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Glossary of Eye Care Terms - E-H

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ectropion An abnormal turning out of an eyelid, typically the lower one, which exposes the inner, conjunctival side of the eyelid; usually due to aging. Additional symptoms include eye or lid pain or discomfort, a red or pink eye or eyelid and overflow tearing.

edema Accumulation of an excessive amount of watery fluid, which causes swelling.


emmetropia The refractive condition where a person can see distant objects clearly with the eyes "at rest" (no focusing effort required) and needs to exert only the normal amount of focusing effort (determined by the distance of an object from the eye) to see near objects clearly. An emmetropic eye has no nearsightedness, farsightedness or astigmatism.

endophthalmitis Inflammation of the interior of the eye, typically caused by an infection from eye surgery or trauma. Endophthalmitis is an ocular emergency. Symptoms include floaters, light sensitivity, eye pain or discomfort, a red or pink eye and vision loss.

endothelium The cornea's inner layer of cells.

enophthalmos The sinking of the eye into the socket. Causes include development problems in utero, trauma and inflammation.

endpiece At left and right of the front of an eyeglass frame, the part that attaches to the temples, usually but not always with a screw mounting.

entropion An abnormal turning in of an eyelid, which causes the lashes to rub on the ocular surface; usually due to aging. Additional symptoms include eye or lid pain or discomfort, foreign body sensation, a red or pink eye, itching, tearing and vision loss.

environmental condition Air pollution, wind and bright light can irritate your eyes and cause symptoms such as burning, dryness and tearing.

enzymatic cleaner A cleaner that removes protein deposits and other debris from contact lenses. It's recommended for use either daily, weekly, or monthly. Some enzymatic cleaners are a small tablet dropped into a solution along with the lens; others come in liquid form.

epiretinal membrane Thin layer of scar tissue on the retina; also called a macular pucker. Epiretinal membranes have a variety of causes, including vitreous detachment, but the cause is often unknown. In its early stages, an epiretinal membrane is often asymptomatic, but some people have blurred vision. You may also develop metamorphopsia.

episclera Outer layer of the eye's sclera that loosely connects it to the conjunctiva.

episcleritis Inflammation of the episclera. The cause is usually unknown, but episcleritis may be associated with some systemic (e.g., autoimmune) diseases. Symptoms include a red or pink eye, eye pain or discomfort, light sensitivity and tearing.

epithelial ingrowth LASIK complication in which epithelial cells grow under the LASIK flap. Often it is temporary and doesn't usually affect vision. But sometimes it does affect vision and requires the flap to be lifted and the cells removed.

epithelium The cornea's outer layer of cells.

esotropia When one or both eyes point inward, so the eyes are "crossed." This is one type of strabismus.

excimer laser An instrument that uses shorter wave (ultraviolet) light to vaporize and remove tissue from the eye's surface during vision correction procedures such as LASIK and PRK.

exotropia When one or both eyes point outward; also called "walleyed." This is one type of strabismus.

extended wear Contact lenses that can be worn during sleep. Some brands are FDA-approved to be worn without removal for up to seven days; Night & Day and PureVision are approved for up to 30 days of continuous wear. Read more about extended wear contact lenses.

eye care practitioner Optometrists (ODs) and ophthalmologists (MDs) both practice eye care, but in different, though often overlapping, areas: In the United States, ODs (Doctors of Optometry) examine eyes for both vision and health problems, prescribe eyeglasses, prescribe and fit contact lenses, and treat some eye conditions and diseases. ODs attend four years of optometry school after attaining their BS or BA college degree. MDs are medical doctors who specialize in the eyes. They examine eyes, treat disease, perform surgery, and prescribe glasses and contacts. Like other physicians, they complete a BS or BA degree, attend four years of medical school, and complete a residency program in their practice specialty. Both ODs and MDs often pursue further subspecialty fellowship training, and they take additional continuing education courses during their careers in order to stay up to date and to maintain state and national board certifications. Other non-doctor eye care practitioners include paraoptometrics, contact lens technicians, and opticians, whose training and continuing education requirements can differ depending on the state in which they practice.

eye herpes See the definition of ocular herpes, or read our eye herpes article.

eye tumor A growth or mass that occurs in or next to the eye. Specific tumors, both benign and malignant, include the dermoid cyst, capillary hemangioma, cavernous hemangioma, choroidal melanoma, retinoblastoma, rhabdomyosarcoma and lymphoma. The cause is dependent on the type of tumor you have. Symptoms can include blurred vision; a bulging eye; double vision; floaters; foreign body sensation; pain or discomfort in the eye, the lid or around the eye; swelling of the lid or around the eye; a red or pink eye; ptosis; vision loss; limited eye or lid movement; a white or cloudy spot on the eye; and an iris defect.

farsightedness See definition for hyperopia.

