A Realistic View of Extended Wear Risk
Before you persuade or dissuade your next patient interested in extended wear, set aside your preconceived notions and learn about the absolute and relative risks.
BY MOHINDER M. MERCHEA, OD
DECEMBER 1998
Extended wear contact lenses offer virtually complete freedom from spectacles without the hassle of the insertion, removal and cleaning ritual associated with contact lens daily wear. Sleeping in contact lenses for one to six nights per week is an attractive option to many patients searching for "normal" vision by a reversible and safe method. However, eyecare practitioners are often reluctant to recommend current extended wear lenses due to concerns about safety. Many are anticipating that the next generation of extended wear lenses will minimize ocular complications and provide a successful alternative to current contact lens wearing refractive surgery candidates.
Over five million people within the United States wear hydrogel lenses for extended wear. However, about 77 percent of contact lens wearers in the United States and 50 to 65 percent worldwide would like to be able to sleep in their lenses overnight, according to one survey. Another survey from the Vision Council of America found that 52 percent of contact lens wearers said their eyecare professional's recommendation is the most important consideration in their purchasing decisions for eyecare products. Therefore, manufacturers will respond with the development and advertising of extended wear contact lenses, but it is the practitioner's perception of the safety of extended wear that will ultimately decide its position within the contact lens market.
Historical Perspective
Continuous wear contact lenses have been offered to the eyecare profession in a variety of forms for over 20 years. Initial trials of extended wear lenses were primarily motivated by the therapeutic use of these lenses for aphakia or as bandage lenses, but it was the cosmetic use of these lenses that provoked a flurry of interest. The FDA approved the first therapeutic soft lenses for extended wear in 1979 and for cosmetic use in 1981.
Much emphasis was placed upon the oxygen transmissibility of materials during overnight wear. The premise that maintaining adequate oxygen levels at the anterior cornea would minimize ocular complications motivated the search for high Dk materials. But the oxygen transmissibility (Dk/L) of hydrogel lenses is dependent on their water content, so a theoretical ceiling existed for hydrogel lenses.
A highly oxygen permeable material called silicone elastomer was investigated for extended wear use in the late 1970s, but despite its high Dk, the lenses were minimally successful due to the inherent hydrophobicity of the material. Silicone-based lenses require surface treatments to improve wettability and reduce deposits, but surface deterioration has been reported frequently along with deposition, drying, discomfort and lens adhesion. Silicone elastomer lenses gained FDA approval for aphakic, pediatric and cosmetic (myopic) extended wear. They are currently available for only adult and pediatric aphakia.
By the mid-1980s, attention shifted to the use of rigid gas permeable overnight wear lenses, but even with their higher levels of oxygen delivery, ocular complications still occurred. Hydrogel contact lens-es experienced much greater acceptance than RGP contact lenses for extended wear use due to the ease of fitting and initial comfort, and RGPs consequently had a minor impact on the extended wear modality.
The introduction of the disposable lens in the United States for extended wear in 1987 was similarly anticipated to reduce ocular complications of overnight contact lens wear, but numerous studies have indicated that contact lens disposability has not significantly reduced serious complications such as infectious keratitis.
Contact lens manufacturers and researchers are now on the verge of developing contact lenses that they claim have the oxygen permeability of silicone plus the comfort and surface characteristics of a hydrogel lens. Demand for these contact lenses will certainly be high, and we must be poised to evaluate and employ these lenses while ensuring the health of our patients.
Mechanisms of Extended Wear Complications
The risks associated with extended wear of contact lenses are numerous, and there are several causal mechanisms that we must address. It has been suggested that the morbidity of infectious and inflammatory keratitis includes hypoxia, post-lens tear stagnation and inflammation and contact lens deposits, and risk factors may include poor hygiene, duration of overnight wear, disposable versus conventional lens use and smoking. To manage adverse reactions and develop preventative measures, it's essential that you have a thorough understanding of these mechanisms.
