Glossary of Ophthalmic Terms

Numbers

20/20: The standard name for normal eyesight. The first “20” refers to the distance in feet between the eye being assessed and the eye chart. The second “20” changes according to your visual acuity and compares the tested eye to a normal eye.

When the second number is higher than 20, it indicates nearsighted vision. For instance, 20/80 means that the person being tested must stand only 20 feet from the eye chart to see clearly what a person with normal vision can see clearly from 80 feet.
When the second number is lower than 20, it indicates vision that is better than 20/20 vision. For instance, 20/16 means that the person being tested can see from 20 feet what a person with normal vision must be only 16 feet away to see clearly.

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AAO: American Academy of Ophthalmology, a professional organization for ophthalmologists.

AAOPT: American Academy of Optometry, a professional organization for optometrists.

AOA: American Optometric Association, a professional organization for optometrists.

ABES: American Board of Eye Surgeons, a professional organization for ophthalmologists. It is affiliated with the American College of Eye Surgeons (ACES)

Ablation: Removal by cutting or abrading

Ablation Zone: The designated treatment area on the cornea where the laser will correct corneal curvature.

ABO: American Board of Ophthalmology, the medical specialty board that certifies ophthalmologists. It offers education and examinations and when an ophthalmologist has completed the 18-month program, he or she is “Board Certified”.

Abrade: To roughen a surface by scraping or rubbing it, removing tiny pieces.

Accommodation: The eye’s ability to switch focus between near and far objects. It is done by the circular ciliary muscle which surrounds the lens and controls its curvature. A flatter curvature focuses distant objects and a steeper curvature focuses nearby objects.

Acuity: Sharpness of central vision. Normal visual acuity is defined as 20/20 vision in relation to the Snellen acuity chart. This is the eye chart used to establish your glasses prescription, with rows of letters that start large at the top and get smaller towards the bottom.

Age-Related Macular Degeneration (ARMD): Deterioration of the macula lutea (also called the yellow spot). It is the small retinal area that gives central vision. If ARMD is not treated it leads to blindness.

AK: Astigmatic Keratotomy, a short surgical procedure to correct astigmatism.

Alcon: A company that manufactures medical products, including the ReSTOR IOL.

Algorithm: A mathematical formula such as those used in preparation for LASIK, PRK, Epi-LASIK or LASEK vision correction. This formula sets the laser’s treatment area for each individual’s eyes.

Allergan: A manufacturer of ophthalmic medical products.

Amblyopia: Use of one eye less than the other, usually in childhood. It causes the less-used eye to atrophy, its visual acuity deteriorating. If amblyopia is left untreated, the brain will eventually be unable to interpret any images from the less-used eye. Also called “lazy eye”.

American Academy of Ophthalmology: See AAO.

American Academy of Optometry: See AAO.

American Board of Eye Surgeons: See ABES.

American Board of Ophthalmology: See ABO.

American College of Surgeons (ACS): A professional organization for surgeons. Members are known as Fellows and may place F.A.C.S. after their name for Fellow of the American College of Surgeons.

American Society of Cataract and Refractive Surgery: See ASCRS.

Ametropia: Eye conditions characterized by impaired refraction such as astigmatism, myopia, and hyperopia.

Amsler Grid: A test for distortions or defects in central vision. It consists of a square divided into many smaller squares, with a single dot in the center.

Aniseikonia: Impaired vision where the left and right retinal images have different sizes. It may be caused by refractive surgery and may also occur naturally.

Anterior: Anatomical term referring to the front part of a structure as opposed to Posterior, referring to the back part.

Anterior Basement Membrane Dystrophy: An abnormality of the Basement Membrane (a thin sub-layer of the cornea that lies below the surface layer – the epithelium). The membrane is uneven and traps cells below it which should be above it. It causes blurriness of vision. Also called Map-Dot-Fingerprint Dystrophy, based on its microscopic appearance, and epithelial Basement Membrane Dystrophy.

Anterior Chamber: The small area between the cornea and the lens. It is filled with Aqueous Humor.

Anterior Ocular Segment: The name for the eye area in front of the lens. It includes the cornea, anterior chamber, iris and ciliary body.

Antimetropia: A conditions where one eye is farsighted and the other is nearsighted.

Antioxidants: Nutrients that destroy or neutralize free radicals.

Aphakia: Name for the condition of having no lens present in the eye.

Aphakic Glasses: Very thick glasses used in past years after a cataractous lens was removed. They provided some central vision but distorted peripheral vision. Ophthalmologists now use an intraocular lens to restore full vision after a cataract surgery.

Aqueous Humor: The fluid in the eye’s anterior chamber. It flows between the cornea and the lens and provides nourishment and moisture.

ASCRS: American Society of Cataract and Refractive Surgery. A professional organization for ophthalmologists who provide cataract and refractive surgeries.

Astigmatic Keratotomy: A surgery for correcting astigmatism that is no longer performed. It involved small incisions made around the corneal periphery to give it a rounder shape. Astigmatism is now treated by LASIK.

Astigmatism: A refractive error occurring when the corneal shape is more oval than round. The oval has two curvatures: one steeper and one flatter. The cornea therefore refracts (bends) incoming light in two ways, creating blurry vision at all distances. It can be mild or severe.

Automated Lamellar Keratoplasty (ALK): An older, pre-LASIK procedure done to correct severe myopia or hyperopia.

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B Scan: An ultrasonic procedure to locate foreign bodies in the eye and to check for abnormalities.

Basement Membrane: A thin layer of cells connecting the corneal surface layer (epithelium) with the middle layer (the stroma).

Bausch & Lomb: A manufacturer of ophthalmic equipment and products, including the Zyoptix system for Custom LASIK.

Best Corrected Visual Acuity: Your best vision, measured as corrected by your contact lenses or glasses.

Bifocals: Eyeglasses with two areas in the lenses: one for distance vision and one for near vision. See also Trifocals.

