Microbial Keratitis and Silicone Hydrogels: Global Studies

Katie Edwards B App Sci (Optom), QUT, Australia Research Optometrist, Vision Cooperative Research Centre (Vision CRC)

Lisa Keay B Optom, UNSW, Australia Research Optometrist, Vision Cooperative Research Centre (Vision CRC)

The relatively recent release of high Dk silicone hydrogel lenses onto the global contact lens market has given patients a convenient, comfortable, and healthy alternative to other forms of vision correction. The elimination of hypoxia with overnight wear of contact lenses is a great accomplishment. Studies around the world, including many reported or reviewed here, have shown that overnight corneal edema levels with silicone hydrogels are considerably lower than those with other low Dk disposable soft lens materials and are in fact similar to the levels found with no lens wear. Predictably, markers of hypoxic stress, such as contact lens-induced corneal striae, microcysts and contact lens-induced endothelial polymegethism are rarely if ever seen with silicone hydrogels compared with extended wear of disposable contact lenses.

Many practitioners and patients alike, encouraged by the advantages of high Dk materials, have embraced the extended wear modality. Since their release a few years ago, we have seen an incredible growth in the use of silicone hydrogels around the world with contact lens sales data currently estimating the number of silicone hydrogel wearers at 900,000. In Australia, there was a 46% growth in value in the silicone hydrogel market in 2001/2002. Globally, silicone hydrogels now comprise 16% of the total contact lens market. However, as practitioners, we have an obligation to our patients to question whether this confidence in this new extended wear modality is warranted. It is widely predicted that the elimination of hypoxia, and it’s related complications, would result in the reduction of the most severe of all contact lens related adverse events, microbial keratitis, but is this true? A number a studies around the world are currently addressing this question.

Microbial Keratitis
As Dr Murali Aasuri discussed in last month’s editorial, microbial keratitis is the most serious of all adverse events encountered with contact lens wear, as it is the only one that is potentially sight threatening. Even in cases where there is no long term effect on visual acuity, there is usually significant pain and discomfort, and sometimes hospitalisation is required.

Historically, microbial keratitis is more prevalent with overnight wear of low Dk soft lenses, however, it can occur with any type of contact lens. While a number of studies indicate that silicone hydrogels have reduced the incidence of hypoxia related changes traditionally associated with lens wear1,2,3,4, and it is theorised that microbial keratitis will consequently also be reduced, there is no data yet available on the true rate of this condition.

The Studies
A number of studies are currently in progress throughout the world, designed to establish the incidence, and relative risk of microbial keratitis with silicone hydrogel lens wear and to determine the associated risk factors. This information is vitally important, not only because it will enable patients and practitioners to make accurate and informed decisions about lens wear but also to take appropriate measures to prevent infection.
As a condition of the USA Food and Drug Administration (FDA) approval for 30-day wear of silicone hydrogel lenses, both Bausch & Lomb and CIBA Vision are conducting studies to gather data regarding the risk of microbial keratitis. These studies will provide the earliest estimate of the incidence of microbial keratitis with 30-day wear of silicone hydrogels as well as information on risk factors and wearer behaviour.

While these studies will soon provide information on the risk of infection in extended wear, they were not designed to assess the risk of microbial keratitis in people wearing silicone hydrogels on shorter wear schedules or for daily wear or to assess the effect on longer term length extended wear. Also, they will not provide comparison with the risk for other lens types and modalities. Two international collaborations are currently underway to determine the relative risk and incidence of microbial keratitis with all modes of lens wear and all lens types.

In the United Kingdom, a study to determine the relative risk of microbial keratitis with silicone hydrogels compared to other lens types and modes of wear is being mounted. Over 24 months, a case-control study will be conducted at Moorfields Eye Hospital casualty department with hospital- and population-based controls.
In Australia and New Zealand a population-based study is underway to determine the incidence of microbial keratitis with all lens types and modalities. The number of cases of infection will be determined through active surveillance of all ophthalmologists and optometrists over 12 months. The number of wearers in the community will be established using a telephone survey. Silicone hydrogels have been available since 1999 in both Australia and New Zealand, and as such, they provide a great opportunity to collect clinical data on “real life” performance of these new products.
In the Australian and New Zealand study, information will also be collected on infection risk factors from both individuals with infections and the population-based controls. Historically, poor contact lens hygiene has contributed to the risk of infection in daily wear. This study will re-examine the relative impact of poor contact lens maintenance, non-compliance to manufacturers’ and practitioners’ advice on contact lens wear, as well as other factors including swimming in lenses, and the effect of environment.

CLPU vs MK
CLPU	MK

Outcomes
These studies will provide valuable data on the rate of microbial keratitis with all lenses and modes of wear, and will assess the risk factors associated with infection. Incidence rates for silicone hydrogel lenses worn overnight, as well as on a daily wear schedule will be determined. Infection rates with extended wear and daily wear of silicone hydrogel lenses will be compared to that with other contact lens modalities, including disposable, conventional and gas permeable contact lenses.

While we may never be able to completely eliminate contact lens related microbial keratitis, it is hoped these studies will confirm our confidence in silicone hydrogel lenses. Just as importantly, the evaluation of risk factors in these studies will also enable practitioners to advise patients about taking appropriate measures to prevent infection. The following 18 to 24 months will be an interesting and exciting time for practitioners with an interest in the fitting of silicone hydrogel lenses.

References

1. Cavanagh H, Ladage P, Li S, Yamamoto K, Molai M, Ren D, Petroll W, Jester J. Effects of daily and overnight wear of a novel hyper oxygen-transmissible soft contact lens on bacterial binding and corneal epithelium: a 13-month clinical trial. Ophthalmol 2002;109:1957-69.

2. Ren D, Yamamoto K, Ladage P, Molai M, Li S, Petroll W, Jester J, Cavanagh H. Adaptive effects of 30-night wear of hyper-O2 transmissible contact lenses on bacterial binding and corneal epithelium: a 1-year clinical trial. Ophthalmol 2002;109:27-40.

3. Keay L, Sweeney DF, Jalbert I, Skotnitsky C, Holden BA. Microcyst response to high Dk/t silicone hydrogel contact lenses. Optom Vis Sci 2000;77:582-5.

4. Dumbleton KA, Chalmers RL, Richter DB, Fonn D. Vascular response to extended wear of hydrogel lenses with high and low oxygen permeability. Optom Vis Sci 2001;78:147-51.