Published: Dec. 13, 2007
Updated: Oct. 5, 2011
Astigmatism is a condition marked by an irregularity in the curvature of the cornea. The cornea is responsible for most of the focusing of light entering the eyes. Normally the cornea has a curvature similar to that of a basketball; with astigmatism, the corneal curvature more closely resembles that of a spoon or football.
Astigmatic keratotomy (AK) is a variant of radial keratotomy. It is an incision surgical procedure, used to correct astigmatism. In AK, transverse incisions are made in the surface of the cornea to smooth out its curvature and make it symmetrical.
A cataract is a cloudiness or complete opacity of the eye’s natural crystalline lens. This condition results in diminished ability to focus. Cataracts may be congenital or caused by trauma, disease, or the aging process. If vision loss is significant, surgical removal of the affected lens may be necessary. In order to replace the refractive power lost when that happens, a synthetic intraocular lens (IOL) may be used to provide missing optical power. Learn more about cataracts.
The cornea is the transparent covering over the front of the eye. It provides most of the eye’s refractive power.
Corneal decomposition is a pathologic condition caused by failure of the corneal endothelium to maintain normal fluid content of the stroma tissue layer of the cornea, and results in shading and clouding.
Corneal scarring is caused by cornea injury, such as abrasion, laceration, burns, contact lens injury, or disease. Depending on the degree of scarring, vision can range from a blur to total blindness. When corneal scarring is dense enough to affect vision, a corneal transplant may be necessary.
Dry eye syndrome is a condition in which there is an unusual dryness of the cornea. It is usually due to a deficiency in normal tear production. Dry eye syndrome can result in a sensation similar to that of having a foreign body in the eye, as well as burning and/or redness of the eye. It can also lead to decreased visual acuity.
Fuchs' dystrophy is a progressive, pathologic corneal disorder characterized by a cloudy and swollen cornea, painful epithelial blisters, and reduced vision. This condition, which is sometimes hereditary, may require a corneal transplant. Learn more about Fuchs' endothelial corneal dystrophy.
Hyperopia occurs when a refractive error causes blurriness while viewing relatively near objects. It is also known as farsightedness.
Keratoconus is a hereditary, degenerative corneal disease that causes a decrease in visual acuity. In patients with keratoconus, the cornea thins and a cone-shaped protrusion occurs in the central portion of the cornea. Management of this disease is aimed at obtaining the best possible vision, and includes the wearing of specialized contact lenses and/or corneal transplant surgery.
LASIK (Laser-Assisted In Situ Keratomileusis) is a form of laser treatment for individuals with higher degrees of myopia (greater than 5 diopters). LASIK is similar to PRK with the exception that a specialized cutting instrument known as a microkeratome is used first to produce a thin flap on the outside of the cornea. This superficial flap is then gently folded back so that the laser may reach and remove deeper layers of corneal tissue. After the application of the cool laser pulses, the corneal flap is folded back into place. LASIK preserves the outermost corneal surface and allows for rapid visual recovery with minimal discomfort and minimal scarring.
Myopia is a refractive error. This causes blurriness when viewing objects that are relatively far away. It is also know as nearsightedness.
Ocular cicatricial pemphigold is a chronic disease that may be progressive and may cause blistering and scarring of the eye’s mucous membranes, leading to adhesions between the palpebral bulbar conjunctive. Ocular cicatricial pemphigoid also causes severe drying and clouding of the cornea and may be devastating to vision.
Penetrating keratoplasty (corneal transplant surgery) involves replacement of a scarred, diseased, or damaged cornea with clear corneal tissue provided from a donor. This treatment is used for corneal decomposition, corneal dystrophies (other than keratoconus) including Fuchs' dystrophy, and corneal trauma/corneal scarring. Learn more about corneal transplant surgery.
Photorefractive keratectomy (PRK) is a non-invasive procedure that permanently corrects certain errors in vision. PRK is accomplished with an eximer laser, a special kind of laser that uses computer-controlled pulses of cool light to remove a layer of tissue less than one micron thick from the other surface of the cornea. In PRK no scalpels are used. Since the cornea is responsible for most of the eye’s focusing power, the change in its curvature achieved with PRK results in improved vision. Currently, PRK can be used to correct myopia and astigmatism. PRK is performed under topical anesthesia, takes only a few minutes to complete, and involves only about 10 to 20 percent of the outermost surface of the eye.
Presbyopia is a condition in which the normal changes in the shape of the lens that occur when looking from a faraway object to a near one (or vice-versa) are limited. Normally, the iris (the muscle around the lens) causes the lens to change shape as one looks at objects at varying distances. This phenomenon (called “accommodation”) allows the eye to focus at different lengths. When accommodation is impaired, the eye is unable to adjust sufficiently to nearby objects. As a result, people with presbyopia may be seen holding reading material at arm’s length in order to discern what is written.
Pteryglum is an abnormal, wedge-shaped growth on the surface of the conjunctiva. This wedge may gradually advance and require surgical removal. (Learn more about pterygium.)
Radial keratotomy (RK) is a surgical procedure used to alter the refractive ability of the cornea. In RK, the surgeon uses a handheld, diamond-tipped blade to make a series of incisions in the surface of the cornea in a radial pattern, while leaving untouched a three to four millimeter circular section in the cornea’s center. This causes the peripheral part of the cornea to relax and the central part to flatten. Since the cornea is responsible for most of the eye’s focusing power, the resulting change in the curvature of the cornea can lead to an improvement in visual acuity. The incisions made in RK penetrate to approximately 90 percent of the depth of the cornea. The refractive surgeons at the Duke Eye Center are concerned about the long term refractive stability and structural integrity of the eye after RK and believe that laser surgery (PRK, LASIK) will largely replace incision refractive surgery. We continue to caution patients who have undergone RK about their increased risk of permanent damage from ocular trauma, including air bag injuries.
Refraction is the bending of light rays in order to focus them. The cornea and the lens achieve refraction of light that passes through the eye. Optimal focusing of light in the eye results in a clear image being received by the brain.
A secondary cataract is any cataract that develops from a known cause, such as chronic iritic or drugs (steroids). This term also designates a clouding of the lens capsule or membrane that supports an intraocular lens after cataract surgery.
Sjogren’s syndrome is a chronic disease of the body’s connective tissue. Dry eyes, dry mouth, and arthritis characterize it.
The Snellen chart is used to assess visual activity. It contains rows of letters of standardized size, each row smaller than the previous one. The patient attempts to read the chart at a specified distance (usually 20 feet). Then, a determination is made of the smallest sized letters the patient is able to identify.
Visual acuity is a measure of the eye’s ability to distinguish details and shapes of increasingly small objects at a fixed distance. The Snellen chart is normally used to determine visual acuity.