Cataract

A cataract is a clouding (opacity) of the lens of the eye that causes a progressive, painless loss of vision. Cataracts are the leading cause of blindness worldwide. Cataracts are common where they affect mostly older adults. Almost one in five people between the ages of 65 and 74 develop cataracts severe enough to reduce vision, and almost one in two people older than 75 have them.

Symptoms & Diagnosis
Because all light entering the eye passes through the lens, any clouding of the lens that blocks, distorts, or diffuses light can cause poor vision. The first symptom of a cataract is usually blurred vision. Glare and halos and, less commonly, double vision can also be early symptoms of cataracts. A person may also notice that colors seem more yellow and less vibrant. Reading may become more difficult because of a worsening ability to distinguish between the light and dark of printed letters on a page. How much vision is changed by a cataract depends on the intensity of light entering the eye and on the location of the cataract. In bright light, the pupil constricts and narrows the pathway through which light enters the eye, and light cannot easily pass around a cataract that is centrally located (nuclear cataract). In dim light, the pupil dilates; bright lights, such as oncoming headlights, are scattered by the edge of the cataract, causing halos and glare, which may be especially disturbing during night driving.
People with cataracts who take drugs that constrict their pupils (certain glaucoma eye drops, for example) may also have greater vision loss. In normal light, however, a nuclear cataract may at first improve vision without eyeglasses for farsighted people. The cataract acts as a stronger lens, thus refocusing light, improving vision for objects close to the eye (near vision). Older people, who generally have trouble seeing things that are close without eyeglasses, may discover that they can read again without eyeglasses, a phenomenon often described as gaining second sight.
Unfortunately, a nuclear cataract eventually blocks and blurs light entering the eye and impairs vision. A cataract at the back of the lens (posterior subcapsular cataract) affects vision more than a cataract in another location because the clouding is at the point where light rays are focused in a narrow beam. This type of cataract impairs vision more in bright light and is more likely to cause glare and halos.
Although cataracts almost never cause pain, rarely they can swell and increase the pressure in the eye (glaucoma), which can be painful. A doctor can usually detect a cataract while examining the eye with an ophthalmoscope (a handheld instrument used to view the inside of the eye). Using an instrument called a slit lamp that allows examination of the eye in more detail, a doctor can identify the exact location of the cataract and the extent to which it blocks light.

Prevention
There are several things people can do to try to prevent cataracts. Consistent use of sunglasses with a coating to filter ultraviolet (UV) light will protect the eyes from bright sunlight and may help. Not smoking is useful and has other health advantages. People with diabetes should work with their doctor to be sure the level of sugar in their blood is well controlled. A diet high in vitamin C, vitamin A, and substances known as carotenoids (contained in vegetables such as spinach and kale) may protect against cataracts. Estrogen use by women after menopause may also be protective, but estrogen should not be used solely for this purpose. Finally, people who are taking corticosteroids for extended periods might discuss with their doctor the possibility of using a different drug.

Treatment
Until vision is significantly impaired, eyeglasses and contact lenses may improve a person's vision. Wearing sunglasses in bright light and using lamps that provide over-the-shoulder lighting may decrease glare and aid vision. Rarely, drugs that keep the pupil dilated may be used to help vision if the cataract is located in the center of the lens.
The only treatment that provides a cure for cataracts is surgery; there are no eye drops or drugs that will make cataracts go away. Occasionally, cataracts will cause changes (such as swelling of the cataract or glaucoma) that lead doctors to recommend the cataract be removed quickly. However, most times people should have surgery only when their vision is so impaired by cataracts that they feel unsafe, uncomfortable, or unable to perform daily tasks. There is no advantage to having cataracts removed before then.
Cataract surgery can be performed on a person of any age and is generally safe even for people with illnesses such as heart disease and diabetes. Usually, the doctor makes a small incision in the eye and removes the cataract by breaking it up with ultrasound and taking out the pieces (phacoemulsificatio n). When all the cataract pieces have been removed, the surgeon replaces the cataract with an artificial lens (intraocular lens). The intraocular lens cannot always be safely placed, however; when this is the case, people must wear thick eyeglasses or contact lenses after the cataract has been removed.
Surgery to remove cataracts is almost always performed under local anesthesia, in which the eye surface is numbed with an injection or eye drops. Rarely, children or adults who cannot hold still during surgery require general anesthesia. The procedure normally takes about 30 minutes, and the person can go home the same day. No sutures are usually needed, because the incision into the eye is small and can seal itself.
People should make arrangements in advance to get extra help at home for a few days after surgery because activity may be restricted (for example, bending over and heavy lifting may be prohibited) and vision changes, such as blurred vision and discomfort with bright light, may occur for a short time after surgery. For a few weeks after surgery, eye drops or ointments are used to prevent infection, reduce inflammation, and promote healing. A person is given eyeglasses or a metal shield to wear to protect the eye from injury until healing is complete, usually a few weeks. The person visits the doctor the day after surgery and then typically one week and one month later. If a person has cataracts in both eyes, many doctors wait several months after the first eye has healed to remove a cataract from the other eye.
Many people notice improved distance vision within a few weeks after cataract surgery. Almost everyone will need eyeglasses for reading, and some people will need eyeglasses to obtain the best possible distance vision as well. The doctor makes calculations before the surgery to decide how powerful the artificial lens should be. Thus, it is possible to go from wearing very thick eyeglasses before the surgery to wearing much thinner eyeglasses after it.
Complications after cataract surgery are rare. A person may develop an infection or serious bleeding in the eye, which can lead to a loss of vision. Eye pressure may become too high, which if left untreated, leads to glaucoma, or the implant can become displaced. The back of the eye (retina) can become swollen or detached. Rarely, people with retinal disorders, such as diabetic retinopathy, may notice their vision worsen after the operation. Proper follow-up with the doctor can lead to early detection and treatment of these unusual complications.
Sometimes people develop a haziness of the tissue (capsule) left behind in the eye when the original lens was removed (secondary cataract). This occurs in about one in four people who have had cataract surgery, months or even years after an artificial lens is implanted. Typically, it is treated by using a laser to make a small opening in the hazy capsule to let light through.
Cataracts usually develop without any apparent cause; however, they can result from injury to the eye, prolonged exposure to certain drugs (such as corticosteroids) or to x-rays (such as with radiation therapy to the eye), inflammatory and infectious eye diseases, and as a complication of diseases such as diabetes. Cataracts also seem to be more common in people with dark eyes, those who have had prolonged exposure to direct sunlight, those with poor nutrition, and smokers. People who have had a cataract in one eye are more likely to develop one later in the other eye. Sometimes cataracts can develop in both eyes at the same time. Babies can be born with them (congenital cataracts), and children can also develop cataracts, usually as a result of injury or illness.
[Merck]