Cataract

A cataract is an opacity of the lens that reduces visual acuity to <= 20/30. Symptoms are glare and painless vision loss. Diagnosis is by slit-lamp examination. Treatment includes appropriate lighting, avoidance of glare, and eventually surgical removal (which is the only cure).

Cataracts are the leading cause of blindness worldwide. The lens is normally transparent until after age 40, when nonspecific opacities may appear. Nonspecific opacities are not usually considered cataracts until visual acuity decreases to <= 20/30. Age is the major risk factor for progression, and almost everyone who lives long enough develops cataracts. Other risk factors include smoking; poor nutrition; corticosteroid therapy; intraocular surgery; glaucoma drugs; and a history of atopic dermatitis, ocular trauma, or exposure to ultraviolet light (as occurs in sunny climates) or to radiation. The cause is thought to be oxidative damage to lens proteins that reduces solubility; eventually, insoluble opacities form in otherwise transparent tissue.

Cataracts are categorized by their location in the lens. Nuclear cataracts form in the central part of the lens and are most common among women, nonwhites, and smokers. Cortical cataracts form peripherally and are most common among people who have darkly pigmented irises or who have had prolonged exposure to sunlight; cortical cataracts are less likely to affect vision than nuclear cataracts. Posterior subcapsular cataracts form in the central part of the lens on the posterior lens capsule. They usually occur at an earlier age, cause the most visual disability, and are likely to form in people who take long-term corticosteroid therapy, who have had ocular trauma, or who have a history of atopy or diabetes.

Symptoms and Signs

The hallmark of all cataracts is painless, progressive vision loss, and glare. The rate of vision loss varies, and the nature of the loss varies by cataract location.

Cortical cataracts are often spokelike and cause glare, especially at night or in dim light when the pupil is slightly dilated. Typically, glare occurs when oncoming headlights are encountered during night driving. These cataracts usually have little effect on visual acuity.

Nuclear cataracts cause glare but initially have little effect on visual acuity. Early in the disorder, they may actually improve close vision in the elderly (called second sight) because the still-transparent cataract increases the refractive index of the lens, magnifying the close field of view. This increased refractive ability can partially compensate for loss of accommodation (presbyopia) and enable people who have been using reading glasses to read without glasses again. However, as nuclear cataracts become denser, they decrease the amount of light reaching the retina and interfere with sight to the point where people cannot see well enough to read, even with glasses.

Posterior subcapsular cataracts are closer to the focal point of the lens. Because all light must pass through the focal point on the way to the retina, these cataracts tend to scatter light to a greater extent, especially if the light is bright and if the patient's pupils are small. Sunglasses help reduce glare. However, over time, these cataracts progress and eventually interfere with vision so that reading becomes difficult, even with glasses. Reading may become difficult because the contrast between the dark letters and light background of the page is reduced. Light that reflects off the page or shines directly in the eye tends to make the pupil constrict and worsen glare.

Rarely, an advanced cataract may swell, and lens material may leak into the anterior chamber, increasing intraocular pressure and causing secondary glaucoma.

Diagnosis

Diagnosis is by slit-lamp examination. Cataracts appear in silhouette against the red reflex when a +10 diopter lens is used. All patients with cataracts require a visual acuity (near and far vision) test to assess effect on vision and a refraction test to assess the degree of refractive shift. The physician must also evaluate the effect of cataracts on function to determine the need for surgical treatment.

Treatment

Treatment usually begins with measures to avoid glare and to improve vision. Patients are advised to wear sunglasses with polarized lenses and, if glare occurs at night, to wear glasses with untinted polarized lenses. Indoor lighting should be bright enough to provide adequate but not excessive light. Using reflected rather than direct light (eg, a lamp that shines light over the shoulder when reading) may help reduce glare. A new prescription for eyeglasses may improve vision. If possible, drugs that cause the pupil to constrict should be stopped, especially in patients with centrally located cataracts. Patients who smoke are advised to stop.

For most patients, cataract surgery is eventually necessary. Cataract surgery is elective and based on the degree of functional impairment due to the cataract, the patient's desire for improved vision, and the likelihood that vision will improve after surgery. There is no advantage to performing surgery before function is noticeably impaired. Patients with glaucoma, age-related macular degeneration, severe diabetic retinopathy, or other neuroretinal disorders may have irreversible vision loss because of the underlying disorder and thus may not be candidates for surgery.

About 98% of 1.5 million cataract extractions done annually in the US involve removal of the native lens and placement of a plastic prosthetic lens. In a few patients, the eye is not medically suitable for placement of a prosthetic lens.

Lens removal can be extracapsular or intracapsular. For extracapsular extraction, the central portion of the anterior capsule is removed, and lens contents are aspirated so that much of the lens capsule remains intact. Extracapsular extraction is almost always preferred to intracapsular extraction because the prosthetic lens can be placed in the posterior chamber, resulting in fewer complications.

Most cataract surgery is done using a local anesthetic. Patients who cannot lie down for long periods (eg, patients with dementia) may require a general anesthetic.

Cataract extraction is safe. The most serious complication is endophthalmitis, which is very rare but can cause blindness. Bacterial or fungal eye infection (endophthalmitis) occurs in about 1/2000 patients, usually 24 to 72 h after surgery, and requires immediate hospitalization and aggressive treatment with IV antibiotics or antifungals and corticosteroids to prevent eye loss. Other complications are uveitis (persistent inflammation of the eye), cystoid macular edema (swelling of the macula or central portion of the retina), retinal detachment, and intraocular lens dislocation.

The posterior lens capsule is usually not removed and may opacify postoperatively, so about 30% of patients need a laser procedure to create an opening in the posterior lens capsule to restore visual acuity to its best level.

Prognosis after treatment is excellent. Visual acuity is restored to >= 20/40 in > 95% of patients. Patient recovery is typically rapid.

This topic was last updated May 2006.