For many people contact lenses are the ideal alternative to spectacles. For sport, special occasions or everyday wear, they are a versatile solution. Contact lenses, like eyeglasses or refractive surgery, can correct your nearsightedness, farsightedness, and astigmatism.
Types of Contact lenses?
Classified by material, there are three types of contact lenses:
These are made from PMMA — also known as Plexiglas or Lucite. These lenses are virtually obsolete and rarely used.
These are by far the most commonly used lenses. They are usually made of a hydrogel polymer and fit snugly onto the cornea making them very comfortable to wear. Some soft lenses have silicone compounds added to allow oxygen permeability, thus permitting significantly increased wearing times, i.e. “Extended wear”.
Rigid Gas Permeable lenses (RGP)
These are smaller and are made from rigid, waterless plastics and initially less comfortable to wear but are especially good for presbyopia and high astigmatism. For some people they allow the sharpest vision, and their oxygen permeability allows them to worn all day. This type of lens should typically last at least a year before replacement is needed. A well known problem with RGP lenses is their tendency to temporarily change the shape of the cornea, so for those contemplating refractive surgery, the lenses may have to be left out for a couple of weeks for the cornea to recover.
From the introduction of soft lenses in 1971 until relatively recently, most lens brands have been made from “hydrogel” plastics. Recently, new silicone hydrogel contact lenses have been introduced. They have become the contact lenses of choice for many eye care practitioners, because they allow more oxygen to pass through the lens to the eye, and they are less prone to dehydration.
How long can I wear contact lenses?
Until 1979, everyone who wore contact lenses removed and cleaned them nightly. The introduction of “extended wear” enabled wearers to sleep in their contacts. Now, two types of lenses are classified by wearing time:
Daily wear — must be removed nightly
Extended wear — can be worn overnight, usually for seven days consecutively without removal
“Continuous wear” is a type of extended wear lens that can be worn for 30 consecutive nights.
What are the different types of contact lens designs?
Many lens designs are available to correct various types of vision problems:
Sphericalcontact lenses are the typical, rounded design of contact lenses, which can correct myopia (nearsightedness) or hyperopia (farsightedness).
Toric Lenses for Astigmatism
Astigmatism is when the cornea is not perfectly spherical or round. Instead of one radius of curvature, it has two running at 90° to each other. Toric lenses allow greater visual clarity for those with astigmatism by correcting this irregularity. They are available in both soft and RGP form, though soft are by far the most commonly used.
Bifocal and Multifocal Lenses
Bifocal and multifocal contact lenses contain different zones for near and far vision to correct presbyopia, These are useful for those into their 40s and beginning to struggle with near tasks. They allow the wearer to achieve clear vision at a variety of distances.
This is more commonly used in the USA than in Britain. Special contact lenses are worn at night before going to bed and they gently mould the cornea whilst the wearer is asleep. Upon waking the lenses are removed, and the temporary cornea shape change allows clearer vision, however, the lenses must be worn every night, or alternate nights or the cornea reverts back to its original shape.
All of these lenses can be custom made for hard-to-fit eyes. Many other additional lens designs are available. Typically these are less common and fabricated for use in special situations, such as correcting for keratoconus
What Other features should I look out for?
Coloured Lenses. Many of the types of lenses described above also come in colors that can enhance the natural color of your eyes — that is, make your green eyes even greener, for example. Or these lenses can totally change the eye’s appearance, as in from brown to blue. They usually consist of a soft contact lens base with coloured iris pattern printed on top.
Special-Effect Lenses. Also called theatrical, novelty, or costume lenses, these take coloration one step further to make you look like a cat, a zombie, or another alter-ego of your choice.
Prosthetic Lenses. Colored contact lenses can also be used for more medically oriented purposes. People with disfigured eyes, as a result of accidents or disease, can use a custom, opaque colored lens to mask the disfigurement and match the appearance of their normal eye.
UV-Inhibiting Lenses. Today, many contacts incorporate an ultraviolet blocker in the lens material, to cut down on UV light that can eventually cause cataracts and other eye problems. You can’t see this blocker by looking at the lens. And since contacts don’t cover your entire eye, UV blockers cannot substitute for traditional sun protection like good quality sunglasses.
Hybrid Lenses. One brand of lenses features a GP center with a soft outer skirt, providing wearers with both the crisp optics of a rigid lens and the comfort of a larger, soft lens.
About Replacement Schemes?
This should be discussed with you by your eye care practitioner. Some lenses are manufactured with a specific wearing time, i.e. daily soft lenses, which are thrown away every day and some which will last a whole year. Replacement schedules may vary from one person to another, for example, a patient who suffers from persistent lens deposits may have to replace sooner.
What problems can contact lenses cause?
Problems may arise for a variety of reasons, but they are usually relatively easy to solve. Here are some of the basic problems:
- Giant Papillary Conjunctivitis
This is characterised by general eye itching and contact lens intolerance. It is readily diagnosed by eversion of the upper eyelid and observing the “cobblestone” appearance underneath. It is generally thought to be due to an immune response to protein deposits on the lens surface. This can be remedied by more frequent lens changes or the use of mast cell stabilisers. Occasionally you may be advised to stop contact lens wear altogether.
- Bacterial Corneal Ulceration
This is a more serious problem associated with contact lens wear. There is typically pain, watering of the eye and occasionally blurred vision, depending upon the location. Diagnosis is usually made by taking a scrape of the corneal surface and identifying the pathogen. Treatment is most commonly in the form of eye drops or eye ointment. For more serious infections, permanent visual impairment may result depending upon the location of the ulcer.
- Corneal Vascularisation
Corneal vascularisation is fairly rare in RGP lenses and almost solely attributed to the long term wear of soft lenses. It is thought that the stimulus for these new vessels is mainly due to the cornea being starved of oxygen. It usually has no symptoms and is characterised by blood vessels gradually invading the cornea. These new vessels are generally thought to be permanent, so it is important to remedy the problem as soon as possible. Possible solutions are to reduce wearing time, fit a silicone hydrogel lens, or change to an RGP. If these solutions fail then you may be advised to stop contact lens wear altogether.
- Corneal Oedema
Corneal oedema or corneal swelling is caused by restricted oxygen supply to the cornea, creating raised lactic acid levels, and is most common with soft lenses and sleeping in lenses overnight. Symptoms are blurred vision, halos, contact lens intolerance and a generally red and watery eye, however occasionally there may be no symptoms at all. This can be alleviated by reducing wearing time, changing to a more permeable lens, or one with a higher water content.
- Dry Eye
Those with reduced or marginal tear film quality may experience dryness and a gritty feeling especially towards the end of the day. The wearing of any contact lens places extra demand upon the tear film, and it is not uncommon to have comfort problems. Aggravating factors include blepharitis and dry, air-conditioned environments. Using artificial tear lubricants appears to alleviate symptoms and for some, RGP lenses are more compatible.
- Acanthamoeba Keratitis
This is probably the most dreaded of all contact lens complications and can have devastating visual consequences. The Acanthamoeba protozoan can commonly be found in soil, tap water and hot tubs and swimming pools, and in its dormant form, is remarkably resistant to amoebicides. The infection may display a characteristic “ring form” on the cornea with extreme pain and redness. Brolene is commonly used as a treatment, but for some a combination of drugs may be necessary to control the infection. As is often the case, prevention is the best solution, so swimming with lenses and using tap water for rinsing is not advised.
As is the case with most things, prevention is the best cure and many problems can be avoided by following instructions and visiting your practitioner regularly.