Class act is a feature of Clinical Correlations written by NYU 3rd and 4th year medical students. These posts focus on evidenced based answers to clinical questions related to patients seen by our students in the clinics or on the wards. Prior to publication, each commentary is thoroughly reviewed for content by a faculty member.
Commentary by Frank Siringo, NYU Medical Student
Soft contact lens wear is the primary risk factor for microbial keratitis, a potentially vision-threatening infection of the cornea, with a relative risk of 80 compared to cases without a predisposing condition (1). Further compounding the risk is overnight contact lens wear, (aka “extended wear” or “continuous wear”), a fact that became evident in the 1980s during aftermarket studies of conventional soft hydrogel lenses, which had been approved for extended wear up to thirty days (2). The incidence of microbial keratitis in this population was found to be 58-100 cases per 10,000 wearers per year (3,4). This is in contrast to a risk of 0.5 cases per 10,000 per year in wearers who take their lenses out every day, (aka “daily wear”).
This experience drove contact lens companies to develop safer materials to decrease the risk of microbial keratitis in extended wear, especially since patient desire for this modality remained strong despite the risk (5). A main focus of research was contact lens oxygen transmissibility. Corneal epithelial hypoxia was thought to be a major risk factor for keratitis, via hypoxia-mediated epithelial devitalization as a route of microbe invasion (6). Silicone, long known to be a prodigious transmitter of oxygen, was then studied as a potential material for contact lens use.
The result was a new breed of soft contact lens, the silicone hydrogel. Oxygen transmissibility is up to four times that of conventional hydrogels (7), allowing near-physiologic oxygenation during extended wear. This market segment includes Focus Night and Day and O2 Optix by Ciba Vision, Acuvue Oasys and Advance by Vistakon, Pure Vision by Bausch and Lomb, and Bioffinity by Coopervision. All of these lenses are FDA approved for extended wear from one to four weeks, depending on brand. The hope was that the risk of microbial keratitis would approach that of daily contact lens wear.
A number of investigators have looked at microbial keratitis in extended wear use of silicone hydrogels. Two large studies with very different methodologies sought to determine the incidence of microbial keratitis over a twelve-month period. One was a multi-center study that relied on hundreds of clinicians and non-standardized assessments to determine clinical severity of the event (8). The other was a single site with homogenous record keeping and examination by a small group of practitioners (9). Although working definitions of microbial keratitis were somewhat different, the rates of microbial keratitis were almost identical at 20 cases per 10,000 patients per year (10,11). Thus the new extended wear hydrogels have a rate of keratitis three to five times less than that seen in the more traditional extended wear lenses. Further, the risk of permanent loss in best-corrected visual acuity following microbial keratitis secondary to extended wear was found to be only 3.6 per 10,000 wearers (12). In contrast, the risk of loss of best-corrected visual acuity approaches 100 per 10,000 patients per LASIK procedure (13), which is considerably higher. Of course, refractive surgery is usually a one-time event, while contact lens wear can be for decades, with risk accrual over time.
In summary, the incidence of microbial keratitis in extended wear silicone hydrogel lenses is less than that seen in earlier hydrogel lenses. However, the rate is still markedly higher than in daily wear, suggesting corneal hypoxia is not the only risk factor for microbial keratitis. These relative risks must be communicated to patients so that they can make informed decisions about the risks and benefits of extended wear.
1. Dart JK, et al. Contact lenses and other risk factors in microbial keratitis. Lancet 1991;338:650-653.
2. Holden BA, et al. Microbial keratitis in prospective studies of extended wear with disposable hydrogel contact lenses. Cornea 2005;24(2):156-61
3. Efron N. Are silicone hydrogels safer? Contact lens and anterior eye 2005;28:153-155.
5. Schein OD, et al. The incidence of microbial keratitis among wearers of a 30-day silicone hydrogel extended-wear contact lens. Ophthalmology 2005;112:2172-2179.
7. Brennan NA. Beyond flux: total corneal oxygen consumption as an index of corneal oxygenation during contact lens wear. Optom Vis Sci 2005;82:467-72.
8. Schein OD, et al.
9. Morgan PB, et al. Incidence of keratitis of varying severity among contact lens wearers. Br J Ophthal 2005;89(4):430-6.
11. Schein OD, et al.
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