Risk Factors for Microbial Keratitis in Contact Lens Wearers

Risk Factors for Microbial Keratitis in Contact Lens Wearers

Risk Factors for Microbial Keratitis in Contact Lens Wearers

By Loretta B. Szczotka-Flynn, OD, PhD, and Kay Downer

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Although relatively rare, microbial keratitis remains a serious complication of contact lens wear. Fortunately, clinicians can reduce the chance of infection by understanding the factors that raise infection risk and taking steps to avoid them. Key elements in risk reduction include selecting a safe lens wear modality and thoroughly educating patients on proper lens hygiene.

Natural Protection Against Infection

Although the eye is continually exposed to environmental pathogens, ocular infection is rare, in large part because of the eye’s natural mechanical, chemical, and immunological defenses. These work together to successfully protect the eye against the vast majority of insults.
   
The tear film forms the outermost barrier protecting the eye; it can rinse away pathogens or sequester them so they cannot cause harm. In addition, enzymes and other proteins floating in the tear film can kill harmful organisms, sometimes on contact.
   
If an organism survives the tear film, the epithelium acts as another barrier. Because of the structure of the epithelium and basal lamina, organisms typically cannot penetrate; thus microbial keratitis is much more likely to occur when there is a break in the epithelium, as from a deep scratch or other corneal injury.
   
In addition to serving as a mechanical barrier, the epithelium also contains Toll-like receptors that are activated by live bacteria or bacterial products such as lipopolysaccharides. When these receptors are activated, they trigger an influx of white blood cells that attack the offending pathogen. The complement system can also be triggered by the epithelium and can provide additional immunological defenses.

Risk Factors

While these defense mechanisms are usually effective, a number of factors can increase the risk of microbial keratitis. Contact lens wear is a major risk factor, of course, but the degree of risk varies depending on the type and mode of lens wear and various patient factors. This article will focus on factors that influence the risk of microbial keratitis among contact lens wearers.

• A break in the epithelium Because the epithelium plays a key role in protecting the eye, any breach in this tissue can significantly increase a patient’s risk of infection. In animal models, microbial keratitis can be induced by inoculation and corneal wounding. However, certain cytotoxic and intracellular bacterial strains may produce their own virulence factors which damage the epithelium and create infection without overt corneal injury.¹

• Overnight lens wear From an epidemiological perspective, overnight lens wear remains one of the most significant risk factors for microbial keratitis. According to a recent study by Stapleton and coworkers, the annualized incidence of microbial keratitis is 19.5 cases per 10,000 among non-silicone hydrogel soft contact lens wearers who wear their lenses overnight, compared to 1.9 cases per 10,000 among non-silicone hydrogel daily-wear soft contact lens wearers.2 The reason for this increased risk is likely that overnight wear allows a stagnant tear layer to collect under the lens, potentially trapping microbes against the corneal surface for an extended period.

• Purchasing lenses via the Internet Interestingly, the Stapleton study found that patients who purchased their lenses over the Internet had a 4.8 times greater risk of developing microbial keratitis compared to patients who purchased their lenses at a doctor’s office. While the lenses themselves are no different, it may be that many of the patients who purchased their lenses online or via mail order did so to avoid an eye exam or their eyecare provider. If this is the case, these patients are not as likely to be reeducated about lens care and hygiene, which may lead to poor lens care habits.

• Poor hygiene Clinicians have long known that poor hygiene contributes to the risk of microbial keratitis, and recent studies have confirmed this finding. In the Stapleton study, poor storage case hygiene increased the risk of microbial keratitis by a factor of approximately 3.7. Biofilms that harbor resistant organisms can develop within patients’ lens cases and can transfer organisms to lenses soaked within the case. In another study, Dart and coworkers found that patients who did not always wash their hands before handling their lenses were about 1.5 times more likely to develop microbial keratitis.3

• Smoking Identified as a risk factor as early as the late 1980s, smoking is believed to triple the risk of microbial keratitis.2 While there are probably a number of reasons for this association, possible explanations include the impact that toxins in smoke have on the ocular surface, changes in the immune system related to smoking, and possible correlations between smoking and poor overall hygiene.

