Ophthalmology Glossary of Terms

A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

Aberration: Distortions, related to astigmatism , that cause the inability of light rays entering the eye to converge (come together) to a single focus point on the retina . Aberration are divided into two main categories: higher-order and lower-order.
Ablation: Surgical removal of tissue, typically using a cool beam laser .
Ablation zone: The area of tissue that is removed during laser surgery.
Abrade: To roughen by rubbing or scraping, thus removing small pieces of surface tissue.
Accommodation: The eye’s ability to switch focus from near objects to far objects. It’s done by tiny muscles attached to the eye’s lens, which pull on the lens to change its convexity. As we age, those muscles become weaker and the lens becomes stiffer, a condition known as presbyopia, where reading glasses become necessary to see close-up objects.
Acuity: Clarity or sharpness of vision, commonly expressed as 20/20 vision in relation to the Snellen acuity chart. This is the eye chart seen at every eye doctor’s office, with the big E at the top.
Acute: Occurring suddenly.
Adnexa: Accessory structures of the eye, including the eyelids, lacrimal apparatus, etc.
Age-Related Macular Degeneration (ARMD): Deterioration, as people age, of the macula lutea (also called the yellow spot), which is a small area on the retina which gives maximum vision. If left untreated, it results in blindness.
AK: Astigmatic Keratotomy, an ophthalmological procedure for correcting astigmatism.
Algorithm: A mathematical formula such as is used when preparing for a LASIK, PRK, LASEK or Epi-LASIK surgery, to set the laser’s ablation area on that particular individual’s eyes.
ALK: Automated Lamellar Keratectomy, a procedure to correct opacities on the corneal surface.
Allegretto: A manufacturer of medical equipment.
Allergan: A company that makes ophthalmic equipment and drugs.
Amblyopia: Dullness or obscurity of sight for no apparent organic reason, therefore not correctable with glasses or surgery. Sometimes called a lazy eye, wherein one eye becomes dependent on the other eye to focus, usually developed in early childhood. Often associated with strabismus .
Ametropia: A generic name for eye conditions characterized by impaired refraction, e.g. myopia, hyperopia and astigmatism.
Amsler grid: A test used to detect defects or distortions in the central visual field. It consists of a square subdivided into many hundreds of small squares by horizontal and vertical lines, with a small dot in the center.
Angle: Drainage area of the eye formed between the cornea and the iris, named for its angular shape, which is why you see the word “angle” in the different glaucoma names.
Anisometropia: Condition of the eyes in which they have unequal refractive power .
Anterior: An anatomical term meaning the front part of a structure as opposed to the posterior, the back part.
Anterior Basement Membrane Dystrophy: The most common corneal dystrophy. An eye condition where the membrane that lies beneath the epithelial cells (surface cells) of the cornea is uneven and traps cells below it which should normally rise above it. It impairs vision, is usually hereditary, and fluctuates in severity over the person’s lifetime. It’s also called Map-Dot-Fingerprint Dystrophy (a name based on how it looks microscopically), and Epithelial Basement Membrane Dystrophy.
Anterior chamber: Space between the cornea and the crystalline lens , which contains aqueous humor .
Anterior ocular segment: Part of the eye anterior to the crystalline lens , including the cornea , anterior chamber , iris and ciliary body .
Antimetropia: A condition where one eye is nearsighted and the other one farsighted.
Antioxidants: Micronutrients that destroy or neutralize free radicals, molecules that have been implicated as one causative factor in the stimulation of abnormal cellular reproduction (cancer) and cellular destruction (aging).
Antireflective coating: Coating on the front or back of glasses lenses, which minimizes the glare for patients who are extremely bothered by glare.
Aphakia: Absence of the crystalline lens in the eye.
Aqueous humor: Transparent fluid occupying the anterior chamber and maintains eye pressure.
Argon laser: device used to treat glaucoma (usually open angle) and diabetic retinopathy using a thermal beam.
ARMD: age related macular degeneration: Destruction and loss of the photoreceptors in the macula region of the retina resulting in decreased central vision and, in advanced cases, blindness.
Asthenopia: Eyestrain.
Astigmatic Keratotomy (AK): Treats astigmatism by flattening the cornea with arc-shaped incisions in its periphery, similar to RK.
Astigmatism: An eye condition where the cornea is oval-shaped rather than round. This oval shape has two curves: the steeper one and the flatter one. Each curve refracts light at a different angle, sending it to focus on the retina in two different places, and creating distorted vision. It can be mild, with just blurriness, or more severe, with ghosting and severe blurring. In a glasses prescription, the second number expresses the degree of astigmatism you have, and the next number is the number of degrees in the angle of refraction. For example, a prescription of –4.00 – 2.00x34o states that you have 2 diopters of astigmatism at an angle of 34 degrees。
Automated Lamellar Keratoplasy (ALK): A refractive eye surgery that treats myopia. The surgeon uses a microkeratome to cut a very thin shaving or flap from the corneal surface. Then a tiny piece even thinner is removed from the underlying tissue, to flatten the cornea a little, and the flap is replaced. It heals up without any need of stitches.
Automated perimeter: Computer-driven device used to plot defects in the visual field (peripheral vision or side vision). Usually, this is a large hemisphere shell into which the patient’s head is placed. Various points of lights, sometimes of different sizes, intensities and colors are projected onto the screen. The patient then indicates whether the light is seen and the response is recorded. The computer then plots the effective visual thresholds within the targeted visual field.
Axis: Optical: a straight line through the centers of both surfaces of a lens. Visual: a straight line from the object of vision to the fovea of the eye.