Hypoxia
The normally avascular cornea produces energy by metabolizing glucose using aerobic pathways. These aerobic pathways rely on atmospheric oxygen levels in the open-eye state and on palpebral conjunctival blood vessels in the closed-eye state. Oxygen can reach the cornea during contact lens wear via two mechanisms -- tear circulation and diffusion through the lens material.
Tear circulation is minimal during lid closure, so the oxygen availability to the anterior cornea depends primarily on contact lens transmissibility. Therefore, the reduced oxygen available in the closed-eye state is further restricted by the presence of a contact lens during extended wear. This results in a state of relative hypoxia and is marked by a switch to anaerobic metabolism.
The pathophysiological changes arising from this hypoxia include corneal edema, microcysts, decreased epithelial mitosis, epithelial fragility, polymegethism and pleomorphism, stromal acidosis, and increased inflammatory activity. Such compromised corneas are predisposed to microbial infection, but there is no definitive proof that hypoxia alone is responsible. Rather, the pathogenesis of infectious keratitis is more likely the result of a combination of factors, so increasing the oxygen transmissibility of contact lens materials may not necessarily abate the associated morbidity rates.
Tear Stagnation
On eyes not wearing lenses, tears remove debris and pathogenic organisms, as well as distribute chemical agents for defense against microbial invaders. The reduction of this tear layer, which has been observed in hydrogel extended wear, results in a reduction in lens mobility. It's possible that this stagnation increases contact time between a compromised corneal surface and microbes or other cellular debris, perhaps initiating an infectious or inflammatory response of the cornea.
RGP lenses are less likely to provoke such a response because debris is removed much more efficiently due to better tear exchange. Yet RGP as well as soft contact lenses have been observed to adhere to the cornea following overnight wear, thereby trapping any debris or microorganisms.
Lens Deposits
Lens surface deposition is known to provoke adverse reactions in extended wear via mechanisms commonly seen with daily wear. Papillary conjunctivitis has been associated with mechanical and immunologic components, while infiltrative keratitis has been linked to immunologic responses to surface deposits. Furthermore, the deposits may provide attachment sites for microorganisms, thereby contributing to infectious keratitis.
Frequent lens replacement, surface treatments, improved cleaning systems and increased cleaning frequency may minimize the interaction between cornea and lens-bound proteins. While these strategies have significantly reduced the incidence of deposit-related complications, such as papillary conjunctivitis, they have not eliminated them altogether, bringing attention to the mechanical effect of the lens on the eye.
Cleaning Regimens and Compliance
Most early investigations into the safety of extended wear contact lenses were unable to determine the connection between complications and contact lens hygiene. Most were confounded by the type of lenses used, frequency of lens care and type of cleaning regimen. Follow-up studies on adverse reactions have implicated behavioral factors such as duration of overnight wear, oversimplification of care regimens, contaminated cases and lack of follow-up care.
Even though the type of disinfection system and frequency of use have been identified as significant, there is insufficient statistical evidence to indicate the frequency of mechanical cleaning as a significant variable. The exact relationship between compliance, contamination and development of adverse reactions is yet to be determined. But most practitioners will agree that lens care should involve surfactant cleaning, rinsing and disinfection, preferably with hydrogen peroxide or a multipurpose solution with minimal preservative concentrations.
Duration of Overnight Wear
The extended wear modality was approved by the FDA for 14 days of continuous wear in 1981 and later for 30 days for cosmetic use. Various studies have indicated an increased risk of complications with increased time of uninterrupted lens wear. In particular, a study by Stapleton, et al. (1993) found an increased risk of ulcerative keratitis with increased duration of overnight wear. Another epidemiological study demonstrated an increased risk for every day of uninterrupted lens wear. The mechanisms relating to duration of wear and complications are multifactorial, but hypoxia, lens deposits, trapped debris and poor hygiene are thought to be relevant.
Following these studies, the FDA reduced the recommended 30 days to a maximum of seven days of continuous wear.
