Cataracts

Definition

A cataract is a clouding of the lens in the eye that affects vision.

Description

When a person looks at an object, light rays are reflected from the object to the person's eye. These light rays are bent and refracted by the cornea, the transparent, dome-shaped window at the front of the eye that provides refracting power, and 65–75% of the eye's focusing power. From the cornea, the light goes through the pupil, the circular opening in the center of the colored iris, and is sharply focused by the crystalline lens on the retina, the multi-layered sensory tissue that lines the back of the eye. It is on the retina that the light focused by the lens produces clear and sharp inverted images, converted to electrical impulses that are then transmitted through the optic nerve to the brain, where the image is perceived in an upright position.

The eye lens consists of four layers, from the surface to the center. The outer surface layer (capsule) is a thin, clear membrane that surrounds a soft, jellylike material (cortex). A layer of cells (subcapsular epithelium) joins the capsule and the cortex and the last layer is the hard center of the lens (nucleus). In a healthy eye, all the layers of the lens are clear, and light passes through unobstructed. However, if the lens material becomes cloudy, yellow, brown, and even opaque, with the result that light is obstructed from entering, the condition is referred to as a cataract.

Cataracts can develop in one or both eyes, but in most cases, they tend to develop in both. Moreover, the condition may or may not affect the entire lens. As a cataract develops, the lens becomes clouded, which scatters the light and prevents a sharply defined image from forming on the retina with the result that vision becomes blurred. Cataracts are classified into three types: nuclear, cortical, and subcapsular.

A nuclear cataract is the most common form of cataract. It forms in the nucleus, at the center of the lens. In its early stages, as the lens starts not to focus light normally, affected persons may become more nearsighted or experience a temporary improvement of their reading vision. However, as the lens gradually turns more yellow and opaque, vision clouds further and deteriorates. As the cataract progresses, the lens may eventually turn brown and causes a decrease in distance vision and in the ability to see in dim light.

A cortical cataract forms whitish spokes on the outer edge of the lens cortex that eventually extend to the center and interfere with light passing through the center of the lens. It may affect both near and distance vision with focusing problems and vision distortions.

A subcapsular cataract usually forms at the back of the lens, in the path of light to the retina. It may occur in both eyes, but tends to be more advanced in one eye than the other. A subcapsular cataract often affects reading vision, also reducing it in bright light.

How quickly cataracts develop varies among individuals and between the two eyes. Most age-related cataracts progress gradually over a period of years.

Demographics

Age-related cataract is a major public health problem. Worldwide it is the major cause of preventable blindness. Cataracts are a leading cause of visual impairment among aging Americans and represent an important quality of life issue. Cataract extractions are the most common surgical procedures performed in the United States, accounting for more

than two million procedures each year. More than 50% of people over the age of 60 have some type of cataract. After age 75, as many as 70% of Americans have cataracts that are significant enough to impair vision.

Causes and symptoms

In young people, the lens changes its shape to adjust for close or distant vision, a process called “accommodation.” With aging however, the eye lenses gradually harden, and lose the ability to accommodate. They become less flexible and less transparent. This is because the protein fibers that make up the lens start breaking down with age. Some of the fibers start aggregating together, clouding small areas of the lens. As the cataract develops, the clouding becomes heavier and spreads to a greater part of the lens. Aging of the lens is not the only cause of cataracts. Some people are born with cataracts or develop them during childhood. Cataracts may also be produced by eye injury or trauma, including exposure to ultraviolet light and radiation. Diseases such as diabetes and hypertension may also lead to cataract formation, as well as eye surgical procedures (secondary cataracts).

Common cataract symptoms include:

• cloudy or blurred vision
• colors that appear faded or yellow
• double vision in one eye
• frequent prescription changes for eyeglasses or contact lenses
• glare. Lamps, or sunlight appears too bright, halos may be seen around lights
• light sensitivity

Diagnosis

Cataracts are detected following a complete eye examinations performed by an ophthalmologist or other eye care practitioner. Tests commonly used to establish diagnosis may include:

• Visual acuity test. This test uses an eye chart that measures how well a person can see letters at a distance. Eyes are tested one at a time, keeping the other eye covered. Using an eye chart that displays progressively smaller letters from top to bottom, the test determines if a person has 20/20 vision (excellent) or less acute vision.
• Dilated eye exam. In this test, drops are placed in the eyes to dilate the pupils. A magnifying lens is then used to examine the retina and optic nerve for signs of damage and other eye problems.
• Tonometry. This test uses an instrument to measure the pressure inside the eye. Drops are used to numb the eye and a special device is used to measure the eye's inner pressure.

