Careless Use of Contact Lenses Can Cause Blindness
16 Feb 2003
It’s estimated that 38 million people in North America wear contact lenses (that’s 76 million eyes). Owners love the benefits. They’re great for sports. They don’t break, fall off or steam up. Most people find them more comfortable than glasses and more flattering. And they’re a great boon following cataract surgery. But placing foreign objects anywhere in the body has a dangerous downside.
I was recently in Washington D.C to attend a research seminar on the prevention of blindness. One of the speakers, Dr. Dwight Cavanagh, Professor of Ophthalmology at the University of Texas Southern Medical School, has been warning for years that users of contact lenses should use them the way porcupines make love, very, very carefully. And that failing to do so can result in serious eye infections.
This time he had a specific warning. He reported that contact lenses wearers are at risk of developing blinding eye infections particularly during the first six months of use. But the risk of bacterial eye infection is not related to whether the lens is hard or soft. Or whether patients wear them daily or continuously. Rather, it depends on what contact lenses are made of.
Bacterial infection of the eye has always been a major concern and the culprit is usually a germ called pseudomonas aeruginosa. The risk of infection for soft lenses is one in 2,500 if they are worn during the day. But this increases to one in 500 if worn overnight.
For rigid lenses the risk of infection is about one in 10,000. And the newer lenses are 10 to 40 times safer. Individually it’s a small risk, but since 100 million people worldwide use contact lenses, this infection strikes many.
Infection is not the only problem. Several years ago Professor Allansmith of The Harvard Medical School reported that debris called "biodeposits" can also build up on the contact lens. This causes irritation and sometimes corneal ulcers, a serious complication that can result in visual loss. It’s amazing corneal ulcers don’t occur more frequently since we blink about 20,000 times every day, the length of three football fields.
In 1994 Dr Cavanagh reported that every lens on the market triggered an increase in the number of bacteria in the eye. Covering the cornea with a lens automatically decreases the flow of oxygen to underlying tissues. It creates a good breeding ground for pseudomonas aeruginosa which tends to thrive when there’s a decreased amount of oxygen. The obvious solution was to manufacture a lens that easily transmits oxygen. Dr. Cavanagh presented his research during the meeting I attended. Patients were given different types of lenses to wear, regular high-oxygen soft lenses, hyperoxygen softlenses and hyperoxygen rigid gas permeable (RGP) lenses. They were then given periodic eye washes to determine the number of bacteria present.
Dr Cavanagh concluded that the hyperoxygen RGP lens was the clear winner as bacterial levels did not increase on the cornea during the trial.
But there was another surprising finding. After three months the level of bacteria sticking to the cornea on patients wearing the other lenses started to drop. And after six months the bacterial levels were the same for all three lenses. In some way the eye adapted to the lens eradicated the bacteria.
I’m sure there will be other improvements to contact lenses. But until the perfect lens is developed Dr. Cavanagh stressed the need to watch contact lens users carefully during the first six months of use.
I haven’t succumbed yet to the contact lens. I would if glasses bothered me or there was a medical need. But I’ve always believed one rarely get something for nothing.
For instance, what about the prospect of corneal thinning? Dr. Cavanagh says, "It’s a new and unexpected problem for those who wear contact lenses."
Dr. Cavanagh explained that when the cornea is covered over by a contact lens the cornea is less able to maintain the usual production of new cells. This means that in some cases the cornea decreases in thickness by 10 percent. Admittedly not a major amount, but in effect, the cornea is saying, I don’t like being covered up.
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