To minimize corneal swelling, contact lenses must provide a healthy level of oxygen transmissibility.
Dr. Fonn: I'd like to dedicate the publication of this roundtable discussion to the memory of Dr. George Mertz, who made such a great scientific contribution to our understanding of the cornea's oxygen requirement.
Now, let's begin. Several indicators tell us eyes are not receiving sufficient oxygen. High correlations between oxygen transmissibility Dk/t and corneal edema have been reported. What does corneal edema tell us about oxygen transmissibility?
Dr. Holden: To look at oxygen transmissibility in the context of corneal edema, we need to keep in mind edema is just one index. Although edema indicates a more significant disturbance to the homeostasis of the cornea than acidosis and limbal redness do, this factor alone can't tell us how to produce a contact lens that doesn't disturb the cornea.
Dk/t is a constant physical measure of
the lens. It's helpful to know this measure for every lens and every power that
involves a design change. However physiological indices are quite different. Biology
is not so neat and clean.
We also need to consider the circumstances behind data related to corneal swelling. The data are fairly consistent in terms of oxygen needs for overnight swelling: If we target no lens edema at 3% or 3.2%, then 125 Dk/t will produce that level of overnight corneal swelling.1 However, studies comparing response to a lens on the eye and no lens on the other eye in the same patient can be confusing because the swelling response is yoked. Only studies of both eyes with lenses versus the same eyes with no lenses offer us a clear picture. Without lenses, the population's mean overnight corneal swelling response is about 3.2%, a result we can achieve with contact lenses of 125 Dk/t or slightly more. There's a great need for further study and review of previous data. In particular, we need to look at the thickness and the Dk/t of the individual lenses used and the bilateral swelling responses in the same group of subjects wearing a variety of lenses.
Dr. Hill: We need to consider a loose correlation between Dk/t and corneal swelling. Dk/t is a constant physical measure of the lens. It's helpful to know this measure for every lens and every power that involves a design change. However, physiological indices are quite different. Biology is not so neat and clean. If we subjected 100 corneas to identical lenses, we'd get 100 different responses. We'd find a mean, but we'd also find significant standard deviations. Thus, it's unwise to use mean values in practice without accounting for the standard deviation. In addition, other physiological tests, such as equivalent oxygen percentage (EOP), can give us earlier information, if needed. Swelling is a more deeply rooted problem and may take several hours to detect.
In the future, we won't use a single-number approach. We're learning we need to make these decisions on a more individual basis for each cornea.
HOW CAN WE PREVENT CORNEAL SWELLING?
Dr. Fonn: Holden and Mertz defined the critical oxygen transmission of a contact lens for daily wear as 24 Dk/t and for overnight wear as 87 Dk/t.2 This paper established the standard for eliminating corneal swelling. However, research now suggests these numbers underestimate even the average cornea's needs. Should we change our view of these criteria?
Dr. Cavanagh: All contact lenses prevent oxygen from reaching the eyes. Anything we put on the eye starts driving the normal oxygen level down from 100%. Eventually, it reaches the milestone of corneal swelling a milestone that may turn into pump function alteration, red eyes or other complications over time. We're trying to determine where this milestone is. We need to aim as close as we can to the 100% that nature intended, and quantify that as well. Of course, some eyes have special needs. A middle-aged patient with florid rosacea, tear-film deficiency, seborrheic blepharitis and atopic allergic disease has a considerably higher-than-normal oxygen consumption rate.
Dr. Holden: The cornea deserves normoxia during the day and as much oxygen as the eyelid can supply at night. Any reduction in oxygen availability will make the cornea function differently. We need to aim for the highest level of oxygen the closest to normal that we can provide and afford.
Teenagers we fit with contact lenses today might be wearing them for 85 years. Over the course of a lifetime, if oxygen levels drop just 2% or 3%, this will be important.
Dr. Wilson: We'll likely get some perspective on target Dk/t numbers in time, particularly as we understand more of what's going on in the eye. Today, we don't fully know the challenges in the nocturnal environment. Variation in blood flow on the back of the lids or differences in rapid eye movements during sleep must play critical roles in making oxygen available, and we'll understand these roles better in the future.
WHAT'S THE "RIGHT" NUMBER FOR SILICONE HYDROGELS?
Dr. Fonn: Silicone hydrogel lenses of 175 Dk/t cause slightly more, but significant, overnight corneal swelling compared to no lens wear. Is this the "right" transmissibility? Would silicone hydrogel lenses with higher transmissibility eliminate overnight corneal swelling, or are other factors at work?
Dr. Sweeney: In our research, we've used corneal swelling to measure the eye's response to various lenses with a Dk/t up to 175. We've also found that individual corneal swelling responses vary and, therefore, so do oxygen requirements. But it appears that 175 Dk/t ensures the "average" patient can avoid corneal edema. Swelling levels with 175 Dk/t were clinically similar to the levels experienced by eyes without lenses. My earlier work has suggested much of the edema is caused by hypoxia, however, temperature, humidity and tonicity changes that accompany lens wear also contribute to the edema response.
Dr. Cavanagh: Dr. Sweeney, you've done a lot of work with gas permeable (GP) lenses. The oxygen requirement may be even higher for soft lenses, particularly because chronic soft-lens wearers have thicker corneas that reduce swelling more slowly.
Dr. Sweeney: Good point. Tear exchange with GP lenses supplements oxygen availability to eyes wearing these lenses. In soft lenses, we need a higher Dk/t to get the same long-term health benefit.
