Cataracts

Description

Causes

Although older age is the primary risk factor for cataracts, experts are still not certain about the exact biologic mechanisms that tie cataracts to aging.

Oxygen-Free Radicals (Oxidants) and Glutathione

Researchers have been focusing on particles called oxygen-free radicals as a major factor in the development of cataracts. They cause harm in the following way.

• Oxygen free radicals (also called oxidants) are molecules produced by natural chemical processes in the body. Toxins, smoking, ultraviolet radiation, infections, and many other factors can create reactions that produce excessive amounts of these oxygen free radicals.
• Oxidants are missing an electron, so they are unstable and tend to chemically bind with other molecules in the body. When oxidants are overproduced, these chemical reactions can be very harmful to nearly any type of cell in the body. At times these reactions can even effect genetic material in cells.
• Cataract formation is one of many destructive changes that can occur with overproduction of oxidants, possibly in concert with deficiencies of an important protective anti-oxidant called glutathione.
• Glutathione occurs in high levels in the eye and helps clean up these free radicals. One theory posits that in the aging eye, barriers develop that prevent glutathione and other protective antioxidants from reaching the nucleus in the lens, thus making if vulnerable to oxidation.

Radiation and Electromagnetic Waves

Sunlight and Ultraviolet Radiation. Sunlight consists of ultraviolet (referred to as UVA or UVB) radiation, which penetrates the layers of the skin. Both have destructive properties that can promote cataracts. The eyes are protected from the sun by eyelids and the structure of the face (overhanging brows, prominent cheekbones, and the nose). Long-term exposure to sunlight, however, can overcome these defenses.

• UVB radiation produces the shorter wavelength, and primarily affects the outer skin layers. It is the primary cause of sunburn. It is also the UV radiation primarily responsible for cataracts. Long-term exposure to even low levels of UVB radiation can eventually cause changes in the lens, including pigment changes, which contribute to cataract development. (UVB also appears to be responsible for macular degeneration, an aging-related disorder of the retina.)
• UVA radiation is composed of longer wavelengths. They penetrate more deeply and efficiently into the inner skin layers and are responsible for tanning. The main damaging effect of UVA appears to be the promotion of the release of oxidants.

Radiation Treatments. Cataracts are common side effects of total body radiation treatments, which are administered for certain cancers.

Electromagnetic Waves. Questions have been raised about the hazards of low-level radiation from computer screens. To date, no study has demonstrated an association between cataract development and video display terminals. It is a good idea, in any case, to sit at least a foot away from the front of a screen.

Smoking

Cataracts are one of the many ill effects caused by smoking. Many studies have implicated smoking in the development of nuclear cataracts. The major damaging effects of cigarette smoke appear to be enhancement of free-oxygen radicals (oxidants), the chemical byproducts in the body that can damage cells, including those in the eye.

Medications

Corticosteroids. Long-term use of oral steroids is a well-known cause of cataracts. Studies have been conflicting, however, over whether inhaled and nasal-spray steroids increase the risk for cataracts. Information on cataract risk from inhaled steroids is important because they are commonly used by asthma patients, and steroid sprays use is increasing among allergy sufferers. Studies have suggested a higher risk for cataracts among middle-aged and elderly patients treated with beclomethasone (Beclovent, Vanceril). However, newer inhaled steroids are available and their effects on the eye are unclear. In children, cataracts are rare, and the benefits of inhaled steroids for asthma far outweigh any small additional risk.

Other Medications Associated with Cataracts.

• Psoralens, a class of drugs used along with light therapy to treat skin disorders, such as psoriasis.
• Antipsychotic medications such as chlorpromazine (Thorazine).
• Glaucoma medications.
• Many others drugs have been weakly associated with cataracts, including but not limited to allopurinol, tamoxifen, amiodarone, tricyclic antidepressants, potassium-sparing diuretics (but not other diuretics), thyroid hormone, tetracyclines, sulfamidase, and mepacrine.

Glaucoma and Other Eye Conditions

Glaucoma. Glaucoma and its treatments, including certain drugs (notably miotics) and filtering surgery, pose a high risk for cataracts. The glaucoma drugs posing a particular risk for cataracts including demecarium (Humorsol), isoflurophate (Floropryl), and echothiophate (Phospholine).

Uveitis. Uveitis is chronic inflammation in the eye, which is often caused by an autoimmune disease or response. Often the cause is unknown. It is a rare condition but carries a high risk for cataracts.

Medical Disorders

A number of medical conditions appear to be associated with a higher risk for cataracts either because of a direct affect or because of the medications used for them, or both. They include the following:

• Diabetes. Cataracts in diabetics appear to form when high levels of blood sugar react with proteins in the eye and begin to form byproducts that accumulate in the lens (sugar cataracts).
• Hypertension.
• Autoimmune diseases, including rheumatoid arthritis, psoriasis, multiple sclerosis, systemic lupus erythematosus, Behcet's disease, and others.

Genetic Factors and Other Causes of Cataracts in Children

Rarely, about once in every 10,000 births, a baby is born with cataracts (called congenital cataracts). The causes include the following:

• Inherited disorders. Hereditary factors are often involved in the development of congenital cataracts in children. Such cataracts are most often due to inborn abnormalities in the structure or shape of the lens, including its capsule. Dozens of variations can affect the lens causing, susceptibility to cataracts. Researchers are also investigating genetic factors that may cause mutations in alpha crystallins-- major proteins in the lens, which form cataracts. (Genetic factors also may play a role in some adult cataract cases. The exact hereditary predispositions have yet to be established.)
• Infection during pregnancy.
• Substance abuse in the pregnant women. Pregnant women who abuse alcohol or drugs increase the risk for cataracts (along with other more serious birth defects) in their infants.

Surgery in children with early-onset cataracts can help correct this problem in many cases, but it should be performed as soon as possible for full benefit. Experts recommend routine examination of the face of a fetus during ultrasound for abnormalities.