Glossary

To surgically remove; to vaporize. In LASIK, an excimer laser is used to ablate a small amount of corneal tissue to reshape the eye and thereby correct vision.

The circular area in the center of the cornea that the excimer laser treats to correct vision. The total LASIK ablation zone is usually 7 to 9 millimeters in diameter and includes a central optical zone (OZ) and a small transition zone between the OZ and the untreated peripheral cornea.

The ability of the eye to increase its refracting power to focus on near objects. In a normal eye, accommodation is needed only to see near objects clearly. No accommodation is required to see distant objects. As we age, our eyes' ability to accommodate diminishes. This normal age-related loss of accommodation is called presbyopia. Because of presbyopia, most LASIK patients over age 40 need reading glasses to see near objects clearly.

Inward misalignment of the eyes ("crossed eyes") associated with focusing effort. Usually associated with uncorrected hyperopia (farsightedness), which requires greater than normal focusing effort for clear vision.

Clarity or sharpness of vision. Visual acuity is typically measured using letter charts and is quantified by means of the Snellen fraction (i.e. 20/20, 20/40, etc.)

LASIK wherein the surgeon uses an IntraLase femtosecond laser (laser microkeratome) to create the corneal flap, rather than using a mechanical (bladed) microkeratome. Also called "blade-free LASIK" and (infrequently) "IntraLASIK".

Poor vision that cannot be corrected with glasses or contact lenses in an otherwise healthy eye. Amblyopia, commonly called "lazy eye," affects up to 5 percent of the population and can be caused by improper eye alignment (strabismus) and/or unequal amounts of farsightedness or nearsightedness in the two eyes.

The surgical removal of the front portion of the clear membrane-like structure (lens capsule) that surrounds the lens inside the eye. Eye surgeons perform the procedure so they can remove the cloudy lens and replace it with a man-made intraocular lens (IOL), while leaving the posterior portion of the lens capsule intact to protect the back of the eye.

The fluid-filled space within the front part of the eye, located between the back surface of the cornea and the front surface of the iris.

Varying slightly from a perfectly spherical (round) arc or curve. Aspheric lenses or laser treatments reduce a specific type of higher-order aberration called spherical aberration.

Unequal curvature of the eye that causes blurred vision at all distances. Astigmatism causes certain aspects of objects to appear more blurred than others, and may produce "ghosting" or double images. LASIK can correct astigmatism.

Any of a varied group of serious, chronic illnesses that can affect almost any part of the body. Autoimmune diseases are characterized by a misdirected immune response in which the body's immune system (which normally protects the body from disease and infection) attacks the very tissues it normally protects. Examples of autoimmune disease include: Type 1 diabetes, rheumatoid arthritis, lupus, multiple sclerosis (MS), Crohn's disease, and Graves' disease.

The best eyesight a person can achieve with corrective eyeglasses or contact lenses.

The best eyesight a person can achieve with corrective eyeglass lenses.

Inflammation of the eyelids, characterized by redness, itchiness, and the accumulation of flaky debris (similar to dandruff) at the base of the eyelashes. Blepharitis is usually caused by staphylococcus bacteria, which may also lead to the formation of a stye. The standard treatment for blepharitis is lid scrubs and topical antibiotic medicine (eye drops or ointments).

High-energy visible light rays with wavelengths between 400 and 470 nanometers (nm). Some research suggests exposure to blue light might be associated with the development of macular degeneration and other eye health problems.

The main protein found in connective tissue in the eye and other body parts.

The ability to distinguish an object from its background of similar brightness and color. Contrast sensitivity is a more sensitive measure of visual function than standard visual acuity testing measured with a high-contrast eye chart.

The clear front part of the eye. The cornea provides about 70% of the focusing power of the eye. LASIK alters corneal curvature to enable the eye to focus light more accurately. The human cornea is about 500 microns (one-half of a millimeter) thick.

