Cataract Overview 

Cataracts are changes in clarity  of the natural lens inside the eye that gradually degrade visual quality. The natural lens sits behind the colored part of the eye (iris)  in the area of the pupil, and cannot be directly seen with the naked  eye unless it becomes extremely cloudy. The lens plays a crucial role in focusing unimpeded light on the retina at the back of the eye. The retina transforms light to a neurologic signal  that the brain interprets as  vision. Significant cataracts block and distort light passing through the lens, causing visual symptoms and complaints. 

The term cataract is derived from the Greek word cataractos, which describes rapidly running water. When water is turbulent, it is transformed from a clear medium to white and cloudy. Keen Greek observers noticed similar-appearing changes in the eye and attributed visual loss from "cataracts" as an accumulation of this turbulent fluid, having no knowledge of the anatomy of the eye or the status or importance of the lens. 

Cataract development is usually a very gradual process of normal aging but can occasionally occur rapidly. Many people  are in fact unaware that they have cataracts because the changes in their vision have been so gradual. Cataracts commonly affect both eyes, but it is not uncommon for cataracts in one eye  to advance more rapidly.  Cataracts are very common, affecting roughly 60% of people over the  age of 60, and over 1.5 million cataract surgeries are performed in the United States each year.
Experts  have estimated that visual disability associated with cataracts accounts for over 8 million physician office visits  a year in the United States. This number will likely continue to increase as the proportion of people over the age of 60 rises. When people develop cataracts, they begin to  have difficulty doing activities they need to do for daily living or for enjoyment. Some of the most common complaints include difficulty driving at night, reading, participating  in sports such as golfing,  or traveling  to unfamiliar areas; these are all activities for which clear vision  is essential. 

Cataract Causes 

The lens is made mostly of water and protein. Specific proteins within the lens are responsible for maintaining its clarity. Over many years, the structures of these lens proteins are altered, ultimately leading to  a gradual clouding of the lens. Rarely, cataracts can present at birth or in early childhood as a result of hereditary enzyme defects, and severe trauma to the eye,  eye surgery, or intraocular inflammation can also cause cataracts to occur earlier in life. Other factors  that may lead  to development of cataracts at  an earlier age include excessive ultraviolet-light exposure, diabetes, smoking, or  the use  of certain medications, such as oral, topical, or inhaled steroids. Other medications that are more weakly associated with cataracts include  the long-term use of statins and phenothiazines. 

Types of Cataracts 

All cataracts are fundamentally  a change in the clarity of the overall lens structure; however, cataracts may result either early in life or as a result of aging, and different portions of  the lens may be more affected than others. Cataracts  that occur at birth or present very early in life (during the first year of life)  are termed congenital or infantile cataracts. These cataracts require prompt surgical correction or they may prevent  the vision in  the affected eye from developing normally. When  the central portion of the  lens is most affected, which is the most common situation, these are termed nuclear cataracts. The outside  of the lens is called the lens cortex, and when opacities are  most visible in this region, the  cataracts are called cortical cataracts. There is an even more specific change that occasionally happens, when the opacity develops immediately next  to the lens capsule, either by the anterior, or more commonly the posterior, portion  of the capsule; these are called subcapsular cataracts. Unlike most cataracts, posterior subcapsular cataracts can develop rather quickly and affect vision more suddenly than either nuclear or cortical cataracts. 

When to Seek Medical Care 

Eye-care professionals may mention during a routine eye exam that you have early cataract development even if you are not yet experiencing visual symptoms.
Although your doctor will be able to tell when you first begin to  develop cataracts, you will generally be the first person to notice changes  in your vision that may require cataract surgery. Clouding of the lens may start to be seen at any age, but it is uncommon before the age of 40. However, a large majority of  people will not begin to  have symptoms from their cataracts until many years after they begin to develop. 

Since  cataract development rarely causes any long-term damage to the eye,  cataract surgery should be considered only when visual symptoms begin to develop. Whenever significant vision problems  are noted, you should schedule an exam by an eye-care professional. Typical symptoms may include blurry vision, difficulty  with glare or night vision, poor color vision, or frequent changes  in eyeglass prescription. 

For an early cataract changes, vision  may be improved by simply changing your eyeglass prescription, using a magnifying lens, or increasing lighting when you  do visually demanding tasks. Eventually, cataracts get to a point where  the only effective intervention is surgery. This decision is made based mainly on the degree  of visual limitation the patient is experiencing.