B scan: An ultrasonic procedure that checks for abnormalities in the eye, and locates foreign bodies within it.
Basement Membrane: A microscopically thin layer of tissue below the epithelial cells (surface cells) of the cornea, connecting those cells to the stroma, which is the middle layer of cells in the cornea.
Best corrected visual acuity (BCVA): Best possible vision a person can achieve with corrective lenses, measured in terms of Snellen lines on an eye chart .
Beta-carotene: Member of the carotinoid family of vitamins, a precursor to vitamin A, thought to be beneficial to the eyes, helpful in treating diseases such as glaucoma.
Bifocals: Glasses designed to correct for both near vision and far vision. A small area at the lower edge of the lens is set for reading or other close-up activities and the rest of the lens gives clearer distance vision according to your prescription. Used by people with presbyopia. See also trifocals.
Binocular: An adjective meaning with both eyes, e.g., binocular vision.
Binocular vision: Simultaneous use of the two eyes. Normal binocular vision yields a stereoscopic image and parallax-induced depth perception.
Black Box Laser: A laser (usually imported to the U.S.) used by some eye surgeons that has been altered so it can do LASIK and other laser eye surgeries, but this alteration is not approved by the FDA. The safety consequences of using a black box laser have not been studied and are completely unknown.
Blepharitis: Chronic inflammation of the eyelids. It can be caused by an allergic reaction to some product, excess oil excreted by eyelid glands, bacterial infection, or poor facial hygiene.
Blind Spot: (a) The place on the retina where the optic nerve enters the eye. No visual cells are on the retina here, so no vision is possible at this place. In this sense, it’s a normal thing. (b) Also refers to any gap in a person’s visual field that corresponds to any area on the retina where visual cells are missing and in this sense, it’s associated with eye disease.
Board Certified: This is the phrase used for approval by the American Board of Ophthalmology (ABO), a non-profit and independent organization that was founded in 1916 to certify ophthalmologists.
Bowman’s membrane: Extremely thin second layer of the cornea , situated between the epithelium and stroma , thought to be responsible for epithelium adhesion.
Broadbeam: The term used for applying the excimer laser to the entire treatment area at one time. It’s the typical way laser treatment is done for myopia, hyperopia and astigmatism. See also Flying Spot and Variable Spot.
Buttonhole Flap: A LASIK problem where the microkeratome cuts through the top surface of the cornea while creating the corneal flap, while lifting it, or while folding it back. It can be caused by loss of the suction which holds the microkeratome against the cornea, or inadequate suction, or by poor matching of microkeratome to patient. If possible, the flap is replaced, and no further surgery is attempted for several months, after it has healed. Sometimes it can cause scarring which impairs vision, but usually there are no permanent ill effects.

Capsular haze: A thin film of scar tissue that occasionally forms on the posterior capsule behind the intraocular lens implant following cataract surgery and removed with a Nd:Yag laser.
Caruncle: Small, red portion of the corner of the eye that contains modified sebaceous and sweat glands.
Cataract: An opaque or cloudy area that forms in the eye’s lens, impeding vision. They tend to occur with aging, but can also be caused by trauma. Cataracts are treated by the lens being removed and an artificial lens implanted. See intraocular lens.
Cataract surgery: Removal of a cataract , replacing it with an intraocular lens implant.
Central Ablation Zone: The ablation zone is the area of the eye treated by the laser in LASIK surgery. Around it is the transition zone, where the treated area gradually merges with the original corneal surface that lies outside the ablation zone.
Central Island: An area of the laser-treated part of the cornea which is erroneously not treated, so that its level remains microscopically higher than the surrounding treated surface. The term island describes its appearance. It causes diplopia (double vision).
Choroid: Part of the uvea, the eye’s middle membrane between the retina and the sclera. It has many blood vessels with smaller ones next to the retina and larger ones next to the sclera. It also has large pigment cells.
Choroid membrane: Dark, vascular , thin skin-like tissue, situated between the sclera and the retina , forming the middle coat of the eye. The choroid membrane nourishes the outer portions of the retina and absorbs excess light.
Chronic: Of long duration, going on for some time.
Closed angle glaucoma: Glaucoma conditions occurring suddenly (acute).
Ciliary body: Part of the eye that connects the choroid membrane to the iris . Produces aqueous humor that fills the front part of the eye and maintains eye pressure.
Ciliary muscle: Muscle attached to the crystalline lens responsible for focus (the same as ciliary body , but used in a different context).
Clear Lens Extraction (CLE): Procedure in which the eye’s natural clear crystalline lens is removed and replaced with an intraocular lens implant , using the same technique as cataract surgery.
Color Blindness: Abnormal color vision where the person confuses red and green. It usually occurs in males as the relevant gene is located on the X chromosome.
Color vision: Ability to perceive differences in color, including hue, saturation and brightness.
Co-management: Collaboration between two or more doctors in caring for a patient. For refractive surgery, usually an optometrist co-manages with an ophthalmologist. The optometrist provides the pre-operative testing and post-operative care, while the ophthalmologist does the surgery itself.
Comprehensive eye exam: Evaluation of the complete visual system.
Coma: A higher order aberration which makes points of light look like comets with blurry tail-like smudges. It can be diagnosed and treated with wavefront-guided LASIK procedures.
Complex Wavefront Retreatment: An off-label use of the excimer laser to do corrective secondary surgery after the original wavefront-guided surgery has left the patient with higher-order aberrations.
Conductive Keratoplasty (CK): Procedure in which a radio frequency probe, rather than a laser , is used to reshape the cornea . It is approved for low to moderate hyperopia in patients over age 40, however it does not appear to have the precision of LASIK .
Cones: A type of retinal cell which is light-sensitive and gives us sharp central vision in bright light. They detect color, and there are three kinds: one absorbs red wavelengths, one absorbs green wavelengths and the third absorbs blue wavelengths. Cones are mostly clustered in the central area of the retina.
Concave Lens: The type of lens used to correct nearsightedness (myopia). Concavity is the hollow type of curve that recedes in the center and raises up at the edges, opposite to convex.
Conjunctiva: Mucous membrane lining the inner surface of the eyelids and covering the front part of the sclera (white part of eye), responsible for keeping the eye moist.
Conjunctivitis: Inflammation or irritation of the conjunctiva . Symptoms can be present in just one eye, or it can affect both eyes and include redness of the eyes or the edges of the eyelids, swelling of the eyelids or itching.
Contact lens: Small, thin removable plastic lens worn directly on the front of the eyeballs, usually used instead of ordinary eyeglasses for correction or protection of vision.
Contact Lens Assisted Pharmacologically Induced Kerato Steepening: A way to undo overcorrection in laser eye surgery (LASIK, PRK and RK). A tightly-fitting contact lens is placed on the eye and anti-inflammatory eyedrops are used. The goal is to make the cornea more steep, after it’s been made too flat.
Contact Lens Disinfectant: A solution for removing bacteria and micro-organisms from contact lenses.
Contact Lens, Daily Wear: Contact lenses designed to be worn only during the daytime.
Contact Lens, Disposable: Contact lenses designed to be worn once and then thrown out, as opposed to the kind that’s removed, cleaned, and reinserted. Depending on the eye doctor’s prescription, disposable contact lenses can be worn for one day, or for up to a week.
Contact Lens, Extended Wear: Contact lenses designed to be worn continuously for up to a week, not being removed for sleep.
Contact Lens, Therapeutic: Special contact lenses designed to help heal the eye and protect it while it heals. They’re often used along with eyedrops specially-prescribed to promote healing.
Convergence: Turning of the eyes inwards so that they are both “aimed” towards a nearobject being viewed. Normally works in harmony with divergence which is used for more distant objects.
Convex Lens: Lenses with the opposite curvature to concave lenses. They bulge outwards like a ball and are used to correct hyperopia and presbyopia.
Cornea: Transparent tissue that forms the front part of the eyeball, covering the iris and pupil. The cornea is the first part of the eye that bends (or refract s) the light and provides most of the focusing power.
Corneal Abrasion: A scrape or scratch on the cornea, the front surface of the eye.
Corneal curvature: Shape of the front of the eye.
Corneal Ectasia: A complication of LASIK similar to the inherited condition of keratoconus. It can happen when the eye was over-treated by a LASIK procedure, so that not enough thickness is left in the cornea to contain the eye’s internal pressure. That pressure pushes against the cornea and causes it to bulge outward. Vision then becomes progressively worse.
Corneal Flap: A small circular piece of the cornea’s surface layer (epithelium) which is cut, all but one section like a hinge, and folded back before the laser treats the stroma. After treatment, it’s folded back into position and heals by itself in a few days. Very occasionally there are complications with this flap. For example, it may have been sized wrongly, or cut too deeply, it might be completely cut instead of retaining a hinge, or it might heal in the wrong position, with a slight wrinkle or with swelling. Many of these complications can be dealt with successfully.
Corneal Haze: An after-effect of Excimer laser surgery, where the cornea develops opaque white cells which cloud the vision to some extent. It can cause glare from bright lights and a vague fogginess of vision. It usually clears up after 6 or 8 months. If it persists, there’s an enhancement procedure which can reduce it. As vision correction techniques improve there’s less incidence of corneal haze.
Corneal mapping, topography: A tool used to see the refractive problems that might be present in the cornea. Corneal topography is used not only for screening all patients before refractive surgery like LASIK but also for fitting contacts.
Corneal Refractive Therapy: A reshaping of the eye with contact lenses, also called Orthokeratology. These lenses are rigid and worn while you sleep, so they gently persuade the eye to change its shape by the time you wake in the morning. The effect lasts only a day or two, so you need to wear these lenses every night. It was approved by the FDA in June, 2002 and is a non-surgical way of temporarily creating the effect created permanently by LASIK surgery.
Corneal relaxing incisions (CRIs): True corneal incisions, such as RK and AK .
Corneal Topographical Map: A map of the cornea that shows its surface profile.
Corneal Topographer: A specialized camera-computer system which photographs and prints out a map of the surface details of a cornea. This is done before any LASIK procedure.
Corneal transplant (penetrating keratoplasty): Surgery to replace the cornea, the clear front area of the eye. Corneal tissue comes from a donor and since the cornea has a small blood supply, there is little risk of rejection, and the new cornea can function very well for years. A corneal transplant can be done to treat Keratoconus, Fuch’s Dystrophy, or damage from a severe infection or injury. It’s a painless outpatient procedure.
CrystalensT: An accommodative intraocular lens used to treat cataracts and presbyopia. Manufactured by Bausch & Lomb and approved by the FDA in November, 2003 for replacing the eye’s natural lens.
Crystalline lens: The eye’s natural lens, which is behind the iris and in a healthy eye is completely clear. Light passes through it and is refracted by the lens to focus on the retina. Tiny muscles attached to the lens change its convexity according to where the eye is focused. As we age, it can start to become cloudy (cataracts), impairing our vision.
Cylinder: Refers to the degree of astigmatism (uneven roundness) present in the cornea.