A relatively non-invasive method to increase the strength of the cornea for the treatment of keratoconus, corneal ectasia and other disorders. After removal of the surface layer of the cornea (epithelium), a solution of riboflavin (vitamin B2) is applied to the underlying cornea tissue and the eye is exposed to UV radiation. The treatment strengthens bonds between connective tissue (collagen) fibers within the cornea to improve the stability of the eye. The corneal epithelium grows back within a few days.

Loss of clarity of the front surface of the eye (cornea) following laser vision correction surgery such as PRK. Corneal haze usually is temporary and may or may not affect visual acuity.

A computerized, highly detailed mapping the front surface of the eye to determine the exact curvature of the cornea. Corneal topography can reveal higher-order aberrations that are not detectable with a standard refraction.

The type of eyeglass or contact lens power required to correct astigmatism

The unit of measurement used to quantify refractive error and lens power. Prescription lenses to correct nearsightedness have a negative (-) diopter value. Lenses to correct farsightedness and presbyopia have a positive (+) diopter value.

The eye that points directly at the object you are viewing. Believe it or not, our eyes don't ever point exactly in the same direction. The eye that points directly at an object is called the dominant eye. The other (non-dominant) eye points slightly to the side of the object. This normal difference in eye direction helps our visual world appear 3-dimensional.

To determine which eye is your dominant eye, perform this simple exercise:

1. Look at a small object in the distance.
2. Extend one arm. (If you are right-handed, extend your right arm.)
3. Put your thumb up and position your thumb directly in front of the object. (Keep both eyes open and don't move your arm or thumb!)
4. Close one eye. If your thumb is still in front of the object, your open eye is your aligned dominant eye. If your thumb has moved significantly off to the side, your open eye is not aligning your thumb with the object. This is your non-dominant eye.

Determining which eye is your dominant eye is important if you're considering monovision LASIK. For comfortable monovision, it's important that your dominant eye is the eye corrected for clear distance vision and your non-dominant eye is the eye made slightly nearsighted for seeing objects up close.

A common condition that occurs when the lacrimal glands don't produce enough tears to keep the eyes moist and comfortable. Symptoms of dry eye include burning, stinging, scratchiness, a feeling something is "in" the eye, and intermittent blurred vision. LASIK temporarily decreases tear production and may cause dry eye symptoms that can last for several weeks. Dry eye symptoms from LASIK are usually managed successfully with the frequent use of artificial tears.

Bulging of the cornea due to a weakening of the tissue. A very infrequent but serious complication of LASIK surgery.

Swelling due to excess fluid. Corneal edema may be present for several weeks after LASIK surgery.

The normal refractive condition of the eye, with no nearsightedness, farsightedness or astigmatism (refractive errors). Typically, an emmetropic eye is capable of 20/20 vision without corrective lenses.

The single layer of cells lining the back surface of the cornea. The endothelium acts like a pump to maintain the proper amount of fluid inside the cornea. Failure of the endothelium leads to corneal swelling and blurred vision.

A complication of LASIK characterized by the growth of cells from the superficial layer of the cornea (the epithelium) under the corneal flap. If not removed, epithelial ingrowth can cause discomfort, blurred vision, and flap complications.

The outer layer of fast-growing cells on the cornea. The epithelium blocks dust, water, bacteria and other foreign materials from entering the cornea. It also provides a smooth surface for the tear film and absorbs oxygen and nutrients from the tears and distributes these nutrients to the rest of the cornea. The epithelium comprises about 10% of the total corneal thickness.

An inward eye turn; cross-eyed

A medical laser used in LASIK and other vision correction surgery. Excimer lasers use very short pulses of ultraviolet (UV) rays to ablate (vaporize) tissue to reshape the cornea, enabling the eye to better focus light. The duration of each pulse of an excimer laser is measured in nanoseconds. A nanosecond is one-billionth of a second. (See also: femtosecond laser.)

A tendency for an outward eye turn (exotropia) without a manifest misalignment of the eyes.

An outward eye turn; wall-eyed

The inability to see near objects as clearly as distant objects and/or the need to use extra focusing effort to see distant objects clearly. Also called hyperopia.