Binocular: An adjective referring to vision with both eyes – binocular vision.

Binocular Vision: Use of both eyes, providing the brain with two slightly different sets of images. The two sets blend to give depth perception.

Black Box Laser: A laser that has been modified to do eye surgeries, but that is not FDA-approved. They are typically imported from another country. They are less expensive than approved lasers, but have not been studied for safety.

Blepharitis: Inflammation of the eyelids. It can have a variety of causes, such as an allergic reaction, bacterial infection, or excess oil produced by eyelid glands.

Blindness: See Legal Blindness.

Blind Spot: A small gap in the visual field caused by lack of light-sensitive cells on the retina where the optic nerve exits the eye. Each eye has a blind spot. See How to Find Your Blind Spot. Can also be used to refer to any gap in the visual field caused by disease-related lack of retinal cells.

Board-Certified: The term for an eye doctor’s approval by the American Board of Ophthalmology (ABO). That doctor is thus legally able to practice ophthalmology in the U.S.

Broadbeam: Application of the excimer laser to the eye’s entire treatment area in one sweep. It is commonly done during LASIK treatments of myopia, hyperopia, and astigmatism. See also Flying Spot and Variable Spot, which are different ways of applying the laser.

Buttonhole Flap: An error in the creation of the LASIK corneal flap. The microkeratome cuts through the flap, detaching it entirely instead of retaining its hinge. When possible, the flap is replaced and allowed to fully heal before any further vision correction is attempted. There are usually no permanent ill effects.

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Cataract: An opaque area in the eye’s lens, usually age-related. Cataracts are progressive and left untreated will cause blindness. They are treated by a Lens Replacement Surgery using an intraocular lens.

Central Ablation Zone: The designated treatment zone for laser vision correction. It is surrounded by the transition zone, which gradually merges with the original corneal surface outside the ablation zone.

Central Island: An error in a LASIK surgery, where a tiny part of the treatment zone is not treated and remains microscopically higher than the surrounding treated area. It causes double vision (diplopia).

Choroid: Part of the eyeball wall. It is part of the middle layer (uvea) and lies between the retina and the sclera, Its blood vessels nourish the outer retinal layers and the sclera.

Ciliary Body: Part of the eyeball’s middle layer (the uvea). It is between the iris (also part of the uvea) and the posterior chamber containing vitreous humor. Fibers are connected to it concentrically that suspend the lens behind the iris.

CK: See Conductive Keratoplasty

CLE: Acronym for Clear Lens Exchange. See Lens Replacement Surgery.

Color Blindness: Confusion of red and green. It is genetic, and because the gene is on the X chromosome, it most often occurs in males.

Co-Management: Collaboration between doctors in the care of a patient. In the case of refractive surgery, an optometrist will often co-manage with an ophthalmologist, providing the initial testing and the post-operative care, while the ophthalmologist performs the surgery itself.

Coma: One of the higher order aberrations. It causes light sources to have a tail-like smudge like a comet. It is diagnosed and treated with Custom LASIK but not with Traditional LASIK.

Complex Wavefront Retreatment: Corrective surgery done after a Custom LASIK procedure which did not correct all the patient’s higher order aberrations. It is an off-label use of the excimer laser used in LASIK.

Cones: One of the two types of light-sensitive cells in the retina. They cluster around the center part of the retina and give sharp central vision in bright light. They also detect color, with one kind of cone absorbing blue, another kind absorbing red, and another kind absorbing green. See also Rods.

Concave Lens: The lens type for correcting myopia. It is curved inward, being thicker around the edge, and is opposite to a Convex Lens, which curves outward and is used to correct hyperopia. A concave lens spreads light rays whereas a convex lens focuses them.

Conductive Keratoplasty: A refractive surgery to treat hyperopia or astigmatism. It uses radio waves delivered through a tiny probe, and shrinks parts of the cornea according to the specific treatment plan.

Conjunctivitis: A contagious inflammation of the conjunctiva, the lining of the eyelids which also covers the sclera. It is itchy and makes the white sclera look pink or red. It is treated with eyedrops.

Contact Lens: Thin, curved lenses placed over the cornea to correct the eye’s refractive errors. They can be flexible or rigid and some are colored. There are many types; some are disposable after one day and others can be worn 24 hours per day for up to a week. Some are therapeutic (also called bandage contact lenses), designed to protect the eye while it heals.

Contact Lens Disinfectant: A cleaning solution for contact lenses that removes bacteria and micro-organisms.

Convex Lens: The lens type for correcting hyperopia and presbyopia. It curves outward like a ball and is the opposite of a concave lens, which is used to correct myopia.

Cornea: The clear front covering of the eye, in front of the iris and pupil. It is a type of lens and refracts light entering the eye. The light is then further refracted by the crystalline lens, behind the cornea, so that it focuses on the retina. Please see our page Eye Anatomy: The Cornea for more detail.

Corneal Abrasion: A scratch or scrape on the corneal surface.

Corneal Ectasia: A potential complication of LASIK. It can happen when a LASIK patient has thinner-than-average corneas or is over-treated. The internal eye fluids exert pressure outwards on the eyeball wall. If too much corneal tissue is removed, there may not be enough thickness left to maintain the eye’s shape against that pressure. The cornea bulges forward, impairing vision.

Corneal Flap: A circular piece of surface corneal tissue that is cut and folded back as on a hinge. This allows laser vision correction to be done beneath the surface, on the middle layer (the stroma). The flap is replaced after treatment and usually heals by itself without incident. Most LASIK complications are related to the corneal flap, either in its creation or its healing. Most of them can be corrected.

Corneal Haze: A possible complication of PRK that occurred more often in the early days and mostly after correction of severe myopia. Opaque white cells develop on the cornea that cloud the vision and cause glare in dim light conditions. In most cases it will clear up within a few months. A second laser procedure can be done to remove the cells.

