Glossary of Eye Care Terms
idiopathic Having an unknown cause. A medical condition that appears suddenly with no apparent explanation is considered idiopathic.
index of refraction A measure of how much a substance reduces the speed of light waves passing through it. The index of refraction (or refractive index) of a substance equals the ratio of the speed of light in a vacuum to its speed in that substance. Lenses or materials with a high index of refraction slow down and refract (bend) light more than materials with a lower refractive index.
insulin A hormone that helps regulate blood sugar levels in the body. People with uncontrolled diabetes are at risk of developing a common and potentially blinding eye disease diabetic retinopathy, caused by high blood sugar.
intraocular lens (IOL) Artificial lens that a cataract surgeon places in a patient's eye after removing the eye's natural lens. Like a contact lens, it has a built-in refractive power tailored specifically to the patient's visual condition.
intraocular pressure (IOP) Eye pressure, as determined by the amount of aqueous humor filling it. High IOP (ocular hypertension) can be a sign of glaucoma.
ion An electrically charged atom.
iris A pigmented membrane that lies between the cornea and the lens; it acts as a diaphragm to widen or narrow the opening called the pupil, thereby controlling the amount of light that enters the eye.
iritis Inflammation of the iris.
ischemia Poor blood flow. Obstructions such as clots in veins and arteries can block blood flow, depriving tissue of oxygen and nutrients. These blockages also can cause "eye strokes" and sudden vision loss.
jaundice Yellow coloring in the skin and eyes caused by high levels of a pigment called bilirubin. Jaundice is associated with a variety of conditions involving the liver, gallbladder and bile ducts, including hepatitis and cirrhosis.
keratectomy Surgical removal of part of the cornea.
keratitis Inflammation of the cornea, caused by an infection or inflammatory process. Symptoms include eye pain or discomfort, light sensitivity, foreign body sensation, grittiness and tearing.
keratoconjunctivitis Inflammation of the cornea and conjunctiva.
keratoconjunctivitis sicca Also called dry eye syndrome. Chronic lack of sufficient lubrication and moisture in the eye.
keratoconus Degeneration and thinning of the cornea resulting in a cone-shaped bulge (a type of irregular astigmatism). The cause is unknown, but may be genetic. The first symptom is blurred vision that doesn't improve enough with glasses (contacts usually work well for a while). You may also have double vision or distorted vision. Read our keratoconus article
keratoplasty Any of several types of corneal surgery, such as shrinking the collagen to reduce farsightedness or transplanting a new cornea to treat keratoconus.
keratometer An instrument that measures the curvature of the eye's clear, front surface (cornea). Keratometers help eye doctors collect information for contact lens fittings and surgical procedures. With keratometry, reflected images also can help identify dry eyes.
keratotomy Incision of the cornea.
lacrimal plug Also called punctal plug. Device to block the lacrimal punctum (an opening at the end of a tear duct), to keep the eye moist.
Lambert-Eaton myasthenic syndrome LEMS is an autoimmune condition that occurs at the junctions of nerve and muscle cells. Disruption of the electrical impulses between these cells produces muscle weakness, a tingling sensation, dry mouth and fatigue.
LASEK (Laser Epithelial Keratomileusis) Procedure that is similar to LASIK, except that the surgeon cuts a flap in the epithelium only, instead of through the epithelium and part of the stroma. LASEK is used mostly for people with thin or flat corneas who are poor candidates for LASIK, which requires more corneal tissue for success.
laser photocoagulation Procedure in which a surgeon uses a laser to coagulate tissue, usually to seal leaking blood vessels and destroy new ones in diseases like macular degeneration and diabetic retinopathy.
LASIK (Laser-Assisted In Situ Keratomileusis) Surgical procedure in which a tiny flap is cut in the top of the cornea, underlying corneal tissue is removed with an excimer laser, and the flap is put back in place. LASIK corrects myopia, hyperopia, astigmatism, and presbyopia through monovision. Read our LASIK procedure article.
lateral rectus muscle Muscle that moves the eye away from the nose.
lead poisoning Condition resulting from excessive levels of lead in the body. Typically, children get lead poisoning from chewing or sucking on lead paint in older homes (lead paint was banned in the 70s). Some people remain symptom-free, but others may suffer convulsions, paralysis, learning difficulties or abdominal and other pain. Some people can also develop vision loss.
