A cataract is an opaque, discolored, or cloudy area within the lens of the eye. The lens is composed of water and protein fibers and is normally clear. The clouding of vision associated with cataracts is caused by clumping of the protein fibers in the lens.
A cataract is a clouding or discoloration in the clear lens of the eye that affects vision. Cataracts may develop in one or both eyes but cannot spread from one to the other. There are three basic types of cataracts caused by aging, which are also called senile cataracts. They are classified by their location in the eye.
- Nuclear. This type develops in the nucleus, the central portion of the lens. Nuclear cataracts are likely to darken, turning a portion of the lens yellow or brown; this type of cataract affects a person's perception of color.
- Cortical. Cortical cataracts develop in the cortex, which is the inner portion of the lens between the nucleus and the capsule. They have a distinctive spoke-like or wedge-shaped appearance and often cause problems with glare.
- Posterior capsular. The capsule is the outermost layer of the lens. A posterior capsular cataract develops within this layer at the back of the lens. People with this type of cataract may see halos around lights or have problems with reading.
- Cataracts that are not caused by the aging process include:
- Congenital. Some babies are born with this type of cataract, sometimes as a result of the mother's exposure to rubella. They do not always interfere with vision, however.
- Traumatic. These develop after an injury to the eye, often years after the accident.
- Secondary. This type of cataract develops as a result of diseases like diabetes, or following surgery for another eye disorder such as glaucoma or retinal detachment.
Cataracts are widespread among older adults in the general population in the United States, although some children are born with cataracts and others develop cataracts in late childhood. About half of people eighty years and older have cataracts or have had surgery to remove cataracts. According to the National Eye Institute, between 300,000 and 400,000 cataracts severe enough to impair vision are diagnosed each year in the United States, and 1.5 million cataract surgeries are performed. Around the world, cataracts due to aging are the single most important cause of vision loss; one-third of cases of blindness in Africa are caused by cataracts.
As far as is known, race is not a factor in the development of cataracts. With regard to sex, different studies have yielded different results as to whether cataracts are more common in women or men.
Although researchers do not completely understand why the protein fibers in the lens clump together to form cataracts, they have identified several risk factors for cataract development:
- Age over fifty
- A history of smoking
- High blood pressure or very low blood pressure
- Exposure to sunlight or other forms of radiation
- A family history of cataracts
- Previous surgery for an eye disorder
- History of injury to the eye or inflammatory diseases affecting the eye
- Long-term use of steroid medications
Cataracts develop slowly and painlessly. Over time, people with cataracts may notice the following signs and symptoms:
- Cloudy, foggy, or filmy vision.
- Need for brighter reading light.
- Increased sensitivity to bright lights and glare; this symptom often affects the person's ability to drive comfortably at night.
- Difficulty telling the difference between some colors (usually blues and purples), or seeing colors as faded or yellowish.
- Need for frequent changes in eyeglass or contact lens prescriptions.
- Seeing halos around lights.
- Seeing double in one eye.
- Increased difficulty in perceiving the contrast between the shape of an object and its background.
Progress in Cataract Surgery
Cataract surgery is now one of the safest surgical procedures, but the high rate of success is relatively recent. In the early twentieth century, cataract removal was a dreaded procedure that all too often had negative effects.
In 1919, after a failed cataract operation on her right eye, American painter Mary Cassatt (1844–1926) underwent a cataract operation in her left eye, before which she wrote to a friend, “I look forward with horror to utter darkness.” After the second surgery, she completely stopped painting. Cassatt's friend and colleague Claude Monet (1840–1926) was reluctant to undergo the surgery because of Cassatt's outcome. By 1905, nuclear cataracts in both eyes had changed his perceptions of colors. Eye drops helped for awile, so he postponed surgery. He finally agreed to it in 1923. Afterward, Monet was very unhappy, writing to his doctor, “I might have finished the [paintings] which I have to deliver in April and I'm certain now that I won't be able to finish them as I'd have liked. That's the greatest blow … and it makes me sorry that I ever decided to go ahead with the fatal operation.” Monet refused to consider an operation on his left eye.
Cataracts are usually diagnosed by a complete eye examination. After the examiner takes a history of the patient's symptoms (including a family history of cataracts and other eye problems), the patient is usually asked to read the letters on an eye chart known as a Snellen chart to test visual acuity, or sharpness of vision. The patient's eye is then dilated with a medication that keeps the pupil wide open, allowing the doctor to see the retina and the optic nerve to make sure that they are not damaged or abnormal.
While the eye is dilated, the doctor uses a device called a slit lamp to examine the various parts of the eye in detail by focusing a beam of light into a very small line or slit, which allows the doctor to determine the size and location of a cataract and to exclude glaucoma or other causes of the patient's symptoms.
The doctor will also usually test the pressure of the fluid inside the eye with a device called a tonometer, which also ensures that the patient does not have glaucoma.
Some cataracts are small enough or growing slowly enough that they may not require treatment, particularly if they are not affecting a person's quality of life. In other cases a change in eyeglass or contact lens prescription may be all that is needed.
