Contact Lenses: An Unobstructed View

Archives Admin · #1 (**permalink**) 04-18-2006, 11:05 AM

Contacts are a popular choice for correcting vision problems. Learn about the various types and some tips for preventing complications.

For people with vision problems, glasses are a common solution. Others opt for laser surgery. But wearing glasses or undergoing surgery aren't for everyone. That's why many people start each day by putting tiny pieces of plastic — contact lenses — on their eyes instead. The use of contact lenses isn't without risk, but proper selection and maintenance can help keep complications to a minimum.

contact lenses are made of many different types of plastic, but they're divided into two main groups: soft and rigid gas-permeable (RGP).

* Daily-wear soft lenses. More than 80 percent of lens-wearers choose this type. They're thin, polymer-plastic lenses that conform to the shape of your eye. Soft contacts contain between 25 percent and 79 percent water. Water is necessary to allow oxygen to pass through the lens and reach the cornea. Water also accounts for the wetness of the lens, which makes them quite comfortable and easy to adapt to. They're more flexible than rigid gas-permeable contacts and are more comfortable from the first wearing. These lenses have a shorter adaptation period and tend to stay in place better, making them a better choice for active lifestyles and sports. Daily-wear soft lenses aren't as durable as gas-permeable lenses, but are more durable than disposable soft lenses. They aren't as effective in correcting some vision problems, including high degrees of astigmatism, as are rigid lenses.

* Rigid gas-permeable (RGP) lenses. These lenses are made of harder plastic materials that don't contain water. Although they're not as flexible as soft contacts, they allow more oxygen to pass through to the cornea than do soft lenses. They offer excellent correction of a wide range of vision problems — such as astigmatism and distorted corneal shape caused by keratoconus or mild corneal scars — and are comfortable for most people after a short period of adaptation. RGP lenses are easy to care for, are less likely than soft lenses to cause infection and are more durable than soft lenses. One disadvantage of RGP lenses is that they may slip off the center of your eye more easily. And, to achieve maximum comfort with RGPs, you have to wear them every day. If you don't wear your soft lenses for several days, they'll still be comfortable when you put them on a week later. If you don't wear your RGPs for a week, you'll probably need an adjustment period before they're comfortable again.

What if I have presbyopia?
As you age, you may experience problems focusing on objects that are up close — a condition called presbyopia. Contact lens options that correct this condition also exist.

* Bifocal lenses. Today's bifocal lenses are available in daily-wear soft and rigid gas permeable materials. These lenses feature two prescriptions on one lens: one to correct distance vision, if that's needed, and the other to correct near vision.

Three different designs for bifocal contact lenses exist: simultaneous, concentric and alternating vision. Simultaneous vision designs fit centered on your cornea with both the distance-vision and near-vision prescriptions within the pupil area. Your own visual system learns to interpret the correct prescription choice depending on the distance you're viewing.

In a concentric design, the center portion of the lens contains the distance prescription and the outside portion contains the near prescription, or vice versa. Alternating vision designs work much like a bifocal eyeglass lens, in that the top of the lens has one prescription and the bottom has the other.

* Monovision. This is another option for people with presbyopia, in which you use a lens with the reading prescription in one eye and a lens with the distance prescription in the other eye. The brain often adjusts to this, and many people feel comfortable with the combined vision from both eyes. The distance vision lens is usually worn in your dominant eye.

* Modified monovision. This option entails wearing a bifocal or multifocal contact lens in one eye and a single-vision (nonbifocal) lens in the other eye. You may have a lens for distance correction in your dominant eye and a bifocal or multifocal lens in the other eye. Both eyes can be used for distance viewing, but only one for reading. Or you may use a bifocal or multifocal lens in the dominant eye and a single-vision lens for reading in the other eye. Both eyes could be used for reading and only one for distance viewing. This may be a good option if you spend a lot of time using a computer. Monovision and modified monovision options are available in both RGP and soft lenses.

These bifocal options require accurate and professional fitting, and getting used to them takes some time. Not everyone is able to use bifocal contacts. But you always have the option of wearing reading glasses over distance-only contacts.

How often do I need to replace my lenses?
contact lenses are made to be worn for a limited period of time before needing replacement. Gas-permeable lenses may be worn for several years. Traditional soft lenses are only worn for about a year. Some soft lenses are designed to be replaced more frequently. These include:

* Disposable lenses. These are soft lenses designed to be worn for a specific period of time, then thrown away and replaced with a fresh pair of lenses. Depending on the lens type and how you care for them, they can be worn for one day or up to three months — during waking hours — before being discarded. The most common replacement recommendation is two weeks. The more frequently you replace your contact lenses, the healthier and more comfortable your eyes are likely to be. Substances like protein, calcium, and lipids — found naturally in your tears — can build up on your lenses. These deposits make your contacts less comfortable than when they were new, and can also make your eyes more prone to infection.

* Extended-wear lenses. These are soft lenses that are approved for more than 24 hours of continuous wear. They are designed to provide adequate oxygen to your cornea even while you sleep, so they can be worn continuously for up to seven days before they need to be removed for cleaning. In 2001, the Food and Drug Administration approved new extended-wear lenses that can be prescribed for extended overnight wear up to 30 days and nights. These new lenses are made of superpermeable silicone hydrogel, which allows far more oxygen to get through to the cornea than do ordinary extended-wear lenses.

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