Contact lenses are small, light-weight plastic devices worn on the eye that correct refractive errors in vision. While they appear to be worn in direct contact with the cornea, they actually float on a layer of tears that separates them from the cornea.
Contact lenses correct or improve the vision of people with nearsightedness (myopia), farsightedness (hyperopia), presbyopia, and astigmatism. In recent years, some people also wear contact lenses not for medical reasons, but rather to change their eye color.
People allergic to certain plastics should not wear contact lenses manufactured from that type of material.
Patients with dry eye or severe seasonal allergies may find contact lenses uncomfortable and may prefer eye glasses. A careful patient history needs to be taken by the physician or contact lens technician to make sure these problems are addressed.
Eye care professionals should ensure that contact lens patients who have disposable or planned replacement lenses keep strictly to their replacement schedules. Contact lenses wear out over time and can damage patients' eyes. Deposits also can build up on the lenses, leading to lid and eye infections.
Patients who have lenses they can sleep in (extended wear lenses) also are advised not to keep their contact lenses in their eyes for longer periods than directed by their physician. Adherence to the schedules recommended by their physicians helps patients avoid infection and long-term damage to the cornea.
People employed in certain occupations may be prohibited from wearing contact lenses, or may be required to wear safety eyewear over the contact lenses. Physicians and employers should be consulted for recommendations.
Ophthalmologists (M.D.s) or optometrists (O.D.s) dispense contact lenses. The prices for lenses vary for the different types. Some physicians offer a "global fee" to their patients that includes the contact lens fitting, lenses, and follow-up visits.
Over 32 million people in the United States wear contact lenses. These lenses provide a field of view unobstructed by eyeglass frames. They do not fog-up or get splattered and are less noticeable than any eyeglass style. On the other hand, they take time to get accustomed to; require more measurements for fitting; require many follow-up visits to the eye doctor; can lead to complications such as infections and corneal damage (but only if not cared for properly or replaced as prescribed); and may not correct astigmatism as well as eye glasses.
Originally, hard contact lenses were made of a material called PMMA. Although still available, it is rarely used because it does not allow oxygen to pass through the lens. The more common types of contact lenses are:
- Rigid gas-permeable (RGP) daily-wear lenses are made of plastic that does not absorb water, but does allow oxygen to permeate from the atmosphere to the cornea. (This is important because the cornea has no blood supply and needs to acquire oxygen from the atmosphere through the film of tears that moves beneath the lens.) They must be removed and cleaned each night.
- Rigid gas-permeable (RGP) extended-wear lenses are made from plastic that also does not absorb water and is more permeable to oxygen than the plastic used for daily-wear lenses. They can be worn for up to a week, then cleaned, and reinserted.
- Daily wear soft lenses are made of plastic that is permeable to oxygen and absorbs water; therefore, they are soft and flexible. These lenses must be removed and cleaned each night, and they do not correct all vision problems. Many patients find it easier to become accustomed to soft lenses, but these lenses are more prone to ripping and do not last as long as rigid lenses.
- Extended-wear soft lenses are highly permeable to oxygen, are flexible by virtue of their ability to absorb water, and can usually be worn for up to one week. They do not correct all vision problems.
- Extended-wear disposable lenses are soft lenses worn continually for up to six days and then discarded, with no need for cleaning.
- One type of disposable soft lenses are worn for one day and then discarded by the patient. Doctors believe these lenses reduce the risk of infection due to poor cleaning habits or over-wearing lenses. They can be price prohibitive for some patients at $1 a day.
- Planned-replacement soft lenses are daily wear lenses that are replaced on a regular schedule, most common- ly every two weeks, monthly, or quarterly. They must also be cleaned.
Soft contact lenses come in a variety of materials. High Dk materials provide high oxygen permeability and allow greater comfort for patients. Advances in toric contact lenses also allow individuals with astigmatism to achieve better visual correction from contact lenses with more comfort.
There are also different kinds of RGP and soft multifocal contact lenses available. There are five basic designs and over one hundred brands that offer variations of these designs. The optics of bifocal contact lenses are improving dramatically, as baby boomers move into presbyopia and more research dollars are devoted to this area. Monovision, where one contact lens corrects for distance vision while the other corrects for near vision, also may be an option for presbyopic patients. However, monovision may affect depth perception and may not be appropriate for everyone. Contact lenses also come in a variety of tints. Soft contact lenses are available that can change eye color. Even though such lenses have no prescription, they must still be fitted and checked to ensure that an eye infection does not occur. People should never wear someone else's contact lenses. This can lead to infection or damage to the eye.
Before contact lenses are prescribed, the patient's eyes are examined by an optometrist or an ophthalmolo- gist. Contact lens technicians, ophthalmic nurses, and ophthalmic technicians also may assist with the exam, although frequently their findings are verified by the physician.
Eyeglass prescriptions, if necessary, are then given to patients. A separate contact lens-fitting exam is necessary if the patient wants contact lenses. This is sometimes performed by the doctor, but in many cases is completed by a contact lens technician.
