Contact lenses Health Dictionary

Very thin, shell-like, transparent discs fitted on the cornea of the eye to correct defective vision. Generally, contact lenses are used to correct myopia (shortsightedness) and hypermetropia (longsightedness). There are several types.

Hard plastic lenses give good vision, are long-lasting and durable, inexpensive, and easy to maintain. However, they are sometimes difficult to tolerate and may fall out. Hard gas-permeable lenses are more comfortable because they allow oxygen to pass through to the eye, but are less durable. Soft lenses are the most comfortable because of their high water content. Disposable soft lenses are for single-use only; extended wear lenses are worn for up to a month.

Other types of lenses include rigid, scleral lenses that cover the whole of the front of the eye and are used to disguise disfigurement due to injury or disease; bifocal contact lenses; and interruptus; hormonal methods, including the use of oral contraceptives, implants, and injections (see contraceptives, injectable); intrauterine devices (see IUDs); postcoital methods (see contraception, emergency); or sterilization of the male (see vasectomy) or female (see sterilization, female).

contraception, barrier methods of

The use of a device and/or a chemical to stop sperm reaching an ovum, preventing fertilization and pregnancy. Barrier methods also help prevent the sexual transmission of diseases such as AIDS, genital herpes (see herpes, genital), and viral hepatitis (see hepatitis, viral).

toric contact lenses with an uneven sur- face curvature to correct astigmatism.

Hard plastic contact lenses may cause abrasion of the cornea if they are worn for too long. Soft lens wearers sometimes develop sensitivity of the eyes and lids. Other problems that may occur with any type of contact lens include infections and redness of the eye.

Contact lenses are lenses worn in contact with the EYE, behind the eyelids and in front of the cornea. They may be worn for cosmetic, optical or therapeutic reasons. The commonest reason for wear is cosmetic, many short-sighted people preferring to wear contact lenses instead of glasses. Optical reasons for contact-lens wear include cataract surgery (usually unilateral extraction) and the considerable improvement in overall standard of vision experienced by very short-sighted people when wearing contact lenses instead of glasses. Therapeutic lenses are those used in the treatment of eye disease: ‘bandage lenses’ are used in certain corneal diseases; contact lenses can be soaked in a particular drug and then put on the eye so that the drug slowly leaks out on to the eye. Contact lenses may be hard, soft or gas permeable. Hard lenses are more optically accurate (because they are rigid), cheaper and more durable than soft. The main advantage of soft lenses is that they are more comfortable to wear. Gas-permeable lenses are so-called because they are more permeable to oxygen than other lenses, thus allowing more oxygen to reach the cornea.

Disposable lenses are soft lenses designed to be thrown away after a short period of continuous use; their popularity rests on the fact that they need not be cleaned. The instructions on use should be followed carefully because the risk of complications, such as corneal infection, are higher than with other types of contact lenses.

Contraindications to the use of contact lenses include a history of ATOPY, ‘dry eyes’, previous GLAUCOMA surgery and a person’s inability to cope with the management of lenses. The best way to determine whether contact lenses are suitable, however, may be to try them out. Good hygiene is essential for wearers so as to minimise the risk of infection, which may lead to a corneal abscess – a serious complication. Corneal abrasions are fairly common and, if a contact-lens wearer develops a red eye, the lens should be removed and the eye tested with ?uorescein dye to identify any abrasions. Appropriate treatment should be given and the lens not worn again until the abrasion or infection has cleared up.

lenses worn directly against the eye, separated from it only by a film of tear fluid. Corneal microlenses cover only the cornea, while haptic lenses cover some of the surrounding sclera as well. Contact lenses are used mainly in place of spectacles, to correct errors of refraction, but also in a protective capacity (see bandage lens). Contact lenses are made of a variety of materials, such as glass, plastic, and silicone; soft gas-permeable lenses have mostly replaced hard Perspex lenses.