Cosmetic wear of contact lenses is increasing in Hong Kong. Contact lenses are presently considered to be worn by 6% of the population for visual correction of myopia. 1 This trend is expected to continue as 80% of the population are myopic and contact lens wear represents a relatively cheap and effective way to provide necessary visual correction for a technologically demanding lifestyle in Hong Kong.
A prospective study was conducted recently in Kowloon and Shatin to establish the incidence of microbial keratitis in Hong Kong within the community and the risk factor involved, in particular, with contact lens wear. 2 An overall incidence figure of expected microbial keratitis has been gained of 0.63 people per 10,000 in the general population and of 3.4 people per 10,000 lens wearers of all types. In addition, this study provided detailed microbiology of the organisms causing infectious keratitis within the sub-tropical climate of Hong Kong. 3
In order to investigate risk factors for CLWs within the prospective study, a separate nested case-control study was conducted to compare matched volunteer asymptomatic lens wearers (volunteers) with our keratitis patients. We took advantage of this situation to also conduct a Health Belief Model Questionnaire with the same population of lens wearers that were studied with a Health (hygiene) Practice Questionnaire. The aim was to consider if CLWs practiced hygiene as they believed they should, and if any differences could be identified between keratitis patients and controls (volunteers) that could be used to influence or educate lens wearers to avoid developing infectious keratitis.
To determine the risk factors for contact-lens-related microbial keratitis, a nested case-control study was carried out within our prospective cohort study 2 by recruiting 135 matched asymptomatic community-based CLW volunteers (with oral consent) from clients attending the laser in situ keratomileusis (LASIK) Clinic (102), the Eye Screening Clinic (12) and optical centers (with the help of the Hong Kong Optometric Association) (6). In addition, 15 hospital-based CLW staff (volunteers) were recruited at the Prince of Wales Hospital, Shatin. Volunteers were not known to represent any specific bias. Those lens-wearing volunteers who could be matched successfully to our CLW patients with microbial keratitis for age, sex, and educational status (graduate/nongraduate) were used as controls.
All CLW keratitis patients and controls (volunteers) were interviewed by the research assistant face to face using the same questionnaires that were designed to examine the use and the care of contact lenses as well as the components of our Health Belief Model (HBM) for perceived threats, barriers, benefits, and self-efficacy (Table 1). This model was based on a previous HBM of lens wear by Sokol et al. in 1990 4 and on another model used in a different speciality. 5
Questions on contact lens health practice (Table 2) included all aspects of contact lens hygiene. Other aspects of contact lens wear, such as wear while sleeping during the daytime and/or night-time, use of extended wear lenses and their relationship to infection, smoking and the use of different types of disinfecting solutions, are considered elsewhere. 2,3
Potential risk factors were evaluated for their association with microbial keratitis by univariate conditional logistic regression analyses. 6 Matched odds ratios, the corresponding 95% confidence intervals, and P values were calculated. Analyses were performed using the LogXact logistic regression software featuring exact methods, version 1.3 (CYTEL Software Corporation, Cambridge, MA).
All together, 206 CLW controls (volunteers) were recruited to take part in the study, of whom 135 were matched for age (<25 years of age [28%], 26–35 [46%], 36–45 [23%]and >45 [3%]), sex (male [20%], female [80%]) and educational status (graduate/nongraduate) with 45 CLW patients with microbial keratitis. 2,3 There was no significant difference in alcohol consumption, history of chronic illnesses or medication, income, housing type, duration of contact lens use, and visual acuity of the right eye. However, keratitis patients were significantly more likely to be smokers and to wear their lenses overnight while sleeping. 2
The responses to the questionnaire for Health Belief are given in Table 1 and compared between the keratitis patients (n = 45) and the matched control (volunteer) group (n = 135). The scores (1–5) in the HBM questions were grouped and analyzed as neutral (score of 3), positive (4 and 5) or negative (1 and 2) (see Table 1). There was no significant difference between keratitis patients and controls (volunteers) in the perceived benefits of optometrists’ instructions generally. When asked specifically, however, keratitis patients scored significantly higher than controls in the perceived benefit of checking with the optometrist initially for the correct method of lens cleaning (Question 2). CLW patients with keratitis were confident that their care of lenses would prevent complications (Question 7). Patients also scored significantly more neutrally in self-efficacy. Patients were significantly less likely to perceive the cost of lens care as a barrier (Question 13).
The responses for Health Practice are given in Table 2, excluding 4 keratitis patients and 7 controls (volunteers) who used daily disposable lenses and 12 controls with missing data. There was no significant difference between keratitis patients and matched controls (volunteers) in cleaning their lens storage cases or in the number who used protein tablets weekly. Keratitis patients, however, were significantly more likely to store their lenses overnight in tap water and not to leave the empty cases to air dry. 3
It should be noted that the response to the HBM questionnaire by CLW patients with microbial keratitis could only be obtained in practice after they had developed the infection. This experience could have made them more cautious and feel less self-efficacious when compared with the controls. 7 This is an element of bias that is impossible to exclude and needs to be borne in mind when considering the results.
