To minimize corneal swelling, contact lenses
must provide a healthy level of oxygen transmissibility.
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
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.
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
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
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
the closest to normal that we can provide and afford.
WILL WE MOVE TO A TOTALLY
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
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?
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
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.
1. Sweeney D, ed. Silicone Hydrogels, ed 2. Philadelphia:
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.
Contact Lens Spectrum, Issue: August 2005