A Glossary Of Contact Lens Terminology

Obscure medical terms can often confuse an already stressful diagnosis. Here we have compiled a very useful glossary of terms that can help you understand the eye, its function and the malfunctions that can sometimes occur.


acanthamoeba keratitis (AY-can-tha-MEE-buh ker-a-TIGH-tiss): A rare but serious sight-destroying inflammation of the cornea caused by a parasite found in contaminated water. Most cases have been traced to the use of home-made saline solution, tap water rinses, and contaminated pools, lakes and hot tubs.

accommodation: A change in the focus of the eye to allow clear vision at various distances.

apex of cornea (Ay-pex): The peak of the cornea, normally located directly over the pupil and visual axis.

aspheric lens (ay-SPHER-ic): A lens that resembles an ellipse, parabola, or other conic section rather than a sphere. It can be designed to improve the fit and comfort of a lens by paralleling the cornea more closely or to provide a progressive bifocal effect.

astigmatism (uh-STIG-ma-tizm): A refractive condition in which the cornea, the lens, or both are ellipsoidal rather than spherical and light is not refracted equally in all meridians.

axis (AK-siss): The principal meridian of a cylindrical lens (in reference to contact lenses, the flattest meridian of a toric contact lens).

bandage lens: See therapeutic lens.

base curve: The central posterior (inside surface) curve of a contact lens. The measurement of the base curve is actually that of the radius of curvature of the sphere from which the lens is made.

benzalkonium chloride (ben-zal-CONE-ee-um KLOR-ide): A quaternary ammonium compound used as a preservative or disinfectant in eye medications and rigid lens solutions.

benzyl alcohol (BEN-zil): A disinfectant and preservative in rigid lens solutions.

blepharitis (blef-er-EYE-tiss): An inflammation of the margins of the eyelids, often caused by the staphylococcus organism.

conjunctiva (kon-junk-TIVE-a): A thin, transparent membrane that lines the eyelids (palpebral conjunctiva) and the sclera (bulbar conjunctiva).

conjunctivitis (kon-junk-ti-VIGH-tiss): An allergic, infectious, or chemically-induced inflammation of the conjunctiva.

cornea (KOR-nee-uh): The transparent, avascular structure that forms the “window of the eye.” It can be compared to the watch crystal over the face of a watch, and is composed of five layers: the epithelium (outermost layer), Bowman’s membrane, the stroma (center layer), Descemet’s embrane, and the endothelium (innermost layer).

corneal edema (Kor-nee-al e-DEE-ma): Swelling and fluid retention in the cornea, usually related to lack of sufficient oxygen in contact lens wearers.

corneal ulcer: A sight-threatening lesion, usually bacterial in nature, and often causing permanent scarring of the cornea.

DK value: A measure of the oxygen transmitted by a contact lens material

DMV remover: A suction cup type of device used to remove a rigid contact lens from the eye.

enzymatic cleaner (en-zi-MAT-ick) A cleaner that will decompose or digest protein, muco-protein, or lipoprotein deposits on a contact lens.

equivalent oxygen percentage (EOP): A lens that is impermeable to oxygen is said to have an EOP of zero; a lens completely permeable to oxygen would have an EOP of 21 (the same as the percentage of oxygen in the atmosphere). The EOP of a contact lens falls between zero and 21, depending upon its material

fenestration (fen-i-STRAY-shun): The drilling of tiny holes in a contact lens to allow tears to circulate more freely and to provide more oxygen to the cornea through the tear exchange.

flare: Light streamers seen by rigid contact lens wearers whose lenses have too small an optical zone or overall diameter or whose lenses center poorly.

fluorescein (FLU-re-seen): A fluorescent dye that is instilled in the eye to evaluate the fit of rigid lenses and to highlight corneal staining, abrasions, and other corneal pathology.

fluorescein pattern: The pattern formed by fluorescein-stained tears flowing under a rigid lens and observed with a Burton lamp or through the cobalt blue filter of a slit-lamp. This pattern shows the relationship between the lens and the cornea (where the lens makes contact and where there is clearance).

gas permeable lens: a lens made of materials that transmit oxygen to the cornea and carry carbon dioxide and heat back to the atmosphere.

ghost image: A shadow-like image alongside letters or objects caused by residual astigmatism, poorly-fitting lenses, or badly-coated lenses.