Leber's congenital amaurosis Inherited condition characterized by vision loss or blindness at birth or shortly thereafter. The exact cause is unknown, but doctors believe that the retina may degenerate, or that its photoreceptors may not develop properly. Nystagmus is a common symptom.
lens 1. The nearly spherical body in the eye, located behind the cornea, that focuses light rays onto the retina. 2. A device used to focus light into the eye in order to magnify or minify images, or otherwise correct visual problems. Eyeglass lenses, contact lenses, and intraocular lenses are examples. Sometimes a lens provides only a cosmetic benefit, as in non-correcting color contacts, theatrical contact lenses, or contacts that hide a disfigurement of the eye. Other times a lens protects the eye, as in safety glasses or sunglasses. A bandage contact lens may be applied after eye surgery. And contact lenses used in ortho-k or corneal refractive therapy are designed to reshape the cornea for better vision when the lens is removed.
lens dislocation Full or partial displacement of the eye's lens. Dislocation is often caused by trauma to the eye or head, but may also be inherited or come as the result of certain systemic conditions, such as Marfan's syndrome or homocystinuria. Blurred vision is a typical symptom, and some people may experience double vision and/or develop glaucoma.
leukocoria White pupil. Causes include congenital cataract, retinoblastoma, intraocular infection, Coat's disease and retinopathy of prematurity.
limbal relaxing incisions A surgical procedure, often performed during cataract surgery, which corrects usually mild astigmatism by flattening the curvature of the eye's clear surface (cornea). Limbal relaxing incisions are inserted at the boundary (limbus) separating the cornea from the white of the eye (sclera). By altering this portion of the eye instead of the center, surgeons are able to preserve the surface and optical qualities of the cornea.
limbus Boundary area connecting the cornea and sclera; the three form the eye's outermost layer.
lipid Organic compound that is oily, fatty, or waxy and commonly found in living cells. Lipids are one component of human tears, forming an oily outer layer that helps keep the eye moisturized by reducing evaporation of the watery and mucus layers beneath it. Lipids can collect on contact lenses, making them uncomfortable.
liver spot Benign dark spot on the skin, usually brown, that is associated with exposure to sunlight. Liver spots tend to increase in size.
low vision Also called partial sight. Sight that cannot be satisfactorily corrected with glasses, contacts, or surgery. Low vision usually results from an eye disease such as glaucoma or macular degeneration.
LTK (Laser Thermal Keratoplasty) Also called Laser Thermokeratoplasty. Surgery to correct mild farsightedness in people over 40; the doctor uses a holmium laser to heat the cornea and shrink its collagen.
lupus erythematosus Inflammatory skin disorder. The exact cause is unknown, but some people appear to have a genetic predisposition to developing lupus, and it is much more common in women than men. Systemic symptoms vary depending on the type of lupus, but red patches on the cheeks are common. When lupus affects the eyes, it can have such symptoms as a red or pink lid, a spot or scale on the lid that may change in pigment (generally losing pigment except for darker color marking the border of the spot), eyelash loss, dry eye syndrome, migraine headaches, uveitis, scleritis, conjunctivitis and retinal vascular occlusion (blockage in the retina's vascular system).
lutein An antioxidant that is found throughout the body, but is concentrated in the macula. Lutein is believed to help protect the eyes from free radical damage caused by the sun's harmful rays.
macula The most sensitive part of the central retina, responsible for visual acuity and color vision.
macular degeneration Disorder characterized by changes in the eye's macula that result in the gradual loss of central vision. The exact cause is unknown, but appears to be related to a genetic predisposition, smoking and several other risk factors. Central vision may be blurred, distorted (metamorphopsia) or shadowy before vision loss occurs.
macular edema Swelling of the central portion of the retina (macula), due to buildup of fluid leaking from retinal blood vessels. Causes temporary or permanent vision loss if untreated.
macular hole Hole in the eye's macula; many doctors believe it can be caused by vitreous shrinkage as we age. Symptoms include blurring or a blind spot in central vision and metamorphopsia.
