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A cataract is a clouding that develops in the crystalline lens of the eye or in its envelope, varying in degree from slight to complete opacity and obstructing the passage of light. Early in the development of age-related cataract the power of the 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. The condition usually affects both the eyes, but almost always one eye is affected earlier than the other.

A senile cataract, occurring in the elderly, is characterized by an initial opacity in the lens, subsequent swelling of the lens and final shrinkage with complete loss of transparency. Moreover, with time the cataract cortex liquefies to form a milky white fluid in a Morgagnian cataract, which 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 some perception of light was restored in the cataractous patients.

Cataract derives from the Latin cataracta meaning “waterfall” and the Greek kataraktes and katarrhaktes, from katarassein meaning “to dash down”.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”. Early Persian physicians called the term nazul-i-ah, or “descent of the water” vulgarised into waterfall disease or cataract—believing such blindness to be caused by an outpouring of corrupt humour into the eye. In dialect English a cataract is called a pearl, as in “pearl eye” and “pearl-eyed”.

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.

If your vision has become cloudy or things you see are not as bright as they used to be, a cataract may have developed in one or both of your eyes. A cataract is a clouding of the eye’s naturally clear lens. Your eye becomes like a window that is frosted or yellowed.The amount and pattern of cloudiness within the lens can vary. If the cloudiness is not near the center of the lens, you may not be aware that a cataract is present.