In ophthalmology, a cataract is any opacity which develops in the crystalline lens of the eye or in its envelope. The name derives from the Latin cataracta meaning "waterfall" and the Greek kataraktes and katarrhaktes, from katarassein meaning "to dash down" (kata-, "down"; arassein, "to strike, dash") . As rapidly running water turns white, the term was later used metaphorically to describe the similar appearance of mature ocular opacities.
Cataracts form for a variety of reasons, including long-term ultraviolet exposure, secondary effects of diseases such as diabetes, or simply due to advanced age; they are usually a result of denaturation of lens proteins. Genetic factors are often a cause of congenital cataracts and may also play a role in predisposing someone to cataracts. Fully half of all people between the ages of 65 and 74 and about 70% of those over 75 have some cataract formation. Cataracts may also be produced by eye injury or physical trauma. A study among Icelandair pilots showed commercial airline pilots as three times as likely to develop cataracts as people with non-flying jobs. This is thought to be caused by radiation coming from outer space. 
Cataracts may be partial or complete, stationary or progressive, hard or soft.
An early technique to remove cataracts was couching, which involved using a thin needle or stick to remove the clouding. This technique is known to have existed in Roman times and continued to be used throughout the Middle Ages and continues to be used in underprivileged "Third World" countries today. In India, however, modern surgery with intraocular lens insertion in Government and Non Government Organisation (NGO) sponsored Eye Surgical Camps have totally replaced this method.
The most effective and common treatment is to surgically remove the cloudy lens. There are two types of surgery that can be used to remove cataracts, extra-capsular and intra-capsular surgery. Extra-capsular surgery consists of removing the lens but leaving the majority of the lens capsule intact. High frequency sound waves (phacoemulsification) are sometimes used to break up the lens before extraction. Intra-capsular surgery involves removing the entire lens of the eye, including the lens capsule, but it is rarely performed in modern practice. In either extra-capsular surgery or intra-capsular surgery, the lens is replaced with a plastic lens (an intraocular lens implant) which remains permanently in the eye.
Previously, polymethylmethacrylate was used as the lens material. Advances have brought about the use of silicone acrylate which is a soft material. This allows the lens to be folded and inserted into the eye through a smaller incision. Acrylic lenses can also be used with small incisions and are a better choice in people who have a history of uveitis or are at high risk of retinal detachment. Acrylic is not always an ideal choice due to its added expense. New FDA approved multifocal intraocular lens implants allow post operative cataract patients the advantage of nearly glass free vision. These new multifocal lenses are not a covered expense under most insurance plans and can cost the patient upwards of $1500 per eye.
Cataract operations are mostly performed under a local anaesthetic and the patient will be allowed to go home the same day. Complications after cataract surgery are possible. In the past many people (up to 50%) developed posterior capsular opacification after initial cataract surgery. This is a thickening and clouding of the lens capsule (which was left behind when the cataract was removed) and it can be easily corrected using a laser to make holes in the capsule for the person to see through. Modern lens design has reduced the chance of capsular opacification to less than 5%. Retinal detachment is an uncommon complication of cataract surgery, but has a higher incidence in people with extreme myopia (nearsightedness).
Although cataracts have no scientifically proven prevention, it is sometimes said that wearing ultraviolet-protecting sunglasses may slow the development of cataracts. Regular intake of antioxidants (such as vitamin C and E) is theoretically helpful, but this is also not proven.
Although statins are known for their ability to lower lipids, they are also believed to have antioxidant qualities. It is believed that oxidative stress plays a role in the development of nuclear cataracts, which are the most common type of age-related cataract. To explore the relationship between nuclear cataracts and statin use, a group of researchers took a group of 1299 patients who were at risk of developing nuclear cataracts and gave some of them statins. Their results suggest that statin use in a general population may be associated with a lower risk of developing nuclear cataract. 
Types of cataracts
- Classified by etiology
- Age-related cataract
- Congenital cataract
- Secondary cataract
- Traumatic cataract
- Classified by location
- Anterior cortical cataract
- Anterior polar cataract
- Anterior subcapsular cataract
- Nuclear cataract
- Posterior cortical cataract
- Posterior polar cataract
- Posterior subcapsular cataract
Associations with systemic conditions
- Chromosomal disorders
- Alport's syndrome
- Cri-du-chat syndrome
- Conradi's syndrome
- Myotonia dystrophica
- Patau's syndrome
- Schmid-Fraccaro syndrome
- Trisomy 18 (Edward's syndrome)
- Turner's syndrome
- Disease of the skin and mucous membranes
- Atopic dermatitis
- Basal-cell nevus syndrome
- Metabolic and nutrition diseases
- Aminoaciduria (Lowe's syndrome)
- Diabetes mellitus
- Fabry's disease
- Hypervitaminosis D
- Wilson's disease
- Infectious diseases
- Congential herpes simplex
- Congenital syphilis
- Cytomegalic inclusion disease
- Toxic substances introduced systemically