The formation of new blood vessels in the body. Also see
angle In glaucoma
terminology, "angle" refers to the drainage channel for
the aqueous humor in the eye; improper drainage can lead
to the high intraocular pressure associated with
glaucoma. In narrow-angle glaucoma, the channel is
blocked, whereas open-angle glaucoma has other causes,
such as the body producing too much aqueous humor.
aniridia Absent or partially absent iris,
typically congenital. Additional symptoms include poor
vision and photophobia.
anisocoria Unequal pupil size. Causes include
glaucoma, head or eye trauma, an intracranial tumor,
infection of the membranes surrounding the brain and
previous intraocular surgery. A small percent of the
population has unequal-sized pupils naturally (without
any known cause).
anisometropia Condition where the eyes have a
significantly different refractive power from each
other, so the prescription required for good vision will
be different for each eye.
anophthalmos Absence of one or both eyes.
Anophthalmos may be congenital or due to trauma,
infection or other causes. Symptoms include reduced
depth perception and peripheral vision.
ANSI The American National Standards Institute
is a private, non-profit organization that coordinates
efforts to develop standards for manufacturing many
different products, including eyeglass lenses. For
example, certain ANSI standards define acceptable levels
of impact resistance for safety eyewear.
ANSI Z87.1-2003 Standard The American National
Standards Institute's Practice for Occupational and
Educational Eye and Face Protection; eyewear that meets
this standard is considered safer than eyewear that does
anterior chamber Part of the eye behind the
cornea and in front of the iris and lens.
antioxidant Substance that inhibits oxidation
and can guard the body from the damaging effects of free
radicals. Molecules with one or more unpaired electrons,
free radicals can destroy cells and play a role in many
diseases. Antioxidant vitamins include B, C, and beta
carotene. It has been theorized that antioxidants can
help prevent macular degeneration and other serious eye
diseases; many studies are being conducted in this area.
anti-reflective coating (AR coating) Thin
layer(s) applied to a lens to reduce the amount of
reflected light and glare that reaches the eye.
apheresis A process in which blood is drawn
outside the body, certain compounds are removed, and the
blood is returned to the body. The technique has various
applications, including: harvesting of needed components
such as plasma or white blood cells; and removing
harmful components such as large proteins, in order to
treat the dry form of age-related macular degeneration
aqueous humor Clear fluid in the front of the
eye, between the cornea and the iris, that provides
nutrients to the cornea and the lens. The fluid is
produced by the ciliary body. Glaucoma causes a
difficulty in draining this fluid, and pressure builds
up. The result is damage to the optic nerve and loss of
arcus An opaque arc or ring around the
peripheral cornea, this represents fatty or oily
deposits in the cornea. It is usually seen in elderly
people and is called arcus senilis. Arcus juvenilis is
seen in people younger than 40 and often indicates high
levels of cholesterol in the blood.
Argyll Robertson pupil Small, irregular pupil,
usually caused by syphilis. Argyll Robertson pupils do
not respond to light.
aspheric Not quite spherical. Aspheric
eyeglass lenses are popular among people who have strong
prescriptions because they are thin and lightweight, and
reduce distortion and eye magnification. Aspheric
contact lenses can work as a multifocal, or to correct a
single-vision problem like astigmatism.
asteroid hyalosis Benign condition in which
flecks of calcium-rich fats become suspended in the
eye's vitreous. The exact cause is unknown, but it may
be due to aging collagen in the vitreous or
decomposition of hyaluronic acid in the vitreous.
Asteroid hyalosis typically is without symptoms, but
some people experience blurred vision or floaters.
astigmatism Condition in which the cornea's
curvature is asymmetrical (the eye is shaped like a
football or egg instead of a baseball); light rays are
focused at two points on the retina rather than one,
resulting in blurred vision. Additional symptoms include
distorted vision, eyestrain, shadows on letters,
squinting and double vision.
atopy Type of allergy where levels of the
antibody immunoglobin E are increased; atopy includes
rhinitis, asthma, hay fever and eczema.
aviator glasses These frames have a large
upside-down teardrop shape and usually have a double-bar
bridge. Originally, these were metal-framed sunglasses
worn mainly by pilots, but now the shape also comes in
plastic and is used for eyeglasses as well.
band keratopathy Opacity of the corneal stroma
and Bowman's membrane. Symptoms include vision loss and
foreign body sensation.
best corrected visual acuity (BCVA) The best
vision you can achieve with correction (such as
glasses), as measured on the standard Snellen eye chart.