Corneal Refractive Therapy: A treatment for myopia that reshapes the cornea while you sleep, using rigid contact lenses. The effect is of short duration so the lenses must be worn each night. Also called Orthokeratology.

Corneal Topography: Corneal contours that are mapped by shining a special light on the cornea and capturing the reflected image with a camera. Computer software then analyzes the image, providing a color-coded map. This is done with Wavefront technology in the diagnosis before a Custom LASIK procedure.

Corneal Transplant: Surgical replacement of the cornea. The new cornea comes from a donor and is seldom rejected by the patient since corneas have very little connection to the bloodstream.

Crystalens: An accommodative intraocular lens made by Bausch & Lomb. Crystalens is one of the choices for replacing the eye’s natural lens in treatments for cataracts and presbyopia.

Crystalline Lens: The eye’s natural lens, positioned behind the iris. It changes its curvature to focus images coming from different distances (a process called Accommodation). With age it can become clouded by cataracts and must be replaced with an intraocular lens to prevent blindness.

CustomCornea®: Part of the trade name LADARVision® CustomCornea®, a Custom LASIK system manufactured by Alcon, Inc. which was withdrawn from the market in 2009.

CustomVue™: The trade name for a Custom LASIK system originally offered by VISX, Inc. VISX was acquired by Advanced Medical Optics (AMO), Inc. in 2005; and AMO has since changed its name to Abbott Medical Optics, also abbreviated as AMO.

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Decentration: Off-center ablation, a complication of eye surgery. In perfect centration, the corneal ablation done in laser vision correction is lined up exactly with the pupil, in the center of the eye. The term decentration is also used in reference to an intraocular lens placed or shifted off-center. Decentration impairs vision.

Degenerative Myopia: Nearsightedness that begins at birth or in childhood and can lead to blindness. It is thought to be hereditary, and is associated with cataracts and retinal changes.

Depth Perception: Ability to assess the relative distances of objects. The dominant eye looks directly at an object and the non-dominant eye looks from a slight angle. The brain processes the two images and estimates the object’s distance.

Diabetes Type 1: Insulin-dependent diabetes. The pancreas is unable to produce insulin and it must be injected regularly to enable conversion of sugars and starches into energy. Also called juvenile diabetes, as it is often found in young people. A diabetic person can develop vision problems when the blood vessels in the retina weaken and start to leak. Please see our page on Diabetic Retinopathy.

Diabetes Type 2: A more common type of Diabetes than Type 1, where the pancreas can produce some insulin, but not enough. Blood sugar levels must be monitored and insulin injected when needed. Some cases can be managed with diet and exercise routines. Also called Adult-Onset Diabetes.

Diabetic Retinopathy: Weakening and leaking in the retinal blood vessels. Swelling occurs and fluid can get behind the retina, detaching part of it. It is progressive and the later stage is called proliferative diabetic retinopathy. The eye is lacking oxygen and in response develops new blood vessels; but these too are weak and leak. Scars may form, vision is impaired, and blindness may be the end result.

Diffuse Lamellar Keratitis (DLK): A rare complication of LASIK where inflammation develops beneath the corneal flap. It blurs the vision. Can be treated with corticosteroid eyedrops. Also called Sands of Sahara.

Dilation: Widening of the eye’s pupil, in the center of the iris. The iris is a muscle which changes the pupil size to allow more or less light into the eye. For some ophthalmology procedures it is dilated with special eyedrops.

Diopter: The unit of measurement for a lens. A convex lens (curving outwards) has positive diopter numbers and a concave lens has negative diopter numbers. A lens of +1 diopter will refract (bend) light rays to a focus one meter away from itself. A lens of +2 will focus them a half-meter away from itself, as it has a steeper curvature. Concave lenses do not focus light, but scatter it.

Diplopia: Double vision.

Dominant Eye: The eye that looks directly at objects. The non-dominant eye sees the objects at a slight angle. Use of two eyes is known as binocular vision and it provides us with good depth perception. Please see How to Determine Your Dominant Eye.

Double Vision: Seeing a relatively faint second image of each object. Also called ghosting.

Dry Eye Syndrome: Chronically insufficient moisture in the eyes. It gives a feeling of burning and/or grittiness and increased light-sensitivity. It can be caused by an eye condition such as Ocular Rosacea. Dry eyes are a temporary side effect of LASIK, but Dry Eye Syndrome is long-term. It can be treated with eyedrops that add moisture, or with punctal plugs that block tear drainage and keep moisture on the eye surface for a longer time.

DSAEK (Descemet’s Stripping Automated Endothelial Keratoplasty): A procedure developed in 2007 to improve corneal transplants. Descemet’s Membrane is the cornea’s very thin innermost layer. DSAEK transplants just this layer rather than the entire cornea. No stitches are needed and recovery time is one to three months instead of one to two years.

Dystrophy: Wasting or weakening of body tissue. There are many dystrophies, and in the eye there can be Epithelial, Stromal, and Endothelial dystrophies, affecting the cornea’s epithelium, stroma, and endothelium respectively. Fuch’s Dystrophy and Keratoconus are endothelial dystrophies. For more on corneal anatomy, please see Eye Anatomy: The Cornea.

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Ectasia: See Corneal Ectasia.

Endothelial Dystrophy: See Fuch’s Dystrophy.

Endothelium: The cornea’s innermost layer, consisting of flat cells adjacent to the anterior chamber. For more details please see Eye Anatomy: The Cornea.

Epikeratome: A surgical tool used in Epi-LASIK to make the corneal flap. It has similarities to the microkeratome used for the LASIK flap, but instead of a thin, sharp blade, it has a blunt separator. The flap in Epi-LASIK is thinner than the LASIK flap.

Epithelial Dystrophies: Inherited dystrophies of the cornea’s surface layer, where the surface tissue deteriorates and impairs vision.

Epithelium: A surface layer of cells that protects the tissue beneath it. The corneal epithelium is where the LASIK flap is created, although it also includes some of the stroma, the middle corneal layer. In IntraLase, epi-LASIK and LASEK, the flaps are made thinner, consisting only of epithelial cells.

