A cataract is an opacity that develops in the crystalline lens of the eye or in its envelope. Early on in the development of age-related cataract the power of the crystalline lens may be increased, causing near-sightedness (myopia), and the gradual yellowing and opacification of the lens may reduce the perception of blue colours. Cataracts typically progress slowly to cause vision loss and are potentially blinding if untreated.
Moreover, with time the cataract cortex liquefies to form a milky white fluid in a Morgagnian Cataract, and can cause severe inflammation if the lens capsule ruptures and leaks. Untreated, the cataract can cause phacomorphic glaucoma. Very advanced cataracts with weak zonules are liable to dislocation anteriorly or posteriorly. Such spontaneous posterior dislocations (akin to the historical surgical procedure of couching) in ancient times were regarded as a blessing from the heavens, because it restored some perception of light in the bilaterally affected patients.
Cataract 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 may later have been used metaphorically to describe the similar appearance of mature ocular opacities. In Latin, cataracta had the alternate meaning, "portcullis", so it is also possible that the name came about through the sense of "obstruction".
Cataracts develop from a variety of reasons, including long-term ultraviolet exposure, exposure to radiation, secondary effects of diseases such as diabetes, and advanced age; they are usually a result of denaturation of lens proteins. Genetic factors are often a cause of congenital cataracts and positive family history may also play a role in predisposing someone to cataracts at an earlier age, a phenomenon of "anticipation" in pre-senile cataracts. Cataracts may also be produced by eye injury or physical trauma. A study among Icelandair pilots showed commercial airline pilots as three times more likely to develop cataracts than people with non-flying jobs. This is thought to be caused by excessive exposure to radiation coming from outer space. Cataracts are also unusually common in persons exposed to infrared radiation, such as glassblowers who suffer from "exfoliation syndrome". Exposure to microwave radiation can cause cataracts.
Cataracts may be partial or complete, stationary or progressive, hard or soft.
There are various types of cataracts, e.g. nuclear, cortical, mature, hypermature. Cataracts are also classified by their location, e.g. posterior (classically due to steroid use) and anterior (common (senile) cataract related to aging).
Cataracts are the leading cause of blindness in the world.
In the United States, age-related lenticular changes have been reported in 42% of those between the ages of 52 to 64, 60% of those between the ages 65 and 74, and 91% of those between the ages of 75 and 85.
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 (extracapsular cataract extraction, or ECCE) and intra-capsular (intracapsular cataract extraction, or ICCE).
Extra-capsular (ECCE) 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 (ICCE) 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 cataractous lens is removed and replaced with a plastic lens (an intraocular lens implant) which stays in the eye permanently.
Cataract operations are usually performed using a local anaesthetic and the patient is allowed to go home the same day. Recent improvements in intraocular technology now allow cataract patients to choose a multifocal lens to create a visual environment in which they are less dependent on glasses. Under some medical systems multifocal lenses cost extra. Traditional intraocular lenses are monofocal.
Complications after cataract surgery, including endophthalmitis, posterior capsular opacification and retinal detachment, are possible.
In ICCE there is the issue of the Jack in the box phenomenon where the patient has to wear aphakic glasses - alternatives include contact lenses but these can prove to be high maintenance, particularly in dusty areas.
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 has not been 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 disease. 
Research is scant and mixed but weakly positive for the nutrients lutein and zeaxanthin. Bilberry extract shows promise in rat models  and in clinical studies.
Types of cataracts
The following is a classification of the various types of cataracts. This is not comprehensive and other unusual types may be noted.
After-cataract - posterior capsular opacification subsequent to a successful extracapsular cataract surgery (usually within 3 months - 2 years) with or without IOL implantation. Requires a quick & painless office procedure with Nd:YAG laser capsulotomy to restore optical clarity.
^ Rafnsson, V; Olafsdottir E, Hrafnkelsson J, Sasaki H, Arnarsson A, Jonasson F. "Cosmic radiation increases the risk of nuclear cataract in airline pilots: a population-based case-control study". Arch Ophthalmol123: 1102-1105.
^ ab SPENCER R, ANDELMAN S. "STEROIDSAREBAD CATARACTS. POSTERIOR SUBCAPSULAR CATARACT FORMATION IN RHEUMATOID ARTHRITIS PATIENTS ON LONG TERM STEROID THERAPY". Arch Ophthalmol74: 38-41. PMID 14303339.
^ Greiner J, Chylack L (1979). "Posterior subcapsular cataracts: histopathologic study of steroid-associated cataracts". Arch Ophthalmol97 (1): 135-44. PMID 758890.
^ ab Sperduto RD, Seigel D. Sperduto RD, Seigel D. "Senile lens and senile macular changes in a population-based sample." Am J Ophthalmol. 1980 Jul;90(1):86-91. PMID 7395962.
^ Kahn HA, Leibowitz HM, Ganley JP, Kini MM, Colton T, Nickerson RS, Dawber TR. "The Framingham Eye Study. I. Outline and major prevalence findings." Am J Epidemiol. 1977 Jul;106(1):17-32. PMID 879158.
^ Epidemiology. 2003 Nov;14(6):707-12. Sun exposure as a risk factor for nuclear cataract
^ http://www.nei.nih.gov/nehep/pdf/NEHEP_5_year_agenda_2006.pdf p.37 quoting Javitt, J. C., F. Wang, and S. K. West. “Blindness Due to Cataract: Epidemiology and Prevention.” Annual Review of Public Health 17 (1996): 159-77.
^ Klein, Barbara; Ronald Klein, Kristine Lee, and Lisa Grady. "Statin Use and Incident Nuclear Cataract". Journal of the American Medical Association295 (23): 2752-2758.
^ Nutrition. 2003 Jan;19(1):21 Lutein, but not alpha-tocopherol, supplementation improves visual function in patients with age-related cataracts: a 2-y double-blind, placebo-controlled pilot study
^ Invest Ophthalmol Vis Sci. 2006 Sep;47(9):3783-6. Lutein and zeaxanthin and the risk of cataract: the Melbourne visual impairment project
^ Invest Ophthalmol Vis Sci. 2006 Jun;47(6):2329-35. Plasma lutein and zeaxanthin and other carotenoids as modifiable risk factors for age-related maculopathy and cataract: the POLA Study
^ J Am Coll Nutr. 2004 Dec;23(6 Suppl):567S-587S Lutein and zeaxanthin and their potential roles in disease prevention
^ Dietary supplementation with bilberry extract prevents macular degeneration and cataracts in senesce-accelerated OXYS rats Adv Gerontol. 2005;16:76-9
^ Yamakoshi J, et al. J Agric Food Chem. 2002 Aug 14;50(17):4983-8.