For example, if your uncorrected eyesight is 20/200, but
you can see 20/20 with glasses, your BCVA is 20/20.
beta blocker Drug that widens or dilates blood
vessels, thus enabling more normal flow of blood.
Topical beta blockers applied as eye drops also can
lessen fluid production and lower internal eye pressure
(intraocular pressure) in eye diseases such as glaucoma
to reduce the possibility of optic nerve damage. Beta
blockers also are used to control high blood pressure
(hypertension). Side effects can include respiratory
beta titanium Titanium alloy. Beta-titanium
eyeglass frames are both strong and flexible.
bifocal Lens with one segment for near vision
and one segment for far vision. The term can apply to
both eyeglass lenses and contact lenses.
blepharitis Inflammation of the eyelid(s),
typically around the eyelashes. Various types of
dermatitis, rosacea and allergic reactions can cause
blepharitis. Symptoms include a red or pink eyelid,
crusty lid or lashes, burning, foreign body sensation,
eye or eyelid pain or discomfort, dry eyelid, dry eye,
eyelash loss, grittiness, stickiness, eyelid swelling
blepharochalasis Excessive, drooping eyelid
skin caused by recurring swelling. Blepharochalasis
typically occurs in young people.
blepharoconjunctivitis Inflammation of the
eyelid and conjunctiva. Infections and allergic
reactions are among the causes. Symptoms include a red
or pink eye, a red or pink eyelid, pain or discomfort of
the eye or around the eye, tearing, burning, eye dryness
and eye stickiness.
blepharospasm Involuntary increased blinking
that progresses to spasms in both eyes. The exact cause
is unknown, but doctors believe it to be a central
nervous system disorder. It can produce a functional
blindness since the patient can't open his or her eyes
long enough to function visually.
botulism Serious illness from a toxin produced
by Clostridium bacteria (usually Clostridium botulinum).
Infant botulism and food-borne botulism are the most
common forms in the United States. Symptoms include
double vision, blurred vision, ptosis, muscle weakness,
difficulty speaking, difficulty swallowing, difficulty
breathing and nausea.
Bowman's membrane Corneal layer between the
epithelium and the stroma.
bridge The part of eyeglasses that extends
across the nose.
cable temple Style of eyeglasses that wraps
around the ear, to keep them well-fastened.
canaliculitis Inflammation of a tear duct (or
ducts), caused by a bacterial infection. Symptoms of
this disorder include discharge, a red or pink eye and a
swollen upper or lower eyelid near the nose.
carotenoid A pigmented substance that adds
color such as red, orange, or yellow to plants.
Carotenoids have antioxidant properties that protect
cells against damage from free radicals, unstable atoms
that can interact with and break down healthy tissue in
different parts of the body including eyes.
cataract Clouding of the natural lens of the
eye, usually caused by aging in conjunction with other
risk factors, such as exposure to the sun's UV rays,
smoking, steroid intake and diabetes. Symptoms include
blurred vision, glare, halos around lights, colors that
are less bright, a cloudy spot in your vision and,
sometimes, temporary vision improvement. Read more about
cataracts and cataract surgery.
cavernous sinus problem The cavernous sinus is
located at the base of the cranium and contains the
carotid artery and cranial nerves. Problems in the
cavernous sinus include tumors, aneurysms and clots.
Typical symptoms include ophthalmoplegia, chemosis and a
bulging eye. You may also experience a red eye and
cellulitis Inflammation of tissue around the
eye. Pre-septal cellulitis affects the lid and other
"outer" areas of the eye, whereas orbital cellulitis
affects the "inner" areas around the eyeball. Pre-septal
cellulitis symptoms include a red, swollen lid, swelling
around the eyes and eye or lid pain or discomfort.
Orbital cellulitis symptoms include a bulging eye and
ophthalmoplegia, as well as a red, swollen lid, swelling
around the eyes, eye or lid pain or discomfort and a
decrease in vision. An orbital cellulitis is an ocular
central island Refractive surgery complication
in which the laser leaves an "island" of corneal tissue
in the concave ablation zone. Symptoms include double
vision and distortion. Read more about potential
complications of LASIK and other kinds of refractive
central serous retinopathy Disorder in which
fluid collects under the central retina (macular area)
and disrupts central vision. The cause is unknown.
Symptoms include blurred central vision and
metamorphopsia. Some patients also develop floaters.
cerebral cortex Outer portion of the brain
where complex functions including certain vision
processes take place.
chalazion A small bump on the eyelid caused by
an obstructed meibomian gland. Additional symptoms
include light sensitivity, tearing and eyelid swelling.