Esotropia: Crossed eyes. Usually one eye looks directly ahead and the other turns inward. Esotropia can be congenital (existing at birth); infantile (diagnosed at about two months of age); accommodative (diagnosed at about two years of age); or partially accommodative. Misaligned eyes should be corrected as soon as possible to avoid amblyopia.

Excimer Laser: The ultraviolet laser used in LASIK and other laser vision correction procedures. It is considered a “cool” laser and because it is ultraviolet it cannot be seen. It is also a pulsed laser with each pulse vaporizing about 1/4000 of a millimeter of corneal tissue, making it a very precise treatment tool. It works by breaking molecular bonds. The cornea has a high water content and the excimer laser light is instantly absorbed by corneal surface water, which prevents the laser from penetrating any deeper.

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FACS: Fellow of the American College of Surgeons.

Farsightedness: The popular name for hyperopia. Also called longsightedness.

FDA: See Food and Drug Administration.

Floaters: Strands or specks floating in the field of vision. They cannot be focused upon because they move when the eyes move. They are the shadows cast on the retina by clumping cells in the vitreous humor. Typically they are more visible against a blank background such as an expanse of snow or the sky. In most cases they are harmless unless flashes of light accompany them – that could suggest a possible retinal detachment.

Flying Spot: A way of applying the excimer laser used in laser vision correction. A flying spot laser continually changes where it sends pulses, never overlapping in the same place. The treatment plan programmed into the computer makes sure that the entire treatment area of the cornea is covered.

Focusing Power of the Eye: The combined action of the cornea and lens in refracting light to focus on the retina. The cornea does most of the refraction and the lens completes it, changing its curvature to accommodate for relative distances. Also called Optical Power.

Food and Drug Administration (FDA): The U.S. federal agency that tests, evaluates, and approves drugs and medical devices. FDA approval is specific to a procedure. For example, the FDA approved excimer lasers first just for treating nearsightedness. Later it approved them for treating farsightedness and astigmatism. See also Off-Label Use.

Fovea: The central part of the macula. The macula is the most sensitive area of the retina, with the highest concentration of cones. It occupies less than one percent of the retina but uses more than 50 percent of the brain’s visual cortex. It gives us our sharpest central vision.

Fuch’s Dystrophy: An inherited and progressive disease of the cornea’s innermost layer, the endothelium. This layer regulates fluid levels in the cornea and removes impurities. When these cells are dysfunctional or dead the eye swells from too much fluid. Fuch’s Dystrophy causes blurry vision, blisters, light sensitivity and decreased depth perception. There is no cure and a corneal transplant will eventually be necessary. Also called Endothelial Dystrophy.

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Ghosting: Double vision. Objects are seen in duplicate with the second image fainter or vaguer than the main one.

Glaucoma: An eye disease related to the optic nerve and the intraocular pressure (IOP). In most cases the IOP is elevated and causes damage to the optic nerve where it exits from the retina. In some cases the IOP is normal, but the damage still occurs to the optic nerve. That damage gradually reduces the visual field, starting around the periphery. Left untreated it will cause blindness. There are several forms of Glaucoma. The most common way of managing glaucoma is with eyedrops to reduce the IOP.

Granular Dystrophy: A hereditary eye disease where crumb-like granules appear in the cornea’s middle layer, the stroma. It is most often diagnosed about the age of 20 and by mid-life vision is progressively impaired as the granules expand, multiply, and penetrate more deeply into the cornea. It can be treated in the early stages with an excimer laser and in the later stages by a corneal transplant.

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Halos: Appearance of a circle of light around light sources. It occurs in dim lighting conditions and can interfere with night driving. It was regarded as a complication of early LASIK procedures (Traditional LASIK), but Custom LASIK is able to more often prevent it.

Haze: Clouding in the cornea. It might be caused by too much moisture in the cornea, by scar tissue, or inflammation.

Higher Order Aberration: Microscopic irregularities in the eye’s contour which cause a variety of night vision problems. They are not treatable with Traditional LASIK. However, the Wavefront technology used in Custom LASIK diagnoses them and presents them in two 3-D colored maps on the computer monitor. They are then incorporated into the treatment plan. Some have only mathematical names and some have been given names such as:

Coma (tail-like streaks on light sources)
Starburst (spiky bright areas around light sources), and
Halos (see above)

Hyperopia: Farsightedness. It is one of the three Lower Order Aberrations, the other two being nearsightedness and astigmatism. It can be corrected with glasses, contact lenses and laser vision correction. Also called Longsightedness.

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iLASIK: A vision correction surgery combining the IntraLase laser made by Abbott Medical Optics (AMO) for flap creation with the VISX CustomVue technology. When AMO acquired VISX in 2006, the technologies of the two companies were combined. The name iLASIK was then given to IntraLase when it uses the AMO femtosecond laser.

Implanted Collamer Lens (ICL): The STAAR Visian ICL™, a phakic intraocular lens (IOL) made by the STAAR Surgical Company and FDA-approved for treating myopia. Phakic IOLs do not require the removal of the natural lens, but instead they modify the lens’ refractive power.

Intacs: An alternative to LASIK for correcting myopia. Intacs are curved plastic implants placed beneath the corneal periphery to create a flatter curvature. They can be surgically removed at any time and their prescription can be changed. They are used as a treatment for keratoconus, a condition where the cornea bulges forward.

IntraLase: A LASIK procedure that uses a super-fast-pulsed laser to create the corneal flap. The IntraLase procedure is able to make the flap thinner than in traditional LASIK, making it a good choice for many people with thin corneas. The same excimer laser is used for treatment as is used in LASIK. Also called IntraLASIK, All-Laser-LASIK, and Blade-Free LASIK.

Intraocular: An adjective meaning “inside the eye”.