maculopathy Any disease of the macula, the most sensitive portion of the central retina responsible for detailed vision and color perception. One example is age-related macular degeneration.
madarosis Eyelash or eyebrow loss. Causes include infections, metabolic disorders, blepharitis, certain drugs, lupus erythematosus and trauma.
medial rectus muscle Muscle that moves the eye toward the nose.
median When applied to numbers, a value that falls exactly in the middle of a specified range. Half of the numbers are above the median, and half of the numbers are below the median. As an example, "five" is the median of a range of numbers from one to nine.
meibomian gland Gland found in the eyelid that produces the oily outer layer of the three-layer tear film that lubricates the eye.
meibomianitis Inflammation of the meibomian glands; rosacea is a common cause. Symptoms include red or pink eyelid margins, a red or pink eye, dryness, burning, blurred vision and a swollen eye.
melanin Pigment that colors the iris of the eye as well as other parts of the body, including skin and hair.
melanosis Condition characterized by melanin (pigment) deposits in the skin or eyes.
meningitis Inflammation of the meninges, membranes that envelope the brain and spinal cord. Viruses and bacteria can cause meningitis. Symptoms include fever, headache, vomiting, a stiff neck and light sensitivity.
meridian One of a number of radially arranged imaginary lines, each of which passes through the center of the pupil when viewing the eye head-on. Generally separated in one-degree increments, meridians are used to determine the location of the most- and least-curved sections of the cornea when prescribing lenses with cylinder power to correct astigmatism. Meridians are also used to describe the shape of corrective lenses.
metamorphopsia Vision problem in which objects appear distorted. For example, straight lines may appear to be wavy, curved or bent, objects may appear to be larger or smaller than they actually are, or closer or farther away than they actually are. Metamorphopsia is typically caused by conditions or diseases that affect the eye's macula and retina.
microaneurysm A weakened area in the walls of tiny blood vessels. In diabetic retinopathy, microaneurysms can occur in the retina from damage related to abnormally high blood sugar levels. As microaneurysms in tiny blood vessels (capillaries) expand, ruptures can result. These ruptures lead to hampered blood flow as well as swelling and leakage that sometimes cause scarring, blind spots and blindness.
microcornea Abnormally small cornea.
microkeratome Small instrument that surgeons use to cut the cornea.
microphthalmia Congenital defect resulting in an abnormally small eye or eyes. The cause is usually unknown. Microphthalmia typically results in blindness or reduced vision, but normal vision is possible if the eyes are nearly normal in size.
migraine Severe headache, sometimes accompanied by nausea and visual disturbances. Visual disturbances alone are also possible; this problem is called an ophthalmic migraine, or migraine without headache. Eye and vision symptoms include blurred vision, ptosis, halos around lights, light flashes, light sensitivity, eye pain or discomfort, vision loss (blind spots in central vision, tunnel vision or overall impaired vision), distorted vision and wavy lines in vision.
milia Raised tiny white bumps on the surface of the skin, often appearing on the eyelid and around the eyes and nose. Milia is caused when dead skin cells do not slough off properly and become trapped at the base of a sweat gland or hair follicle, forming a small keratin cyst. They are commonly seen in newborns, but also can affect adults of all ages.
minification Making objects appear smaller; the opposite of magnification. Most lenses for nearsightedness make objects look smaller, and when the lenses are in eyeglasses, they also make the wearer's eyes look smaller. Aspheric lenses reduce this minification effect, for a more natural look.
monochromatic Refers to one wavelength of light, as opposed to the many wavelengths of light found in varying colors.
monofocal Type of spectacle lens, intraocular lens (IOL) or contact lens design that has only one area through which the eye focuses. A multifocal lens has more than one focal area, enabling sight at multiple distances, typically for people with presbyopia.
monovision Vision correction method for those with presbyopia in which one eye is corrected for near vision and the other for far, either through contact lenses or refractive surgery. Monovision eliminates the need for reading glasses, but does have some drawbacks, including decreased depth perception. Read our article about monovision with contact lenses.
mucin Lubricant such as saliva that protects body surfaces. In the eyes, mucin is a tear layer that helps moisten and protect the eye's surface.