The abbreviation for the U.S. Food and Drug Administration. In the United States, the FDA is the government agency that evaluates and approves medical devices, including laser systems for LASIK and other types of refractive surgery.

A medical laser used in LASIK and other vision correction surgery. Femtosecond lasers use ultra-short pulses of low-energy infrared rays to cleave tissue bonds (creating microscopic bubbles) at a precise depth below the surface of the cornea. The duration of each pulse is measured in femtoseconds. A single femtosecond is one-billionth of a millionth of a second. Femtosecond lasers are used to create the corneal flap in LASIK surgery prior to the reshaping of the eye performed with an excimer laser.

A faint, second image surrounding the primary image of an object. Uncorrected astigmatism after LASIK can be a source of ghost images and blurred vision.

Scatter from bright light that decreases vision. Corneal edema can be a source of glare and decreased vision for several weeks after LASIK.

An eye disease characterized by damage to the optic nerve that transmits visual information from the eye to the brain. Left untreated, glaucoma can cause severe vision loss and even blindness. Glaucoma usually, but not always, is associated with increased fluid pressure inside the eye.

Luminous rings appearing around lights. Corneal edema or optical imperfections in the eye after LASIK may cause halos, either temporarily or permanently.

Diffuse clouding of the cornea that causes the sensation of looking through smoke or fog.

Optical imperfections of the eye other than nearsightedness, farsightedness and astigmatism. HOA cannot be corrected with conventional eyeglasses or contact lenses. Standard LASIK may increase HOA. Customized, wavefront-guided LASIK can reduce higher-order aberrations and therefore may provide better vision than eyeglasses, contact lenses or standard LASIK.

The inability to see near objects as clearly as distant objects and/or the need to use extra focusing effort to see distant objects clearly. Also called farsightedness.

The body's normal reaction to trauma, infection or a foreign substance, characterized by pain, heat, redness, swelling and/or loss of function. Some inflammation of the cornea is normal after LASIK. You will be given an anti-inflammatory eye drop medication to use for a week or so after LASIK surgery to control post-operative inflammation.

A document disclosing the risks, benefits and alternatives to a surgical procedure. You will be given an Informed Consent form to review and sign prior to LASIK surgery.

Inside the eye. Cataract surgery is an intraocular procedure; LASIK and PRK (which reshape the surface of the eye) are not.

A man-made implant designed to replace the eye\'s natural lens, which is located behind the pupil and iris. IOLs are used in cataract surgery and refractive lens exchange (RLE) surgery to improve vision.

Within the central tissue of the cornea (stroma).

The abbreviation for intraocular lens implant. An artificial lens that is implanted inside the eye. IOLs are used for refractive surgery (phakic IOL implantation) or to replace the eye's natural lens if it becomes cloudy (cataract surgery).

The colored ring of tissue located behind the cornea and directly in front of the lens of the eye. The central, circular opening in the iris is the pupil. The iris is what gives a person their "eye color."

In wavefront-guided LASIK, a computerized method to identify and take into account possible rotation of the eye when the patient is lying down for surgery. Iris registration uses landmarks on the colored part of the eye (iris) to ensure perfect alignment of the excimer laser treatment of the cornea.

A hand-held card with six paragraphs of different-sized print that is used to test near visual acuity. The paragraph with the smallest-sized print is the J1 paragraph. The paragraph with the next larger print size is J2 and so on, until the largest paragraph (J6). The card is typically held at a testing distance of 14 inches from the viewer's eyes. Near visual acuity of J2 on the Jaeger card at a test distance of 14 inches is considered normal and the equivalent of 20/20 distance visual acuity as measured by a Snellen chart.

The surgical removal of corneal tissue.

Inflammation of the cornea.

The prefix meaning "cornea" or "corneal".

A degenerative disorder of the eye characterized by a progressive bulging outward of the clear front surface of the eye (cornea) into an irregular cone shape. Keratoconus (KC) often causes visual impairment that cannot be corrected with eyeglasses or contact lenses, and severe cases may require a corneal transplant to restore vision. The National Keratoconus Foundation estimates the condition affects one of every 2,000 people in the United States.