Intraocular Lens (IOL): An artificial lens used to replace the eye’s natural lens in treatment for Cataracts and Presbyopia. There are several to choose from, each using different technology. Some are multifocal and some are accommodative, meaning that they are controlled by the ciliary muscle which controlled the natural lens.

Intraocular Pressure (IOP): The pressure inside the eye. The eye is filled with fluid that exerts outward pressure on the eyeball wall. If the pressure becomes too high it can damage the optic nerve where it exits the retina, a condition called Glaucoma. Please see Eye Anatomy: Internal Eye Fluids.

Iris: The eye’s colored part. The iris is a circular muscle that controls the size of the pupil, the opening that allows light to enter the eye’s interior. The iris is part of the eyeball wall’s middle layer, the uvea.

Iris Registration: The trade name for a hardware addition to the VISX STAR system for CustomVue LASIK, now owned by Abbott Medical Optics (AMO). It is a tracking device that keeps the excimer laser centered on the eye so that even if the eyes move during treatment, vision correction will still be done accurately.

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Keratectomy: Surgical removal of corneal tissue. Many medical terms referring to the cornea begin with “kerat—“, from the Greek word for “horn”.

Keratitis: Inflammation of the cornea. It can be caused by bacteria or a virus and can impair vision by leaving scars on the cornea.

Keratoconus: An inherited corneal disease. The cornea gradually becomes thinner and less able to maintain its shape against the pressure of the fluids inside the eye. It bulges forward, blurring vision and may eventually require a corneal transplant.

Keratometry: Measurement of the cornea’s curvature, done with a keratometer.

Keratoplasty: Corneal transplant.

Keratomileusis: A precursor to LASIK. First devised by eye surgeon Jose Barraquer in Colombia in the 1950s. Instead of the hinged corneal flap that eye surgeons now create for LASIK, he completely removed a surface flap. Then he froze it, reshaped it, and replaced it on the corneal surface.

LASIK stands for Laser-Assisted in situ Keratomileusis, where the “in situ” refers to the fact that the treatment is done directly on the cornea rather than on a separated part of it. Barraquer went on to develop the microkeratome, and a modified version of it is still used to create the hinged flap in LASIK.

Keratotomy: A surgical incision in the cornea. There was an early treatment for myopia called Radial Keratotomy in which a series of radial incisions were made around the corneal periphery to flatten its curvature. It is no longer performed.

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Lacrimal Gland: The gland that produces tears to moisturize the eye. It is located above the outer corner of the eye. Please see The Tear System for more information.

LASEK: See Laser Assisted Epithelium Keratomileusis.

Laser: A man-made light that is directional and parallel rather than scattered like a flashlight. It has a single wavelength, which gives it a specific color -- white lights contain all colors. The term “laser” was originally an acronym for “Light Amplification by the Stimulated Emission of Radiation”.

Laser-Assisted in situ Keratomileusis: The spelled-out name of LASIK.

Laser Assisted Epithelium Keratomileusis: The spelled-out name of LASEK. LASEK is an alternative treatment to LASIK, offered to those whose corneas are too thin for LASIK. It creates an extra-thin corneal flap using an implement called a trephine and a diluted alcohol solution.

Laser Thermal Keratoplasty (LTK): A refractive surgery for mild hyperopia. A Holmium laser slightly heats the corneal periphery to shrink it. This steepens the corneal curvature. LTK is also used to treat presbyopia.

LASIK: Laser-Assisted in situ Keratomileusis, a refractive surgery that corrects myopia, hyperopia, and astigmatism. It uses an excimer laser to reshape the cornea to change the way it refracts light. Please see Traditional LASIK and Custom LASIK for more detail.

Lattice Dystrophy: A progressive eye disease where abnormal protein fibers develop on the cornea’s middle layer, the stroma. They enlarge over time and become more opaque. This blocks light from entering the eye and clouds the vision. In some cases they cluster at the upper part of the stroma, just below the epithelium where the corneal nerve endings are. This can be very painful. Early treatment can be done with an excimer laser and in some cases a corneal transplant is required.

Legal Blindness: Visual acuity of 20/200 or worse (that is, severe myopia), or tunnel vision of 20 degrees diameter in the better eye. A person found to be legally blind can apply for government disability benefits, but is not necessarily totally incapacitated.

Lens: Any curved, transparent structure that refracts light. In the eye, the natural lens is behind the iris. It is able to change its curvature depending on how distant an object is. Controlled by the circular ciliary muscle around it, its curvature becomes steeper for close objects and flatter for distant objects. This is known as accommodation.

The cornea is also a lens. In a normal eye, the cornea and natural lens bend incoming light so as to focus it clearly on the retina.

Lower Order Aberrations: The name for myopia, hyperopia, and astigmatism. These are the vision problems treated by LASIK and its alternative procedures such as PRK. They are refractive problems, meaning that they stem from how the eye is bending light. See alsoHigher Order Aberrations.

Low Vision: Vision impairment that interferes with daily activities but cannot be corrected by glasses or contact lenses. Less severe than legal blindness.

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Macula: The central area of the retina that gives us sharp, central vision. In the macula’s center is the fovea, which has no nerve cells or blood vessels, but only light-sensitive cells. These cells are called cones, and they see sharp detail in bright light, and also detect color. For more detail please see Eye Anatomy: The Retina.

Macular Degeneration: See Age-Related Macular Degeneration (ARMD)

Macular Dystrophy: Macular degeneration that is hereditary rather than age-related.

Macular Edema: Swelling of the macula, the central area of the retina. Fluid pools in and around the macula, causing blurry vision, light sensitivity, and a pinkish color to the vision. It can happen after an eye injury or disease and usually lasts a few months.

Meesmann’s Dystrophy: An inherited and rare eye disease where small cysts develop on the corneal surface. They cause irritation and can rupture. The condition is progressive and in some cases a corneal transplant becomes necessary.

Micron: One-millionth of a meter.