GPC (giant-cell papillary conjunctivitis): An inflammation of the lining of the eyelids, generally induced by the presence of denatured protein on the patient’s contact lenses rubbing against the lids. Symptoms of this auto-immune or allergic condition include itching, mucus secretion, a foreign body sensation, lens clouding, and lens slippage.

hyperopia (high-per-OH-pee-uh): A refractive state of the eye in which parallel rays of light would come to focus behind the retina (if they were not intercepted by it). This can result from a cornea that is too flat, an eyeball with too short an axial (front-to-back) length, a crystalline lens that is too thin, or a combination of these factors. Hyperopia is commonly referred to as farsightedness.

hypoxia (high-POX-ee-a): Insufficient oxygen reaching the cornea.

infiltrates (IN-fil-trates): Collections of white blood cells and lymphocytes in the cornea. Causes include viral infection, hypoxia, and solution sensitivity.

K-readings: The measurement of the flattest and steepest meridians of the patient’s cornea.

keratitis (ker-uh-TIGH-tiss): An inflammation of the cornea that can be caused by mechanical irritation, solution sensitivity, allergy, infection, or other disease process.

keratoconjunctivitis (KER-uh-toh-kon-junk-ti-VIGH-tiss): An inflammation involving both the cornea and the conjunctiva.

keratometer (ker-uh-TOHM-i-ter): An instrument used to measure the curvature of the two principle meridians of the central cornea.

keratoplasty (ker-uh-toh-PLASS-tee): Corneal transplant.

lacrimal gland (LACK-ri-mul): A gland that produces tears.

lysozyme (LIGH-so-zime). An antibacterial enzyme in tears that forms protein deposits on contact lenses.

Mixed Solution Syndrome. A toxic red eye caused by mixing contact lens solutions with incompatible preservatives or active ingredients and wearing a lens that has been soaked in the resulting solution.

myopia (migh-OH-pee-uh). A refractive state of the eye in which parallel rays of light come to focus in front of the retina. This may result from a cornea that is too steep, a crystalline lens that is too thick, an eyeball whose axial (front-to-back) length is too great, or a combination of these factors. Commonly referred to as nearsightedness.

neovascularization (nee-oh-VAS-cue-lar-i-ZAY-shun). The ingrowth of abnormal blood vessels in the normally avascular cornea, often due to hypoxia in contact lens wearers.

optical zone (OZ). The central area of a contact lens within which the power is ground or molded.

overrefraction. A refraction performed over a patient’s contact lenses to determine what power is needed to provide optimum visual acuity.

Overwearing Syndrome. An acute corneal reaction to hypoxia, often seen in PMMA contact lens wearers, with clinical findings that include central epithelial erosions, pronounced injection (redness), lid edema, and constricted pupils.

pachometer (pak-AH-mi-ter). An instrument used to measure corneal thickness.

pancreatin (pan-kree-AT-in). A substance containing enzymes obtained from the pancreas of the hog. These enzymes are used to digest or decompose tear proteins, muco-proteins, and lipo-proteins that have accumulated on the surface and within the matrix of a contact lens.

papain (PAP-ayn). An enzyme obtained from the papaya tree and used to decompose protein deposits on contact lenses.

photokeratoscope (foh-toh-KER-i-tuh-scope). An instrument for the measurement of corneal curvature that photographs a concentric(bull’s-eye) target reflected off the patient’s cornea. The data obtained from measuring the distance between the concentric circles is fed into a computer, which provides readouts of the central and peripheral corneal curvature in several meridians.