Chalazia are usually not painful unless they become
chemosis Conjunctival swelling that is often
caused by an allergy.
choroid Layers of blood vessels located
between the sclera (white of the eye) and the retina;
they provide nourishment to the back area of the eye.
choroidal neovascularization Abnormal growth
of new blood vessels in the choroid. Choroidal
neovascularization is commonly associated with macular
degeneration, but it can occur as a result of other eye
conditions as well. Symptoms include vision loss and
ciliary body Part of the eye between the iris
and the choroid; the three form the uvea. The ciliary
body's main functions are accommodation, aqueous humor
production and holding the lens in place.
CK (Conductive Keratoplasty) Procedure in
which a surgeon uses radio waves to heat collagen in the
cornea's periphery to shrink it and reduce hyperopia
(farsightedness). CK is also used to treat presbyopia.
clip-on Type of glasses that attaches to your
regular glasses, such as clip-on sunglasses.
CMV retinitis (cytomegalovirus retinitis)
Serious eye infection usually found in those with immune
problems, such as AIDS patients; symptoms include
floaters, blind spots, blurry vision and vision loss.
cohort A term used in clinical studies to
define a set of people who have something in common such
as similar backgrounds, experiences, and/or health
collagen Fibrous protein in bones and
connective tissue, it is also present in the eye. One
type of vision correction surgery heats collagen around
the edges of the cornea (which lets light into the eye).
This procedure reshapes the cornea, helping it focus
light right onto the retina, for clearer vision.
coloboma Cleft, usually due to incomplete
embryologic development in utero. An iris coloboma is
the most common eye coloboma; the pupil will often look
like a keyhole or upside-down pear. Colobomas can also
affect other eye structures, such as the eyelid, retina
and optic nerve; only iris and eyelid colobomas are
visible with the naked eye. Additional symptoms such as
poor vision may occur, but are not readily apparent from
a parent's perspective.
color blindness Partial or total inability to
distinguish specific colors. Color blindness is
inherited, and is much more common in men than in women.
computer vision syndrome Collection of
problems, mostly eye- and vision-related, associated
with computer use. Symptoms include eyestrain, dry eyes,
blurred vision, red or pink eyes, burning, light
sensitivity, headaches and pain in the shoulders, neck
cone A photosensitive receptor in the retina
that helps you to see color.
conjunctiva Mucous membrane that lines the
visible part of the eye and the inner surface of the
conjunctivitis Inflammation of the
conjunctiva, characterized by a pink eye. The cause is
either infectious or allergic; the term "pink eye"
really refers to the viral variety, but is commonly used
for any type of conjunctivitis. Other symptoms include
burning, discharge, dryness, itching, light sensitivity,
eye pain or discomfort, stickiness, tearing and
contact lens drops Eyedrops for contact lens
wearers; regular eyedrops can discolor contact lenses.
contact lens problem Contact lens problems can
range from minor to sight-threatening, and include
protein build-up, debris on the lens, a ripped or nicked
lens, infections and more. Symptoms can include frequent
blinking, blurred vision, burning, discharge, foreign
body sensation, itching, light sensitivity, eye pain or
discomfort, a red or pink eye or lid and eyelid
contrast sensitivity The ability of the eye to
detect the line of demarcation between an object and its
background or an adjacent object.
convergence Eyes' ability to turn inward.
People with convergence insufficiency have trouble
(eyestrain, blurred vision, etc.) with near tasks such
cornea The clear part of the eye covering the
iris and pupil; it lets light into the eye, permitting
corneal abrasion A loss of the epithelial
layer of the cornea, typically due to minor trauma
(contact lens trauma, a sports injury, dirt or another
foreign body, etc.). Symptoms include blurred vision,
foreign body sensation, grittiness, light sensitivity,
eye pain or discomfort, a red or pink eye and tearing.
corneal dystrophy One of a group of
conditions, usually hereditary, in which the cornea
loses its transparency. The corneal surface is no longer
smooth. Common forms include map-dot-fingerprint
dystrophy, Fuch's dystrophy and lattice dystrophy.
Symptoms include blurred vision, foreign body sensation,
light sensitivity, eye pain or discomfort and vision
corneal edema Swelling of the eye's cornea;
causes include intraocular surgery, corneal dystrophies,
high intraocular pressure and contact lens
complications. Symptoms include vision loss, halos
around lights, a white or cloudy spot on the eye,
photophobia, eye pain and foreign body sensation.
corneal erosion Recurrent breakdown of the
corneal epithelium, typically caused by a previous
corneal abrasion or by map-dot-fingerprint dystrophy.