Dr. Holden: I agree that once the eyes are open, recovery is quicker with GP lenses because of the additional tear exchange. But boundary effects seem to cause a greater barrier to oxygen transmission through GP lenses. Data from GP lens studies often are examined beside soft-lens data, which makes optimal oxygen transmissibility difficult to assess.
Any reduction in oxygen availability will make
the cornea function differently. We need to go for the highest level of
WILL WE MOVE TO A TOTALLY INDIVIDUAL APPROACH?
Dr. Fonn: Is it possible to determine an individual's oxygen requirement?
Dr. Sweeney: Rather than create hard-and-fast rules, we should give some guidance to practitioners about what oxygen levels they should consider. We can determine the average oxygen needs and help our colleagues understand that every cornea is different. By delivering the most oxygen we can through the lenses, we also can provide for patients with above-average oxygen needs.
Dr. Cavanagh: We can't apply the average oxygen needs to all patients throughout their lives, because the cornea's oxygen needs tend to increase with age. The need increases even more for a patient who has background inflammation or drying. A 21-year-old woman might be able to wear a variety of contact lenses, but the same woman at 50 might give up on the lenses she's worn for years. Maybe 175 Dk/t won't even do it. We'll learn a great deal from longitudinal studies of all types of patients.
Dr. Hill: It's the definition of aging itself. We begin to lose our backup systems. Our tolerances narrow. We respond less rapidly and maybe less effectively to assaults, which means we're more vulnerable. The eyes lose some resilience, just as we do.
Dr. Fonn: What is the role, then, for the oxygen numbers we're finding?
Dr. Hill: Averages are informative, but I think we're missing a comprehensive picture. We can't just define healthy contact lens wear for most cases. The definition needs to encompass eyes with higher oxygen needs, perhaps those with a genetic predisposition to dystrophies, or a history of trauma or toxicity. Age might be a factor. Even altitude changes can affect oxygen levels. All our studies include a small percentage of these interesting fringe corneas. In this varied population, the single-number strategy we've used for years isn't the best one. To cover everyone, we'd need many numbers one for corneal acidosis, one for increase of oxygen uptake, one for swelling, and more for other functions we discover. We can work with good probabilities related to average numbers, but this carries a certain risk. Focusing on the individual eye with quick, accurate, reliable tests will be the new frontier in the next 5, 10 or 15 years. That individual focus will help us answer the ultimate question: What is healthy contact lens wear?
|Can We Reverse Damage Caused by Long-term Wear of Low-Dk/t Lenses?|
Dr. Fonn: Many patients are still wearing low-Dk/t lenses. Dr. Sweeney, what signs do you see from chronic wear of these lenses?
Dr. Sweeney: Studies have shown and we've seen in practice that chronic wearers of 24 Dk/t lenses have obvious redness even after lens discontinuation. Some have prominent neovascularization, while others show signs of corneal exhaustion or intolerance to lens wear.
Dr. Fonn: Can we reverse corneal damage?
Dr. Holden: Brien Saks (personal communication,1985) reported patients living at high altitudes with vascularization with low-Dk soft lenses were refitted with silicone elastomer lenses. Some had a 5-diopter change in refractive error, and the vessels cleared because they had constant normal oxygen tension. Vessels shrink and clear of blood, so redness is one problem that certainly decreases with high-Dk/t soft lenses. When patients switch to high-Dk/t lenses, their eyes are white again, and they notice.
Dr. Papas: In the literature 10 years ago, you could read that some limbal redness was actually acceptable for soft lens wear. That's not the case anymore.
Dr. Fonn: A year ago, a study was published showing that refitting long-term low-Dk/t lens wearers with polymegethism resulted in some recovery.1 What about reversing polymegethism?
Dr. Holden: During the Gothenburg study,2 over 12 months, we couldn't find a statistically significant recovery in polymegethism when patients discontinued lens wear in one eye. Some studies3 from Japan suggested that polymegethism decreased somewhat and the endothelium became slightly more regular.
Dr. Cavanagh: I think patients with 24-Dk/t soft lenses are unable to recover to baseline corneal thickness while the lenses are off at night. Over time, the endothelial pump function might become compromised. We don't measure this with polymegethism, but I recall Japanese data3 that show this effect. This has to be detrimental.
Dr. Hill: Reversibility is an interesting concept. I think some people are genetically less able to reverse these processes. The question is: If we keep leaning on the cornea, at what point do we reach irreversible damage? It's a probability game that we don't know much about at this time. Certainly, reversibility is an area we'll need to study.
1. Odenthal MT, Gan IM, Oosting J, et al. Long-term changes in corneal endothelial morphology after discontinuation of low gas-permeable contact lens wear. Cornea. 2005;24:32-38.
2. Holden BA, Sweeney DF, Vannas A, et al. Effects of long-term extended wear contact lens wear on the human cornea. Invest Ophthalmol Vis Sci. 1995;11:1489-1501.
3. Mishima S. Clinical investigations on the corneal endothelium-XXXVIII Edward Jackson Memorial Lecture. Am J Ophthalmol. 1982;93:1-29.
1. Sweeney D, ed. Silicone Hydrogels, ed 2. Philadelphia: Butter-worth Heinemann;2004.
2. Holden BA, Mertz GW. Critical oxygen levels to avoid corneal edema for daily and extended wear contact lenses. Invest Ophthalmol Vis Sci. 1984;25:1161-1167.