Disposable versus Conventional Lenses
The concept of disposability was first introduced in Denmark, in the form of the Danalens. The concept was adopted internationally in the late 1980s with the hope that replacement of contact lenses as frequently as every week would reduce many of the complications of extended wear.
Initial research indicated an increased risk for infectious keratitis with disposable lens use, but these studies were confounded by the varying amounts of overnight use and differing care regimens, and the association turned out to be insignificant. Other studies have shown that the prevalence of all types of complications taken together was higher for conventional hydrogel contact lens extended wear than for either disposable extended wear or conventional daily wear. Furthermore, it was shown that no significant difference existed between disposable extended wear and conventional daily wear for all extended wear complications.
Therefore, disposability of contact lenses has resulted in fewer complications and unscheduled visits for daily wear, but has demonstrated limited influence against the incidence of infectious or sterile keratitis in extended wear.
Smoking
Smoking was found to increase extended wear complications by about four to one compared to daily wear, where its effect was minimal. Though the mechanism of interaction has not been clearly defined, behavioral characteristics are thought to be a commonality between smoking and the development of adverse reactions with overnight wear.
Complications of Extended Wear
Corneal Edema
Stromal edema during extended wear, a response to insufficient oxygen levels at the anterior cornea, is characterized by the presence of haze, striae and stromal folds. This causes a metabolic shift in energy production, resulting in lactate build-up and an influx of fluid into the stroma. The amount of overnight swelling is directly related to the Dk/L of the lenses. The critical Dk/L needed to limit overnight edema to four percent, the amount observed with no contact lens wear, is 87 x 10-9 (cm x mlO2)/(s x ml x mmHg). This criterion cannot be achieved with current hydrogel contact lens materials and designs.
A hydrogel lens of 34 x 10-9 Dk/L causes eight percent overnight edema, from which the cornea is capable of recovering completely during daytime hydrogel lens wear. A thin high water content lens can achieve this Dk/L, but causes significant corneal desiccation due to dehydration, and is therefore of limited success.
Microcysts
Another useful index for corneal hypoxia is the presence of epithelial microcysts, which are pockets of cellular debris or apoptotic cells resulting from a shift in epithelial metabolic activity (Fig. 1). They are inclusions showing reversed illumination, and are located across the entire cornea, migrating from deep epithelium anteriorly. Microcysts typically occur after three months of extended wear.
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Microcystic response may increase temporarily after discontinuing extended wear due to the return of normal metabolic activity and the increased rate of waste removal. Microcystic response indicates compromised epithelium, and reduced wear time is necessary if the microcysts number greater than 50 or are accompanied by marked epithelial disruption.
Compromised Epithelial Integrity
Chronic hypoxia seen with hydrogel extended wear lenses may lead to the disruption of tight junctions and hemidesmosomes in the corneal epithelium and mitosis. Superficial punctate staining is a consequence seen in about 86 percent of extended wear cases. Corneal abrasions subsequent to extended wear have a dual mechanical and hypoxic etiology. Epithelium loosely adherent to the basement membrane is removed along with the lens, resulting in a full thickness epithelial defect. The abrasions will make the patient more prone to microbial infection, so lens wear must be discontinued and a broad spectrum antibiotic should be used.
Corneal Vascularization
Corneal vascularization may be attributed to chronic hypoxia, which causes corneal edema and stromal tissue disruption, but other factors include the accumulation of corneal metabolic waste such as lactate, the release of vasostimulators from disrupted epithelium, or vasoproliferative factors. Vascularization may be superficial or deep stromal. Cessation of contact lens wear causes blood columns to empty, resulting in ghost vessels, but the vessels will refill if the cornea is returned to a hypoxic state.
