A cataract is a clouding of the normally clear lens of your eye. For people who have cataracts, seeing through cloudy lenses is a bit like looking through a frosty or fogged-up window.

Clouded vision caused by cataracts can make it more difficult to read, drive a car (especially at night) or see the expression on a friend's face.

Most cataracts develop slowly and don't disturb your eyesight early on. But with time, cataracts will eventually interfere with your vision.

At first, stronger lighting and eyeglasses can help you deal with cataracts. But if impaired vision interferes with your usual activities, you might need cataract surgery. Fortunately, cataract surgery is generally a safe, effective procedure.

At first, the cloudiness in your vision caused by a cataract may affect only a small part of the eye's lens and you may be unaware of any vision loss. As the cataract grows larger, it clouds more of your lens and distorts the light passing through the lens. This may lead to signs and symptoms you're more likely to notice.

Types of Cataracts by Location

Cataracts are classified by doctors according to the location of the opacity, or clouding, of the lens. Nuclear Sclerotic Cataract: A nuclear sclerotic cataract is the most common type of age-related cataract. This type of cataract causes a gradual yellow cloudiness and hardening of the central part of the lens called the nucleus. Changes in vision are usually gradual. In some cases, patients may see an actual improvement in near vision before their vision deteriorates to a significant degree. Referred as "second sight," this stage is usually only temporary.

Cortical Cataract: A cortical cataract generally appears as a cloudy opacity in the part of the lens called the cortex. The cortex consists of the peripheral, or outer part, of the lens. These cataracts often resemble wheel spokes that point inward toward the center of the lens. Light tends to scatter when it hits the spoke-like opacities.

Posterior Subcapsular Cataract: Often referred to as a PSC, a posterior subcapsular cataract is an opacity that develops on the back surface of the lens, directly underneath the lens caspsular bag that houses the lens. This type of cataract causes light sensitivity, blurred near vision, and glare and halos around lights. They are more common in diabetic patients and patients who have taken steriods for extended periods of time.

Types of Cataracts by Origin

Eye doctors also classify cataracts according to their origin.

Age-related Cataract: Most cataracts develop as we age. Although signs can be seen as early as your 40s to 50s, cataracts usually do not become significant until the late 60s or 70s.

Secondary Cataracts: Cataracts can sometimes develop after undergoing eye surgery, such as surgery for glaucoma or retinal surgery. Patients with diabetes sometimes develop cataracts earlier than normal. Also, patients who are taking steroids for an extended period of time may develop cataracts.

Traumatic Cataract: Cataracts sometimes result from direct injury or trauma to the eye. Cataracts may develop immediately or years after an event that damages the eye. Traumatic cataracts often occur after blunt trauma to the eye or from exposure to certain chemicals.

Congenital Cataract: Some children are born with cataracts. In some cases, the inherited cataract is not significant enough to affect vision. If significant, however, the cataract should be removed in order to avoid vision problems, such as strabismus or amblyopia.

Radiation Cataract: Although rare, cataracts sometimes form after exposure to certain types of radiation. This type of cataract may be caused from exposure to ultraviolet light from the sun and other forms of radiation.

Symptoms and signs

Signs and symptoms of cataracts include:

  • Clouded, blurred or dim vision
  • Increasing difficulty with vision at night
  • Sensitivity to light and glare
  • Seeing "halos" around lights
  • Frequent changes in eyeglass or contact lens prescription
  • Fading or yellowing of colors
  • Double vision in a single eye

What Causes Cataracts?

The lens inside the eye works much like a camera lens, focusing light onto the retina for clear vision. It also adjusts the eye's focus, letting us see things clearly both up close and far away.

The lens is mostly made of water and protein. The protein is arranged in a precise way that keeps the lens clear and lets light pass through it.

But as we age, some of the protein may clump together and start to cloud a small area of the lens. This is a cataract, and over time, it may grow larger and cloud more of the lens, making it harder to see.

No one knows for sure why the eye's lens changes as we age, forming cataracts. But researchers worldwide have identified factors that may cause cataracts or are associated with cataract development. Besides advancing age, cataract risk factors include:

  • Ultraviolet radiation from sunlight and other sources
  • Diabetes
  • Hypertension
  • Obesity
  • Smoking
  • Prolonged use of corticosteroid medications
  • Statin medicines used to reduce cholesterol
  • Previous eye injury or inflammation
  • Previous eye surgery
  • Hormone replacement therapy
  • Significant alcohol consumption
  • High myopia
  • Family history

One theory of cataract formation that's gaining favor is that many cataracts are caused by oxidative changes in the human lens. This is supported by nutrition studies that show fruits and vegetables high in antioxidants may help prevent certain types of cataracts (see below).