Microkeratome: An oscillating surgical blade used to create the corneal flap at the beginning of a LASIK procedure. Originally developed by Jose Barraquer, remembered as “the father of modern refractive surgery”. See also Keratomileusis.

Monocular: Adjective referring to vision from one eye.

Monovision: Correction of one eye for near vision and the other for distance vision. Used as a treatment for presbyopia, where the second eye may be left as is for distance vision.

Myopia: Nearsightedness, a refractive error and one of the eye’s lower order aberrations. When the corneal curvature is too steep for the length of the eyeball (front to back), light is refracted too sharply and focuses in front of the retina. This causes blurriness of distance vision. Depending on its severity it may also cause blurriness of intermediate vision. Myopia can be corrected by LASIK. Also called shortsightedness.

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Nasolacrimal ducts: Part of the tear system. They carry tears out of the eyes and into the nasal passages. They can become obstructed, most often in a child, causing tears to accumulate in the eyes and overflow. Left untreated this can cause infection.

Nearsightedness: Myopia.

NearVision CK: A trade name for Conductive Keratoplasty (CK), a treatment for hyperopia.

Nystagmus: Rapid and uncontrollable eye movements. There are many types of nystagmus. The two basic types are Early Onset Nystagmus (usually diagnosed within two months of birth) and Acquired Nystagmus (related to neurological conditions appearing in later life).

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Ocular Herpes: Herpes simplex, type 1. It causes cold sores and can also infect the eyes. When minor it affects only the corneal surface and heals well. In severe cases it penetrates more deeply, causes corneal scarring, and leads to vision loss, possibly to the point of blindness. Sometimes it occurs on the retina (Herpes Retinitis) or in the eyeball wall (Herpes Uveitis).

Ocular Migraine: A migraine occurring around the eye area. A headache may or may not be part of it but it does cause nausea, vomiting and double vision. Typically it affects one eye where vision can become grayed out or even temporarily lost. Also called Retinal Migraine.

Off-Label Use: Use of a medical device or drug for a purpose not specifically FDA-approved. A physician can make a judgment call and it is legal. An example was use of the excimer laser for LASIK when it had been approved for PRK but not yet for LASIK.

Ophthalmic: Adjective meaning relating to the eye.

Ophthalmologist: A physician who specializes in diagnosing and treating vision and eye diseases. Ophthalmology is one of the specialties acknowledged and overseen by the American Board of Medical Specialties (ABMS). The ABMS examines and certifies each ophthalmologist to practice in the U.S.

Ophthalmoscope: An instrument for examining the eyes. It is hand-held and contains a light to make the eye’s internal structures visible to the physician. It was first devised in 1915.

Optical Zone: The eye area that light passes through. Light enters through the cornea and travels through the aqueous humor, pupil, lens, and vitreous humor and then stops at the retina.

Optical Ablation Zone: The corneal area of the optical zone that is treated in laser vision correction.

Optic Disc: The eye’s blind spot. It is the area of the retina where the optic nerve fibers exit the eye and where blood vessels enter and exit. There are no light-sensitive cells in the optic disc. Also called the optic nerve head.

Optician: A technician qualified by the American Board of Opticianry (ABO) to fit eyeglasses. An optician may grind the lenses from raw materials and will also fit them in frames and check their accuracy. In some states opticians can also fit contact lenses.

Optic Nerve: The bundle of nerve fibers that carries vision data to the brain’s vision center. Each separate fiber is connected to a retinal cell and they combine at the eye’s optic disc to exit the eye. The bundle that makes up the optic nerve is about one quarter of an inch wide.

Optometrist: A doctor of optometry (O.D.) Optometry is not a branch of medicine but is a health care profession. An optometrist diagnoses eye diseases and vision problems, and prescribes remedies (glasses, contact lenses and drugs). Optometrists do not perform surgery but do provide post-surgical care. The education required is a B.A. followed by four years of optometry school. To practice optometry, the student must pass an exam administered by the National Board of Examiners in Optometry (NBEO). Some optometrists also complete a residency in a sub-specialty such as pediatric eye care or ocular diseases.

Overcorrection: A complication of refractive surgery. The vision problem is overtreated, so that, for example, a myopic eye becomes slightly hyperopic. The over-correction will usually resolve by itself within a few months, but if it doesn’t, a follow-up surgery can correct it.

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Pachymetry: Testing the cornea’s thickness with a pachymeter. This is done in determining LASIK candidacy and for some disorders where excess fluid thickens the cornea.

Penetrating Keratoplasty (PK): A full-thickness corneal transplant. The central part of the cornea (known as a button) is replaced with one from a donor. This can restore vision if corneal problems were causing vision impairment or blindness, for example keratoconus. Other treatments would be tried first, such as different types of contact lenses. In some cases PK has been replaced by a procedure that transplants only the cornea’s innermost layer, the endothelium.

Perioperative: Adjective relating to the period of time between hospital admission for surgery and discharge. A perioperative nurse cares for the patient during that time.

Peripheral Vision: Side vision; not the sharp central vision of direct focus. It is provided by the light-sensitive cells on the retina outside the macula It is less detailed but includes night vision. Without peripheral vision, there would be only tunnel vision and that person would be legally blind.

Phacoemulsification: The procedure of breaking up a cataractous lens before removing it. It is done with ultrasound, delivered with a probe through a tiny incision outside the visual field. The lens pieces are then removed using suction.

Phakic: Adjective referring to an eye that has its natural lens in place. Phakic IOLs are IOLs positioned in front of, or behind, the natural lens to improve vision.

Phoropter: An instrument for testing visual acuity. It has many lenses through which a patient looks at a digital vision chart and is asked to read the smallest line of letters seen clearly. This provides the data used for glasses or contact lens prescriptions. The digital chart is set at 20 feet away, and then 16 inches away, to test for both distance and near vision.

