Chapter 6: Cornea
CONTACT LENSES
Glass contact lenses were first described in 1888 by Adolf Fick and were then used for the treatment of keratoconus by Eugene Kalt. Poor results were achieved until 1945, when Kevin Tuohy of Los Angeles produced a plastic precorneal lens with a diameter of 11 mm. Since that time, advances in contact lens technology have produced several different varieties of lenses, which are broadly divided into two types: rigid and soft lenses. The basic requirement for success of contact lenses is to overcome the effect on oxygen supply to the cornea from wearing an occlusive lens. The optical features of contact lenses are discussed in Chapter 21.
Rigid (Hard) Lenses:
A. Standard Hard Lenses:
These direct descendants of Tuohy's lens are made of polymethylmethacrylate (PMMA, Perspex), are impervious to oxygen, and thus rely on pumping of tears into the space between the lens and the cornea during blinking to provide oxygen to the cornea. They are smaller than the corneal diameter. Always for daily wear, these lenses are easy to care for, are relatively inexpensive, and correct vision efficiently, particularly if there is significant astigmatism. Unfortunately, many persons cannot tolerate them. Corneal edema due to corneal hypoxia and spectacle blur (poor vision with spectacle correction after a period of contact lens wear) are common complaints
B. Gas-Permeable Hard Lenses:
These are rigid lenses made from cellulose acetate butyrate, silicone acrylate, or silicone combined with polymethylmethacrylate. They have the advantage of high oxygen permeability, thus improving corneal metabolism, and greater comfort, while retaining the optical properties of rigid lenses. They are generally used on a daily wear basis but can be used on an extended-wear (24-hour) basis in exceptional circumstances. In keratoconus, the gas-permeable lens has become the lens of first choice.
Soft Lenses
A. Cosmetic Soft Lenses:
Hydrogel lenses, based on hydroxymethyl methacrylate (HEMA), are considerably more comfortable than rigid lenses but are flexible and thus conform to the surface of the cornea. Regular astigmatism can be partially corrected by incorporating cylinder into the soft lens; irregular astigmatism is poorly corrected. The oxygen permeability and water content values vary among different types of hydrogel lens. They are more difficult to care for and more expensive than rigid lenses. Complications are also more common and include ulcerative keratitis, (particularly if the lenses are worn overnight), immune corneal reactions to deposits on the lenses, giant papillary conjunctivitis, reactions to lens care solutions (especially those containing the preservative thimerosal), corneal edema, and corneal vascularization.
Cosmetic soft contact lenses are usually worn on a daily wear basis. For aphakic correction, it is occasionally necessary to resort to extended wear because of the patient's inability to insert and remove the lenses themselves. Extended wear increases the risks associated with use of contact lenses.
B. Disposable Soft Lenses:
These lenses are designed to be discarded daily or sometimes after extended wear for 1 week. They eliminate the need for contact lens solutions and theoretically reduce the risk of ulcerative keratitis by minimizing bacterial adherence to the lens surface. They do appear to be safe if worn on a daily wear basis, but overnight wear, even for one night, is associated with a significant risk of ulcerative keratitis.
C. Therapeutic Soft Lenses:
The use of therapeutic soft contact lenses has become an indispensable part of the ophthalmologist's management of external eye disease. The lenses form a soft barrier between the outside and the cornea, providing protection against trichiasis and exposure. Lenses with high water content can act as a "stent" for epithelial healing, such as in the treatment of recurrent erosions. Patients with pain due to epithelial disease, such as in bullous keratopathy, particularly benefit from therapeutic soft contact lenses. Lenses with low water content can be used to seal small corneal perforations or wound leaks. In all cases of therapeutic contact lens wear, infection can occur. Antimicrobial coverage may be indicated if epithelial defects exist.
Contact Lens Care
It is essential that all contact lens wearers be made aware of the risks associated with contact lens wear-particularly those patients choosing the high-risk varieties such as extended-wear lenses for cosmetic optical correction purely on the grounds of convenience. All wearers must be under the regular care of a contact lens practitioner. Many of the chronic complications of contact lens wear are asymptomatic in their early and easily treated stages. Any contact lens should be removed immediately if the eye becomes uncomfortable or inflamed, and ophthalmic attention must be sought immediately if symptoms do not rapidly resolve.
Contact lenses require regular cleaning and disinfecting, and in the case of soft and gas-permeable lenses removal of protein deposits is required. Disinfection regimens include heat, chemical soaking, and hydrogen peroxide systems. All are effective if used according to the manufacturer's instructions, though heat systems may be preferable for combating resistant organisms such as Acanthamoeba. Soft and gas-permeable lenses are much less durable than hard lenses; contact lenses vary in tolerance to disinfection.
There is a significant trend among soft lens wearers toward the use of nonpreserved contact lens care systems because of the development of preservative- related hypersensitivity reactions. It is important that such individuals be aware of the ability of organisms such as Pseudomonas and Acanthamoeba to survive in nonpreserved saline solutions, such as may be found in their contact lens storage cases. The use of nonpreserved contact lens solutions requires much greater vigilance in the regular disinfection of lenses and lens storage cases.
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10.1036/1535-8860.ch6