mucormycosis Fungal infection typically occurring in the sinuses or lungs and mainly acquired by those with compromised immune systems and by diabetics. Symptoms include sinusitis, eye and facial pain, fever, a bulging eye and vision loss.
multifocal Type of spectacle lens, intraocular lens (IOL) or contact lens design that includes more than one area through which the eye focuses. such as bifocals or trifocals. Examples are bifocals or trifocals. This enables sight at multiple distances, typically for people with presbyopia.
myasthenia gravis Weakness of the voluntary muscles, believed to be autoimmune in nature. Symptoms include double vision and eyelid ptosis; patients sometimes have non-eye symptoms as well, such as difficulty swallowing or using the arms and legs.
myelin A sheath made of proteins that covers nerve fibers. Myelin is essential to transmission of nerve impulses carrying information to and from various parts of the body. When myelin is destroyed or damaged in the optic nerve, the result is optic neuritis, with vision loss or distortions.
myokymia Common eyelid twitch typically brought on by stress or fatigue.
myopia Also called nearsightedness. Condition in which the length of the eye is too long, causing light rays to focus in front of the retina rather than on it, resulting in blurred distance vision. Additional symptoms include eye strain, poor night vision and squinting.
nanometer A measurement of length equal to one-billionth of a meter. Abbreviated as nm. Wavelengths of light are measured in nanometers. Eyeglass and sunglass lens manufacturers use nanometers to describe the different types of light that may pass through or be blocked by a lens, whether it be visible light, ultraviolet light, blue light, etc. For example, visible light has a wavelength range of 400 to 700 nm. The amount of light transmission itself is described as a percentage. For example, a dark sunglass lens might allow only 12 percent of visible light to pass through to the eye.
nearsightedness Also called myopia. Condition in which visual images come to a focus in front of the retina, resulting in defective vision of distant objects.
neovascularization Abnormal growth of new blood vessels, such as in an excessive amount, or in tissue that normally does not contain them, or of a different kind than is usual in that tissue. Also see angiogenesis.
neuronal ceroid lipofuscinosis Rare, hereditary, degenerative disease in which the body does not store pigments called lipofuscins properly; the disease is characterized by vision loss, seizures and dementia. Types include Santavuori-Haltia (infantile), Jansky-Bielschowsky (late infantile), Spielmeyer-Vogt (juvenile) and Kufs' disease (adult). Some types may also be called Batten disease.
neurons Nerve cells that form intricate networks through which sensory information is transmitted via electrical impulses sent throughout the body. Neurons are part of the central and peripheral nervous systems, which support functions such as thought, sight, perception, hearing, taste, feeling, speech and movement.
neuroretinitis Inflammation of the optic nerve and retina, commonly caused by an infection. Symptoms include blurred vision, headache, floaters, eye pain or discomfort, vision loss and loss of color vision.
nevus Birthmark, freckle or mole that is often brownish, but can be other colors as well. A nevus can occur on the skin or inside the eye and can become a melanoma, a type of cancerous growth. If your eye doctor discovers a nevus within your eye, he or she will want to check it regularly to see if it grows or becomes a melanoma that requires treatment.
nickel Metallic element used mainly in alloys. Many eyeglass frames are made of nickel alloy, so people who are allergic should choose a hypoallergenic substitute, such as titanium.
nose pad One of a pair of pads, usually clear, that rest on either side of your nose and help to support your glasses.
nystagmus Rapid and involuntary eye movement that is oscillating and non-chaotic. Blurred vision may result. Nystagmus typically affects infants and has a variety of causes.
OD Abbreviation for "oculus dexter," the Latin term for "right eye." Or, doctor of optometry.
ocular herpes Recurrent viral infection that can cause inflammation and scarring of the cornea. It is not sexually transmitted. There are various types of ocular herpes, ranging from herpes keratitis to more serious forms that can lead to blindness. See our eye herpes article for more details.
ocular hypertension Condition in which the intraocular pressure of the eye is elevated above normal and which may lead to glaucoma.
ocular migraine Visual phenomena that may accompany a migraine headache or that may occur without any headache. They include light flashes, spots, wavy lines, flickers, zig-zagging lights, semi-circular or crescent-shaped visual defects and distortions of shapes. See our ocular migraine article for more details.
