How Much Is Enough Oxygen?

BY TED KASSALOW, O.D., F.A.A.O.
APR. 1996

For decades now, polymer scientists have been striving to bring us contact lenses that transmit an ever-increasing amount of oxygen to the cornea. Sometimes these products have a downside that occasionally offsets the intended benefit of the product. We tend to forget the tremendous variability of our patients and their individual oxygen needs.

One of the advantages of growing long in the contact lens tooth is that we can recall the time when PMMAs were the only game in town. We could design a lens larger or smaller, flatter or steeper, thinner or thicker; we could increase or decrease the bevel, and thin and reshape the edge. Skilled practitioners were rewarded with happy patients.

We still see many PMMA patients who wouldn't dream of changing their modality. Of course, most of us have seen a number of these patients with various degrees of edema in their corneal epithelium, but subjectively, all is well. The point is, we had very good success with a lens that transmitted zero oxygen. This is not to say that the hydrogel or gas permeable materials weren't welcome. They increased our success rate, particularly among those of us who only prescribed contact lenses occasionally.

EXTENDED WEAR MIRACLE

It seemed like a miracle when the 71 percent water Permalens reached our shores. Patients finally had the luxury of sleeping with contact lenses on their corneas for a month at a time. Yahoo!

But it wasn't long before we realized that a month was far too long. Many patients experienced discomfort, blood vessels infiltrating the cornea, lens coating -- a precursor of all the former symptoms -- and corneal ulcers from mild to severe. Our solution (years before the FDA voiced an opinion) was to reduce our patients' extended wear time to one or two weeks.

The next step, the 55 percent water lens supplied sufficient oxygen for daily wear and extended wear, oxygen permeability equal to Permalens by virtue of thinness, but featured less coating and better visual acuity. This was followed by the 38 percent water CSI lens, made of a thin daily wear and a very thin extended wear product that featured high oxygen transmission, less coating, excellent visual acuity and better physiology.

IMPROVED DK

When the rigid gas permeable lens joined the oxygen race, it seemed to be an answer to a contact lens practitioner's dream. But there were downsides. The lens warped easily, visual acuity was more variable and the lens coated more. As we read this, even more products are being introduced with still higher Dk numbers and greater wettability. In fact, we expect a 200+ Dk material to be arriving before long.

I hope you can fathom what I'm leading up to. Yes, we want and need a large variety of products to custom fit each patient. Even though we use the improved products most often, in cases where the patient is not responding satisfactorily, an older material may be just the thing. We all have anecdotes about patients who were unwillingly dragged from their PMMAs to gas perms and then returned comfortably to their time-tested, zero-oxygen PMMAs. There were even a few PMMA wearers who were able to achieve extended wear status, thus illustrating the tremendous variability in corneal oxygen demand.

There are still some aphakes who did not have implants and are doing beautifully with their PMMAs. The aphake's low oxygen demand leads to a much more forgiving fitting situation.

In summary, "new and improved" is usually, but not always, better for some patients. If we customize as much as possible and don't lose sight of the successes our patients have enjoyed with older generation products, we'll have happy contact lens patients who just might show their appreciation by telling their friends. CLS

Dr. Kassalow has been in practice in New York for 37 years and is a diplomate of the Cornea and Contact Lens Section of the AAO.