photophobia (foh-toh-FOH-bee-uh). Sensitivity to light.

polymer (PAH-luh-mer). A material formed by the joining of many small molecules (monomers). The polymer may be formed from units of the same monomer or different monomers.

polymethylmethacrylate (pah-lee-METH-il-meth-ACK-ruh-late). (Commonly referred to as PMMA). A material similar to Lucite® or Plexiglas® used for the manufacture of non-gas permeable rigid contact lenses.

presbyopia (prez-bee-OH-pee-uh). From the Greek term for “old sight.” A hardening of the normally flexible crystalline lens of the eye that results in the gradual loss of accommodation (the ability to focus at near) as we get older.

pseudomonas aeruginosa (soo-doh-MOH-nos uh-roo-jin-OH-suh). A bacterium that causes sight-threatening corneal ulcers, found more commonly in extended wear patients than in patients who remove and disinfect their contact lenses on a daily wear basis or who do not wear contact lenses.

punctate staining. An abnormal stippling of the corneal epithelium that stains when fluorescein dye is instilled in the eye. It may be caused by desiccation, solution sensitivity, deposits on the contact lens, hypoxia, mechanical irritation, or infection.

refraction. A change in the direction of rays of light when they pass obliquely from one transparent medium to another of a different density.

residual astigmatism (ree-ZID-you-uhl uh-STIG-muh-tizm). The amount of astigmatism that remains after a patient has been fit with contact lenses.

sagittal depth (SA-juh-tuhl). A measurement of the height of a lens, in millimeters, from a flat surface to the highest point of its curvature.

serratia marcescens (suh-RAY-shuh mar-SESS-ins). A bacterial organism associated with infection in contact lens wearers. The organism has been found on lenses, in lens cases, and in lens care solutions that have been mishandled by patients.

silicone acrylate (SIL-uh-kohn AC-ruh-late). Silicone and oxygen are made into a siloxane which is then copolymerized with PMMA to create silicone/acrylate, an oxygen permeable material used in the manufacture of rigid lenses.

spectacle blur. Vision that is temporarily distorted or out-of-focus when a contact lens wearer, particularly a rigid contact lens wearer, switches from contact lenses to glasses. The blur may be caused by corneal edema, mechanical molding, or a combination of the two.

sphere (sfeer). A curved surface that has the same radius of curvature in all meridians.

staining. Retention of fluorescein dye by damaged corneal tissue.

superpermeable lens. A rigid contact lens that transmits very high amounts of oxygen.

surfactant cleaner (ser-FACK-tent). A detergent-like cleaner that emulsifies debris and removes foreign substances from the surfaces of a contact lens.

tear exchange. The pumping in of fresh, oxygenated tears between the contact lens and the cornea and the pumping out of deoxygenated tears and metabolic wastes with each blink. This is necessary to maintain normal corneal metabolism.

thimerosal (thigh-MER-uh-sall). A mercury compound, used as a preservative in some contact lens solutions, that has been implicated as the cause of many red eyes and other adverse reactions in contact lens wearers.

toric lens. A cylindrical contact lens used for the correction of astigmatism.

visual acuity. Sharpness of vision. Normal visual acuity is expressed by the fraction 20/20. The numerator indicates the patient’s distance from the Snellen eye chart. The denominator indicates the size of the letters he is able to read. A visual acuity of 20/200 means that from a distance of 20 feet, the patient can read letters of a size that someone with normal vision can read from 200 feet away. Visual acuity of 20/15 means that from a distance of 20 feet, the patient can read letters that someone with normal vision would have to ome within 15 feet of the chart to read. He is considered, in this instance, to have better-than-normal vision.

wetting agent. A substance that enables tears to spread evenly over the surface of a rigid contact lens and helps to cushion the lens on insertion.

wetting/soaking solution. A combination solution for rigid contact lenses that contains both wetting agents and disinfecting chemicals.

Here is a list of medical terms concerning eye shapes and contact fitting that can help you understand the eye a little better.