onchocerciasis Commonly called "river blindness," onchocerciasis is caused by a parasitic worm, which is spread in the human bloodstream through bites from blackflies and buffalo gnats found in parts of Africa, South America, and Central America. The worm's offspring cause inflammation, bleeding, and other problems in the eye. Without a 15-year regimen of annual doses of Mectizan, blindness will result. (Information supplied by ORBIS International.)
ophthalmologist A medical doctor (MD) who specializes in the eye. Ophthalmologists perform eye exams, treat disease, prescribe medication, and perform surgery. They may also write prescriptions for eyeglasses and contact lenses.
ophthalmoplegia Eye muscle paralysis. Causes include stroke, multiple sclerosis, a tumor, thyroid disease, migraines and progressive supranuclear palsies. Symptoms can include limited eye movement, blurred vision, double vision, nystagmus and ptosis. Tolosa-Hunt Syndrome, often called "painful ophthalmoplegia," is characterized by intense pain behind the eye and a headache.
opsoclonus Rapid and involuntary eye movement that is irregular and chaotic; sometimes called "dancing eyes." Typically seen in Opsoclonus-Myoclonus Syndrome.
optic The rounded, central portion of an intraocular lens (IOL) used in cataract surgery. Optic also more generally refers to eyes or vision.
optical coherence tomography A method of imaging that, in ophthalmology, uses light waves to provide cross-sectional views of interior eye structures. Also known as OCT.
optic nerve head Also called optic disk. Circular area where the optic nerve enters the retina, and the location of the eye's blind spot.
optic nerve problem The optic nerve (second cranial nerve) is the part of the eye that carries stimuli from the rods and cones to the brain. Problems such as inflammation (optic neuritis), tumors and swelling can lead to symptoms such as: blurred vision, loss of color vision, floaters, headache, eye pain or discomfort, nausea and vision loss.
optician In the United States, opticians are not doctors, but in some states they must complete training and be licensed. And in some states they can, after special training, become certified to fit contact lenses. (Please visit the Opticians Association of America website for licensing requirements for various states.) Most opticians sell and fit eyeglasses, sunglasses, and specialty eyewear that are made to an optometrist's or ophthalmologist's prescription. Many also have equipment on the premises so they can grind lenses and put them in frames without ordering from a lab.
optometrist Doctors of optometry (ODs) examine eyes for both vision and health problems, prescribe glasses, and fit contact lenses. They can prescribe many ophthalmic medications and may participate in your pre- and postoperative care if you have eye surgery. ODs must complete four years of post-graduate optometry school for their doctorate.
orbit Eye socket.
orbital cellulitis A sudden infection of the tissues immediately surrounding the eye, resulting in painful swelling of the upper and lower eyelid, and possibly the eyebrow and cheek. Other symptoms include bulging eyes, decreased vision, fever, and eye pain when moving the eyes. Bacteria from a sinus infection are a common cause; other causes include a stye on the eyelid, bug bites or a recent eyelid injury. Orbital cellulitis is a medical emergency and prompt IV antibiotic treatment often is needed to prevent optic nerve damage, permanent vision loss or blindness and other serious complications.
orbital pseudotumor An inflammatory mass in the tissues around or behind the eye that looks like and mimics the symptoms of a tumor. The cause is unknown. The primary symptom is a painful, bulging eye. You may also experience pain or discomfort around the eye.
orphan drug A drug designated for treatment of a rare condition or disease that typically affects fewer than 200,000 U.S. residents. When the FDA grants orphan drug status, a company may be qualified for special grants, tax breaks or other incentives to help defray research and production costs. Without these incentives, companies would be unable to profit from development of orphan drugs because of limited demand.
orthokeratology (ortho-k) Procedure in which a doctor fits you with special gas permeable contact lenses to reshape your cornea and correct errors like nearsightedness. Often, patients wear the lenses just at night.
OS Abbreviation for "oculus sinister," the Latin term for "left eye."
osteopetrosis Rare, hereditary disease in which the bones are too dense. Common symptoms include bone pain and fractures. Retinal degeneration may occur; it results in vision loss.