Decentration: A complication of eye surgery. When centration is perfect, the corneal ablation, or the position of the intraocular lens being implanted, is in the center of the eye, lined up with the pupil. Then vision is perfectly centered through the ablated area and pupil, so that we see clearly and fully, like looking through the center of your glasses. When decentration happens, the ablation has been done off-center, or the lens’ position is shifted, so that vision is partially corrected and partially as it was originally.
Degenerative Myopia: Nearsightedness thought to be hereditary, which may start at birth, or in later childhood. It’s a more severe form of myopia and can lead to blindness. It’s associated with to cataract formation and with retinal changes and can lead to retinal detachment.
Depth perception: Ability of the vision system to perceive the relative positions of objects in the visual field.
Detached retina: A retinal detachment occurs when the retina is pulled away from its normal position in the back of the eye.
Diabetes mellitus: Chronic metabolic disorder characterized by a lack of insulin secretion and/or increased cellular resistance to insulin, resulting in elevated blood levels of simple sugars (glucose) and including complications involving damage to the eyes, kidneys, nervous system and vascular system
Diabetes type I (IDDM): Insulin-dependent diabetes, also known as juvenile diabetes because it’s often diagnosed in young people. The pancreas, which normally would produce insulin, is unable to do so, so insulin must be injected so the patient can convert sugars and starch into energy. A person with Diabetes, Type 1 or 2, is susceptible to vision problems because the tiny blood vessels in the eyes weaken and start to leak, damaging the retina. See Diabetic Retinopathy.
Diabetes type II (NIDDM): The more common of the two types of Diabetes, where the pancreas can produce insulin, but not enough. Insulin may or may not have to be injected for the person to convert sugars into energy. Sometimes Type 2 can be well-managed by sticking to the right diet and exercise routines.
Diabetic retinopathy: Damage done to the small blood vessels that feed the retina. In the early stage it’s known as background diabetic retinopathy. The small arteries in the retina weaken and leak, which often causes swelling and impaired vision.
The later stage is called proliferative diabetic retinopathy. Since the arteries are not functioning well to bring oxygen to the eye’s cells, retinal cells become ischemic (lacking in oxygen). The eye develops new blood vessels (a response known as neovascularization) but these are weak and also start leaking, so the problem becomes worse.
Eventually, as the body continues to grow extra blood vessels, which continue to leak, scars can form, and the retina can detach from its membrane at the back of the eye, causing blindness.
Dilated, dilation: Enlargment of the pupil (space in the middle of the iris).
Diopter: The unit of measurement for a lens. Positive diopter numbers indicate the lens is convex (curving outwards) and negative numbers indicate it’s concave. A one-diopter lens will bend straight light rays to focus them one meter away from itself. A two-diopter lens will bend them to focus only a half-meter from itself. A lens with any minus number doesn’t focus the light rays, but scatters them.
Diplopia: Condition in which a single object is perceived as two; also called double vision.
Divergence: Turning of the eyes outwards so that they are both “aimed” towards the object being viewed. Normally works in harmony with convergence.
Dominant Eye: The eye that looks directly at an object. The non-dominant eye looks at it from a slight angle, and this combination creates depth perception.
Double vision: Same as diplopia.
Dry eye Syndrome: Insufficient moisture in the eye, which gives a feeling of grittiness, burning, stinging, or uncomfortable dryness, and extra sensitivity to light. It can be a symptom or complication of another condition, or caused by certain medications. It is also a common temporary result of a LASIK procedure. It can be treated with moisturizing eyedrops, eyedrops which stimulate more tear production, or punctal plugs which block the drainage into the nasal passages and sinuses. passages and sinuses. passages and sinuses.
DSAEK: DSAEK (Descemet’s Stripping Automated Endothelial Keratoplasty) is a new procedure devised in 2007 for improving a corneal transplant. Descemet’s Membrane (named after French physician Jean Descemet (1732-1810), is the basement layer, the innermost of the cornea’s five layers. DSAEK transplants only this very thin layer rather than the entire cornea. The replacement layer from a donor stays in place without the need for any sutures. Recovery from DSAEK is one to three months, as compared to the one to two years’ recovery required by a full corneal transplant.
Dystrophy: Weakening or wasting of body tissue, such as happens in Epithelial Dystrophies and Stromal Dystrophies, where abnormalities appear in different corneal membranes, causing loss of function and often impairing vision.

