A cataract is an eye disease in which the clear lens of the eye becomes cloudy or opaque, causing a decrease in vision.
The lens is a portion of the eye that is normally clear. It focuses rays of light entering the eye onto the retina, the light-sensitive tissue at the back of the eye. In order to get a clear image onto the retina, the portions of the eye in front of the retina, including the lens, must be clear and transparent. The light initiates a chemical reaction within the retina. The chemical reaction, in turn, initiates an electrical response which is carried to the brain through theoptic nerve. The brain then interprets what the eye sees.In a normal eye, light passes through the transparent lens to the retina. The lens must be clear for the retina to receive a sharp image. If the lens is cloudy from a cataract, the image striking the retina will be blurry and the vision will be blurry. The extent of the visual disturbance is dependent upon the degree of cloudiness of the lens.
Most cataracts are related to aging. Cataracts are very common in older people. By age 80, more than half of all Indians either have some degree of cataract or have already undergone cataract surgery in one or both eyes. By age 95, this percentage increases to almost 100%. A cataract can occur in either or both eyes. Individuals with a cataract in one eye usually go on to develop a cataract in the other eye as well. A cataract is not contagiousand cannot spread from one eye to the other or from person to person. Cataracts do not cause the eye to tear abnormally. They are neither painful nor make the eye itchy or red.
Although vision can be restored in most people with cataracts, age-related cataracts are still the most common cause of blindness in the world,
Causes of cataracts
The lens is made of mostly water and protein. The protein is arranged in a specific way that keeps the lens clear and allows light to pass through it to focus a clear image onto the retinal surface. As we age, some of the protein may clump together and start to cloud a small area of the lens. This is our understanding of the cause of an age-related cataract. Over time, the cataract may become denser or cloud more of the lens, making it more difficult to see through. A cataract is not a growth or tumor.
There are many causes of non age-related cataracts or secondary cataracts. Secondary cataracts are a result of similar changes to the protein of the lens, also resulting in visual blurring or visual loss.
1.Blunt or penetrating injury to the eye may cause secondary cataracts, either immediately after the injury or some weeks to years afterward. A cataract following an injury may appear and then not increase in density (be stationary) or be progressive. Eye surgery for other conditions can also cause cataracts.
2.Excessive exposure to ionizing radiation (X-ray), infrared radiation (as in glass blowers), or ultraviolet radiation cause secondary cataracts
3..Diabetes is associated with the development of secondary cataracts.
4.Inflammatory disease of the eye, such as iritis or uveitis, may cause or accelerate the development of cataract in the involved eye.
5.There are many genetic illnesses that are associated with the development of secondary cataracts. These include myotonic dystrophy, galactosemia,homocystinuria, Wilson's disease and Down syndrome, plus many others. Congenital infections with herpes simplex, rubella, toxoplasmosis,syphilis, and cytomegalic inclusion disease may also result in cataracts.
6.There are many medications which, when taken over a long period of time, can cause secondary cataracts. The most common of these are oralcorticosteroids, such as prednisone, which are used for a wide variety of medical conditions.
7.The term "congenital cataract" is used when a baby is born with any clouding of the lens. This may be present in one or both eyes, be stationary or be progressive. Causes include genetic disorders or intrauterine developmental disorders, both often associated with other physical abnormalities of the baby.
8.Atopic dermatitis, other diseases of the skin and mucous membranes,
9.hypothyroidism, and hyperparathyroidism are associated with the early development of cataracts.
Patients who develop cataracts in both eyes at an early age often have family members who have also developed cataracts prematurely, implying a genetic cause, even in the absence of a recognized underlying disease.
In the same way Cataract develop in the eye of Animals also as their eyes are no different from human eyes.