FDA (Food & Drug Administration) A U.S. government body that oversees medical devices and medications, including contact lenses, intraocular lenses, excimer lasers and eye drops. In the United States, these products must be approved by the FDA before they can be marketed.

femtosecond laser Device that creates bursts of laser energy at an extremely fast rate measured in terms of a unit known as a femtosecond (one quadrillionth of a second). These ultra fast energy pulses precisely target and break apart tissue or other substances at a molecular level, without damaging adjacent areas.

fibromyalgia Condition that produces long-term pain all over the body and/or at tender points in soft tissues such as muscles, tendons and joints. The cause is unknown, but proposed triggers include trauma, sleep problems, an infectious microbe, depression, chronic back pain and hypothyroidism.

fixation In terms of vision, the eye's ability to maintain gaze upon an object.


flap and zap Slang for LASIK. Learn more about the LASIK procedure.

floaters A dark or gray spot or speck that passes across your field of vision and moves as you move your eye. Floaters are very common and may look like clouds, strands, webs, spots, squiggles, wavy lines or other shapes. As your eye ages, the gelatinous vitreous humor begins to liquefy in the center of the gel. Floaters are caused by the undissolved vitreous humor that floats in the liquid vitreous. Sometimes, a "shower of floaters" is a sign of a serious condition, particularly if you also see flashes of light. Read our article on spots and floaters in the eye.

fluorescein Compound that becomes a bright, fluorescent yellow-green when in contact with alkaline substances. A fluorescein dye solution can help eye doctors see corneal lesions or conduct tests for eye dryness.

fluorescein angiography An imaging test that involves first injecting fluorescent yellow-green dye into the veins. When the dye reaches interior regions of the eye, it provides opportunity for high contrast photography or other imaging of blood vessels. Fluorescein angiography particularly is useful in diagnosing conditions such as age-related macular degeneration, which in advanced forms can be characterized by abnormal growth of blood vessels in the retina.

forceps Surgical tool shaped like tongs, used for gripping.

foreign body Something in or on the eye that doesn't belong there. Symptoms include foreign body sensation, eye pain or discomfort, a red or pink eye, tearing, frequent blinking, blurred vision, discharge, light sensitivity and vision loss.

foreign body sensation Sensation that something is in your eye.

fovea A depression in the retina that contains only cones (not rods), and that provides acute eyesight.

free radicals Atoms or molecules with unpaired electrons that are highly chemically reactive. Free radicals are capable of causing tissue damage and accelerating the effects of aging through a process called oxidation.

frequent replacement contact lenses Also called planned replacement. Technically, this is any contact lens that is thrown away after a moderately short period of time. Among most eye care practitioners, "disposable" usage ranges from one day to two weeks, while "frequent replacement" lenses are discarded monthly or quarterly.

fungal keratitis eye infection The source of a 2006 outbreak of fungal eye infections among contact lens wearers is a fungus known as Fusarium, found in places such as soil, water, and organic matter including plants. Ordinarily, it is rare for this fungus to invade and damage the eye. But symptoms can be severe, and if untreated, the infection may become so eye-damaging that a corneal transplant is required. For more information, please see our fungal keratitis eye infection article.

geographic atrophy Deterioration of tissue in the central portion of the retina, often associated with aging. Geographic atrophy (GA) is considered the end stage of a "dry" form of age-related macular degeneration, which occurs when tissue in the retina begins to break down and form yellowish spots known as drusen. GA can cause significant central vision loss.

glaucoma Disease characterized by elevated intraocular pressure, which causes optic nerve damage and subsequent peripheral vision loss. Most people have no initial symptoms of chronic (open-angle) glaucoma, but you can develop peripheral vision loss, headaches, blurred vision, difficulty adapting to darkness and halos around lights. Other forms of glaucoma (e.g., closed-angle glaucoma) may have additional symptoms such as eye pain, a pupil that doesn't respond to light, redness, nausea and a bulging eye. Learn more about glaucoma.

glycemic index A method of ranking foods in terms of how quickly they affect blood sugar levels. For example, foods with high glycemic index rankings (processed foods such as white flour, sugar, etc.) can cause unhealthy spikes in blood sugar. Foods with lower glycemic index rankings, such as whole grains, create more stable blood sugar levels.

