact lens case cleaning instructions reinforced in our study, positive behavior changes (e.g., rub and rinse lens case, and air-dry face down) were observed. This highlights the significance of describing detailed hygiene practices when advising lens wearers in clinical practice. It is possible that the compliance level may drop off over time, Therefore, periodic reinforcement of hygiene practices might be warranted to ensure maintenance of good hygiene behavior.27,30
The clinical relevance of the lens case bioburden reduced by the alternative guideline is yet to be established. Some contact lens disinfecting solution product inserts recommend lens cases be replaced every month, and this forms the basis of our evaluation time point. It is worth keeping in mind that if contact lens cases had been used for longer than 1 month, a common practice in lens wearers (Fig. 3),20,27,31,32 the level of contamination reduced by following the alternative guideline might differ from the 1 month's results.
The solution used in the study is a commercially available cleaning system, which consisted of a lens case with smooth internal surface. We are aware that the effectiveness of applying shearing force (rubbing and tissue-wiping) in removing contaminants may vary according to the lens case internal surface.17 Further research is needed to determine the effectiveness of the alternative guidelines in wider demographics and using a variety of cleaning products.
This study demonstrated that the alternative cleaning guidelines, incorporating rubbing and tissue-wiping of lens cases is more effective in removing contaminants from lens cases than the current manufacturer's guidelines. The findings of this study should benefit practitioners by enabling them to give evidence-based advice on how to clean contact lens cases and potentially reduce the incidence of contact lens-related inflammatory events and infection associated with contact lens case contamination.
Brien Holden Vision institute
Level 4, North Wing Rupert Myers Building Gate 14, Barker St
University of New South Wales
Sydney, 2052 New South Wales
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