Contact lenses are a safe and effective vision correction option and have positively impacted our patients’ lives for many years. However, there exists an ever present risk of corneal infection or microbial keratitis (MK), which occurs at higher rates in contact lens wearers compared with non-lens wearers who have no other risk factors. We will explore in this article what current research tells us about the risk of MK with contact lens wear in general and then relate this specifically to specialty contact lens wear.

Comparing Modalities of Contact Lens Wear and Risk of MK

The study that first opened our eyes to the relative risk of MK associated with contact lens wear was published in 1989 by Schein et al. The authors stated that ulcerative keratitis was the most serious adverse effect of contact lens use.

They conducted a case-control study estimating the relative risk of ulcerative keratitis among users of extended wear as compared with daily wear soft contact lenses. This relative risk among the population-based controls was 3.90 (95% confidence interval [CI], 2.35 to 6.48), and among the hospital-based controls it was 4.21 (95% CI, 1.95 to 9.08). Thirty-eight percent of the patients who wore extended-wear lenses used them only during the day, and 11% of those who wore daily wear lenses occasionally wore them overnight.

When lens wearers were distinguished according to their overnight use of lenses, the users of extended wear lenses who wore them overnight had a risk 10 to 15 times as great as the users of daily wear lenses who did not, and the users of daily wear lenses who sometimes wore them overnight had nine times the risk of the users of such lenses who did not. For the users of extended wear lenses, the risk of ulcerative keratitis was incrementally related to the extent of overnight wear. The authors concluded that soft contact lenses worn overnight carry a significantly greater risk for ulcerative keratitis compared with soft lenses worn only during the day.

Liesegang (1997) reviewed available epidemiologic data pertaining to contact lens-related MK. The incidence rates for bacterial MK range from approximately 2/10,000 per year for rigid contact lenses to 2.2 to 4.1/10,000 per year for daily wear soft contact lens to 13.3 to 20.9/10,000 per year for extended wear soft contact lenses. The risk with therapeutic contact lenses, however, was much higher: approximately 52/10,000 per year. The authors stated that the most significant risk factors include overnight wear, smoking, male gender, and socioeconomic status.

Confirming studies subsequently have been published that showed similar rates of MK. A study from the Netherlands (Cheng et al, 1999) found an estimated annualized incidence of MK of 1.1 per 10,000 (95% CI, 0.6 to 1.7) users of daily wear GP lenses, 3.5 per 10,000 (2.7 to 4.5) users of daily wear soft lenses, and 20.0 per 10,000 (10.3 to 35.0) users of extended wear soft lenses (p < 0.00001 for comparison between all groups).

A study from Australia (Stapleton et al, 2008) found that the annualized incidence of MK per 10,000 wearers was 1.2 (CI, 1.1 to 1.5) in daily wear GP wearers; 1.9 (CI, 1.8 to 2.0) in daily wear soft contact lens wearers; 2.2 (CI, 2.0 to 2.5) in soft contact lens wearers (occasional overnight use); 2.0 (CI, 1.7 to 2.4) in daily disposable contact lens wearers; 4.2 (CI, 3.1 to 6.6) in daily disposable contact lens wearers (occasional overnight use); 11.9 (CI, 10.0 to 14.6) in daily wear silicone hydrogel lens wearers; 5.5 (CI, 4.5 to 7.2) in silicone hydrogel lens wearers (occasional overnight use); 19.5 (CI, 14.6 to 29.5) in overnight wear soft contact lens wearers; and 25.4 (CI, 21.2 in 31.5) in overnight wear of silicone hydrogel contact lenses. Loss of vision occurred in 0.6 per 10,000 wearers.

Risk factors included overnight lens wear, poor storage case hygiene, smoking, internet purchase of contact lenses, < 6 months of wearing experience, and higher socioeconomic class. Although there have been significant developments in contact lens material science since the first MK incidence studies were published, it appears that the dramatic increase in oxygen permeability of contact lenses has had little effect on the incidence rates of MK.

The authors of this study stated that incidence estimates for soft contact lens use were similar to those previously reported. New lens types have not reduced the incidence of disease. Overnight use of any contact lens is associated with a higher risk compared to daily use.

In another study evaluating MK in wearers of high-Dk silicone hydrogel contact lenses worn for 30-day continuous wear (Schein et al, 2005), the authors found that the incidence of visual acuity loss due to MK among users of the silicone hydrogel contact lens was low; however, the overall rate of presumed MK with the wearing schedule of as many as 30 nights was similar to that previously reported for traditional extended wear soft lenses worn for fewer consecutive nights.