Treatment

In the early stages, when symptoms are mild, new eyeglasses, brighter lighting, anti-glare sunglasses, or magnifying lenses may prove effective. When vision loss interferes with everyday activities, surgery can be performed to remove the cloudy lens and replace it with an artificial lens.

Nutrition/Dietetic concerns

Several research reports show that the antioxidant properties of vitamins C and E may protect against the development and progression of cataracts. The Nutrition and Vision Project (NVP), a cooperative effort of Harvard and Tufts University researchers, found that higher intakes of vitamin C led to a reduced risk for cortical and nuclear cataracts. Results also showed that people who took vitamin C and E supplements for more than ten years had decreased progression of nuclear cataracts. Research also suggests that the carotenoids lutein and zeaxanthin, the only carotenoids found in the lens and which are also antioxidants , may also protect against cataracts.

QUESTIONS TO ASK YOUR DOCTOR

• How is a cataract treated?
• How do I know if cataract surgery is right for me?
• Is cataract surgery effective?
• Are there different types of cataract surgery?
• What are the risks of cataract surgery?
• Can problems develop after surgery?
• After surgery, when will my vision be normal again?
• What can I do to protect my vision?

Therapy

The only effective therapy for a cataract is surgery to replace the lens with a clear intraocular lens (IOL) implant, made of plastic, acrylic or silicone. Sometimes cataracts are removed without reinserting implant lenses. Two surgical procedures can be used to remove cataracts:

• Phacoemulsification: In this procedure, the surgeon removes the cataract but leaves most of the outer layer (capsule) in place. The surgeon makes a tiny incision where the cornea meets the thin, transparent tissue that covers the outer surface of the eye (conjunctiva) and inserts a needle-like probe that uses ultrasound waves to break up the cataract and suctions out the fragments. The lens capsule remains in place to provide support for the IOL implant.
• Extracapsular cataract extraction: This procedure requires a larger incision, made where the cornea and the white of the eye (sclera) meet. The surgeon opens the lens capsule, removes the hard nucleus in one piece and aspires out the softer lens cortex, leaving the capsule in place.

Once the cataract has been surgically removed, a clear artificial IOL is implanted into the empty lens capsule. It requires no care and becomes a permanent part of the eye.

Prognosis

The success rate of cataract surgery in restoring vision is excellent. Nine out of ten people that undergo the surgery regain good vision. Improved vision is achieved if other vision-limiting problems are not present.

KEY TERMS

Antioxidant —Any substance that reduces damage due to reactive oxygen such as that caused by free radicals.

Cataract —Opacity or cloudiness of the eye lens, which may prevent a clear image from forming on the retina.

Carotenoids —Red to yellow pigments responsible for the characteristic colour of many plant organs or fruits, such as tomatoes, carrots, etc.

Conjunctiva —The thin, transparent tissue that covers the outer surface of the eye.

Cornea —Transparent front part of the eye that covers the iris, pupil, and anterior chamber and provides most of an eye's optical power.

Dilated pupil —Enlarged pupil, resulting from contraction of the dilator muscle or relaxation of the iris. Occurs normally in dim illumination, or may be produced by certain drugs or trauma.

Iris —Pigmented tissue lying behind the cornea that gives color to the eye and controls amount of light entering the eye by varying the size of the pupil.

Lens —Transparent, biconvex crystalline tissue that helps bring rays of light to a focus on the retina.

Opaque —Impenetrable by light, neither transparent nor translucent, not reflecting light.

Pupil —Variable—sized black circular opening in the center of the iris that regulates the amount of light that enters the eye.

Retina —Light—sensitive tissue at the back of the eye.

Sclera —Tough, opaque tissue that serves as the eye's protective outer coat. Also called “the white of the eye.”

