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Two major studies examining microbial keratitis (MK) in contact lens wearers are about to be published in a forthcoming issue of Ophthalmology.

Both look at the potential risk factors among a population attending eye hospitals and are aimed at studying the contact lens population now that silicone hydrogels (SiH) and daily disposable lenses (DDs) have started making inroads into the market. It might be assumed that both these modalities might decrease the incidence of MK, but that does not seem to be the case.

Indeed, not only does MK of any severity appear more likely among SiH and DD lens wearers, it also appears with sufficient frequency to suggest significant non-compliance with instruction on the part of the patient.

UK study

The first study, led by John Dart, looked at a population of 367 contact lens wearers attending Moorfields Eye Hospital with proven or presumed MK.

For comparison there were control groups comprised of 1,069 patients with hospital conditions unrelated to contact lenses and 639 contact lens wearers from the local area.

Rather controversially for some, the study claims to be the first to look at infection rates among a large population including data for SiH and daily disposables. The preceding study at Manchester Royal Eye Hospital by Efron and Morgan is not paid much heed.

In looking at the potential risk of MK for different types of lens, a number of risk factors were built into the regression model, such as demographics, lens wear, type and disposal, aftercare frequency, showering and smoking. Planned replacement soft lenses were taken as the referent to which infection risk rates were compared.

Contrary to what might be expected, DDs carried a 1.5 times increased risk of MK of all severities compared to planned replacement soft lenses. For severe to moderate MK the difference was negligible.

As might be expected, RGP lenses showed a five times reduced risk. SiH lenses were found to have no significant influence on MK rates.

Sub-group analysis looked at the risk of MK among different types of DDs. Bearing in mind that the study began in 2003 when the formulation of some DDs differs from that today, CIBA Dailies showed a 3.24 times increased risk of MK, while B&L Soflens One Day gave a 2.48 times increased risk and other (own brand), a 2.1 increased risk. Acuvue 1-Day was found to have a reduced risk of 0.68 times compared to planned replacement soft lenses. When just the severe and moderate MK cases were looked at, the differences remained the same, except for the Soflens which now showed insignificant difference in risk terms from the referent lenses.

Planned replacement lenses used only on a daily wear basis showed no difference in risk. When all the planned replacement lenses were looked at - including those used for occasional overnight and overnight wear - differences existed with Acuvue 2, showing a 3.47 times increased risk, Biomedics 55 UV a 3.47 times increased risk, while Soflens 66 showed significantly less risk.

So there is a significant increased risk of MK with DD lenses. This is likely related to the remarkable finding that 30 per cent of DD lens users are using lenses for occasional or regular overnight wear, an 'off-indication use' that increases the risk of MK.

The difference in risk between brand types of DDs suggests contact lens design and/or polymer factors. It is suggested that the increase in risk for CIBA Dailies may be due to the relative difficulty in removal of this lens resulting in corneal compromise.

Newer versions of this lens include a number of moisturising components which are likely to reduce this effect.

Australian study

The UK study highlighted a number of risk factors for MK, such as not washing hands (1.49 times), wearing lenses for cosmetic purposes only (2.19 times risk), male gender (1.48 times) and overnight wear.

A study from Sydney, however, looked at 285 cases of MK and 1,798 controls to identify the most likely risk factors.

Again the paper cites itself as 'the first study to describe the incidence and risks associated with contemporary contact lens types', despite the prominence of other studies such as the Manchester Hospital research. Wearing daily wear conventional hydrogels showed a very low incidence of MK (1.9 to 2 per 10,000).

After controlling for other risk factors, use of SiH had a higher, but not statistically significant, risk of MK than hydrogels.

This study showed no increased or decreased risk of MK with DD lenses compared with planned replacement daily wear (whereas the UK study showed increased risk for DDs).

Other risk factors included where the lenses were bought, with internet sale lenses showing a 4.76 increased MK risk than when bought from an optometrist. This may merely reflect lack of compliance or care with this patient group.

A puzzling finding was that the patients in a higher socio-economic group showed a 2.66 times increased risk than those in lower socio-economic groups.

So, in summary, overnight wear is still the major risk factor for developing microbial keratitis.

The improvements in infection rates that might have been expected from the phasing in of daily disposable lenses and silicone hydrogel lenses appear not to exist. Compliance with instructions is key in avoiding infection.

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