Not many people think about it but contact lenses are classified as medical devices. Millions of people use them. Thousands have problems because of them. The best way of ensuring safe contact lens wear is to explain the dangers of contact lens wear on a physiological basis.
The front surface of the eye is the cornea. It is clear. It has to be clear or otherwise light couldn’t reach the retina to be sensed. But clear is a problem for living tissue. Because in order to be clear, no blood can be in the tissue. And blood is what normally brings living tissue its oxygen.
So if there is no blood in the clear cornea, where is that living tissue getting its oxygen from? The answer is directly out of the air. No other tissue in your entire body does that. The whites of your eyes has its own blood supply and doesn’t need to do that, but your cornea does.
What are you doing to the cornea when you wear a contact lens? You are putting a hunk of plastic between it and its source of oxygen. So anytime you wear a contact lens you are stressing your cornea at least a little bit.
If you sleep in your contacts, you are giving your eyes a double whammy. Now between the cornea and its oxygen is an eyelid and a hunk of plastic. Studies done several years ago showed that oxygen tension dropped from the 21% it is in air to 3% at the level of the cornea when one slept in their lenses. This is not good enough to keep those cells happy and healthy.
So you have sick cells, a nice warm juicy environment, a contact lens that you have been handling with your fingers. You could have gotten some bacteria on them. What happens if you get the wrong bacteria, like PSEUDOMONAS? You could get a bacterial corneal ulcer.
A bacterial corneal ulcer is a sight-threatening condition. If you a bad one, you need to be put on a strong antibiotic drop every 15 minutes for the next 48 hours around the clock. That should be enough to kill the bacteria. But when the ulcer heals, a scar will be left behind. Scars are not clear, they are opaque. If the scar happens to lie on your visual axis, you will have lost some vision that can’t be replaced by anything short of a corneal transplant. And other studies have shown that people who make it a habit of sleeping in their contact lenses get a sight-threatening corneal ulcer at a 15 fold higher rate than those who take them out at night.
It is nice to sleep in your lenses. You don’t have to handle them in the morning. You can see as soon as you wake up. It is convenient. But you are trading convenience for risk. If you take your lenses out at night you cut your chances of developing a sight-threatening corneal ulcer by a factor of 15. I think the risk outweighs the convenience.
Having said all that, I can tell you that recently there has been a breakthrough in contact lens materials … the silicon hydrogels. All soft contact lenses let some oxygen through. Some do it better than others. Silicon hydrogels do it a lot better than others.
We can measure how much oxygen goes through a lens. That measurement is a technical term … Dk/t. All you really need to know about Dk/t is that the higher the number the more oxygen goes through.
Prior to the silicon hydrogels about the best Dk/t we could get was 30. There are 6 silicon hydrogels out on the market now. Their Dk/t are as follows:
Acuvue Advance – 88
Bausch and Lomb PureVision – 105
Ciba O2 Optix – 138
Acuvue Oasys – 148
Cooper Biofinity – 160
Ciba Focus Night and Day – 175
Of those lenses, my lens of choice is the Acuvue Oasys. People who wear it tell me it is the most comfortable. However, I should note that Cooper Biofinity is the newest and I have very little experience with it. I have heard good things about it as well.
Is this good enough to sleep in? My answer is unfortunately, “No”. These lenses do seem to have solved the oxygen problem, but there is a hazard from wearing the lenses so long. The lenses are still made of plastic and a lot of things like to bind to plastic. One of the things that like to bind to plastic is bacteria. If you get some bacteria on the lenses and leave it in the eye for a long period of time, you still increase the risk of corneal ulcers. Preliminary studies suggest that these lenses still do not solve the eye infection problem.
There you have it. I highly recommend the silicon hydrogels. You stress your cornea much less wearing these lenses than you do the old-style lenses. However, I still recommend you remove your lenses at night.