Emmetropia: Twenty-twenty vision.
Endothelium: A lining of flat cells inside the cornea, facing on to the anterior chamber.
Enhancement: An additional LASIK procedure, used in the refinement of Personal Best Vision.
Epikeratome: A surgical tool similar to a microkeratome, used to make the corneal flap in an Epi-LASIK procedure. It has a blunt separator where the microkeratome, used in LASIK procedures, has a very thin and sharp blade. It’s another way of making the corneal flap and does not involve the use of alcohol, as LASEK procedures do. Each way of making the corneal flap has its advantages and disadvantages and consultation with an experienced eye surgeon will determine which would be best for you.
Epi-LASIK: A variation on the basic LASIK treatment, where instead of using a microkeratome to cut the corneal flap before applying the laser, an epikeratome is used. This instrument gently separates the epithelial cells. Epi-LASIK is done for people whose corneas are too flat for traditional LASIK.
Epithelial Dystrophies: A group of inherited dystrophies where the surface layer of the cornea atrophies and vision is impaired. See Anterior Basement Membrane Dystrophy, Reis-Buckler’s Dystrophy, and Meesmann’s Dystrophy.
Epithelium: Cellular tissue that covers the outer surface of the eye. Consists of one or several layers of cells with only little intercellular material.
Esophoria: Position of the eyes in an over-converged position compensated by the external eye muscles so that the eyes do not appear turned inward.
Esotropia: Inward turning of the eyes; crossed eyes. Usually one eye looks straight ahead and the other turns inward.
Excimer laser: Laser used in LASIK surgery that operates in the ultraviolet wavelength, producing a cool beam.
Exophoria: Position of the eyes in an over-diverged position compensated by the external eye muscles so that the eyes do not appear turned outward.
Exotropia: Position of the eyes in an over-diverged position so that non-fixating eye is turned outward. One eye looks straight ahead and one turns outward.
Extracapsular cataract surgery: Surgery in which the cataract is removed in one piece through a larger incision, which usually requires several stitches.
Extraocular muscles: Six muscles that control eye movement. Five originate from the back of the orbit; the other one originates from the lower rim of the orbit. Four move the eye up, down, left and right, the other two control the twisting motion of the eye when the head tilts. All six muscles work in unison; when they do not function properly, the condition is called strabismus .
Eye chart: Technically called a Snellen chart, a printed visual acuity chart consisting of Snellen optotypes, which are specifically formed letters of the alphabet arranged in rows of decreasing letter size.
Eyelid: Either of two movable, protective, folds of flesh that cover and uncover the front of the eyeball.

Farsightedness: Common term for hyperopia .
FDA: Abbreviation for the Food and Drug Administration. It is the United States governmental agency responsible for the evaluation and approval of medical devices.
Femtosecond laser: Used in the IntraLASIK procedure to make a safer and more precise flap than the older mechanical microkeratome technology, it uses a longer wavelength, smaller spot, and shorter duration per pulse than the excimer laser used to reshape the cornea .
Field of vision: Entire area which can be seen without shifting the gaze.
Flap: Part of the cornea consisting of epithelium , Bowman’s membrane and some stroma , cut with a remaining hinge and lifted up as part of the LASIK procedure.
Flashes & floaters: Light spots or streaks and dark moving specks due to the vitreous traction on the retinal (light flashes) and solid vitreous material or blood (floaters).
Fluorescein angiography: Diagnostic test by which the veins deep inside the eye are examined. Dye is injected into a vein in the arm and circulated by the blood to the back of the eye, allowing for visual examination.
Flying Spot: A method of applying the excimer laser light used in refractive surgery. A flying spot laser is used, which has a 1mm: 2mm diameter light beam. The computer that controls the laser is programmed to send pulses to changing spots on the cornea, with no spots overlapping. Part of the ophthalmic laser system is an eye tracker, which responds to all eye movement during surgery. So even if the eye moves while it’s being treated, the entire targeted area will be treated, because the treatment plan has determined where the laser should shine, in the series of flying spots that will cover it entirely, and the whole system moves to correspond with any eye movements during surgery.
Focusing power of the eye: The combined action of the cornea and the lens to refract light on to the retina.
Fovea: Small depression in the retina , the point where vision is most acute.
Fuch’s Dystrophy: A progressive, inherited eye disease (dystrophy) in which the cornea loses cells from the endothelium which normally remove fluids and impurities from the eye. Without those cells, the eye retains too much fluid and begins to swell. By changing the cornea’s curvature, this makes vision blurry, especially first thing in the morning, since while the eyes are closed in sleep, no moisture can evaporate from them. It causes other symptoms, e.g. blisters, light sensitivity, pain, and decreased depth perception. There is no cure, but there are some ways to minimize symptoms. A corneal transplant eventually becomes necessary. Also called Endothelial Dystrophy.
Fundus: Furthest point at the back of the eye, consisting of the retina , choroid membrane , sclera , optic disc and blood vessels, seen by means of the ophthalmoscope.