Measurement of the curvature of the anterior surface of the eye.

The surgical reshaping of the cornea. LASIK reshapes the cornea by removing (vaporizing) small amounts of corneal tissue with a laser.

A surgical incision (cut) of the cornea.

The acronym for Light Amplification by Stimulated Emission of Radiation. A laser is an instrument that produces powerful pulses of light energy that can vaporize tissue.

(also laser posterior capsulotomy or YAG laser capsulotomy) A laser procedure performed weeks or months after cataract surgery if the posterior portion of the natural lens capsule (which remains in the eye during cataract surgery) later becomes cloudy and causes vision problems. (Posterior capsule opacification, or PCO, also is called a "secondary cataract.") The simple, noninvasive procedure is performed with a special laser called a Nd:YAG (neodymium-doped yttrium aluminium garnet) laser, and the energy emitted from the laser creates a hole in the posterior lens capsule to eliminate the cloudiness and restore clear vision. 

LASIK is the most popular type of laser vision correction surgery. The term LASIK is an acronym for Laser In-situ Keratomileusis or Laser-assisted In-situ Keratomileusis.

The procedure involves 2 surgical steps:

1. A thin flap of corneal tissue is created with a microkeratome or an IntraLase laser.
2. An excimer laser is used to reshape the underlying cornea.

LASIK is popular because it is a very brief and safe procedure, it involves little or no discomfort, results are almost immediate, and visual outcomes are typically good to excellent.

The clear biconvex structure inside the eye (located directly behind the pupil and iris) that provides some of the eyes focusing power. About the size of an M&M chocolate candy, the lens is able to change shape, allowing the eye to focus on objects at different distances. This ability to change shape and focusing power is called accommodation. As we age, the lens becomes less flexible and the eye loses its ability to accommodate. This normal age-related loss of near focusing ability is called presbyopia and typically begins after age 40. LASIK patients who are presbyopic need reading glasses to see near objects clearly.

The clear membrane-like structure that surrounds the lens inside the eye.

Arc-shaped incisions created in the periphery of the cornea to normalize corneal curvature. Limbal relaxing incisions (LRI) commonly are performed after refractive cataract surgery to reduce pre-existing corneal astigmatism and reduce the need for glasses after surgery.

An eye disease characterized by degeneration of cells in the most sensitive part of the retina (macula) that controls central vision and color perception. Advanced AMD can cause legal blindness and make driving, reading and other critical tasks difficult or impossible. The exact cause of macular degeneration is unknown, but risk factors include a genetic predisposition, smoking, obesity and aging.

Inflammation of glands in the eyelid (called Meibomian glands) that secrete an oily substance into the tear film to reduce the evaporation of tears. Also called Meibomianitis or Meibomian gland dysfunction (MGD), Meibomitis is a contributing cause of dry eye syndrome (DES).

Chronic inflammation of the Meibomian glands can lead to cysts in the eyelid called chalazions. Bacterial infection of a Meibomian gland is called a stye.

Treatment of Meibomitis usually consist of warm compresses, lid hygiene, and antibacterial eye drops or ointments.

Named after Heinrich Meibom, German anatomist and physician (1638-1700).

A precision cutting tool used to create the corneal flap in LASIK surgery.  Usually refers to a mechanical microkeratome that contains a disposable oscillating blade. Recently the term laser microkeratome has been coined to describe the IntraLase femtosecond laser, a blade-free surgical device that uses laser energy to create the corneal flap. The average human cornea is about 530-540 microns thick (a little over 1/2 of a millimeter). Mechanical and laser microkeratomes used for LASIK create corneal flaps that are typically 100 to 180 microns thick.

The unit used to measure corneal thickness. One micron is equal to 1/1000 of a millimeter. The central corneal thickness of the average human eye is about 530 to 540 microns – a little over 1/2 of a millimeter. (The center of the cornea is usually its thinnest point.)

Containing one lens power. Also called "single vision." Monofocal lenses contain a single lens power for all distance; multifocal lenses contain two or more lens powers.

