Update on Microbial Keratitis with Silicone Hydrogels

Introduction

According to the results of a recent poll conducted on this website, 45% of 432 respondents have seen microbial keratitis (MK) with silicone hydrogel lens wear. This percentage was much higher than expected. Data collected by the CCLRU (see below) indicate that the incidence of MK is much lower with silicone hydrogel lens wear. This article provides an explanation as to why we think these numbers differ.

What We Know About the Incidence of MK with Silicone Hydrogel Lenses

Although the high oxygen transmissibility of silicone hydrogel lenses is thought to contribute to a decreased risk of microbial keratitis (MK) with contact lens wear, MK still occurs with this lens type. Practitioners must always remain alert to the potential signs and symptoms of adverse events and must continue to educate their patients on the necessity for compliance to their wear schedule and hygiene procedures, irrespective of lens type or wear modality.

As detailed in this month's editorial, " CW worldwide, an update on silicone hydrogel use" , the use of silicone hydrogels has doubled over the past year, making up 16% of the world-wide contact lens market. Over this time, the CCRLU, with enormous help from the contact lens industry, has continued to actively collate anecdotal reports of MK from practitioners and members of the ophthalmic community at international meetings. The CCLRU is also able to include cases identified through pilot surveillance studies conducted in the UK, and in Australia and New Zealand. To date we are aware of 57 cases of MK, with the majority coming from the United Kingdom, Australia, and the USA (Figure 1). Of the 40 cases for which wear schedule is known, 35 were wearing silicone hydrogels on an extended or continuous wear basis and 5 were wearing their lenses for daily wear.

Figure 1

The relatively well-defined nature of the contact lens wearing population in Australia enables us to estimate the incidence of MK with some degree of confidence by consulting anecdotal reports and current lens sales data. Using 55,000 wearer eye years as a denominator, we estimate that the incidence of infection with silicone hydrogels is 1 in 3235 wearers per year. It is certain, however, that these numbers have been underestimated; we should be able to provide an accurate estimate of the risk of MK with silicone hydrogels once we see the results of post-market studies being conducted by CIBA Vision and Bausch & Lomb, and population-based studies currently being conducted in Australia, New Zealand and the UK.

The visual outcomes for the 57 patients reported through CCLRU surveillance are:

•  20: No change
•  4: Loss of one line of VA or less
•  5: Loss of 2 lines
•  2: Keratoplasty required
•  26: No data

What We Know About the Incidence of MK with Daily, Disposable Lens Wear

Daily disposable lens wear is generally considered to be the safest form of daily wear, though no population-based surveillance studies have been conducted to assess the relative risk of this modality and anecdotal surveys suggest that quite a large number of practitioners have seen MK with daily disposable lens wear (Figure 2). This is far greater than the established incidence of MK with daily wear of soft lenses would suggest (1:5000).

Figure 2

This higher than expected rate of MK with daily disposables may be a result of the potential for misdiagnosis of MK, particularly because of the many similarities between MK and contact lens induced peripheral ulcer (CLPU). Although practitioners must always be conservative in the way they diagnose and manage MK, it is also important that they learn to be confident in their ability to differentiate between this event and non-infectious adverse events such as CLPU. Accurate diagnosis of adverse events associated with contact lens wear contributes to improved patient management and satisfaction, decreased chair-time, and more efficient reporting. See THE GUIDE TO DIFFERENTIAL DIAGNOSIS HERE - PDF 317KB

MK vs. CLPU

The most prominent difference between MK and CLPU is the aggressive worsening of signs and symptoms with MK after the removal of lenses compared to an improvement in signs and symptoms with CLPU. Although MK is often seen as a large, irregular lesion in the central or paracentral region with diffuse infiltration and small satellite lesions, it can not be categorically defined by one sign. Infectious lesions can occur in the peripheral regions of the cornea and may be small, depending on the stage at which a patient presents. CLPU, on the other hand, is almost always characterized by a small and circular focal infiltrate in the periphery to mid-periphery of the cornea and may have surrounding diffuse infiltration. More detail on the differentiation between these two events is described in Clinical Diagnosis of Microbial Keratitis and Contact Lens-Induced Peripheral Ulcer . Any patient that presents with a central infiltrate and overlying loss of epithelium or that has irregularly-shaped focal infiltrates with raised edges or satellite lesions should be treated as if they have been diagnosed with MK until proven otherwise. See Management Algorithm for Contact Lens-Associated Bacterial Keratitis.

The threat of MK should not deter any practitioners from prescribing contact lens wear, though they should also never become complacent. Practitioner vigilance and good patient education are essential components of practitioners' duty to ensure an excellent standard of care.