Giant Papillary Conjunctivitis
A hyperemia and hypertrophy of the upper tarsal conjunctiva is characteristic of giant papillary conjunctivitis (GPC) (Fig. 2). It is typically accompanied by mucous discharge, itching and contact lens intolerance. The incidence of GPC is about 20 percent in hydrogel extended wear with conventional lenses, and about six percent with disposable extended wear. The etiology of GPC secondary to hydrogel extended wear may be a hypersensitivity reaction to lens deposits, or it may be a mechanical response to the lens itself. This dual mechanism may explain why use of disposable contact lenses has not been able to completely eliminate GPC. Management involves discontinuing lens wear, changing cleaning regimens or contact lens material, and instilling a topical soft steroid or non-steroidal anti-inflammatory agent.
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Contact Lens Induced Acute Red Eye
Contact lens induced acute red eye (CLARE) reaction is also identified as "tight lens syndrome." Onset is sudden, usually upon awakening, with pain, tearing and photophobia. Clinical presentation includes conjunctival and limbal hyperemia, and subepithelial infiltrates (Fig. 3). CLARE is an acute inflammatory response to toxic substances released by post-lens cellular debris breakdown in the closed-eye environment during hydrogel lens extended wear.
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Advise patients to immediately discontinue lens wear until resolution. There is a 50 percent chance of recurrence if extended wear is resumed. In order to minimize the possibility of developing a CLARE reaction, refit the patient into a lens with sufficient movement to remove debris behind the lens, and emphasize mechanical and enzymatic cleaning.
Infiltrative (Sterile) Keratitis
Sterile infiltrates appear as amorphous collections of polymorphonuclear cells from limbal vessels in the epithelium or stroma. The presence of infiltrative keratitis has been reported with daily and extended wear, but the condition seems to be more prevalent in extended wear.
Corneal infiltrates are also more commonly reported with disposable contact lens extended wear compared to conventional lens extended wear. It is thought that this increased risk for infiltrates in disposable extended wear is due to an increased tendency of these patients to wear their lenses for six to seven nights per week, compared to an often shorter duration for conventional extended wearers in these studies. Mechanism of action is likely a hypersensitivity reaction to bacterial components adherent to lens surfaces. Cessation of wear usually resolves the inflammatory reaction; otherwise a topical antibiotic-steroid combination may be employed.
Infectious Keratitis
Corneal ulcers are the most serious adverse reactions associated with contact lens wear. They present as epithelial defects accompanied by underlying stromal infiltration and surrounding edema. Patient symptoms can vary from mild foreign body irritations to severe pain, mucopurulent discharge and an anterior chamber reaction. Management of corneal ulcers includes topical treatment with fluoroquinolones or fortified antibiotics, cycloplegics, oral analgesics and discontinuation of contact lens wear until resolution. Culturing is also advised.
It's important to differentiate infectious corneal ulcers from sterile peripheral ulcers. These small (<2mm) epithelial defects are accompanied by stromal infiltration in the middle to peripheral cornea (Fig. 4). The absence of severe pain, mucopurulent discharge and anterior chamber reaction is indicative of a sterile ulcer. Discontinuing contact lens wear results in a rapid resolution without the need for medication, but a conservative approach is best when the alternative is misdiagnosing a sight-threatening corneal ulcer.
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Who Says Extended Wear Is So Risky?
Both extended and daily wear contact lens types are associated with some risk of developing minor complications such as conjunctivitis or more severe entities such as corneal ulcers. Eyecare practitioners and patients embrace the advantages of daily wear contact lenses as a convenient and safe means of correcting refractive errors.
The complications of daily wear are accepted as rare and are considered to be of limited public health concern. However, many practitioners perceive the overnight wear of contact lenses as an unfavorable modality that places the patient at undue risk. This may be due to a misperception of the concepts of absolute and relative risk for complications with extended wear versus daily wear contact lenses.
