Extended Wear and Daily Disposables

TWO SIDES OF THE CONTACT LENS COIN: Extended Wear Update

By Jason J. Nichols, OD, MS, FAAO
July 2001

Extended wear is one of today's hottest contact lens topics. Its popularity has increased mainly through use of highly oxygen transmissible (high-Dk) materials. Overnight wear of low-Dk lenses has time and again proven to be the primary factor associated with microbial keratitis during contact lens wear. However, new contact lens materials may possess the oxygen transmissibility needed to overcome this barrier to safe overnight wear of contact lenses.

Silicone hydrogel materials have revolutionized our perspective on overnight lens wear. Although only one of these lenses is approved in the United States (Purevision, Bausch & Lomb), many in the field anticipate CIBA Vision's Focus Night and Day lens. Both lenses have oxygen transmissibilities that exceed the Holden and Mertz minimum requirement for no residual corneal swelling during overnight lens wear. Investigators outside the United States have been examining the safety and efficacy of these lenses when worn on a long-term basis. Investigators are also examining the long-term wear of high-Dk rigid gas permeable materials.

Superior epithelial arcuate lesion (SEAL) seen in extended wear.

Many in the field now use the term "continuous wear" rather than extended wear to denote contact lens wear for up to 30 days and nights or even longer. I will discuss some issues and results of recent studies on extended contact lens wear reported over the past year. Although many of the referenced studies utilize a 30-day and night wear schedule, lenses are currently approved for only seven days/six nights of wear in the United States.

Patient Preference

Why overnight lens wear? Last year we reported on an ongoing study at The Ohio State University examining patient preferences when comparing daily disposable and disposable extended wear modalities. Practitioners consider both options convenient for the patient, yet it wasn't known if patients preferred one over the other. In this cross-over clinical trial, patients wore each modality for 30 days. At the completion of the study, patients reported a significant preference for the disposable extended wear modality. When asked why, patients responded with one answer: convenience. Patients who preferred extended wear to daily disposable lens wear were about 30 times more likely to suggest convenience as compared to comfort, lens handling, ocular health and vision.

	These study results indicate that patients prefer extended wear over daily disposable wear, mainly for convenience.

Ask patients if they want to wear lenses overnight. If a patient expresses interest, he is about 10 times more likely to prefer disposable extended wear to the daily disposable modality. This study further confirms our thoughts that patients desire a permanent, convenient visual correction. The introduction of new silicone hydrogel and RGP materials will likely be the link to safe extended wear for these patients.

Silicone Hydrogel Material

Silicone hydrogels combine revolutionary polymer chemistry with an old polymer, silicone. Oxygen is more permeable in silicone than in water. These lenses, primarily composed of silicone, have high oxygen transmissibilities yet low water contents. By reducing hypoxia, a major factor in contact lens-related keratopathies, silicone hydrogel lenses may provide safe overnight lens wear for appropriate patients.

Studies at the University of Texas Southwestern Medical Center (Dallas) are examining the effects of extended wear and its impact on the corneal epithelium. One study examined the effects of oxygen transmissibility on bacterial binding to the corneal epithelium. In this study of 247 human subjects, each were assigned to either six-night or 30-night extended wear using low- and high-Dk/t RGP and hydrogel lens materials. Outcomes included the adherence of Pseudomonas aeruginosa (PA) to shed corneal epithelial cells. There was a significant increase in PA binding for all hydrogel lens types during the first three months of lens wear, although the low-Dk/t lens-wearing epithelial cells showed greater binding than the high-Dk/t lens-wearing eyes. This binding activity returns to baseline levels after nine months of lens wear. There was no difference in PA binding between the six-night and 30-night hydrogel groups. Neither the low-Dk/t or high-Dk/t RGP lens groups showed increased PA binding to exfoliated epithelial cells. The authors also found an increase in epithelial cell surface size regardless of lens type or wear schedule.

Peripheral corneal/limbal staining.

Another study from this laboratory examined the effects of the overnight wear of high-Dk/t hydrogel lenses (balafilcon A, CIBA Vision) and eyelid closure on rabbit corneal epithelium proliferation. They found that both silicone hydrogel lens wear and eyelid closure significantly suppress the corneal epithelial proliferation rate even though the silicone hydrogel lens possessed an oxygen percentage similar to that of the open-eye condition (about 20 percent). The investigators conclude that factors other than hypoxia may be associated with a suppressed mitotic activity of the epithelium in this environment.

Another report recently published in the British Journal of Ophthalmology studied the effect of high-Dk/t hydrogel lenses on corneal epithelial size and viability. Subjects wore Focus Night and Day lenses (lotrafilcon A, Dk/t = 175, CIBA Vision) on a 30-day and night schedule, Acuvue lenses (etafilcon A, Vistakon) on a seven day/six night schedule or no lenses at all. Epithelial cell size increased only in the low-Dk/t (Acuvue) lens group. There was no difference between any groups in terms of epithelial cell viability. Although these results differ from previous studies, this research may provide further evidence that high-Dk/t lens wear is safe for the epithelium.

