A cataract is a clouding of the eye’s natural lens, which lies behind the iris and the pupil.

Cataracts are the most common cause of vision loss in people over age 40 and is the principal cause of blindness in the world. In fact, there are more cases of cataracts worldwide than there are of glaucoma, macular degeneration and diabetic retinopathy combined, according to Prevent Blindness America (PBA).

Today, cataracts affect more than 22 million Americans age 40 and older. And as the U.S. population ages, more than 30 million Americans are expected to have cataracts by the year 2020, PBA says.

Types of cataracts include:

• A subcapsular cataract occurs at the back of the lens. People with diabetes or those taking high doses of steroid medications have a greater risk of developing a subcapsular cataract.
• A nuclear cataract forms deep in the central zone (nucleus) of the lens. Nuclear cataracts usually are associated with aging.
• A cortical cataract is characterized by white, wedge-like opacities that start in the periphery of the lens and work their way to the center in a spoke-like fashion. This type of cataract occurs in the lens cortex, which is the part of the lens that surrounds the central nucleus.