Ghosting: A name for double vision. The eye sees objects in duplicate, with the second image fainter than the main one.
Giant papillary conjunctivitis: Type of conjunctivitis wherein bumps or ridges form on the inside of eyelids, which make wearing contact lenses uncomfortable; in fact, this condition is often caused by overwearing of certain contact lenses
Glare: Scatter from bright light that decreases vision.
Glaucoma: An eye disease where pressure builds up inside the eye. If it isn’t diagnosed and treated, it can damage the optic nerve, reducing the field of vision gradually, until blindness results. It’s treated with special eye drops which lower the pressure.
Gonioscopy: Viewing procedure utilizing a mirror/lens device placed directly upon the cornea that is used to view the drainage area called “the angle” through which aqueous fluid exits the eyeball.
Granular Dystrophy: A hereditary eye condition where pale gray granules appear in the stromal layer of the cornea, like little crumbs. It’s usually detected by the time a person is about 20. By age 40 or so, vision will be increasingly impaired as those granules, or lesions, expand, increase in number, coalesce, and penetrate more deeply into the stroma. It can be treated in earlier stages with an excimer laser, or in other ways that remove the granules, and later on by a corneal transplant.

Halos: A visual condition where a light source appears with a blurry circle of light around it, rather than having visible edges. It can be a complication of refractive surgery, and can also occur naturally. It makes night vision difficult.
Haptics: The arms of an intraocular lens , which holds it in place once inserted inside the eye.
Haze: Clouding of the cornea. It can be caused by inflammation, too much moisture, scar tissue, or some kind of foreign substance as from a medication.
Heterophoria: Constant tendency of one eye to deviate in one or another direction due to imperfect balance of ocular muscles.
Higher Order Aberration: Irregularities of the eye that are not refractive.
Holmium laser: A laser which operates in the infrared wavelength, producing a hot beam. It is used in Laser Thermokeratoplasty surgery and more commonly in surgical procedures involving the disintegration of stones and fibrous tissue ablation .
Hyperopia: Also called farsightednesss, hyperopia is the inability to see near objects as clearly as distant objects, and the need for accommodation to see distant objects clearly.
Hypoxia: Deficiency of oxygen supply to a tissue.

ICL (Implanted Collamer Lens): The name used for the STAAR Visian ICL™, created by the STAAR Surgical Company and approved by the FDA in 2005 for treating myopia. It is a refractive lens for use in a phakic eye (an eye with its natural lens intact) and is positioned in front of or behind the natural lens to modify the lens’ refractive power.
iLasik: The name for vision correction surgery which combines the Intralase technology manufactured by Advanced Medical Optics, Inc. (AMO) and the VISX Excimer technology originally marketed by the company VISX which was acquired by AMO in 2006. It is an Intralase procedure using the VISX hardware.
Image: Light reflected into the eye, off objects in front of the eye. This light contains all the information about the objects (such as color, shadow. motion and detail) that are translated to the brain and allow you to “see” (know about the objects).
Inflammation: Body’s reaction to trauma, infection, or a foreign substance, often associated with pain, heat, redness, swelling, and/or loss of function.
Informed Consent Form: Document disclosing the risks, benefits, and alternatives to a procedure.
In Situ: Term meaning “in place”.
Intracapsular cataract surgery: Cataract surgery in which both the lens and capsule are completely removed, a rarely used procedure.
IntraLase: A type of laser used to create the corneal flap in an IntraLASIK procedure. In a traditional LASIK surgery, the flap is created by a hand-held device called a microkeratome, with an oscillating blade. In IntraLASIK, it’s created by the IntraLase™ FS laser, a cool light which passes through the corneal surface to the exact location beneath the surface which your LASIK surgeon has programmed into the computer. This ensures that the flap is not cut too deeply or unevenly. For the treatment itself, the excimer laser is used as in traditional LASIK procedures.
IntraLASIK: A variation on a LASIK procedure. See IntraLase.
Intraocular lens: An artificial lens which is implanted in the eye to replace the natural crystalline lens. Intraocular lenses are used to treat cataracts and presbyopia. Traditionally they were monofocal, so that you needed glasses for either close-up vision or distance vision. Newer ones are multifocal, with different areas designed for different distances, or can accommodate in a way similar to how the natural lens accommodates for distance.
Intraocular lens implant (IOL): Permanent, artificial lens surgically inserted inside the eye to replace the crystalline lens following cataract surgery or clear lens extraction .
Intraocular pressure (IOP): Fluid pressure within the eye created by the continual production and drainage of aqueous fluid in the anterior chamber .
Iridotomy: Treatment for closed-angle glaucoma, one of the many types of glaucoma, usually done with a laser .
Iris: Colored part of the eye. Elastic, pigmented, muscular tissue in front of the crystalline lens that regulates the amount of light that enters the eye by adjusting the size of the pupil in the center.
Ischemia: Restriction or blockage of blood flow through a blood vessel. Ischemia is a causative agent of certain heart attacks and strokes and is involved in various types of visual field losses.
Intacs: Surgically implanted plastic half rings that change the shape of the cornea .

Keratectomy: Surgical removal of cornea l tissue.
Keratitis: Inflammation of the cornea caused by bacteria or a virus. Can leave scarring and cause loss of vision.
Keratotomy: Surgical incision (cut) of the cornea . .
Keratoconous: Rare, serious, degenerative cornea l disease, in which the cornea thins and assumes the shape of a cone.
Keratometry: (kehr-uh-TAH-mih-tree) Measurement of the cornea’s curvature. It’s done with a keratometer.
Keratoplasty: See corneal transplant.
Keratomileusis: Carving of the cornea to reshape it.
Keratoplasty: Surgical reshaping of the cornea .