If the cataract is large or opaque enough to interfere with the patient's vision, surgical removal is the only effective treatment. There are no medications that can dissolve or clear up cataracts. The two basic types of cataract removal are called small incision cataract surgery and extracapsular surgery. In small incision surgery, the ophthalmologist makes a small incision along the side of the cornea and inserts a device that uses ultrasound waves to break up the clouded lens, which is then removed by suction. This technique is also called phacoemulsification or simply phaco; it can be completed in as little as thirty minutes.
In extracapsular surgery, used primarily to treat cataracts that are too dense to be broken up by phacoemulsification, the ophthalmologist makes a longer incision in the cornea and removes the lens all in one piece.
In most cases, the ophthalmologist inserts a new artificial lens, called an intraocular lens (IOL), inside the lens capsule after the cataract is removed. Some patients cannot use an IOL because they have another type of eye disorder. They are given a new eyeglass prescription or soft contact lenses.
Cataract removal can be done in an outpatient center for eye surgery or a hospital. Most people do not need to stay overnight, but patients should ask a friend or family member to drive them home and help with home care after outpatient surgery. They will need to use eye drops to reduce inflammation and the risk of infection for a few days, and they will not be allowed to bend too far forward or lift heavy objects for several days while the eye heals.
Cataract surgery is successful in improving the patient's vision in 90–95 percent of cases. The chief risks are infection, persistent inflammation, changes in the fluid pressure inside the eye, and an increased risk of retinal detachment—a condition in which the retina separates from the tissues at the back of the eye. In a few cases the patient's IOL will need to be removed or replaced. Serious complications occur in fewer than one per 1,000 cataract surgeries.
WORDS TO KNOW
Capsule: The outermost layer of the lens of the eye.
Cornea: The transparent front part of the eye where light enters the eye.
Cortex: The part of the lens underneath the capsule.
Nucleus: The innermost part of the lens of the eye.
Ophthalmologist: A doctor who specializes in diagnosing and treating eye disorders and can perform eye surgery.
Phacoemulsification: A technique for removing cataracts by breaking up the lens of the eye with ultrasound waves and removing the pieces of the lens by suction.
Retina: The light-sensitive layer of tissue at the back of the eyeball.
Retinal detachment: A disorder in which the retina pulls away from its underlying tissues at the back of the eye.
Senile cataract: Another term for cataracts caused by the aging process.
Snellen chart: A series of letters arranged in lines on a chart to be viewed from a distance of 20 feet (6.1 meters) used to measure visual acuity (clearness of vision). Dutch ophthalmologist Hermann Snellen invented it in 1862.
Tonometer: An instrument used by an ophthalmologist to measure the pressure of the fluid inside the eye.
Cataracts cannot always be completely prevented, but their development can be slowed by a combination of lifestyle changes and good eye care:
- Quitting smoking and using alcohol only in moderation.
- Protecting the eyes by using sunglasses or regular glasses designed to screen out ultraviolet light.
- Eating a balanced diet rich in fruits and vegetables. Some researchers think that the vitamins in these foods may slow the growth of cataracts, although this theory has not been proven.
- Following the doctor's recommendations for such other conditions or diseases as high blood pressure and diabetes.
Current research on cataracts includes the possibility of developing medications to treat cataracts. Other research being conducted by the National Eye Institute concerns genetic studies that may help doctors understand how cataracts develop and studies of the effects of sunlight exposure on the lens of the eye.
SEE ALSO Glaucoma; Rubella
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Straus, Eugene W., and Alex Straus. Medical Marvels: The 100 Greatest Advances in Medicine. Amherst, NY: Prometheus Books, 2006. Cataract surgery is the subject of Chapter 71.
Gugliotta, Guy. “Simulations of Ailing Artists' Eyes Yield New Insights onStyle.” New York Times, December 4, 2007. Available online at http://www.nytimes.com/2007/12/04/science/04impr.html?ex=1354424400&en=ab1fa1d4e2c15aeb&ei=5088&partner=rssnyt&emc=rss (accessed June 10, 2008).
American Optometric Association (AOA). Cataract. Available online at http://www.aoa.org/cataract.xml (accessed June 10, 2008). The page includes a play list of five videos that explain the different types of cataract as well as the symptoms; the videos take about four minutes total to play.
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EyeCare America. Cataract Surgery. Available online at http://www.eyecareamerica.org/eyecare/treatment/cataract-surgery/video.cfm (accessed June 9, 2008). This is a seven-minute video prepared by the American Academy of Ophthalmology about cataract surgery. It includes patient interviews.
Mayo Clinic. Cataracts. Available online at http://www.mayoclinic.com/health/cataracts/DS00050 (updated May 20, 2008; accessed June 9, 2008).
Medical University of South Carolina. Cataract Animation. Available online at http://www.muschealth.com/gs/AnimationList.aspx#anim7 (accessed June9, 2008). This is an animation with voiceover and printed text describing the development and characteristics of cataracts. It takes about 2 minutes and 30 seconds to play.
National Eye Institute (NEI). Facts about Cataract. Available online at http://www.nei.nih.gov/health/cataract/cataract_facts.asp (updated April 2006; accessed June 9, 2008).