Before prescribing contact lenses, a technician performs an evaluation. He or she conducts a written and oral interview with the patient to determine if the patient is a viable contact lens candidate. The technician then assesses the technical aspects of the patient's ocular status, since good eye health is required to wear lenses. The patient's palpebral aperture and visual iris diameter is measured to determine the appropriate diameter for the contact lens. The technician also tests the patient's tear quantity. A poor tear film is a contraindication for contact lenses use. Using instrumentation and information gained from the patient's eye exam, the technician determines what type of lens is best for the patient. For example, RGPs are commonly given to astigmatic patients because they provide clearer vision for these patients.
The contact lens technician selects the lens material and design, then determines the best trial lens. Based on the patient's experience with the trial lenses, the technician then determines the lens parameters. The physician reviews these findings to make the recommendation for the proper contact lenses. He or she ascertains the proper fit, and measures visual acuity and over-refraction to determine the proper lens prescription. Because a contact lens is an FDA-approved medical device placed directly onto the eye (unlike eye glasses), it cannot be issued until the eye's proper health has been ascertained via a follow- up examination. Unlike eyeglass prescriptions, a contact lens prescription includes not only the prescription, but also the contact lens material, diameter, brand, and curvature.
Before the patient is sent home with the lenses, the technician will give a detailed demonstration of inserting, removing, and cleaning the lenses. A written list of detailed instructions is usually dispensed with the lenses.
The doctor and technician schedule several return visits for the patient to evaluate the lens fit and performance. At these visits, eye care professionals determine whether the lens design or material needs to be modified.
Patients may be allergic to certain solutions that are used to clean or lubricate the lenses. For that reason, physicians instruct patients not to change solutions without prior approval. Contact lens wearers also are advised to seek immediate attention if they experience eye pain, a burning sensation, red eyes, intolerable sensitivity to light, cloudy vision, or an inability to keep the eyes open.
To avoid infection, it is important for contact lens wearers to exactly follow the instructions for lens insertion, removal, and cleaning. Soft contact lens wearers should never use tap water to rinse their lenses or to make up solutions. All contact lens wearers should always carry a pair of glasses and a contact lens case with them,
in case the contact lenses have to be removed due to eye irritation.
Wearing contact lenses increases the risk of corneal damage and eye infections, if the lenses are not properly cleaned, if the lenses are inserted or removed with dirty hands, or if the lenses are worn for a longer period of time than recommended. However, there is little increased risk of eye infection or damage if the lenses are worn as instructed and cared for properly. Dry eye and seasonal allergy symptoms may be exacerbated by wearing contact lenses.
The normal expectation is that people will achieve 20/20 vision while wearing corrective lenses.
Health care team roles
Nursing and allied health professionals play an important role in contact lens examination and fitting. With advances in technology, technicians now have duties that formerly only a physician performed.
Contact lens technicians take the lead role in the contact lens fitting by recording the pertinent patient history, measuring the eye for the proper lens fit, and testing the tear quantity.
Advanced and intermediate level ophthalmic technicians perform refractions and determine the patient's depth perception. These professionals also may perform corneal topography (mapping).
Some of these professionals seek certification through the American Board of Opticianry and the National Contact Lens Examiners or other organizations. These organizations offer seminars and testing that helps professionals keep current with technological advances.
Physicians have become increasingly dependent on these technicians and assistants as managed care dictates they see more patients per day. By delegating these task to qualified personnel, the physicians can see more patients per day without jeopardizing patients' ocular health.
Technicians emphasize the importance of cleaning contact lenses and adhering to any planned replacement schedule. They stress that while patients may save money by wearing lenses longer or skimping on cleaning solution, these actions increase the risk of eye infections or damage. Younger patients also are cautioned that sharing colored contact lenses with others can lead to eye infections. The importance of inserting and removing contact lenses with clean hands also is emphasized to all contact lens wearers.
Optometrists and ophthalmologists receive training in optometry schools or medical residencies for these procedures. Contact lens technicians may complete special training courses or be trained by the physician.
Astigmatism—A vision condition that occurs when the cornes is slightly irregular in shape. This irregularity prevents light from focusing properly on the retina.
Cornea—The clear outer covering of the front of the eye.
Index of refraction—A constant number for any material that is an indicator of the degree of the bending of the light caused by that material.
Lens—A device that bends light waves.
Permeable—Capable of allowing substances to pass through.
Polycarbonate—A very strong type of plastic often used in safety glasses, sport glasses, and children's eye glasses. Polycarbonate lenses have approximately 50 times the impact resistance of glass lenses.
Polymer—A substance formed by joining smaller molecules. Examples of polymers include plastic, acrylic, cellulose acetate, cellulose propionate, nylon, etc.
Presbyopia—A condition affecting people over the age of 40 where the system of accommodation that allows focusing of near objects fails to work because of age-related hardening of the lens of the eye.
Retina—The inner, light-sensitive layer of the eye containing rods and cones; transforms the image it receives into electrical messages sent to the brain via the optic nerve.
Ultraviolet (UV) light—Part of the electromagnetic spectrum with a wavelength just below that of visible light. It is damaging to living material, especially eyes and DNA.
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