Graves' ophthalmology Autoimmune eye disorder usually associated with abnormalities of the thyroid gland; symptoms include eyelid retraction, bulging eyes, light sensitivity, eye discomfort, double vision, vision loss, a red or pink eye and a limited ability to move the eyes.

hard contact lenses Rarely worn now, these are the small, hard lenses made of PMMA material that many people wore in the '70s and '80s. Compared with modern soft lenses and rigid lenses, they are less healthy to wear long-term, since the material doesn't allow oxygen to reach the surface of the eye.

hemifacial spasm Involuntary muscles twitches on one side of the face, typically caused by compression of the seventh (facial) cranial nerve by a neighboring blood vessel somewhere in the brain.

hemorrhage Profuse bleeding.

herpes of the eye See the definition of ocular herpes, or read our eye herpes article.

heterochromia Condition where one eye is a different color from the other, or one eye is more than one color. Read more about two different colored eyes.

higher-order aberration Irregularity of the eye other than a refractive error (myopia, hyperopia or astigmatism). Higher-order aberrations sometimes affect your vision (such as decreasing contrast sensitivity), and sometimes do not. For more information, please see our article on higher-order aberrations.

high-index Type of lens with a higher index of refraction, meaning that light travels faster through the lens to reach the eye than with traditional glass or plastic. It is denser, so the same amount of visual correction occurs with less material (whether glass or plastic) — so the lens can be thinner. Read more about high-index lenses.

histamine A protein that can be released as part of the body's immune system responses during an allergic reaction. Presence of histamine can lead to inflammation and swelling, which is why antihistamines often are prescribed for allergy symptoms.

histiocytosis Abnormal proliferation of histiocytes (immune system cells). Common symptoms include bone tumors and skin rashes. If histiocytosis affects the eyes, it causes bulging.

homocysteine An amino acid containing sulfur. A high homocysteine level in the blood is a possible risk factor for heart disease. One major study indicates that lowering homocysteine levels through vitamin B supplementation might help prevent diseases associated with impaired function of small blood vessels, such as macular degeneration. More studies are needed to verify such a link.

Horner's syndrome Condition characterized by a small pupil, ptosis and an abnormal lack of facial perspiration (all on the same side of the face); Horner's syndrome is caused by injury to the sympathetic nerves of the face.

hybrid contact lenses Contact lenses that have a central optical zone made of rigid gas permeable (GP) plastic, surrounded by an outer "skirt" of silicone hydrogel or regular hydrogel material. Hybrid contacts are designed to provide the crisp optics of GP contact lenses and wearing comfort comparable to that of soft lenses.

hydrogel A hydrophilic ("water-loving") plastic material that is rigid when dry but becomes soft and pliable when moist. Hydrogels used for soft contact lenses range from approximately 38 to 75 percent water by weight when fully hydrated.

hyperopia A refractive error where a person must exert a greater-than-normal focusing effort to see distant and near objects clearly. Depending on the farsighted person's age and degree of hyperopia, objects may be clear or blurred without glasses. Other symptoms of hyperopia include headaches, eye strain, and avoidance of reading and other near tasks. Also called farsightedness. Learn more about hyperopia.

hypoglycemia Abnormally low level of sugar (glucose) in the blood, which causes dizziness, hunger, shakiness and other symptoms. Skipping or delaying meals, too-small meals, or a high level of physical activity can cause hypoglycemia. Too much alcohol or certain drugs can also cause it. In diabetics, it can be caused by too much insulin.

hypotony Low intraocular pressure, often caused by eye surgery or trauma (e.g., open globe injury). Symptoms include blurred vision and eye pain or discomfort.

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Page updated June 2017