Prevention

Most cataracts occur with age and cannot be prevented. However, vision can be protected and delay the onset of cataracts by wearing sunglasses and a hat with a brim to block ultraviolet sunlight. Smoking should also be avoided and good nutrition has been shown to lower the risk of age-related cataract. Eating green leafy vegetables and fruits provide a good source of antioxidants. People aged 60 or older should have a comprehensive dilated eye exam at least once every two years.

Caregiver concerns

The American Academy of Ophthalmology estimates that, by age 65, 33% of Americans have some form of eye disease that impairs vision. Unfortunately, most people in this age group are unaware of these disorders because there are often no warning symptoms or they believe that poor sight is a natural consequence of aging. However, by detecting eye disease early through annual exams, seniors can preserve their sight longer. In the United States, the Seniors EyeCare Program ensures that every senior has access to medical eye care and promotes annual, dilated eye exams. It raises awareness about age-related eye disease, including cataracts, provides free eye care educational materials and facilitates access to eye care, at no cost to seniors. People eligible for a referral through the program receive a comprehensive, medical eye exam and up to one year of free treatment for any disease diagnosed during the initial exam.

Resources

BOOKS

Gimbel, Howard, and David Chang. Cataracts: A Patient's Guide to Treatment. Omaha, NE: Addicus Books, 2004.

Harvard Medical School. The Aging Eye. New York, NY: Free Press (Simon & Schuster), 2001.

Jae Yong, Kim. Essentials of Cataract Surgery. Thorofare, NJ: Slack Inc., 2007.

Mayo Clinic. Mayo Clinic On Vision And Eye Health: Practical Answers on Glaucoma, Cataracts, Macular Degeneration & Other Conditions. Scottsdale, AZ: Mayo Clinic Trade Paper, 2002.

Watts, Mark. Cataract: What You Need To Know. London, UK: Sheldon Press, 2006.

PERIODICALS

Chiu, C. J., and A. Taylor. “Nutritional antioxidants and age-related cataract and maculopathy.” Experimental Eye Research 84, no. 2 (February 2007): 229–245.

Ciulla, T. A., and B. R. Hammond. “Macular pigment density and aging, assessed in the normal elderly and those with cataracts and age-related macular degeneration.” American Journal of Ophthalmology 138, no. 4 (October 2004): 582–587.

Datta, S., et al. “The importance of acuity, stereopsis, and contrast sensitivity for health-related quality of life in elderly women with cataracts.

” Investigative Ophthalmology & Visual Science 49, no. 1 (January 2008): 1–6.

Foss, A. J., et al. “Falls and health status in elderly women following second eye cataract surgery: A randomised controlled trial.” Age and Ageing 35, no. 1 (January 2006): 66–71.

Hejtmancik, J. F., and M. Kantorow. “Molecular genetics of age-related cataract.” Experimental Eye Research 79, no. 1 (July 2004): 3–9.

Kulmala, J., et al. “Lowered vision as a risk factor for injurious accidents in older people.” Aging Clinical Experimental Research 20, no. 1 (February 2008):25–30.

Zubenko, G. S., et al. “Reduced age-related cataracts among elderly persons who reach age 90 with preserved cognition: a biomarker of successful aging?” The Journals of Gerontology A 62, no. 5 (May 2007): 500–506.

OTHER

Cataract. NIH Senior Health. April 9, 2007 (February 20, 2008) http://nihseniorhealth.gov/cataract/toc.html.

Cataracts. EyeCare America. May 2007 (February 20, 2008) http://www.eyecareamerica.org/eyecare/conditions/cataracts/index.cfm.

Cataracts. Mayo Clinic. May 19, 2006 (February 20, 2008) http://www.mayoclinic.com/health/cataracts/DS00050.

Cataract. National Eye Institute. April 2006 (February 20, 2008) http://www.nei.nih.gov/health/cataract/cataract_facts.asp.

ORGANIZATIONS

American Academy of Ophthalmology (AAO), P.O. Box 7424, San Francisco, CA, 94120-7424, (415) 561-8500, (415) 561-8500, [email protected], http://www.aao.org.

EyeCare America, 655 Beach St., San Francisco, CA, 94109-1336, (877) 887-6327, (800) 222-3937, http://www.eyecareamerica.org/eyecare.

National Eye Institute, 2020 Vision Place, Bethesda, MD, 20892-3655, (301) 496-5248, [email protected], http://www.nei.nih.gov/.

Monique Laberge Ph.D.