Corneal Ulceration: Incidence Versus Relative Risk
Several epidemiological studies have investigated the association between corneal ulceration and contact lens wear. In the United States, the annualized incidence rates of contact lens induced corneal ulcers per 10,000 lens wearers is reported to be from 4.10 to 5.2022 for daily wear, and from 18.20 to 20.90 for extended wear. Another more recent U.S. study concluded an annualized incidence of 14 per 10,000 extended lens wearers. An epidemiological study of hospitalized cases from Sweden have found rates to be about 0.46 for daily wear and 3.34 for extended wear per 10,000 wearers when conventional and disposable lenses are taken together. Furthermore, studies have shown no difference in incidence of corneal ulcers between extended wear of conventional and disposable hydrogel lenses. Another study, which investigated all cases of infectious keratitis in Sweden over a three-month period, found annualized incidence rates per 10,000 wearers of 2.16 (daily wear) and 12.00 (extended wear).
The above studies indicate that extended wear is a risk factor for infectious keratitis, and that contact lens disposability does not influence ulcer rates. Possible reasons why Sweden has lower incidence rates include a greater degree of patient education which stresses mechanical cleaning with surfactants, as well as a requirement for newly fit patients to have ophthalmologic health assessment visits within six months. It's important to note that these studies, which report absolute incidence, demonstrate that the absolute risk of ulcerative keratitis is low.
Compared to daily wear conventional hydrogel lenses with a relative risk of 1.0, the relative risks associated with extended wear are greater by a factor of between two and six. Furthermore, it has been shown that this relative risk increases by approximately a relative risk of 1.0 for each additional consecutive night of wear.
Without examining the absolute incidence of a disease, we can't comprehend the relative risks to determine precisely how common or rare it is. Additionally, the perceived risks are subject to personal biases and value judgments. For eyecare practitioners, the personal biases are usually the legal and public health implications; for consumers, the biases are the concerns of the moment, which are highly susceptible to media sensationalism. This situation is not aided by the eyecare community, which has published fewer contact lens-related articles in the past five years in ophthalmic literature than in the previous five years, many of which are negative.
In summary, the relative risk of ulcerative keratitis for daily wearers is about 60 times that of non-contact lens wearers. The risk increases by about two to six times with extended wear. Apparently, the eyecare community and public are willing to accept a 60-fold increase in risk for daily wear, but an additional two- to six-fold increase is unacceptable with extended wear. We must supply potential extended wear patients with all contact lens alternatives, as well as with the accompanying absolute and relative risk ratios, so that they can make informed decisions.
Extended Wear in the 21st Century
Extended wear of contact lenses appeals to many patients who want "normal vision" without the burden of removing, cleaning and disinfecting contact lenses. Millions of existing extended wearers throughout the world are still not wearing lenses optimized for safety. Millions more are opting for refractive surgery, despite the fact that the relative risk for vision loss is five to 15 times higher for procedures such as photorefractive keratectomy than for extended wear. Both of these groups would benefit from an extended wear contact lens capable of minimizing the complications listed in this article.
Bausch & Lomb plans on introducing its foray into the second generation of extended wear hydrogel lenses, PureVision (Balafilcon A). PureVision is already approved in the European Union for seven-day extended wear, and 30-day extended wear studies are in progress. Surface treatments are used to convert a hydrophobic lens surface into a hydrophilic one with a Dk/L of 110, combining the oxygen transmissibility of silicone, the fluid transport capacity of HEMA lenses, and the deposit resistance, uniform wettability and dehydration resistance of a traditional hydrogel lens. CIBA Vision has a similar product under investigation, and many other manufacturers are not far behind. Patient demand is already high and this will only grow due to the marketing sure to follow the release of these materials in the United States.
Providing patients with a safe method of refractive correction is our ultimate responsibility. It is also our responsibility to inform our patients completely about the risks and benefits for all options to correct refractive error, including extended wear. It is clear that a demand exists, and as more overnight wear options are introduced into the market, we must be prepared to meet that challenge.
To receive references via fax, call (800) 239-4684 and request document #43.
Dr. Merchea is in the second year of his cornea and contact lens graduate fellowship at The Ohio State University College of Optometry in Columbus.