What Is a Cataract?
Derived from the Latin word meaning waterfall, the term “cataract” arose from the ancient misconception that evil liquids flowing into the eye were the cause of cataract symptoms.
Cataracts can occur at any age (in fact, babies can be born with them), but they are most common later in life. In the United States, about 70 percent of people over age 75 have cataracts. It is estimated that more than 20 million Americans over age 40 (approximately 17 percent) have had a cataract.
In most individuals, vision loss from cataracts can be corrected by surgery. In fact, cataract surgery is the most commonly performed surgical procedure in the United States. The American Academy of Ophthalmology estimates that 1.6 million cataract operations are performed each year in the United States. Not all cataracts, however, affect vision significantly or require treatment.
Types of cataracts
The lens is made of protein fibers arranged in a specialized way that results in its transparency. The lens has four layers: At the center is the nucleus, which is surrounded by the next layer, the cortex. Surrounding the cortex is the lens epithelium. The layer at the surface is the lens capsule.
In healthy eyes, light rays reflected from an object enter the eye through the cornea and lens, which together focus the light onto the retina to produce a sharp image. When a cataract develops, however, light rays are no longer precisely focused. Instead, the rays are scattered before reaching the retina.
The three common types of cataracts are defined by where they occur in the lens: nuclear, cortical, and posterior subcapsular (in the rear of the lens capsule). It is possible for a person to have more than one type of cataract in the same eye.
• Nuclear cataracts are the most common type. The incidence of nuclear cataracts increases with age and cigarette smoking.
• Cortical cataracts also become more common with age. The development of this type of cataract is related to lifetime exposure to ultraviolet light.
• Posterior subcapsular cataracts are most likely to occur in younger people. This type of cataract is often the result of prolonged use of corticosteroids (such as prednisone), inflammation, trauma or diabetes.
The extent and rapidity of vision damage depend not only on the size and density of the cataract, but also on its location in the lens. For example, a cataract on the outside edge of the cortex has little effect on vision because it does not interfere with the passage of light through the center of the lens. But a dense nuclear cataract causes severe blurring of vision.
Symptoms of cataracts
Cataracts form painlessly. The most common symptom is cloudy or blurry vision. Everything becomes dimmer, as if seen through glasses that need cleaning. Most often, both eyes are affected, though vision is usually worse in one eye. Other symptoms include glare, halos, poor night vision, a perception that colors are faded or that objects are yellowish and the need for brighter light when reading.
In some cases double vision occurs. Double vision is caused by passage of light through a lens that has irregular areas of opacity, splitting the rays of light from a single object and focusing them on different parts of the retina.
Another symptom of cataracts is the need for frequent changes in eyeglass and contact lens prescriptions. The symptoms can develop over months or almost imperceptibly over many years.
Paradoxically, in the early stages of a nuclear cataract, vision may temporarily improve in some people. For example, a person who previously needed reading glasses for presbyopia is able to read without them. This change, referred to as second sight, occurs because the cataract alters the shape of the lens, making it better able to focus on nearby objects. Over time, however, the improvement is lost, as the cataract’s progression impairs vision.
People with cortical or posterior subcapsular cataracts often have worse vision in bright light; for example, they may have problems with night driving because of oncoming headlights. Bright light causes the pupils to contract and restricts the passage of light to the center of the lens (the part of the eye that may be most affected by the cataract).