Symptoms include blurred vision, foreign body sensation
and eye pain or discomfort.
corneal implants Devices (such as rings or
contacts) placed in the eye, usually to correct vision.
corneal opacity A cloudy spot in the cornea,
which is normally transparent. Causes include corneal
scar tissue and infection. Symptoms include halos around
lights, photophobia, vision loss and a white or cloudy
spot on the eye.
corneal ring Type of vision correction surgery
where a doctor inserts a tiny plastic ring into the
cornea (which lets light into the eye). This ring
reshapes the cornea, helping it to focus light better
onto the retina so you can see better. The ring can be
adjusted and even removed if desired.
corneal topography A corneal topographer
shines light onto the surface of the eye, then measures
the reflected light to create a map of the cornea's
curvature as well as any irregularities. The map is used
for evaluations related to refractive surgery, contact
lens fitting and corneal disease management. It is
especially useful for measuring astigmatism. The color
map uses blue and green to represent flatter areas of
the cornea, while red and orange represent steeper
corneal ulcer An infected corneal abrasion.
Frequently found in extended wear contact lens wearers.
A corneal ulcer is an ocular emergency. Symptoms include
light sensitivity, eye pain or discomfort, a red or pink
eye, a white or cloudy spot on the eye and tearing.
cranial nerve One of the 12 pairs of nerves
that go from the brain to other parts of the head. Those
that affect the eyes and vision are the second cranial
nerve (optic nerve), third (oculomotor), fourth
(troclear), sixth (abducens) and seventh (facial). The
optic nerve carries stimuli from the rods and cones to
the brain. The third, fourth and sixth cranial nerves
work with the eye muscles to control eye movement. The
seventh cranial nerve works with the facial muscles to
control facial movement (specifically the closure of the
cranial nerve palsy Palsy (full or partial
paralysis) of the third, fourth or sixth cranial nerves
can result in difficulty moving the eye with such
symptoms as eyes that don't point in the same direction,
reduced depth perception, double vision, ptosis, vision
loss, a dilated pupil that doesn't respond to light and
head tilting. Causes include head trauma, diabetes,
tumors, aneurysms, infarction (tissue death) and more.
In most cases, the cause of paralysis of the seventh
cranial nerve is unknown (termed "Bell's palsy").
Symptoms include weak facial muscles, difficulty closing
the eye, infrequent blinking, earache, acute hearing,
facial drooping, ectropion, tearing, eye dryness,
blurred vision and a burning feeling in the eye.
crossed eyes Type of strabismus (a
misalignment of the eyes) where one or both eyes point
inward, toward the nose.
cystoid macular edema (CME) Swelling of the
eye's macula, caused by an excessive amount of fluid.
dacryoadenitis Inflammation of the tear gland,
typically caused by a viral or bacterial infection.
Symptoms include a dry eye, a red or pink eyelid,
swelling of the lid or around the eyes and ptosis.
dacryocystitis Inflammation of the
nasolacrimal (tear) sac, typically caused by
dacryostenosis. Symptoms include discharge, a sticky
eye, eye pain or discomfort, a red or pink eye, swelling
around the eye and tearing.
dacryostenosis Blocked tear duct, which is
characterized by a lot of tearing; you may also have a
discharge or a sticky eye.
daily wear These soft contact lenses are worn
every day for six months up to a couple of years. They
require daily cleaning and disinfecting, as well as a
periodic enzymatic soak (usually once a week).
decentered ablation Also called decentration.
Refractive surgery complication in which the laser is
not centered on the pupil when it removes tissue.
Symptoms include glare, double vision and halos.
dermatochalasis Excessive, drooping eyelid
skin caused by a loss of elasticity in aging skin.
Descemet's membrane Corneal layer between the
stroma and the endothelium.
diabetic retinopathy Leaking of retinal blood
vessels in advanced or long-term diabetes, affecting the
macula or retina. Most people have no symptoms at first,
but can develop blurred near vision, double vision,
floaters, retinal/vitreous hemorrhages and
metamorphopsia. In later stages, you can also suffer
diopter Unit which measures the refractive
(light-bending) power of a lens; eyecare practitioners
use it in eyeglass and contact lens prescriptions. A
negative number refers to nearsightedness, while a
positive number refers to farsightedness. For example,
someone with -8.00 diopter lenses is very nearsighted,
while someone with +0.75 diopter lenses is only slightly
diplopia Also called double vision. When two
images of the same object are perceived by one or both
eyes. Read our article on double vision and also review
eye symptoms for a list of conditions that can cause
disposable contact lenses Technically, this is
any contact lens that is thrown away after a short
period of time. Among most eyecare practitioners,
"disposable" usage ranges from one day to two weeks,
while "frequent replacement" lenses are discarded
monthly or quarterly.
