Overnight wear of new high-Dk materials results in healthy epithelia

Contact Lenses and Eyewear

Issue: May 2002

ByJennifer L. Smythe, OD, MS, FAAO

Part 2: How to best fit high-Dk lenses for continuous wear

The desire for clear, comfortable vision 24 hours a day without inconvenience is what motivates our patients to seek alternatives to spectacles for vision correction. The three “Cs” — clarity, comfort and convenience — are what has driven the interest in refractive surgery, the resurrection of orthokeratology and contact lens manufacturers to make safe extended wear their ultimate goal. Technological advances in lens material chemistry and manufacturing have now made it possible to add a fourth “C” — “continuous” wear — to the equation.

New products, new materials

Two high-Dk products have received Food and Drug Administration approval for up to 30 nights of continuous wear. The CIBA Vision Focus Night & Day, a fluorosiloxane hydrogel, was launched in the United States in October 2001. The PureVision lens from Bausch & Lomb, a silicone hydrogel, has been on the market for more than 2 years and also received FDA approval in November 2001 for up to 30 nights of continuous wear. A summary of the material characteristics and parameter availability appears in the accompanying chart.

chart The new high-Dk silicone and fluorosiloxane hydrogels offer up to six times greater oxygen transmission than conventional HEMA-based lenses. The increase in oxygen transmissibility is accomplished not by increasing water content, but rather by incorporating silicone or silicone and fluorine. In contrast to the previously marketed silicone elastomer lenses, the surface treatments on the new high-Dk lenses offer consistent wettability and deposit resistance.

Many practitioners have met the idea of 30-night continuous wear with justifiable reservation. Fear of the worst-case scenario is frequently expressed. During the heyday of extended wear in the late 1980s and early 1990s, the media sensationalized microbial keratitis even though the incidence of this complication was low, approximately 1 in 500 with overnight wear. Today, we must ask ourselves what we really need to know to alleviate our reservations and offer continuous wear as a viable option for our patients.

Perhaps part of the answer can be found by examining the cornea’s physiological response to overnight wear with the new high-Dk materials. It has been generally held that corneal hypoxia during overnight wear of contact lenses brings about physiological changes that predispose the cornea to infection. Let’s address several key markers related to the cornea’s physiological response to continuous wear.

Overnight corneal swelling

An obvious and measurable impact of overnight wear of a contact lens is corneal swelling, which has been linked to oxygen deprivation. Thus, it has been accepted that if a lens could be shown to cause no more swelling than occurs without the lens, the oxygen transmissibility of the lens must be adequate. Several studies have been conducted evaluating overnight corneal swelling with these new materials, confirming that induced overnight swelling is significantly reduced in comparison to conventional low-Dk extended wear materials and is statistically similar to no lens wear levels.

Fonn and colleagues, at the Centre for Contact Lens Research (CCLR) Unit in Waterloo in 1998, have reported overnight corneal swelling for a group of 20 patients as 2.7% with Focus Night & Day vs. 1.4% with no lens wear. Similar results were presented by Mueller and colleagues (2001) at Pacific University where overnight corneal swelling was measured using the Orbscan (Orbtek, Salt Lake City) on 12 contact lens neophytes. They reported no statistically significant difference in overnight corneal swelling between the Focus Night & Day and no lens wear.

Corneal epithelial response

Microcysts are small, irregular-shaped inclusions that have been described as accumulated cellular debris or degenerated epithelial cells. Holden and colleagues at the Cooperative Research Centre for Eye Research and Technology (CRCERT) in Australia in 1987 have proposed a hypoxic mechanism for the development of microcysts and, furthermore, have shown that the lower the Dk/t of a lens, the higher the overnight corneal edema and subsequently the greater the number of epithelial microcysts. Microcysts are typically observed in significant numbers after 2 to 3 months of conventional, low-Dk extended wear.

Clinical trials at the CRCERT have shown lower numbers of microcysts in silicone hydrogels compared to conventional low-Dk lenses, and, furthermore, the levels were comparable to the numbers observed in spectacle lens wearers. It is important to note that a “rebound” effect is commonly observed when refitting low-Dk hydrogel lens wearers into silicone hydrogels where the number of microcysts temporarily increases before they decrease significantly. In essence, this occurs in response to a “wake-up” call or return to normal epithelial metabolism and mitotic rate and the subsequent removal of entrapped cellular debris.

