Approximately 125 million people worldwide wear contact lenses.1 Apart from daily disposable and extended wear users, the rest of the lens wearers require disinfection of their contact lenses on a daily basis. Thus, hygiene practices are critical not only to contact lenses but also to contact lens cases.
Current contact lens case guidelines involve rinsing the cases with disinfecting solution and allowing them to air-dry.2 However, the majority of instructions available are not detailed or explicit, such as describing where to place the contact lens cases while air-drying, and whether to position the lens cases face up or face down. A review of current contact lens care guidelines3 has demonstrated that contact lens case hygiene instructions vary between advisory bodies, such as contact lens practitioners and industry sources, particularly regarding the contact lens case air-drying position and location.
Some optometrists recommend contact lens cases be stored away from the toilet/bathroom, while the Food and Drug Administration has recommended lens cases be positioned face down while air-drying.2 However, these recommendations have not been experimentally evaluated.
As most contact lens wearers carry out lens insertion and removal in the toilet/bathroom, it is likely that that the majority of the lens cases will be stored in these locations after use. Several studies have confirmed that the aerosol effect of toilet flushing results in airborne bacteria and viruses settle on adjacent surfaces, a considerable distance away.4–6 However, it has also been suggested that toilets actually have lower levels of bacterial contamination owing to the use of strong antiseptic cleaning agents compared to other areas.7 The conflicting and limited experimental evidence warrants investigation of the impact drying-environment has on lens case contamination. Such a study is necessary to develop clear and explicit instructions to promote better patient compliance8 and lens case hygiene.
This study was undertaken to evaluate the impact of air-drying lens cases face up and face down and in humid (toilet and bathroom) and non-humid (office and bedroom) environments. We hypothesized that lens cases air-dried face down have lower contamination rate that those lens cases air-dried face up.
Contact Lens Case Preparation
Ninety-seven unused contact lens cases (Alcon Ltd, Fort Worth, TX) were used in this study. Before the experiment, lens cases were autoclaved to ensure their sterility. Under aseptic conditions, one lens well (test) was opened and filled with 2 ml of sterile phosphate buffered saline (PBS). The PBS remained in the lens case for 5 min and was then removed. These lens cases were air-dried at room temperature in different locations: two humid (toilet/bathroom) and two non-humid (office and bedroom) environments. At each location, lens cases were further divided into two subgroups, positioned face up or face down on a clean facial tissue. The lids of the lens cases were retained in a sterile sealed plastic bag. After 24 hours, the contact lens cases were recapped and transported to the microbiology laboratory for microbial investigation. The microbial sampling was initiated within 2 h of lens case collection. The test well was sampled with a PBS-moistened calcium alginate swab. Swabs were vortexed in 2-ml PBS with 1% (w/v) sodium hexametaphosphate. Subsequently, 0.4-ml aliquots were spread onto chocolate agar and Sabouraud agar (Oxoid Australia Ltd, Australia) and incubated in 5% CO2 for 48 h at 37°C for bacterial and in air for 7 d in room temperature for fungus recovery. Each sample was given a unique code, and researchers were masked to the lens case air-drying positions and locations. Total numbers of viable organisms were enumerated and morphologically different bacterial colonies were identified, using standard techniques.9 Fungi (yeasts and molds) were identified by morphologies of their colonies on agar plates and their conidia.
The study was undertaken at two households and on two different desks. Each household had similar test settings: first, a toilet separated from the shower/bath facility and, second, a bathroom that did not contain a toilet. Lens cases were air-dried at different locations:
- Toilets: lens cases were placed on a wash basin within 1 meter of the toilet bowl.
- Bathrooms: lens cases were placed on a wash basin approximately 1.5 meters away from the shower.
- Bedrooms: lens cases were placed on a bedroom bench/dressing table.
- Office: lens cases were placed on two different desks.
The total number of viable organisms and the number for each microbial type were enumerated as colony forming unit/lens well. The differences in lens case contamination rates between drying conditions were analyzed using the χ2 test.
Based on a pilot study, a sample size of 43 in each group (humid and non-humid) was determined to be required to detect a 25% difference in the level of lens case contamination between humid and non-humid environment, with the significance level set at 0.05 and power = 0.8. In this study, a total of 97 samples (48 and 49 in each group) were included.
