Contact Lens Care & Compliance

Defenses Against MK

By Michael A. Ward, MMSc, FAAO

Contact lens-associated microbial keratitis (MK) is rare, but remains a significant complication of contact lens wear. It has been spotlighted in recent years due to multipurpose solution (MPS)-associated corneal infections. Two major outbreaks occurred that were associated with MPS products: Fusarium keratitis in 2006, which was associated with ReNu with MoistureLoc (Bausch + Lomb), and Acanthamoeba keratitis in 2007, which was associated with Complete Moisture Plus (Abbott Medical Optics). These are no longer on the market.

The 2006 outbreak of Fusarium keratitis was the largest outbreak of any fungal keratitis on record, and it was MPS product-specific. The frequency of Fusarium keratitis has returned to the (very rare) pre-MPS-product-launch levels. The 2007 outbreaks of Acanthamoeba keratitis are less well understood, and the levels of Acanthamoeba keratitis do not seem to have returned to pre-MPS-product-launch levels.

Of all potential corneal pathogens, bacteria are the most rapidly destructive. Pseudomonas aeruginosa has remained the most common organism isolated in contact lens-associated MK.

When considering the significant percentages of lens cases that are contaminated with bacteria and other microbes (50 to 92 percent, depending on the author), it is a wonder that we don't see more ocular infections. Willcox et al (2010) reported case contamination ranges from 76 to 92 percent among daily wear silicone hydrogel wearers using a variety of common lens care products.

Natural Defenses

The cornea is rarely infected without trauma, local disease, or lens wear. Contact lens wear is not a sterile event: our eyes are not sterile, our fingers are not sterile, only new lenses are sterile—until they are touched.

Our innate ocular defenses provide the most significant barriers to infection. Our ocular defenses include:

1. An intact epithelial surface, which is the eye's primary defense against the outside world.
2. Anti-microbial activities of resident microbes; these are turf-protecting local floral bacteria that kill invading foreign bacteria.
3. Decreased ocular temperature; the ocular surface is cooler compared to the rest of the body, and pathogens typically live within narrow temperature ranges.
4. Mechanical actions of blinking coupled with the constant irrigation of the mucin-laden tear film that can trap particles and pass them into the lacrimal drainage system.
5. Tear components such as lysozyme and complement fixation/activation components that can activate microbe-lethal properties of our immune system.
6. Anti-microbial peptides (broad spectrum antibiotics) and toll-like receptors (TLR) on the ocular surface that can sense microbial components and activate cytokines and chemokines, which recruit neutrophils and other white blood cells and can thus modulate our immune system; over-stimulation of this system may lead to ocular tissue damage.

Patient and Practitioner Roles

Fortunately, current-generation dual disinfectant multipurpose solution and peroxide lens care products are fully competent against Fusarium, Acanthamoeba, and all bacterial challenge organisms.

However, just killing microbes is not the answer. Personal hygienic practices play a significant role in general infection risk. For contact lens wearers, this includes proper hand washing, not sleeping or swimming in lenses, daily rubbing and rinsing, use of fresh solutions, lens case care—all the points that we continue to stress. Practitioners need to also take some responsibility and only fit contact lenses on patients who have the capacity and desire to exercise personal responsibility for safe contact lens wear. CLS

For references, please visit www.clspectrum.com/references.asp and click on document #184.

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Mr. Ward is an instructor in ophthalmology at Emory University School of Medicine and Director, Emory Contact Lens Service. He is also a consultant to AMO and B+L. You can reach him at mward@emory.edu.