The treatment of presbyopia by correcting one eye for clear distance vision and intentionally making the other eye mildly nearsighted to improve near vision (near objects will be clear, but distant objects will be blurred in this eye). Though monovision reduces the clarity of a person's distance vision to some degree, it allows older patients to see near objects quite well without reading glasses. Monovision can be performed with contact lenses or LASIK surgery.

The inability to see distant objects as clearly as near objects. Commonly called nearsightedness.

The inability to see distant objects as clearly as near objects. Also called myopia.

A medical doctor (M.D.) or Doctor of Osteopathy (D.O.)specializing in the diagnosis and medical or surgical treatment of vision problems and eye disease. Ophthalmologists are eye surgeons trained and licensed to perform LASIK and other types of refractive surgery.

A diagnostic tool that uses an infrared beam of light, a special camera, a video monitor and a computer to get cross-sectional images of the eye. OCT is similar to ultrasound, except that it uses light rather than sound.

A Doctor of Optometry (O.D.) – a professionally licensed eye doctor who provides routine eye care and prescribes eyewear, contact lenses, low vision aids and vision therapy for adults and children. Optometrists also may treat eye diseases – such as infections, allergic conditions and glaucoma – and provide pre-operative and post-operative care for eye surgery patients. Optometrists, however, are not licensed to perform eye surgery.

A complication of refractive surgery where the achieved amount of treatment is more than desired, usually resulting in a reversal of refractive error. For example, a LASIK surgery overcorrection of nearsightedness may result in the patient's refractive error changing from nearsightedness to farsightedness. Overcorrections can usually be successfully treated with a second LASIK surgery (often referred to as a LASIK enhancement).

Method of cataract extraction in which a vibrating ultrasonic probe is used to break up the lens into small particles that can then be removed from the eye with suction.

Refers to an eye that possesses its natural lens. Phakic IOL implantation is a refractive surgery procedure that involves inserting an artificial lens inside the eye in front of the eye's natural lens.

A heightened and usually uncomfortable reaction to sunlight and other sources of light. Visual photosensitivity (also called photophobia) can have many causes including inflammation of the cornea.

Clouding of the natural lens capsule behind the intraocular lens implanted in cataract surgery. PCO usually can be successfully treated with a laser procedure called a YAG capsulotomy.

The narrow fluid-filled space within the eye, located between the back surface of the iris and the front surface of the lens.

The normal age-related loss of near vision caused by reduced elasticity of the lens inside the eye. Presbyopia usually becomes apparent after age 40. Refractive surgery cannot reverse presbyopia, but the near vision of presbyopic patients can be improved somewhat with monovision LASIK and other presbyopia-correcting surgical procedures.

The abbreviation for photorefractive keratectomy. PRK is similar to LASIK in that it uses an excimer laser to reshape the cornea. But a corneal flap is not created in PRK: the corneal epithelium is removed and the laser energy is applied directly to the surface of the cornea. Though outcomes with PRK are similar to LASIK, visual recovery takes longer and there may be significant discomfort for several days after PRK. For these reasons, PRK is not as popular as LASIK.

The insertion of small stoppers (plugs) into the puncta – the openings of tear drainage ducts located in the upper and lower eyelids near the nose. Punctal occlusion is used to treat dry eye syndrome. By partially blocking (occluding) the drainage of tears away from the eyes, the eyes stay more moist. The plugs used for punctal occlusion can be temporary (made of a material that dissolves after several days) or permanent (made of silicone). Permanent plugs can be easily removed at a later date if necessary or they can remain in the puncta indefinitely.

The clear opening in the center of the iris that allows light to reach the retina. The pupil changes in size in response to changes in lighting: it dilates (gets larger) in dim lighting and constricts (gets smaller) in bright light. Pupil size may affect your vision after LASIK. If you have a pupil that dilates larger than the diameter of the LASIK treatment zone on the cornea, you may experience more glare at night. If you have a very small pupil, you may not be a candidate for certain types of wavefront-guided custom LASIK that have pupil size restrictions.