Lens cases positioned face up while air-drying had a significantly higher rate of contamination (34/48, 71%) compared with lens cases air-dried face down (6/49, 12%) (p < 0.001). For lens cases air-dried face up only, more contamination occurred when lens cases were placed in humid environments (toilet and bathroom) than in the non-humid environments (office and bedroom) (p = 0.01). However, the rate of lens case contamination between the locations was similar when lens cases were air-dried face down (Fig. 1) The contamination rate of lens cases placed at matching locations/positions between the two households showed no significant difference (p = 0.7) and neither was any difference seen between the two desks within one office (p = 0.8).
Total number of microorganisms recovered from lens cases ranged from 0 to 275 colony forming unit (median = 0 ± 10, mean = 12 ± 35) per lens well. The most frequently recovered microorganisms from the contaminated lens cases were coagulase-negative Staphylococci, followed by fungi, Bacillus spp. and Micrococcus spp (Table 1). Thirty-three percent (13/40) of lens cases were contaminated with multiple microorganisms (median = 1 type, maximum = 3 types).
DISCUSSION AND CONCLUSIONS
The current contact lens case hygiene guidelines from the various advisory bodies and manufacturers are a heterogeneous collection that may confuse lens wearers. In particular, there are limited guidelines regarding how to air-dry lens storage cases.3 To our knowledge, this is the first published study to evaluate the impact of air-drying lens cases in different positions and locations.
For the lens cases air-dried face up, they were exposed to the open air for 24 h; therefore, it is expected that air-borne contaminants had the opportunity to land in the lens well compared with lens cases positioned face down during air-drying. In addition, when lens cases are air-dried face down, not only does this minimize the exposure to air-borne contaminants but also promotes faster drying of the excessive water from lens wells as suggested by the Food and Drug Administration.2 Some concerns may arise in relation to the likelihood of promoting fungal growth because of lack of dynamic airflow when lens cases were positioned down. However, our study did not find any significant difference in the level of fungal recovery between the face up and face down group. We propose that even though the lens case was positioned face down, the rim of the lens case was not completely sealed off by the surface, and transient airflow is still permitted to an extent that did not encourage fungal growth.
Among the air-drying locations, lens cases air-dried face up in the toilet/bathroom were found more frequently contaminated than those in other locations in this study. For the toilet setting, lens cases were left to air-dry within 1 meter of the toilet bowl and lens cases air-dried face up showed the highest contamination rate. This is in agreement with the literature that an appreciable amount of droplet contamination is found in the vicinity of the toilet.4–6 Closing the toilet bowl lid only minimized the dispersal of larger-sized microorganisms5; smaller-sized microorganisms still managed to escape and remain suspended in the air for several hours.6 Furthermore, surface contamination was found up to 80 cm above the seat level,6 and the air-borne contaminants produced by the flushing were still present 12 min after the flush.5
As these lens cases were sterilized before the experiment, the number of recovered microorganisms is low and likely to be airborne microorganisms in the indoor environments. This finding is consistent with the literature that coagulase-negative Staphylococci, Bacillus spp. and Micrococcus spp. are the predominant organisms in indoor air10 and are common contaminants of lens cases during use.11–15
It is worth noting that if organic soil, biofilms, and nutrients exist as in a used lens case,16–18 the number of viable microorganisms may increase over time while the case is left air-dry. The clinical significance of this has not yet been established, and future studies may wish to include organic soil and rinse the lens case with disinfecting solution to better reflect actual in use conditions.
In the study, we did not note any significant differences in the level of cleanliness between each test locations. This perhaps minimized potential confounders (such as one location dirtier than the other) and presented a comparison solely on the impact of lens case air-drying environment (humid vs. non-humid) and positions (up vs. down) as we intended to do. However, we acknowledge that the level of cleanliness and quality of indoor air may be very diverse among households and workplaces in general.
In conclusion, this study specifically examines the effect of air-drying lens cases positioned up and down while placed in different environments: toilet, bathroom, office, and bedroom. Small numbers of microorganisms may contaminate lens cases from the environment while lens cases are air-dried, particularly when lens cases are air-dried face up. On the basis of this study, we would recommend that lens cases be air-dried face down to minimize airborne contamination.
We thank Francis Chen, BOptom (Ocularthera Gradcert), for his assistance in this study.
Institute for Eye Research
The University of New South Wales
Level 3, North Wing, Rupert Myers Building Gate 14
University of New South Wales
Sydney, 2052, NSW
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