As adults get on in years, many come to a gradual realization that they’re just not seeing as clearly as they once did. It may be that they’ve gotten noticeably worse at seeing far-away objects or focusing on things up close, or it may just be a generalized sense that, more and more, they’re viewing the world “through a glass darkly.” The reason is that, in one or both eyes, these people have developed cataracts.

Cataracts are the leading cause of blindness globally and the leading cause of vision loss in the U.S., with 17% of Americans—that’s more than 20 million people—age 40 or older having a cataract in one or both eyes and more than half of adults 80 or older having, or having had, at least one cataract. Because of their ubiquity, cataracts are often a topic of discussion. Yet most of us have only a vague understanding of what they are. We generally know that cataracts are some kind of clouding-over of the eye, but most of us don’t know exactly how cataracts are formed and why some people get them and some don’t.

Anatomy of the Eye

Our vision depends on the passage of light rays through various parts of the eye. The lens is located on the inside of the outer eye. Light going through the eye follows this path:

  • First it passes through the cornea.
  • Then it goes through the aqueous humor, a layer of fluid.
  • Next it traverses the pupil, an opening in the center of the iris. The iris is what gives the eye its color.
  • Next light penetrates the lens, which, when healthy, is a perfectly clear film that bends light rays to help bring images into focus. This is where cataracts form.
  • From the lens, the light continues into the back of the eye, first through another liquid, this one more viscous, known as the vitreous humor.
  • Then to the retina with its rods and cones, where visual data is converted into an electrical signal that’s then transmitted to the brain’s visual cortex via the optic nerve.

Cataracts

Cataracts are white or cloudy patches that form on the lens. We’ve known about them for thousands of years, with evidence dating back to ancient Egypt, and we’ve been using essentially the same name for them since Roman times. Cataracta, in Latin, means “waterfall,” with the name presumably arising because this form of visual impairment can feel like viewing the world through a thin watery mist.

Cataracts impair vision by not allowing sufficient light to pass through the lens to the retina and by impairing the lens’s ability to focus. When people describe what it’s like living with cataracts, they often reach for metaphors like, “looking through stained glass,” “looking at the world through a dirty window,” “looking through frosted glass,” and others. If this sounds like your vision, you know what can happen to the images you’re trying to see:

  • They become blurry
  • Colors look faded
  • Lights have halos around them
  • Images appear doubled, and
  • It’s harder to see at night

All of these are possible symptoms of cataracts.

While the causes of cataracts aren’t fully understood, cataracts typically develop as adults get older, starting from about age 50. You can have cataracts in one or both eyes, and at the same time or one at a time. They don’t “spread” from one eye to the other, however. A person could have a severe cataract in one eye and a moderate one in the other, a moderate one in one eye and none in the other, and so on.

What are the Three Types of Cataracts?

The type of cataract you may have is based on the part of the lens affected. It’s possible for any one, two, or all three of the areas of the lens to be impacted at once, meaning that a single person could have all three types of cataract in one or both eyes.

The lens has three parts:

  • Capsule. This is a very thin outer layer to the lens; it’s a thin, protective membrane.
  • Cortex. This is the “meat” of the lens.
  • Nucleus. This is the center of the lens.

The types of cataracts correspond to these three areas of the lens:

  • A posterior subcapsular cataract forms at the back of the cortex closest to the retina
  • A cortical cataract is located in the lens’s cortex
  • A nuclear cataract is located in the lens’s nucleus. Nuclear cataracts typically form as a result of a process known as nuclear sclerosis, in which the protein fibers of the nucleus of the lens become compressed, dense, hardened, yellow, and cloudy. Sometimes people use the terms “nuclear cataract” and “nuclear sclerosis” interchangeably, but technically a nuclear cataract doesn’t appear unless the process of nuclear sclerosis becomes severe enough to cause the lens proteins to clump and distort light.

Cataract treatment

In the early stages of a cataract, most people can manage by working under brighter lighting or by getting a stronger prescription for their glasses. But as the cataract continues to develop, it can make life difficult and even dangerous, especially with driving, fall risk, and home accidents.  At that point, cataract surgery is recommended. Fortunately, today’s cataract surgeries are quick, safe, and effective.

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