For CLW controls (volunteers), the subjects came from selected groups within the community within the same cohort area as the patients; this included clients of the LASIK Clinic, with only 15 out of 135 being CLW hospital staff (volunteers). With the exception of visual acuity and smoking, 2 there was no significant difference in other demographic characteristics or socioeconomic factors tested between CLW keratitis patients and CLW controls (volunteers); it is believed that this latter group is not open to bias.
The development of microbial keratitis in contact lens wear is expected to be multifactorial. We have reported elsewhere in a multivariate conditional logistic regression analysis that that overnight use of lenses, poor compliance with cleaning procedures, and being a smoker are significant independent variables. 2 Our results for a statistically significant three-fold increase in incidence of microbial keratitis with overnight wear of contact lenses (daily wear 3.09 per 10,000 CLW; overnight wear 9.30 per 10,000 CLW) 2 was similar to that of Poggio et al. 8 in the New England (USA) cohort study, who found a significant five-fold increased rate (daily wear 4.2 per 10,000 CLW; overnight wear 21.8 per 10,000 CLW). Schein et al. 9 in the New England cohort study, researched the lens-care index based on the frequency of four basic hygiene tasks (disinfection, daily cleaning, rinsing, and enzymatic cleaning). A significant difference was just present (P = 0.05) between CLW keratitis patients and community-based controls; the evidence of a protective effect was strongest for cleaning the lens case. 9 In contrast, we found similar results for cleaning of the lens storage case between the keratitis patients and volunteer controls (Table 2) suggesting that this factor does not apply in Hong Kong.
We found smoking to be a risk factor for contact-lens-wear-associated microbial keratitis, 2 as did Schein et al. 9 in the New England cohort study, but this may represent a marker for other factors not studied. We speculated that CLWs use second and third fingers to hold the lenses when placing them on the cornea. The same two fingers are commonly used to hold cigarettes while smoking. It is not certain whether contamination with nicotine or other cigarette chemicals plays a role in the development of microbial keratitis among CLW smokers or whether this matter is related to other types of behavior.
Patients were more likely than controls (volunteers) to perform their lens care hygiene in the toilet or bathroom (Table 2). Although toilets and bathrooms harbor many water-borne organisms, including Pseudomonas aeruginosa and Acanthamoeba, and a clean, dry area is the preferable site microbiologically, it is certain that the proper use of contact lenses and the practice of hygiene in the care of contact lenses are most important in the prevention of contact-lens related infections.
There was no significant difference between keratitis patients and controls (volunteers) in the perceived benefits of optometrists’ instructions generally; however, when asked specifically, keratitis patients scored significantly higher than controls in the perceived benefit of checking with the optometrist initially for the correct method of lens cleaning. Patients also scored significantly more neutrally in self-efficacy. Nevertheless, controls (volunteers) were more likely than patients to practice, as advised, on the proper use and care of contact lenses, e.g., with regard to overnight wearing. Patients were also significantly more likely to store their lenses overnight in tap water and not to leave the empty cases to air dry, although there was no significant difference between the two groups in using tap water to clean the lenses/cases or in the number who used protein tablets weekly. 3 Thus, the apparently higher appreciation of official advice and neutrality in self-efficacy seen in our patient group could reflect a bias resulting from the fact that the HBM questionnaire was used after the diagnosis of microbial keratitis. 7 Storing lenses in tap water would reduce the cost of contact lens wear, but patients in our study significantly found cost a lesser barrier than the controls (volunteers) (Question 13, Table 1). Nevertheless, practitioners should be certain that people can afford the hygiene costs of looking after contact lenses and, if not, should fit them with spectacles instead. Disinfection of contact lenses has been found to be most effective with multipurpose solutions in both the UK 10 and Hong Kong. 3 The cost of these solutions can be considerable.
Contact lens related infection is multifactorial—smoking (usually heavy), extended wear of the lens with overnight sleeping, and lack of effective lens disinfection are all avoidable risk factors 2,3 The good news from this study is that both the majority of CLW keratitis patients and controls (volunteers) consider that advice given by their family, friends or news media (TV and radio) is relatively unimportant to them (Table 1, HBM questions 9–11, scoring 1–3). Thus, there is considerable opportunity for ophthalmologists and optometrists to educate and influence CLWs in the best ways to prevent infection. Hygiene instruction is of utmost importance today even though a new era has begun of extended-wear, high–oxygen-transmissible silicone-hydrogel lenses. The majority of CLWs will still use hydrogel lenses, some on a weekly disposable (reusable) basis, and care is required to reduce and prevent infection. Education can therefore be effective, and practitioners can improve their instructions to both new and old CLWs by printing booklets explaining clearly the “Dos” and “DO NOTs” of hygiene required to avoid infection.
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