Photoablation: Literally “removal using light”. The removal of corneal tissue done in laser vision correction. It is done according to a precise treatment plan and reshapes the cornea. This is effective for correcting myopia, hyperopia, and astigmatism.

Photorefractive Keratectomy (PRK): An alternative treatment to LASIK. PRK is sometimes recommended for those whose corneas are too thin or too steep for LASIK because in PRK, there is no corneal flap. Instead, corneal epithelial tissue is removed. A bandage contact lens is worn for protection during recovery and the cornea heals by itself.

Phototherapeutic Keratectomy (PTK): A treatment for corneal irregularities. An excimer laser is used to make the corneal surface more smooth, clarifying vision. It can be done during laser vision correction.

Pink Eye: See Conjunctivitis

Posterior Chamber: The area between the lens and the retina. It is filled with a clear gel called vitreous humor and is the largest area of the eyeball.

Presbyopia: Progressive loss of vision which begins in mid-life. Near vision becomes blurry, making reading glasses necessary. Over time, the blurriness extends to intermediate vision, making computer glasses useful. Bifocals are worn to improve near vision and distance vision if necessary. Presbyopia can be treated in a variety of ways, some still experimental.

PRK: See Photorefractive Keratectomy.

PTK: See Phototherapeutic Keratectomy.

Punctal Plugs: A method for treating dry eyes. They are plugs made of silicone, plastic, or collagen, inserted into the eye’s puncta to slow or prevent tear drainage. Absorbable plugs are used for short-term treatment and non-absorbable plugs for long-term treatment. Placement of the plugs takes less than a minute and is painless.

Punctum: An opening in the eye for tear drainage. They are in the inner corners of the eyes, two in each upper lid and two in each lower lid.

Pupil: The opening in the center of the iris. The iris is a circular muscle which contracts and relaxes to change the pupil size according to ambient light. Pupil dilation admits more light and pupil contraction admits less. Pupil dilation is often part of an eye examination.

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Radial Keratotomy (RK): A procedure for treating myopia by making a series of radial incisions around the cornea’s edge. This flattens the corneal curvature so that light can focus on the retina instead of in front of it. RK is no longer performed, as it has been superseded by LASIK.

Reading Glasses: Eyeglasses that increase visual acuity for close-up distances. When presbyopia develops in mid-life, reading glasses become necessary, even if the person still has 20/20 distance vision.

Refraction: Bending of light rays. Two eye structures refract light: the cornea and the lens. In a 20/20 eye they refract incoming light the right amount for a clear focus on the retina.

Refractive Error: Refraction of light such that it does not focus on the retina and therefore gives blurry vision. A myopic eye refracts some incoming light too sharply, giving blurry distance vision. A hyperopic eye refracts some too little, giving blurry near vision. An astigmatic eye has an oval-shaped cornea with two curvatures, one steeper and one flatter. This type of eye refracts light at two angles, giving blurriness at all distances. Refractive error is expressed in diopters. See Your Prescription for more information.

Refractive Lens Exchange (RLE): A surgery that removes the natural lens and replaces it with an artificial lens called an intraocular lens. This is a treatment for cataracts, presbyopia, and severe hyperopia. Also called Clear Lens Exchange and Lens Replacement Surgery.

Refractive Surgery: A surgery to permanently change the eye’s optical power. It can be performed either on the cornea or the lens, the eye’s two focusing structures.

Regression: Reversion by the cornea to its original refractive error, after LASIK or PRK.

Reis-Buckler’s Dystrophy: An inherited eye condition where multiple opacities form in the cornea, just below the surface. It causes scarring, along with impaired vision, increased light sensitivity and eye irritation. It tends to occur between the ages of about eight and twenty, although it can occur later in life. It requires surgery and Phototherapeutic Keratectomy has been used with some success. Also called Buckler’s Dystrophy and Buckler’s Syndrome.

Retina: The inside back surface of the eye, filled with light-sensitive cells. It converts images received in light rays into nerve impulses. The optic nerve fibers carry these to the brain’s vision center. Please see Eye Anatomy: The Retina for more details.

Retinal Detachment: A separation of the retinal nerve layer from the layer below it that contains retinal blood supply. Fluid can then seep into the gap and vision is lost in this area. When retinal detachment is diagnosed and treated early, vision can be restored. Retinal detachment is more likely among diabetics who develop Diabetic Retinopathy. For more about the retina, please see Eye Anatomy: The Retina.

Retinal Vein Occlusion: Blockage of a vein carrying blood away from the retina. Blood may back-up and cause increased pressure within that vein and its branches (capillaries). They may then leak on to the retina. Retinal vein occlusion can be a cause of retinal detachment.

Retinitis Pigmentosa (RP): A hereditary degeneration of the retina, usually leading to legal blindness. The peripheral vision is gradually reduced, causing tunnel vision. There are three main forms:

Dominant – where about 50 percent of family members develop RP
X-linked – where males suffer the disease in alternate generations and females are carriers
Recessive – where RP occurs occasionally without a full family history

Rigid Gas-Permeable (RGP) Contact Lenses: Hard contact lenses that allow air to pass through them. The air brings oxygen and thus these contact lenses are more comfortable and healthy for the eyes than the older hard lenses used several decades ago. They contain silicone, which is flexible and makes RGP contact lenses less hard than the older ones.

Rods: One of the two types of retinal light-sensitive cells. Rods provide vision in low light conditions and do not detect color. See also Cones, which give bright-light sharp central vision. For more details, please see the page on Eye Anatomy: The Retina.

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Sands of Sahara: See Diffuse Lamellar Keratitis

Sclera: The “white” of the eye. It is a tough, protective layer of the eyeball wall and joins with the cornea at a junction known as the limbus. It continues around the back of the eye to connect with the sheath over the optic nerve.

Scotopic Pupil Size: The pupil size in low light conditions.