1. The bending of light as it passes from one medium (e.g. air) to another (e.g. the cornea).
2. The test used during an eye exam to determine your eyeglasses prescription.

1. Relating to how light is bent or focused as it enters the eye. 2. Caused by refractive errors.

Imperfections in the focusing power of the eye. Usually refers to myopia (nearsightedness), hyperopia (farsightedness) and/or astigmatism, but may also may be used to describe higher order aberrations (HOA) higher-order aberrations that cannot be corrected with eyeglasses or contact lenses.

The ability of an object (such as the cornea) to bend light as light passes through it.

The fine layer of sensory tissue that lines the inner surface of the back of the eye. The retina acts like a camera photoprocessor to capture images. It transforms images into electrical signals that are sent to the visual part of the brain via the optic nerve.

The abbreviation for radial keratotomy, an older refractive surgery technique popular in the 1980s. RK was designed to correct myopia (nearsightedness) by flattening the cornea with radially-arranged blade cuts. Because modern LASIK surgery is safer and more effective than radial keratotomy, RK is no longer a widely-used surgical procedure.

A mathematical measure used to describe the magnitude of combined wavefront errors or higher-order aberrations. Generally, as the RMS value increases, the quality of visual images decreases.

The tough outer coating of the eye (the "white" of the eye) that is continuous with the clear cornea at the front of the eye.

The chart of different-sized letters used to test distance visual acuity. The Snellen chart has be the standard method used to test visual acuity since the 1880s.

(of a contact lens or intraocular lens) Having the same curve in all meridians, enabling correction of nearsightedness or farsightedness, but not astigmatism.

Mean amount of refractive error in all meridians of an eye.

If an eye has no astigmatism, the spherical equivalent equal to the amount of nearsightedness or farsightedness, expressed in diopters (D).

In eyes with astigmatism, the spherical equivalent is determined by multiplying the amount of astigmatism by 0.5 and adding this to the amount of nearsightedness or farsightedness.

For example, the spherical equivalent of an eye that has –2.00 D of myopia and –1.00 D of myopic astigmatism is: –2.00 + (–1.00 x 0.5) = –2.50 D.

A measure of eye teaming ability important to depth perception.

Misaligned eyes, including esotropia (crossed eyes), exotropia (wall-eyed), hypertropia (upward misalignment) and hypotropia (downward misalignment).

The middle, thickest layer of the cornea. In LASIK, an  excimer laser is used to ablate (vaporize) a small amount of stromal tissue to reshape the cornea for better vision.

(of a contact lens or intraocular lens) Having two curves instead of one to enable correction of astigmatism as well as nearsightedness or farsightedness.

A complication of refractive surgery where the achieved amount of treatment is less than desired. Undercorrections can usually be successfully treated with a second LASIK surgery (often referred to as a LASIK enhancement).

The treatment of vision problems or improvement in visual function by a supervised, non-surgical program, which may include eye patching, vision "exercises," and visual stimulation. Also called vision therapy.

A program of visual activities or ocular stimulation specifically designed to improve visual acuity, contrast sensitivity or other visual abilities.

The clearness of vision; the ability to distinguish details and shapes. Visual acuity usually is measured with an eye chart (called the Snellen chart) consisting of rows of capital letters that decrease in size.

A highly-detailed computerized measurement of the total refractive errors of the eye, including higher order aberrations (HOA) that cannot be corrected with eyeglasses or contact lenses. Wavefront-assisted custom LASIK provides an added level of precision to the excimer laser treatment and therefore offers the potential for better visual acuity than standard LASIK (which uses an eyeglasses prescription to program the laser).

An advanced type of LASIK surgery designed to correct nearsightedness, farsightedness and astigmatism, and also produce sharper vision by inducing less spherical aberration (a specific type of higher-order aberration) than conventional LASIK.

Wavefront-optimized LASIK sometimes is confused with wavefront-guided LASIK (also called custom LASIK), which is designed to correct nearsightedness, farsightedness, astigmatism and a variety of pre-existing higher-order aberrations of the eye, with the intent of producing sharper vision than that attainable with eyeglasses or conventional LASIK.