Defining and Understanding MK in Contact Lens Wear

Diagnostic criteria for MK can have a dramatic impact on study incidence rate outcomes. Efron and Morgan (2006) looked at this important issue. The authors applied a number of diagnostic criteria sets to a data set of cases and examined a variety of lens wearing modalities and lens types. They found that the choice of criteria for diagnosing contact lens-associated MK has a significant impact on calculations of the incidence of this condition.

An interesting study looked at the relative risk of silicone hydrogel overnight wear contact lens-associated MK as compared to other life risks (Szczotka-Flynn et al, 2009). A comparative ratio was defined as the incidence of MK in a population of silicone hydrogel extended wear contact lens users divided by the incidence of other disease or occurrence in a given population at risk.

The authors found that the risk of silicone hydrogel lens-related MK is about 1.5 to 16 times less risky compared to certain nonfatal disruptive occurrences in the general population, and it is about the same as the risk of developing breast cancer. Compared with other ocular conditions, the risk of MK with silicone hydrogel lenses is about the same as developing late-stage age-related macular degeneration or retinal detachment after cataract extraction on an annual basis; it is more than 200 times greater than developing eye or orbit cancer; it is about 7, 20, or more than 30 times less than proceeding to penetrating keratoplasty in keratoconus, developing nuclear cataract, or experiencing a corneal inflammatory event during low-Dk extended wear, respectively.

They concluded that although the risk of MK with modern day silicone hydrogel contact lens extended wear has not changed since the 1980s, when put in perspective with other life risks, it is a relatively rare occurrence.

What about those “Specialty” Contact Lenses?

We have quite a bit of data related to the incidence rates of MK in standard contact lens wear both for daily wear and extended wear; however, little is known about MK rates for what some would call “specialty” contact lenses. Again, a definition would be in order to identify what lenses would be classified as such. Lenses such as hybrid designs, scleral GPs, custom-designed soft lenses, and piggyback/tandem lens systems would arguably fall into this category. Unfortunately, there is virtually no information pertaining to MK rates for these types of lenses based upon well designed and controlled clinical trials.

One study looked at the performance of a high-Dk rigid GP lens design worn for up to 30 days of continuous wear (Albright et al, 2010). In this study of 507 participants over a 24-month follow-up period, there were no incidences of MK.

Another study looked at the success rate for scleral GP contact lenses fit for patients following penetrating keratoplasty (Severinsky et al, 2014). In this study, a total database of 31 consecutive patients fit with scleral GP lenses was retrospectively reviewed. Demographic data, etiology prior to lens fitting, visual outcomes, follow-up time, and complications were analyzed. The authors found that during the studied period, 10 eyes (30.0%) presented at least one graft rejection episode, and two eyes (6%) had an episode of MK.

One other study looked at the performance of a particular GP scleral lens used to treat patients who had persistent epithelial defects that did not respond to other traditional therapies (Ciralsky et al, 2015). The study was based on a retrospective review of eight eyes of seven consecutive patients who had persistent epithelial defects refractory to traditional therapies. The standardized treatment regimen consisted of 1) 24-hour scleral lens wear until re-epithelialization was achieved; 2) brief daily device removal, cleaning, disinfection, and reservoir fluid replacement; 3) addition of a benzalkonium chloride (BAK)-free fourth-generation fluoroquinolone antibiotic drop to the reservoir; and 4) transition to long-term, daytime scleral wear upon re-epithelialization.

All eight eyes exhibited resolution of the persistent epithelial defect. No eyes developed MK. Four eyes exhibited recurrences; all recurrences promptly responded to reinstitution of continuous wear.

Concluding Remarks

MK remains the most serious potential adverse event associated with contact lens wear. This topic has been significantly studied over the past few decades. Results indicate that the rates of MK, especially those that result in vision loss, are quite low and demonstrate an acceptable risk/reward ratio.

The most significant risk factor for MK in contact lens wear appears to be continuous wear of lenses (regardless of the degree of oxygen permeability). Other less impactful risk factors have also been identified. By controlling these risk factors, the likelihood of developing MK with contact lens wear is quite minimal.

The question remains, however, regarding the relative risk of MK with the use of specialty contact lenses, especially when fit on diseased eyes. The data on that topic is weak, and further study may be required. On the other hand, perhaps the lack of data is a result of MK rates that are too low to report. Only well-constructed long term studies will find an answer. CLS

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