Merck Manual

Please confirm that you are not located inside the Russian Federation

honeypot link



Leila M. Khazaeni

, MD, Loma Linda University School of Medicine

Reviewed/Revised Jan 2024
Topic Resources

A cataract is a clouding (opacity) of the lens of the eye that causes a progressive, painless loss of vision.

Cataracts are the leading cause of blindness worldwide. Cataracts are common in the United States, where they affect mostly older adults. Almost 1 in 5 people between the ages of 65 and 74 develop cataracts severe enough to reduce vision, and almost 1 in 2 adults older than 75 has cataracts. Fortunately, people in the United States can often have their cataracts treated before they cause blindness.

The Lens

Cataracts usually occur with aging or develop without any apparent cause. However, other risk factors include the following:

People who have had a cataract in one eye are more likely to develop one later in the other eye. Sometimes cataracts can develop in both eyes at the same time. Babies can be born with them (congenital cataracts Congenital Cataract A congenital cataract is a painless clouding of the lens that is present at birth or shortly after birth. (See also Cataract in adults.) Congenital cataracts have many causes. They may be inherited... read more ), and children can also develop cataracts, usually as a result of injury or illness.

Did You Know...

  • Cataracts are the leading cause of blindness worldwide.

Symptoms of Cataract

Cataracts usually develop slowly over years. Because all light entering the eye passes through the lens, the cataract (clouding of the lens) can block and scatter light, which causes poor vision. Early symptoms may include

  • Seeing halos and starbursts around lights (glare)

  • Difficulty reading because of a worsening ability to distinguish the contrast between the light and dark of printed letters on a page

  • Needing more light to see well

  • Problems distinguishing dark blue from black

  • Blurred vision

  • Colors seeming more yellow and less vibrant

  • Less commonly, mild double vision (also called ghost images)

How Cataracts Affect Vision

On the left, a normal lens receives light and focuses it on the retina. On the right, a cataract blocks some light from reaching the lens and distorts the light being focused on the retina.

How Cataracts Affect Vision

How much vision is changed by a cataract depends on the intensity of light entering the eye and on the location of the cataract.

With a cataract in the center (inside) of the lens (nuclear cataract), the following symptoms are most common:

  • Worsened distance vision

  • Initially, improved near vision because the cataract acts as a stronger lens, thus refocusing light

People who have needed reading glasses since their mid-40s may discover that they can see up close again without their glasses, a phenomenon called "second sight." This effect is temporary and goes away as the cataract becomes more opaque.

With a cataract near the back of the lens (posterior subcapsular cataract), the following symptoms are most common:

  • Blurred vision (less visual acuity) when the pupil constricts (for example, in bright light or during reading)

  • Loss of contrast

  • Halos and starbursts (glare) from bright lights or car headlights while driving at night

People who take medications that constrict their pupils (for example, certain glaucoma eye drops) may also have greater vision loss.

Did You Know...

  • Sometimes a cataract in a certain part of the lens can cause near vision to improve for a while, and a person can read again without needing reading glasses.

Diagnosis of Cataract

  • A doctor’s examination with an ophthalmoscope

  • Slit-lamp examination

A doctor can usually detect a cataract while examining the eye with an ophthalmoscope Ophthalmoscopy A person who has eye symptoms should be checked by a doctor. However, some eye disorders cause few or no symptoms in their early stages, so the eyes should be checked regularly (every 1 to 2... read more (a hand-held light with a magnifying lens that can shine into the back of the eye).

A doctor can identify the exact location of the cataract and the extent to which it blocks light by using an instrument called a slit lamp Slit-Lamp Examination A person who has eye symptoms should be checked by a doctor. However, some eye disorders cause few or no symptoms in their early stages, so the eyes should be checked regularly (every 1 to 2... read more (an instrument that enables a doctor to examine the eye under high magnification). The slit lamp allows examination of the lens and other parts of the eye in more detail.

Treatment of Cataract

  • Eyeglasses and contact lenses to improve vision

  • Surgery to remove the cataract and place an intraocular lens

Until vision is significantly impaired by a cataract, eyeglasses Eyeglasses Refractive errors can be corrected with glass or plastic lenses mounted in a frame (eyeglasses) or with a small lens made of plastic floating or resting on the cornea (contact lens). Good vision... read more and contact lenses Contact Lenses Refractive errors can be corrected with glass or plastic lenses mounted in a frame (eyeglasses) or with a small lens made of plastic floating or resting on the cornea (contact lens). Good vision... read more may improve a person's vision. Wearing sunglasses in bright light and using lamps that provide over-the-shoulder lighting may decrease glare and aid vision. Rarely, drops that keep the pupil dilated for a long time may be used to help vision if the cataract is small and located in the center of the lens.


