What Is a Cataract?

A cataract is a clouding of the lens of the eye that makes objects look as though you were viewing them through foggy glasses or a frost-covered window. The condition is called cataract or cataracts - either term is correct. It can occur in one or both eyes. If both eyes are affected, the cataracts may progress (become larger or more opaque) at different rates, but the condition is not contagious and does not spread from one eye to the other. Cataracts are not painful and do not irritate the eyes or cause them to produce tears.

Simulation of cataract.  A street scene with an approximation of what somebody with cataract sees compared to normal vision

The lens is a transparent, layered structure that lies behind the iris - the colored part of the eye. (Learn more about the parts of the eye in The Anatomy of the Eye) For a visual image to be processed, the lens must focus light on the retina, the back part of the eye (read How the Eye Works). The clearer the lens is, the sharper your vision will be. A hazy lens will cause the brain to perceive filmy images, rather than crisp and distinct ones.

Types of Cataract

Age-related cataracts

Cataracts are categorized according to their cause, and there are several types of cataract, as we’ll explain below. But unless otherwise specified, the term usually refers to age-related cataracts. The lens is made up primarily of water and protein fibers, which are arranged in a way that optimizes visual clarity. But year by year, the lens becomes thicker and heavier, and these protein fibers begin to break down. The proteins form clumps that distort light as it penetrates the lens and reaches the retina (see How the Eye Works). A cataract can form in any part of the lens, but the closer it is to the center, the more likely it is to obscure vision.

Other types of cataract

Secondary. Age-related cataracts is considered to be a primary disease because it evolves by itself, rather than as the unintended consequence of a surgical procedure, as a medication side effect, or as a result of some other condition. A secondary cataract is one that can be linked to another cause, such as those listed below:

  • Eye surgery, such as for glaucoma or for a previous cataract in the same eye
  • Use of medically approved medications, such as corticosteroids (for example, prednisone), chlorpromazine, and phenothiazine
  • Other conditions, such as diabetes, retinitis pigmentosa, hypoparathyroidism, atopic dermatitis, and uveitis

Congenital. A baby born with cataracts is said to have congenital cataracts, although the condition may be either inherited (genetically transmitted) or congenital (acquired in the womb by means of illness or an infection such as rubella [measles]). Cataracts that develop during early childhood is also usually called congenital cataracts.

Traumatic. A traumatic cataract is one that occurs after an injury, either right away or years after the event. Almost any kind of traumatic injury can be responsible:

  • Blunt trauma, like getting punched in the eye during a fist fight
  • Penetrating trauma, such as when a shard of glass or metal pierces the eye during an auto accident
  • Alkaline chemical burns
  • Electrical trauma, such as being shocked with a Taser (stun) gun

Radiation. The word radiation simply refers to radiant heat, such as the heat of the sun’s rays. Radiation cataracts, sometimes called toxic cataracts, is a form of the disease caused by ultraviolet radiation from sunlight or by exposure to other forms of radiation. For example, the development of cataracts among workers who cleaned up the Chernobyl nuclear reactor accident was linked to their radiation exposure at the plant. Even radiation therapy intended to shrink a cancerous tumor can cause cataracts as a side effect.

Classification of Cataract

The lens of the eye is made up of three layers the nucleus, or core, of the lens is surrounded by a cortex. This cortex, or cortical layer, is in turn enveloped by a capsule, or capsular layer. It might help to picture the lens as a candy-coated chocolate-covered almond. The almond is the nucleus, the chocolate is the cortex, and the candy shell is the capsule that surrounds the chocolate-covered almond. When a vision specialist diagnoses a cataract, he or she classifies it according to type and according to its location on the lens:


When the nucleus of the lens hardens or becomes opaque (clouded), it's called a nuclear cataract. This type of cataract tends to have a muddy, dark-brown or a milky, opalescent appearance and poses the greatest threat to visual acuity because it lies behind the pupil, obscuring central vision. People with this kind of cataract tend to become more nearsighted as the cataract distorts the light entering the pupil. They may actually have better vision in dim light because pupil dilation allows them to see around the dense area of the lens.


A cataract on the cortex of the lens is called a cortical cataract. This type of cataract has a distinctive spoked or striated appearance. Nearly two-thirds of all cataracts fall into this category.

Posterior subcapsular

A lacy, dense area that forms just in front of the posterior lens capsule (that is, the back of the lens capsule) is called a posterior subcapsular cataract (PSC). PSCs often make the person sensitive to glare and make it difficult for them to focus on objects in the near distance. A PSC often combines with one of the other types of cataract as they progress to more advanced stages.

Other types
When two or more types of cataract combine in a person with advanced cataract disease, the area is referred to as a mixed cataract. When the lens becomes completely veiled, the cataract is said to be mature or ripe. A hypermature cataract may cause pain or inflammation that necessitates surgery.

Weighing Your Options

If a vision care provider discovers that you or a friend or loved one has a cataract (read Diagnosing Cataracts), it’s time to take stock of your options (see Treatment and Care of People with Cataracts). But take heart - remember, most cataracts progress slowly and do not cause low vision for several years after they’re detected.

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