Lacrimal gland: The small structure in each eye which produces tears. It’s above the outer corner of the eye, and lacrimal ducts run from the inner corner to the nose. See also punctum and nasolacrimal duct obstruction.
LASEK: Laser Epithelial Keratomileusis, a refractive surgery in which the epithelium is cut with a fine blade, called a trephine, and involves displacing the cornea l epithelium as a sheet and then replacing it to (theoretically) act as a natural bandage.
Laser: Device that generates an intense and highly concentrated beam of light. Acronym for: Light Amplification by Simulated Emission of Radiation. (Also see: holmium laser , argon laser , Nd:YAG laser , femtosecond laser , and excimer laser )
Laser Thermokeratoplasty (LTK): Holmium ‘hot’ beam laser, instead of the ‘cool’ beam excimer laser, is used to treat farsighted patients and is very limited in its application; the effects are not long lasting.
LASIK: Laser Assisted In-Situ Keratomilieusis, a refractive surgery in which Excimer laser ablation is performed under a flap on the cornea to correct refractive errors .
Lattice Dystrophy: A hereditary corneal condition where abnormal protein fibers appear on the stroma.
Lazy eye: Amblyopia, an eye condition noted by reduced vision not correctable by glasses or contact lenses and is not due to any eye disease.
Legal Blindness: A definition of blindness which enables a person to apply for government disability benefits. It can be either a visual acuity of 20/200 (or worse), with corrective lenses, in the better eye, or tunnel vision in the better eye of 20 degrees in diameter. This level of blindness is severe, but does not necessarily prevent a person from functioning at all.
Lens: Same as the crystalline lens . Double convex, clear part of the eye, behind the iris and in front of the vitreous humor. Serves to refract the various rays of light so as to form an image on the retina .
Lenticular: Special non- cataract lenses for patients who have cataracts.
Lid speculum: A surgical tool that holds the eyelids open and which allows the surgeon to gain access to the eye with minimal pressure on the globe.
Limbal relaxing incisions (LRI): Small incisions placed on the far peripheral aspect of the cornea resulting in a cornea that is more round, for correcting astigmatism .
Limbus: Thin area that connects the cornea and the sclera .
Lower Order Aberrations: The name for myopia, hyperopia, and astigmatism. These are the conditions measured to determine a prescription for glasses or contact lenses, and that can be corrected with refractive eye surgery such as LASIK. The higher order aberrations are more numerous and still be researched and some examples are: halos, glare, and double vision.
Low vision: Condition occurring when ordinary eyeglasses or contact lenses are unable to bring a patient’s sight up to normal sharpness.
LTK (Laser Thermal Keratoplasty): Holmium ‘hot’ beam laser, instead of the ‘cool’ beam excimer laser, is used to treat farsighted patients and is very limited in its application; the effects are not long lasting.
Lutein: Member of the carotinoid family of vitamins, similar to beta-carotene, thought to be beneficial to the eyes, helpful in treating diseases such as glaucoma.

Macula: An extra-sensitive area roughly in the center of the retina, which enables us to see fine detail and do activities such as reading. The center of the macula is the fovea, which has no nerve cells or blood vessels to interfere with vision, and this makes it the most sensitive area of the macula.
Macular Degeneration: A deterioration of specialized cells in the macula of the retina which normally detect light and color to give us sharp central vision.
Macular Dystrophy: A hereditary type of macular degeneration.
Macular edema: Pooling of fluid in and around the macular area of the retina, which causes swelling and impairs vision.
Manifest Refraction: The eye examination which determines a person’s degree of refractive error.
Meesmann’s Dystrophy: A rare hereditary eye condition where tiny cysts appear on the corneal surface which can rupture.
Meridian: Orientation of a particular curve, often used in relation to the cornea .
Mesopic Pupil Size: The size of the eye’s pupil (black circular area inside the iris) in medium lighting conditions, such as a typically lit room, or daylight.
Micron: A measurement of length equal to one-millionth of a meter.
Microkeratome: Mechanical surgical device that is affixed to the eye by use of a vacuum ring. When secured, a very sharp blade cuts a layer of the cornea at a predetermined depth.
Miosis: Pupillary constriction.
Monocular: An adjective for vision out of one eye. Also the name of a type of telescope.
Monovision: Purposeful adjustment of one eye for near vision and the other eye for distance vision.
Mydriasis: Pupillary dilation.
Myopia: Also called nearsightedness or shortsightedness, the inability to see distant objects as clearly as near objects.

Near point of convergence: Maximum extent the two eyes can be turned inwards.
Nearsighted: Common term for myopia. The inability to see distant objects as clearly as near objects.
Neodymium YAG Laser: Laser used to treat Posterior Capsular Opacification (PCO) as well as open angle glaucoma Selective Laser Trabeculoplasty
Neovascularization: Often associated with diabetes, involves the formation of new blood vessels, often fragile and inappropriate for the location.
Nerve fibers/axons: Extensions of photoreceptors that form the nerve bundle that is called the optic nerve.
Neuro-ophthalmology: Subspecialty that treats the nervous and vascular systems that involve the eye.
Normal vision: Occurs when light is focused directly on the retina rather than in front or behind it.