Dk/t Dk is the oxygen permeability of a
contact lens material; t is the thickness of the contact
lens design. Dk/t is a measurement of a contact lens's
double vision Also called diplopia. When two
images of the same object are perceived by one or both
eyes. Read our article on double vision and also review
eye symptoms for a list of conditions that can cause
drooping eyelids Also called ptosis. Condition
in which the upper eyelid(s) only sag. It can be present
at birth or caused by a later problem with the muscles
lifting the eyelid, called levators.
drugs Many drugs, both legal and illegal, can
affect your eyes and vision. These include eyedrops,
other topical eye medications, pills and more. Symptoms
can include blurred vision, burning, dry eyes, eyelash
loss, floaters, halos around lights, light sensitivity,
pupils that are dilated, small or unresponsive to light,
peripheral or general vision loss and jaundice.
druse Small yellow or white deposit in the
eye. Drusen are sometimes signs of macular degeneration.
dry eye Lack of sufficient lubrication and
moisture in the eye. Most dry eye complaints are
temporary and easily relieved; dry eye syndrome, also
called keratoconjunctivitis sicca, is chronic and needs
more advanced treatment by an eyecare practitioner. Read
our article on dry eye syndrome.
dry eye syndrome Chronic dryness due to
reduced quality or quantity of the eye's tear film, or
due to increased evaporation of the existing tear film.
Dry eye syndrome has many causes, including aging,
certain systemic diseases and long-term contact lens
wear. Additional symptoms include foreign body
sensation, eye pain or discomfort, burning, grittiness,
itching, light sensitivity, frequent blinking, a red or
pink eye and tearing. Read our article on dry eye
dry skin Skin that is generally dry may
include dry eyelids.
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
edema Accumulation of an excessive amount of
watery fluid, which causes swelling.
emmetropia The condition of an eye with normal
vision, meaning that light rays correctly are focused at
the inner back of the eye (retina) where images are
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
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
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;
epithelial ingrowth does not usually affect vision.
epithelium The cornea's outer layer of cells.
excimer laser An instrument that uses shorter
wave (ultraviolet) light to vaporize and remove tissue
from the eye's surface during vision correction
extended wear Currently, these contact lenses
are FDA-approved to be worn without removal for up to
seven days (or 30 days in the case of one brand),
meaning some people will be comfortable sleeping with
them in their eyes. Thirty-day contact lenses are
sometimes referred to as "continuous wear."
eyecare practitioner Optometrists (O.D.s) and
ophthalmologists (M.D.s) both practice eyecare, but in
different, though often overlapping, areas: In the U.S.,
O.D.s (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. O.D.s attend four years of
optometry school after attaining their BS or BA college
degree. M.D.s 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 O.D.s and
M.D.s 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 eyecare practitioners include
paraoptometrics, contact lens technicians, and
opticians, whose training and continuing education
requirements can differ depending on the state in which
eye herpes See the definition of ocular
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 Also called hyperopia. To
farsighted people, near objects are blurry, but far
objects are in focus.
FDA (Food & Drug Administration) A United
States government body that oversees medical devices,
including contact lenses, intraocular lenses, excimer
lasers and eyedrops. In the U.S., 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
fixation In terms of vision, the eye's ability
to maintain gaze upon an object.
flap and zap Slang for LASIK.
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
spots and floaters article.
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
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.
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 eyecare 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.
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.
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
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 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.
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
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 higher-order aberration article.
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.
histiocytosis Abnormal proliferation of
histiocytes (immune system cells). Common symptoms
include bone tumors and skin rashes. If histiocytosis
affects the eyes, it causes bulging.
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.
hyperopia Also called farsightedness.
Condition in which the length of the eye is too short,
causing light rays to focus behind the retina rather
than on it, resulting in blurred near vision. Additional
symptoms include eyestrain and squinting.
hypotony Low intraocular pressure, often
caused by eye surgery or trauma (e.g., open globe
injury). Symptoms include blurred vision and eye pain or