According to the Free Dictionary, senile nuclear sclerosis describes a type of cataract that is characterized by a gradual hardening of the nucleus of the lens. This type of cataract is usually brown or black in color and usually occurs in both eyes.
According to MedicineNet.com, cataracts are an abnormality of the lens of the eye. Normally clear, the lens focuses the light that enters the eye through the pupil onto the light-sensitive retina, initiating a chemical reaction that allows an image to be perceived by the brain. A lens that contains a cataract becomes increasingly cloudy, distorting the light that reaches the retina, interfering with vision and eventually causing blindness. Cataracts can be corrected surgically; however, because many third world countries lack surgical services, cataracts remain the most common cause of blindness in the world.
MedicineNet.com explains further that most cataracts are associated with aging, explaining the use of the word "senile" in "senile nuclear sclerosis." Over 50 percent of Americans develop cataracts by the age of 80, and 100 percent of the population have them by the age of 95. Senile cataracts occur most often in the central portion of the lens, or the nucleus, while those in younger people generally are found in other parts of the lens. Non-age-related cataracts are called secondary cataracts and have many causes, including diabetes, inflammatory conditions, medications such as corticosteroids, infections and penetrating injuries to the eye.