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Abstract

A pilot study identified associations between keratitis and disposable soft contact lens (DSCL) wear (Matthews et al., 1992), and a large series of Acanthamoeba keratitis cases showed an increasing frequency in parallel with the growing popularity of these lenses (Bacon et al., 1993). A 12-month prospective case-control study was conducted to establish the relative risks (RR) of microbial keratitis, sterile keratitis and other types of contact lens (CL) complications amongst lens types and wear schedules currently available. A three-year retrospective case-control study was also performed, in order to evaluate lens type and other risk factors for Acanthamoeba keratitis (AK).

For both studies, CL wearers presenting as new casualty patients at Moorfields Eye Hospital during the study period completed a questionnaire detailing CL use and hy-giene practices. AK cases identified retrospectively were sent a postal questionnaire. Lens-related disorders were classified according to pathogenesis, and patients attending with disorders unrelated to CL wear were used as controls. RR with 95% confidence interval (Cl) and p values were calculated, and, for microbial, sterile and Acanthamoe-ba keratitis, multivariable analysis was performed. Daily wear (DW) and extended wear (EW) soft CLs were analysed separately, using conventional soft CL (SCL) as the referent.

In the prospective study there were 94 microbial keratitis cases, 174 sterile keratitis cases, 866 patients with other lens-related complications and 778 patients with disorders unrelated to lens wear. Multivariable analysis showed that, compared to conventional SCL worn with the same wear schedule, both DW and EW DSCL use carried an excess risk of microbial keratitis (RR: 3.51, 95% CL 1.60-7.66, p=0.002; RR: 4.76, 95% Cl: 1.52-14.87, p=0.007, respectively), and EW-DSCL use was also associated with an increased risk of sterile keratitis (RR: 3.53, 95% Cl: 1.01-12.28, p=0.048) com-pared to conventional EW-SCL. Both DW and EW DSCL showed a reduced risk of toxic keratitis compared to DW-SCL (RR: 0.41, 95% CL 0.18-0.82, p=0.009; RR: 0.14, 95% Cl: 0.00-0.84, p=0.023, respectively). For the remaining disease categor-ies there was no significant difference in risk between DSCL and conventional SCL, although small numbers limited statistical analysis. In the retrospective study, 31 AK cases and 240 controls were included in the multivarible analysis of risk factors amongst DW soft CL users. Although an excess risk of AK with DSCL was identified (RR: 3.82, 95% Cl: 1.01-14.48, p=0.049), the predominant risk factors were the use of chlorine-based or omitted disinfection (RR: 14.63, 95% Cl: 2.8-76, p=0.001; RR: 55.86, 95% Cl: 10.0-302, p<0.001, respectively), which were significantly more common amongst DSCL wearers.

These studies show that DSCL are associated with increased risks of suppurative kerati-tis, including Acanthamoeba keratitis, and appear to have a limited protective effect against other types of acute lens-related disorders. Patient and practitioner factors, such as the use of these lenses as a panacea for previous CL complications, and the possibil-ity of poor lens fitting standards as a result of the commercial pressure on practitioners to fit these lenses, may be contributing to these excess risks. It is likely, however, that properties of certain DSCL are partly responsible. Further research needs to be directed at the effects of their fitting characteristics, high level of manufacturing defects, inter-action with tear proteins, and increased in vivo dehydration on the likelihood of CL- related corneal infection. A UK populattion based incidence study is needed to deter-mine the absolute risk of microbial keratitis with CL wear modalities currently avail-able. Such data would enable practitioners and patients to make informed decisions regarding choice of CL use. Meanwhile, patients should be advised to minimise the risk by employing optimal disinfection and case hygiene routines, and by restricting over-night use to exceptional situations.

Publication Type:	Thesis (Doctoral)
Subjects:	R Medicine > RZ Other systems of medicine
Departments:	School of Health & Psychological Sciences > Optometry & Visual Sciences School of Health & Psychological Sciences > School of Health & Psychological Sciences Doctoral Theses Doctoral Theses

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