A cataract is a clouding or opaque area over the lens of the eye– an area that is normally transparent. As this thickening occurs, it prevents light rays from passing through the lens and focusing on the retina. This clouding is caused when some of the protein which makes up the lens begins to clump together and interferes with vision.
In its early stages, a cataract may not cause any problems. The cloudiness may affect only a small part of the lens. However, over time, the cataract may grow larger and affect more of the lens, making it harder to see. As less light reaches the retina, it becomes increasingly more difficult to see. While cataracts cannot spread from one eye to another, many people develop cataracts in both eyes.
According to the National Eye Institute, part of the National Institutes of Health, cataract types are subdivided accordingly:
- Age-related cataracts. The majority of cataracts are related to aging.
- Congenital cataracts. Some babies are born with cataracts or develop them in childhood, often in both eyes. Some congenital cataracts do not affect vision, but others do and need to be removed.
- Secondary cataracts. Secondary cataracts develop primarily as a result of another disease occurrence in the body (for instance, diabetes). Secondary cataract development has also been linked to steroid use.
- Traumatic cataracts. Eye(s) that have sustained an injury may develop a traumatic cataract either immediately following the incident, or several years later.
- Medication Induced Cataracts. The following medications can contribute to the formation of cataracts. There are likely to be other possible causes, so ask your doctor about your symptoms.
Other sources, including the American Academy of Ophthalmology, describe the different types of cataracts according to the cataract location on the eye lens, including:
- Nuclear cataract. This is the most common type of cataract, and the most common type associated with aging. Nuclear cataracts develop in the center of the lens and can induce myopia, or nearsightedness
- Cortical cataract. This type of cataract initially develops as wedge-shaped spokes in the cortex of the lens, with the spokes extending from the outside of the lens to the center. When these spokes reach the center of the lens, they interfere with the transmission of light and cause glare and loss of contrast. This type of cataract is frequently developed in persons with diabetes, and while it usually develops slowly, it may impair both distance and near vision so significantly that surgery is often suggested at an early stage.
- Subcapsular cataract. A subcapsular cataract usually starts as a small opacity under the capsule, at the back of the lens. This type of cataract develops slowly and significant symptoms may not occur until the cataract is well-developed. A subcapsular cataract is often found in persons with diabetes, myopia, retinitis pigmentosa, and in those taking steroids.