Ocular herpes: A recurrent viral infection caused by the herpes simplex virus. Ocular herpes represents the most common infectious cause of corneal blindness in the United States.
Ocular hypertension: Elevated fluid pressure. The normal pressure is about 10 to 21mmHg, with the majority of people falling between 13 and 19. Over 21 is considered suspicious. Over 24 cautiously concerned – warranting immediate investigation. Over 30 is considered urgent and a potential emergency situation.
OD: Abbreviation standing for “oculus dextrum” meaning: right eye.
ONH: Optic nerve, optic nerve head. A bundle of nerve fibers about the diameter of pencil that pass through the back of the eyeball and connects to the nerve fiber layer of the retina . It can be observed directly with an instrument called an ophthalmoscope.
Open angle glaucoma: Glaucoma conditions of long duration (chronic).
Ophthalmologist: An ophthalmologist is either a medical doctor (MD) or an osteopathic physician (D.O.) who is qualified and especially trained to diagnose and treat all eye and visual system problems, both medically and surgically, as well as diagnose general diseases of the body.
Ophthalmoscope: Instrument used to examine the interior of the eye: it consists of a perforated mirror arranged to reflect light from a small bulb into the eye.
Ophthalmoscopy: Examination of the internal structures of the eye using an illumination and magnification system.
Optic disc: The head of the optic nerve that is formed by the meeting of all retina l nerve fibers.
Optic nerve: Bundle of nerve fibers that connect the retina with the brain. The optic nerve carries signals of light to the area of the brain called the visual cortex, which assembles the signals into images called vision.
Optician: Experts who designs, verifies and dispenses lenses, frames and other fabricated optical devices upon the prescription of an ophthalmologist or an optometrist.
Optometrist: Eye care professional, graduate of optometry school, provides non-surgical visual care. Specifically educated and trained to examine the eyes, and determine visual acuity as well as other vision problems and ocular abnormalities. An optometrist prescribes glasses and contact lenses to improve visual acuity.
Orbit: Boney socket containing the eyeball, fat, extraocular muscles, nerves and blood vessels.
Orthokeratology (OK): Non-surgical procedure using contact lenses to alter the shape of the cornea to effect a change in the refractive error.
Orthoptics: Exercises designed to help the eye muscles work together to improve visual perception.
OS: Abbreviation standing for “oculus sinistrum” meaning: left eye
Overcorrection: Occurence in refractive surgery where the achieved amount of correction is more than desired; in LASIK , typically due to a patent’s over-response to the laser ablation.

Pachymeter: Instrument that measures the distance between the top of the cornea l epithelium and the bottom of the cornea l endothelium used as diagnostic testing device measuring for cornea l thickness.
Pachymetry: Exam for measuring cornea l thickness.
Papilledema: Non-inflammatory swelling/elevation of the optic nerve often due to increased intracranial pressure or space-occupying tumor.
PD: Used on prescriptions to indicate the distance between the pupils of both eyes.
Pellucid marginal degeneration: A bilateral, noninflammatory, peripheral corneal thinning disorder, which is characterized by a peripheral band of thinning of the inferior cornea.
Peripheral vision: Ability to perceive the presence, motion, or color of objects outside the direct line of vision.
Personal Best Vision: Best possible vision for each individual as corrected.
Phacoemulsification cataract surgery: Cataract removal procedure which involves making a tiny incision, about 1/8″ long. A pen-like instrument, inserted through the opening, is used to emulsify and aspirate the clouded lens material, using gentle sound waves. Then an intraocular lens is inserted into place.
Phacofracture cataract surgery: Cataract surgery in which the lens is removed through a small incision by “fracturing” it into several small segments, rarely used today.
Phakic Intraocular Lenses (IOLs): Placed inside the eye without removing the natural lens, and performs much like an internal contact lens.
Phoropter: A common device found in most eye doctor’s offices, with mulitple lenses, used to measure refractive errors. A phoropter calculates the prescription required for corrective lenses.
Photocoagulation: Focusing of powerful light rays onto tiny spots on the back of the eye, producing heat, which seals retina l tears and cauterizes small blood vessels.
Photophobia: Sensitivity to light.
Photoreceptors: Microscopic light-sensitive cells that are located in the retina called rods and cones. There are approximately 7 million cones and 125 million rods
Photo Refractive Keratectomy (PRK): Surgery in which a small area on the cornea l epithelium (surface cells) is gently polished away. The laser then reshapes the cornea l surface in exactly the same way as for LASIK surgery.
Pingecula: Irritation caused degeneration of the conjunctiva resulting in a thickening and yellowing of the normally thin transparent tissue.
Pink eye: Type of conjunctivitis, commonly seen in children.
Posterior capsule: The thin membrane in the eye that holds the crystalline lens in place.
Posterior chamber: The back section of the eye’s interior.
Posterior optical segment: Part of the eye posterior (behind) to the crystalline lens, including the vitreous, choroid retina and optic nerve.
Posterior Vitreous Detachment (PVD): Separation of the vitreous body from its attachment from the retina l surface due to shrinkage from degenerative or inflammatory conditions or trauma. It is an age-related condition.
Prelex: Surgical procedure that attempts to correct presbyopia.
Presbyopia: Inability to maintain a clear image (focus) as objects are moved closer. Presbyopia is due to reduced elasticity of the lens with increasing age.
Prescription: Amount of vision correction necessary, written in a form that can be utilized during the manufacture of corrective lenses or to configure a laser machine.
PRK: Acronym for Photo-Refractive Keratectomy, which is a procedure involving the removal of the surface layer of the cornea ( epithelium ) by gentle scraping and use of a computer-controlled excimer laser to reshape the stroma .
Progressive lenses: Bifocal or trifocal lenses which appear to be single vision with no distinct lines between the various focal lengths.
Punctal occlusion: Treatment for dry eye in which plugs are inserted into the punctum in order to retain lubricating tears naturally produced by the eye.
Punctum: The hole in the upper and lower eyelids through which tears exit the eye. In patients with dry eyes, temporary or permanent plugs may be inserted to help keep tears in the eye. Tears flow through the punctum to the nose, which is why people often experience a runny nose when crying.
Pupil: Black circular opening in the center of iris through which light passes into the crystalline lens. It changes size in response to how much light is being received by the eye, larger in dim lighting conditions and smaller in brighter lighting conditions.
Pupillary response: Constriction and dilation of the pupil due to stimulation by light or accommodation.

Radial Keratotomy (RK): Outdated procedure once used to correct mild to moderate myopia, whereby making a series of spoke-like incisions around its periphery flattens the cornea.
Refract: To bend aside, as in “the crystalline lens refract s the light as it passes through”, or to measure the degree the eyes or lenses bend light, as in “the doctor refract s a patient’s eyes”.
Refraction: Test to determine the refractive power of the eye; also, the bending of light as it passes from one medium into another.
Refractive errors: The degree of visual distortion or limitation caused by inadequate bending of light rays, includes hyperopia, myopia, and astigmatism .
Refractive power: Ability of an object, such as the eye, to bend light as light passes through it.
Refractive surgery: Type of surgery (such as LASIK) that affects the refract ion of vision.
Retina: Layer of fine sensory tissue that lines the inside wall of the eye, composed of light sensitive cells known as rods and cones . Acts like the film in a camera to capture images, transforms the images into electrical signals, and sends the signals to the brain by way of the optic nerve.
Retinal Detachment: Condition wherein retina breaks away from the choroid membrane, causing it to lose nourishment and resulting in loss of vision unless successfully surgically repaired.
RK: Abbreviation for “ radial keratotomy “, an outdated procedure once used to correct mild to moderate myopia , whereby making a series of spoke-like incisions around its periphery flattens the cornea .
Rods: One of the two types of light-sensitive cells, located primarily in the side areas of the retina (also see cones). There are about 125 million rods, which are responsible for visual sensitivity to movement, shapes, light and dark (black and white) and the ability to see in dim light.
Routine eye exam: To test the overall condition of the eye and prescribe corrective measures such as glasses, contact lenses or LASIK.

