As a result of the risks associated with contact lens wear, contact lens research and development has focused in the past on producing lenses that allow higher levels of oxygen to pass through to the eye. Researchers assume that if a material could be created that would allow almost as much oxygen to get to the cornea as compared to not wearing any lens at all, these painful infections and other contact lens related inflammations would be minimized or eliminated once and for all.
Oxygen Deprivation Syndrome
Around 1995, high oxygen contact lenses consisting of silicone were re-introduced to the public. These new lenses allowed as much as seven times the amount of oxygen as current lenses to pass through to the cornea and eye, even during sleep.
So, where are we now? Have these high-oxygen silicone lenses helped to eliminate complications? The answer is both yes and no. Silicone lenses do provide a healthy environment for the eye overall. Many patients who over-wear their regular soft contact lenses can develop "oxygen deprivation syndrome." Oxygen deprivation syndrome is a term coined by the eye care industry to represent patients who do not have an infection, but have swelling in their corneas and have developed neovascularization, or "new blood vessel growth" in their eyes. These abnormal blood vessels are attempting to feed oxygen to the cornea through blood flow instead of from the atmosphere where the cornea normally receives its oxygen. These patients often have red eyes and their vision fluctuates heavily because of the swelling. As a result, it is very difficult for an eye doctor to test their vision during an eye exam. However, when doctors re-fit them into one of the new silicone-based lenses, almost magically, many of those signs or symptoms resolve quickly.
Research Doesn't Show Reduced Infection Risk
What about infection? Do these lenses eliminate infections as well? Recent studies were completed by both Australian and United Kingdom researchers to find out if the new silicone hydrogel lenses would reduce infection risk as a result of the improved oxygen permeability. These studies agreed that silicone lenses did not reduce the risk of bacterial keratitis for contact lens wearers. Current research is focusing on what clinical doctors have felt for years: Contact lens-related infections may be caused by factors other than oxygen, such as tear film stagnation, changes in the surface of the cornea, and the slower turnover of corneal cells induced by contact lens wear. It is important to understand that oxygen transmission is still a large factor, but may not be the only factor that contributes to infection.
One risk factor seems to turn up in almost every study completed on contact lens-related keratitis...sleeping in contact lenses. The single, largest risk factor for permanent vision loss is wearing lenses overnight. Your risk for developing infection is five times higher if you sleep in contact lenses. Other risk factors for developing serious eye infections include smoking, purchasing lenses via the internet, low socio-economic status, improper cleaning, extended wearing times, and young age. The UK study found that risks varied significantly depending on the brand of contact lens. In this study, researchers looked at whether single use, daily disposable contact lenses have a lower infection rate than two week or monthly disposable lenses. Interestingly, daily disposable lens wearers had a 1.5 times higher risk of developing keratitis. However, the type of bacteria or "bug" was much less nasty. In other words, the organisms that caused the infections in daily disposable lens wearers were not as likely to cause severe vision loss. In fact, none of the daily disposable wearers had end results of vision worse than 20/40.
The current brands of silicone hydrogel lenses available, in order of highest oxygen transmissibility to lowest:
- Focus Night & Day by Ciba
- Air Optix Aqua by Ciba (enhances comfort for people who wear lenses daily)
- Biofinity by Coopervision
- Acuvue Oasys by Vistakon (designed to be more wettable than the others, beneficial for people who have dry eyes)
- PureVision by Bausch and Lomb
- Acuvue Advance by Vistakon
There are also two toric (astigmatism correcting) silicone hydrogel lenses on the market:
- PureVision Toric by Bausch
- Acuvue Advance for Astigmatism
Although these lenses are a bit more pricey than traditional lenses, they do provide additional benefits to wearers.
So the controversy continues: Is it better to re-use a lens that must be disinfected and stored in potentially unclean cases that harbor bacteria or to simply dispose of a lens every day? Everyone’s lifestyle, biochemistry and physiology differs, so what works for one patient may not work for the next. Ask your eye care professional for advice.
Source: Sindt, Christine W., O.D. Silicone Hydrogel Contact Lenses. Review of Optometry, Sept 2007.