• Being new to lens wear Finally, the Stapleton study reported that patients newer to lens wear were at greater risk of developing microbial keratitis than patients who had worn lenses for 6 months or longer. This finding corresponds well with studies by Ren and coworkers in which the authors found that there was greater binding of Pseudomonas aeruginosa to exfoliated corneal epithelial cells during the first 3 months of soft lens extended wear.4 Such data suggests that the corneal epithelium undergoes an adaptive change in response to contact lens wear, which may reduce the risk of infection over time.

Preventing Microbial Keratitis
   
Knowing the risk factors for microbial keratitis gives us a good place to start when developing strategies for prevention. While some of the risk factors listed above are difficult or impossible to avoid, many can be reduced by selecting a safer lens modality and educating patients about the importance of proper lens care and hygiene.
   
In terms of lens selection, rigid gas permeable (GP) lenses are associated with a much lower rate of microbial keratitis than soft lenses, making them an excellent option for patients who want the safest lenses possible. While there are probably many reasons for this safety advantage, the constant tear flushing that occurs when rigid lenses move on the eye likely plays a significant role in reducing the risk of infection.
   
For patients who prefer soft lenses, clinicians can reduce the risk of microbial keratitis by keeping the patient in daily rather than extended wear. Daily disposable lenses may also be of benefit for some patients, although researchers have found no safety advantage for daily disposable lenses compared to soft lenses that are replaced at intervals such as 2 weeks.^2,3 However, disposable lenses make it easier for patients to comply with their lens care routine, which may make these lenses a better option for individuals who would otherwise have difficulty with lens care.
   
Finally, proper lens hygiene is essential to reduce the risk of microbial keratitis. Patients need to wash their hands before handling their lenses, keep their lens cases clean, replace their cases at regular intervals, and clean their lenses carefully. Since peroxide cleaning systems are most effective against Acanthamoeba, they are recommended for patients who are at highest risk for microbial keratitis.
   
Alternatively, if patients use a multipurpose solution, they should use a “rub and rinse” technique (even for solutions labeled as “no-rub”). Patients should also know to use fresh solution every time they store their lenses and allow the lens case to dry between uses. Also, since exposure to tap water may be a risk factor for Acanthamoeba, patients need to wash and dry their hands before handling their lenses, use only lens solution—not water—to clean the case, and avoid swimming or showering in their lenses.

THE BOTTOM LINE
A rare but serious complication of contact lens wear, microbial keratitis is associated with a number of risk factors, including a break in the epithelium, overnight lens wear, purchasing lenses over the Internet, poor lens case or hand hygiene, smoking, and being new to contact lens wear. While some of these risk factors are difficult to avoid, clinicians can make lens wear safer for many patients by selecting GP lenses or instructing soft lens patients to wear their lenses on a daily (rather than extended) wear basis, and by stressing proper lens care and hygiene.

Loretta Szczotka-Flynn, OD, PhD, is an associate professor in the department of ophthalmology and visual sciences at Case Western Reserve University and director of the contact lens service at the University Hospitals Eye Institute in Cleveland, OH. Kay Downer is senior editor for Refractive Eyecare.

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REFERENCES

1.    Choy MH, Stapleton F, Willcox MD, Zhu H. Comparison of virulence factors in Pseudomonas aeruginosa strains isolated from contact lens- and non-contact lens-related keratitis. J Med Microbiol. 2008 Dec;57(Pt 12):1539-46.
2.    Stapleton F, Keay L, Edwards K, et al. The incidence of contact lens-related microbial keratitis in Australia. Ophthalmology. 2008 Oct;115(10):1655-62.
3.    Dart JK, Radford CF, Minassian D, Verma S, Stapleton F. Risk factors for microbial keratitis with contemporary contact lenses: a case-control study. Ophthalmology. 2008 Oct;115(10):1647-54,1654.e1-3.
4.    Ren DH, Yamamoto K, Ladage PM, et al. Adaptive effects of 30-night wear of hyper-O(2) transmissible contact lenses on bacterial binding and corneal epithelium: a 1-year clinical trial. Ophthalmology. 2002 Jan;109(1):27-39;discussion 39-40.