Sjogren’s Syndrome: A systemic, chronic autoimmune disease. White blood cells damage glands that produce moisture, such as those in the eyes and mouth, causing dryness. Most sufferers are female and typically it occurs in mid-life. In many cases it occurs with other autoimmune diseases such as Rheumatoid Arthritis and Lupus. It may affect many body organs such as the kidneys, liver, blood vessels, and digestive tract, and symptoms include joint pain and fatigue as well as dryness.

Slit Lamp: A well-lit microscope for examining the eye’s interior. It has a selection of magnification settings and can be reduced to a slit to provide detailed views of the eye’s structures and fluids.

Snellen Chart: The standard vision chart used to determine your vision prescription. It has a large letter at the top and rows of increasingly small letters beneath. It is named for Herbert Snellen, the Dutch ophthalmologist who first devised it in 1862. Originally it was in the form of a chart on the wall but now it is digital inside the phoropter in every eye doctor’s office.

Starbursts: One of the Higher Order Aberrations. It occurs in night vision where light sources look blurry and have spikes projecting around them. It can occur naturally and can also be a complication of laser vision correction.

Strabismus: Misaligned eyes. One eye turns out, down, or up while the other looks straight ahead. Sometimes both eyes turn. Strabismus can be ongoing or intermittent and causes impaired vision, including reduced depth perception. Also called crossed eyes or turned eye.

Stroma: The middle layer in the eye’s cornea, and the thickest. It is the layer where laser vision correction is done, as it is stable, whereas the surface (epithelium) is continually renewing itself.

Stromal Dystrophies: A group of hereditary conditions that impair the middle layer of the cornea, the stroma. The three main ones are Macular Dystrophy, Granular Dystrophy and Lattice Dystrophy.

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Tear Duct: A canal which distributes tears in the eye. Tears are produced by the lacrimal gland and carried to the corneal surface, where they are spread by blinking. They drain out into nasal cavities through tiny openings called puncta. Please see our page on The Tear System for more information.

Topography: Surface contours, as in corneal topography, which are measured by a corneal topographer.

Toric Contact Lenses: Contact lenses designed to correct both astigmatism and either myopia or hyperopia. They have two curvatures and a mechanism to prevent them from rotating on the eye. They can be hard or soft.

Transition Zone: The area between the optical ablation zone and the untreated area on the cornea. The LASIK laser removes most tissue from the ablation zone and less at the edges towards the untreated zone.

Trephine: A surgical tool used in LASEK to create a corneal flap. It is able to create a thinner flap than the microkeratome used in LASIK. This allows laser vision correction to be done for those with thinner corneas than average.

Trifocals: Eyeglasses with three powers of correction: near, intermediate, and far. Bifocals have two powers of correction (far distance for most of the lens and near distance in the lower part). Trifocals also use the central area to correct for intermediate distance. Both bifocals and trifocals are used to correct presbyopia.

Tunnel Vision: Loss of peripheral vision, with only central vision remaining. Tunnel vision can be caused by Glaucoma or Retinitis Pigmentosa (RP) and can make a person legally blind.

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Undercorrection: A complication of laser vision correction that results in vision that is less than expected or desired. It sometimes occurs when the cornea heals more quickly than usual, undoing some of the correction. See also Overcorrection and Regression.

Uvea: The center layer of the eyeball wall. It lies between the sclera on the outside and the retina on the inside. The iris is part of the uvea.

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Variable Spot: One of the ways the excimer laser can be used during laser vision correction. The controlling computer is set to focus the laser on spots of different sizes all across the treatment area. See also Flying Spot, where the spots are of equal sizes, and Broadbeam, where laser application is smooth.

Vision Therapy: Treatment for weak visual skills, typically prescribed and planned by an optometrist. There are thought to be 20 visual skills, such as spatial relations, visualization, visual memory, and accommodation. If one or more is weak, daily activities can be affected, such as computer use, reading, and sports. Also called eye exercises, orthoptics, and visual training.

Visual Acuity: Central vision. It is clear vision with sharp details and clear color.

Visual Field: The visible field when the eyes look directly ahead. It includes both central and peripheral vision.

VISX: The manufacturer of the STAR S4 IR™ Excimer Laser System that performs CustomVue™ laser vision correction. In 2006 it was acquired by Advanced Medical Optics, Inc. (AMO), which has since changed its name to Abbott Medical Optics.

AMO is the manufacturer of many medical products including the IntraLase femtosecond laser, the ReZoom™ and Tecnis® IOLs and the Verisyse™ Phakic IOL. AMO has combined its own technology with the VISX technology and one result so far has been iLASIK.

Vitreous Detachment: Separation of vitreous gel [link to Internal Fluids] from the retina. When the vitreous pulls on the retinal surface layer, containing light-sensitive cells, it sometimes tears it and causes retinal detachment. This tends to occur more with older people and those with diabetes.

Vitreous Humor: The transparent gel in the eye’s posterior chamber, between the lens and the retina.

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Wavelength: A term used of light waves, referring to the distance between the top of one wave and the top of the next. Each wavelength corresponds to a specific color. The excimer laser used in LASIK has a wavelength of 193 nm (nanometers), which makes it an ultraviolet laser, beyond the violet end of the color spectrum and not visible to our eyes.

Wavefront: Technology used in the eye’s diagnosis before Custom LASIK. Wavefront technology detects and maps each eye’s higher order visual aberrations as well as the three lower order aberrations of myopia, hyperopia, and astigmatism. Custom LASIK corrects the higher order aberrations whereas Traditional LASIK does not.

Wavelight: A German manufacturer of ophthalmic equipment, such as the Allegretto Wavelight LASIK system. Their U.S. business is handled by Alcon, Inc.

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YAG Laser: A laser used after cataract surgery to vaporize clouding that sometimes develops on the lens capsule. YAG stands for Yttrium-Aluminum-Garnet, referring to the medium that generates this laser. It is an infrared, short-pulsed laser with a wavelength of 1064 nm.

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Zyoptix: Brand name for the Bausch & Lomb Custom LASIK system.