The only treatment that provides a cure for cataracts is surgery. There are no eye drops or medications that will make cataracts go away. Cataract surgery can be done on a person of any age and is generally safe even for people with illnesses such as heart disease and diabetes.

In most cases, people should have surgery only when their vision is so impaired by cataracts that they feel unsafe, uncomfortable, or unable to perform daily tasks. There is no advantage to having cataracts removed before then. Very rarely, cataracts cause changes, such as inflammation in the eye or increased pressure in the eye (glaucoma Glaucoma Glaucomas are a group of eye disorders characterized by progressive optic nerve damage (often, but not always, associated with increased eye pressure) that can lead to irreversible loss of vision... read more ), that lead doctors to recommend the cataract be removed quickly.

Surgery to remove cataracts is almost always done using a local anesthetic (an injection or eye drops) to numb the eye surface. People usually also receive a sedative. Rarely, children or adults who cannot hold still during surgery require general anesthesia.

During surgery, the doctor usually makes a small incision in the eye and removes the cataract by breaking it up with ultrasound and taking the pieces out of the lens capsule (phacoemulsification). Sometimes doctors use a laser (called a femtosecond laser) during certain parts of cataract surgery, such as making incisions and entering and softening the cataract so that it is easier to remove with ultrasound. Where phacoemulsification is not available, doctors can simply remove the lens during surgery without the use of ultrasound. This technique is called conventional extracapsular cataract extraction.

When all the cataract pieces have been removed, the surgeon usually places a plastic or silicone lens (intraocular lens) in the lens capsule. However, the intraocular lens cannot always be safely placed. When lens placement is not possible, people must wear thick eyeglasses or contact lenses after the cataract has been removed.

Ocular Implant


The procedure normally takes about 30 minutes, and the person can go home the same day. No sutures are usually needed because the incision into the eye is small and seals itself.

People should make arrangements in advance to get extra help at home for a few days after surgery because activity may be restricted (for example, bending over and heavy lifting may be prohibited). Vision changes, such as blurred vision and discomfort with bright light, may occur for a short time after surgery.

For a few weeks after surgery, corticosteroid, nonsteroidal anti-inflammatory drug (NSAID), and antibiotic eye drops are used to reduce inflammation, prevent infection, and promote healing. Sometimes antibiotics, corticosteroids, and/or NSAIDs are injected into the eye at the end of surgery, a step that reduces the need for eyedrop use after surgery. People are given eyeglasses or a plastic shield to wear while sleeping to protect the eye from injury until healing is complete, usually a few weeks. Rubbing the eye, heavy lifting, and excessive forward bending are avoided. People visit the doctor the day after surgery and then typically about 1 week and 1 month later. If a person has cataracts in both eyes, many doctors wait several months after the first eye has healed to remove a cataract from the other eye.

Many people notice improved distance vision within a few weeks after cataract surgery. Almost everyone will need eyeglasses for reading, and some people will need eyeglasses to obtain the best possible distance vision as well.

Newer intraocular lenses with multiple focusing powers (multifocal lenses) may allow a person to have good near and distance vision without needing glasses, although some people may lose contrast and experience starbursts and halos at night with these lenses. The doctor makes calculations before the surgery to decide how powerful the artificial lens should be. Thus, it is possible to go from wearing very thick eyeglasses before the surgery to wearing much thinner eyeglasses after it. Some people with the newer multifocal lenses no longer need eyeglasses or need them for only certain things, such as driving at night, reading in dim light, or for midrange vision, such as viewing a computer monitor. Some other lenses can correct astigmatism What Is Astigmatism? What Is Astigmatism? in the eye and that can also reduce the need for glasses after surgery.


Major complications after cataract surgery are rare. Proper follow-up with the doctor can lead to early detection and treatment of complications. Complications include:

In some people, the thin, transparent back layer (capsule) deliberately left behind in the eye after the original lens was removed becomes clouded, impairing vision. This problem, called a secondary cataract (or posterior capsule opacification), occurs in about 1 in 4 people who have had cataract surgery, months or even years after an artificial lens is implanted. Typically, it is treated by using a laser to make a small opening in the hazy capsule to let light through.

Prevention of Cataract

There are several things people can do that may help prevent cataracts, including the following:

Estrogen use by women after menopause may also be protective, but estrogen should not be used solely for this purpose. Finally, people who are taking corticosteroids for extended periods might discuss with their doctor the possibility of using a different medication.

quiz link

Test your knowledge

Take a Quiz!