Schirmer test: Test for dry eyes, which use a thin strip of filter paper placed at the edge of the eye.
Sclera: White part of the eye. Tough covering that (with the cornea) forms the external, protective coat of the eye.
Scotoma: Area of partial or complete loss of vision surrounded by an area of normal vision, as what can occur in advanced ARMD or glaucoma.
Single vision: Lenses with only one focal length.
Slit-Lamp: Ophthalmic instrument producing a slender beam of light used to illuminate and examine the external and internal parts of the eye.
Sloan eye chart: A common chart used to test visual acuity with black letters of various sizes against a white background.
Snellen eye chart: Most common chart used to test visual acuity with black letters of various sizes against a white background.
Snellen lines: Snellen optotypes arranged in horizontal rows called “lines”.
Snellen optotypes: Specifically formed letters of the alphabet arranged in rows of decreasing letter size on the Snellen chart.
Sphere: Focusing power of the corrective lens.
Stereoscopic vision: Ability to see in three-dimension.
Stereopsis: Ability to perceive three-dimensional depth.
Strabismus: Condition occurs when the muscles of the eye do not aligned properly and binocular vision is not present. Patients with a history of strabismus may develop double vision after refractive eye surgery.
Stroma: Middle, thickest layer of tissue in the cornea.
Suppression: Inability to perceive all of part of objects in the field of vision of one eye.
Suspensory ligament of lens: Series of fibers that connect the ciliary body of the eye with the lens, holding it in place, also known as zonules.
Sympathetic ophthalmia: Inflammation of one eye following inflammation in the other eye.

Tonometry: Procedure for the measurement of intraocular pressure. A test for glaucoma.
Topography: A tool used to see the refractive problems that might be present in the cornea. Corneal topography is used not only for screening all patients before refractive surgery like LASIK but also for fitting contacts.
Toric: Lens (eyeglasses, intraocular lens, or contact lens) that is the warped (astigmatic) opposite to that of the eye, thereby canceling out the error.
Trabecular meshwork: Drainage channels located inside the eye.
Trabeculoplasty: A procedure for the treatment of glaucoma, using a laser (Argon or Nd:YAG ). Trabeculoplasty remodels the trabecular meshwork in order to increase drainage of aqueous and lower the intraocular pressure.
Trifocals: Lenses containing three focal lengths, usually arranged with the focus for distance above, intermediate distance in the middle, and near vision below.
Twenty-twenty, 20/20 vision: To have 20/20 vision means that when you stand 20 feet away from the Snellen eye chart you can see what the majority of people can see at that same distance.

UCVA: Uncorrected visual acuity.
Uveal tract: Pigmented, middle layers of the eye, which include the choroid, ciliary body and iris.
Uveitis: Inflammation of any portion of the uveal tract.
Ultrasound waves: Sound waves above 20,000 vibrations per second, above the range audible to the human ear, used in medical diagnosis and surgery.
Ultrasonography: Recordings of the echoes of ultrasound waves sent into the eye and reflected from the structures inside the eye or orbit. Ultrasonography is used to make measurements and to detect and localize tumors and retina l detachments.
Ultraviolet radiation: Radiant energy with a wavelength just below that of the visible light. UV-c is the shortest wavelength at 200-280 nm and is absorbed by the atmosphere before reaching the surface. UV-b, at 280-315 nm is the burning rays of the sun and damages most living tissue. UV-a, at 315-400 nm is the tanning rays of the sun and is somewhat damaging to certain tissues. UV radiation has been described as a contributing factor to some eye disease processes, which result in ARMD and cataract s and causes exposure keratitis.
Uncorrected visual acuity (UCVA): Best possible vision a person can achieve without corrective lenses measured in terms of Snellen lines on an eye chart.
Undercorrection: Occurence in refractive surgery where the achieved amount of correction is less than desired; in LASIK , typically due to a patient under-responding to the laser treatment.

Vascular: Having to do with transporting blood.
Vision: The ability of the brain to see and interpret what is in front of the eyes.
Vision therapy: Orthoptics, vision training, eye exercises. Treatment process for the improvement of visual perception and/or coordination of the two eyes, for more efficient and comfortable binocular vision.
Visual acuity: Clearness of vision; the ability to distinguish details and shapes, which depends upon the sharpness of the retina l image.
Visual cortex: That part of the brain responsible for vision.
Visual field: Area or extent of space visible to an eye in a given position of gaze. There is a central visual field – the area directly in front of us, and a peripheral visual field – our “side vision”. The fields of each eye partly overlap. We do not perceive the blind spots from each eye because the area that is missing in one eye is present in the other.
VISX CustomVue Procedure: WaveScan-driven laser vision correction with the potential to produce better vision than is possible with glasses or contact lenses, and enable surgeons to measure and correct unique imperfections in each individual’s vision.
VISX STAR S4 Excimer laser System: Highly advanced laser technology platform, the VISX STAR S4 combines Variable Spot Scanning (VSS) and ActiveTrak 3-D Active Eye Tracking along with the WavePrint.
Vitreous humor, fluid, or body: Jelly-like, colorless, transparent substance occupying the greater part of the cavity of the eye, and all the space between the crystalline lens and the retina.
Vitrectomy: Surgical removal of vitreous humor that is diseased or has lost its transparency.

Wavefront: Wavefront technology produces a detailed map of the eye. The information is transferred to the laser via computer software.

YAG laser surgery: Properly called Nd:Yag laser capsulotomy, a procedure using a Nd:YAG (neodymium-yttrium-aluminum-garnet) laser, used primarily to treat secondary cataract s (capsular haze) that occur subsequent to the primary cataract procedure, or to relieve increased pressure within the eye from acute angle-closure glaucoma via a peripheral iridotomy. It can also be used to treat open angle glaucoma in a procedure called selective laser trabeculoplasty.