Vascular response

The filling and engorgement of the limbal capillary vessels is another response to corneal hypoxia; and a reduction in the phenomenon with high-Dk materials enhances the safety of contact lens wear. Papas and colleagues at the CRCERT in Australia in 1997 have shown that high-Dk lenses do not induce limbal hyperemia. This is desirable to our patients because, cosmetically, their eyes appear “whiter.”

More importantly, limbal hyperemia has been suggested as a precursor to corneal vascularization. Recently, Dumbleton and colleagues at the CCLR in 2001 compared neovascularization in 59 conventional, low-Dk lens wearers to 32 subjects wearing Focus Night & Day on a 30-night continuous wear schedule. They reported an increase in neovascularization in the low-Dk group but no change in the high-Dk lens group. These researchers also showed that the wearers of the high-Dk lenses who initially presented with high levels of hyperemia or neovascularization from their previous habitual lenses displayed a decrease in bulbar and limbal hyperemia and neovascularization.

Increased oxygen permeability

It is clear from the extensive published research and the worldwide experience that the side effects of hypoxia have been nearly eliminated with continuous wear of silicone hydrogels. Furthermore, Ren and colleagues (2002) have shown that high-Dk soft lens continuous wear produces significantly less binding of Pseudomonas aeruginosa to corneal epithelium than with extended wear of conventional low-Dk materials. It is also interesting to note that their findings do not indicate a difference in bacterial binding between a 6-night vs. 30-night continuous-wear schedule of the high-Dk soft lenses.

So, if we have healthier epithelia from greater oxygen permeability and less bacterial binding, will microbial keratitis still occur with continuous wear of silicone and fluorosilicone hydrogel lenses? As of this January, 13 cases of microbial keratitis have been reported worldwide with the high-Dk soft lenses. None of the cases have resulted in a loss of best-corrected visual acuity, and a history of non-compliance such as ocean swimming or performing a tap water rinse has been reported in several of the instances. At this point, the risk of microbial keratitis appears to be approximately 1 in 20,000 with continuous wear of the high-Dk contact lenses, a 38 times lower risk than extended wear of conventional low-Dk soft lenses.

One final piece of the puzzle will be to meet the challenge of eliminating tear stagnation and the proverbial “cesspool” that develops with overnight wear of any lens that covers the limbus 360°. The stiffer modulus of the new lens materials and surface properties allow greater on-eye movement than previous-generation materials. Although inflammatory adverse events still occur, they appear at significantly lower rates.

For Your Information:

The Focus Night & Day lens is available from CIBA Vision Corp., 11460 Johns Creek Pkwy., Duluth, GA 30097-1556; (800) 241-5999; Web site: www.cibavision.com.

The PureVision lens is available from Bausch & Lomb, 1400 N. Goodman St., PO Box 450, Rochester, NY 14603-0450; (800) 553-5340; (585) 338-6000; fax: (585) 338-6896; Web site: www.bausch.com.

References:

Mueller N, Caroline P, Smythe J, et al. A comparison of overnight swelling response with two high Dk silicone hydrogel contact lenses. Optom and Vis Sci. 2001;78(2)125.

Holden BA, LaHood D, Sweeney DF. Prediction of extended wear microcyst response on the basis of mean overnight corneal response in an unrelated sample of non-wearers. Am J Optom Physiol Opt. 1987;64,83.

Sweeney D, Keay L, Jalbert I et al. Clinical performance of silicone hydrogels. In: Sweeney D, ed. Silicone Hydrogels: the Rebirth of Continuous Wear Contact Lenses. Butterworth Heinemann. 2000:113.

Papas EB, Vajdic CM, Austen R, et al. High-oxygen transmissible soft contact lenses do not induce limbal hyperaemia. Curr Eye Res. 1997;16:942-948.

Dumbleton K, Chalmers R, Richter DB, Fonn D. Vascular response to extended wear of hydrogel lenses with high and low oxygen permeability. Optom Vis Sci. 2001;78(3).

Ren D; Yamamoto M, Ladage P, et al. Adaptive effects of 30-night wear of hyper-0₂ transmissible contact lenses on bacterial binding and corneal epithelium. Ophthalmology. 2002;109(1):27-40.