	Epithelial microcysts.

The Cornea and Contact Lens Research Unit headed by Brien Holden, PhD, has also actively researched the corneal response to high-Dk/t silicone hydrogel lens wear. Microcysts occur in the corneal epithelium in response to corneal hypoxia. Some recent research from the CCLRU examined how high-Dk/t hydrogel lens wear affects the microcystic response. Subjects wore either high Dk/t silicone hydrogel contact lenses (Focus Night and Day) on a 30-day and night basis or low Dk/t contact lenses on a seven day/six night basis for 12 months. Following this time, the patients originally assigned to the low-Dk/t lens were crossed-over to the high-Dk/t lens to assess the change in the microcystic response. Over 12 months of lens wear, the number of microcysts did not change in the high Dk/t lens-wearing group but significantly increased in the low Dk/t lens-wearing group. After crossing-over to the high Dk/t contact lenses, patients originally assigned to low Dk/t lens wear showed a 50 percent increase in the number of microcysts after the first week followed by a return to normal levels after three months of high Dk/t contact lens wear. These results indicate that high Dk/t lens wear is associated with reduced corneal hypoxia.

Silicone Hydrogel Complications

	Contact lens-related papillary conjunctivitis, or GPC.

A very small number of patients in silicone hydrogel lens wear have developed "mucin balls," a form of tear film debris in continuous lens wear. In a recent report originating from the Center for Contact Lens Research (University of Waterloo, Ontario, Canada), investigators described mucin balls as "spherical, gray, translucent bodies between the back surface of a hydrogel contact lens and the corneal epithelium, which result in a depression of the ocular surface after lens removal." High modulus (stress/strain) associated with silicone hydrogel may induce a sheering force during overnight wear, producing mucin balls. Although not fully understood, mucin balls have no known clinical significance to the patient.

The CCLR recently explored the issue of refractive error change with overnight lens wear. Many reports indicate that low-Dk/t lens wear leads to myopic progression due to edema associated with corneal hypoxia. Many think that corneal steepening, possibly by changes in its posterior elevation, causes the increase in myopia. Myopic creep associated with low-Dk/t lenses might be eliminated with these new materials. Over a nine-month period, the investigators found an increase in myopia during extended wear of only low-Dk/t materials (seven day/six night) but not during the wear of high-Dk/t silicone hydrogels on a continuous basis (30 days and nights). They conclude that the wear of silicone hydrogel materials, even for periods of up to 30 days, has no impact on refractive error.

RGP Extended Wear

The Berkeley Contact Lens Research group is currently conducting the ongoing Contact Lens Extended Wear Study (CLEWS). Previous investigators have shown that hydrogel extended wear presents an increased risk for microbial keratitis ­ the most significant risk associated with contact lens wear. The aim of this National Eye Institute-funded contact lens clinical trial is to determine how hypoxia and acidosis associated with rigid gas permeable extended wear affects the cornea. More specifically, the investigators are trying to quantify the changes in corneal pH during extended wear of contact lenses of varying oxygen transmissibilities and to ultimately determine the impact of corneal acidosis on contact lens-induced corneal complications. There were 411 patients enrolled in this study. Each was randomized to either high-Dk/t or medium-Dk/t RGP contact lenses worn on a seven-day/six-night schedule for 12 months. The high Dk/t material fitted to each subject is FluoroPerm F151 with Dk/t's ranging from 37 to 92. The authors presented their initial results at ARVO (2000). Although the overall rates of successful extended wear were equal in both groups (62 percent), the rate of failure due to corneal complications was significantly higher in the medium-Dk/t group. As expected, overnight swelling was higher in the medium-Dk/t group (6.1 percent) as compared to the high-Dk/t group (4.8 percent). The hypoxia associated with medium-Dk/t RGP materials may explain the higher success rate of the high-Dk/t group. The authors further note that the corneal complication rate in the medium-Dk/t group is small compared to hydrogel extended wear, which suggests that factors other than hypoxia may cause corneal morbidity during hydrogel extended wear.

In addition to the Berkeley studies, another multi-center study (Pall, ARVO 2001) is underway examining how RGP extended wear affects corneal physiology. This research group is comparing the effects on corneal physiology of 30-day continuous wear using a high-Dk/t RGP material (tisifilcon A, Dk=161) to seven days/six nights of hydrogel extended wear (etafilcon A, Dk=28). Initial results indicate that there is no increase in corneal swelling of the RGP continuous wear group as compared to the hydrogel lens wear group. These initial results may prove that RGP continuous wear causes less hypoxic corneal stress than hydrogel extended wear.

The Future

Although extended wear lenses are currently approved in the United States only for wear periods of seven days/six nights, studies suggests that a 30-day wear schedule with either silicone hydrogel or high-Dk/t RGP materials may again be appropriate. These new materials may effectively and safely correct refractive error on a more permanent basis for our patients.