OU Abbreviation for "oculus uterque," the Latin term for "each eye," used in vision correction prescriptions to indicate both eyes. Also an abbreviation for "oculi unitas" or "oculi uniter," meaning both eyes working simultaneously together.
overconvergence Condition in which the eyes come too far inward when focusing on a near object, resulting in blurring.
overflow tearing Common congenital condition in infants and developmental condition in older adults caused by a blocked tear duct. In infants, a membrane blocks the tear drainage duct, resulting in excessive tears and mucus. In adults, the cause of the blockage is usually unknown, but can be related to poor eyelid function.
papilla Small bump where the optic nerve exits the eye.
papilledema Swelling with accompanying compression of the optic nerve head, which can be a medical emergency. Causes of papilledema can include bleeding near the vicinity of the optic nerve and abnormally high cerebrospinal fluid (CSF) pressure. Autoimmune disorders, trauma and infections of the central nervous system also can cause papilledema. Optic nerve swelling can result from central retinal vein occlusion or may occur as a side effect of medications such as lithium, tetracycline and corticosteroids.
papilloma Usually benign tumor, such as a wart or a skin tag; papillomas may be raised or flat, and can be a variety of colors, such as skin-colored, yellow, pink, brown or black. Eye papillomas are typically on the eyelid, but may also appear on the conjunctiva. The cause of papillomas is felt to be viral.
Parinaud dorsal midbrain syndrome Inability to look up, typically associated with a brain lesion, characterized by nystagmus and pupil unresponsiveness to light. Causes include hydrocephalus ("water on the brain") and tumors of the pineal gland.
Parkinson's disease Neurological disorder characterized by tremors, muscle rigidity, a shuffling walk and a mask-like appearance in the face. Parkinson's may also cause infrequent blinking.
pars plana Posterior part of the eye's ciliary body.
Patau syndrome Also called Trisomy-13. Condition caused by an extra, third copy of chromosome 13. Symptoms include severe mental retardation, a small head, microphthalmia, a cleft lip or palate, heart defects and extra fingers or toes; many patients also have an iris coloboma and retinal dysplasia (abnormal development). The majority of infants with Patau syndrome die within the first year.
pediculosis Lice infestation, typically caused by contact with an infected person or infected bedding. When lice infest the eyelid and eyelashes, they can cause such symptoms as visible lice (white or gray), eggs called nits (white or gray) or feces (reddish-brown), blue bite marks, blepharitis and conjunctivitis. Some people also develop keratitis.
penetrating keratoplasty A transplant procedure in which a circular area of surface eye tissue is removed from a healthy donor cornea and transferred to a recipient. A penetrating keratoplasty or corneal transplant may be needed in case of eye damage from injury or from eye diseases such as keratoconus.
peripheral vision The edges of your visual field.
phacoemulsification Also called "phako," this in-office cataract surgery procedure involves using a device with a vibrating, ultrasonic tip to break up the cataract, then suctioning the pieces out with a tiny needle. Read our article about cataract surgery.
phakic An eye that still has its natural lens. When an eye is aphakic, usually the lens has been removed during cataract or other eye surgery.
phoropter Device that provides various combinations of lenses used for tests of vision errors in eye examinations.
photoablation Procedure in which a surgeon uses ultraviolet radiation to remove tissue.
photochromic Able to change lens color or darkness/density depending upon the degree of exposure to light.
photocoagulation Use of heat from a high-energy laser to seal off bleeding in damaged tissue. Photocoagulation also may prevent formation of abnormal blood vessels (neovascularization) in eye diseases such as age-related macular degeneration or diabetic retinopathy. Photocoagulation also may be used to reattach a detached retina.
photokeratitis "Sunburn" of the cornea; symptoms include discomfort, blurred vision, and light sensitivity. The temporary vision loss that can result is called "snow blindness."
photophobia Discomfort from sun or other light. Photophobia has many causes.
photopsia Flashes of light often noticed in the edges of the visual field. Photopsia can have many causes, including mechanical (rather than visual) stimulation of light-sensitive cells (photoreceptors) in the retina. For example, a detached retina can cause photopsia when the retina pulls away or detaches from tissue in the inner back of the eye. A vitreous detachment with accompanying photopsia can occur when the eye's gel-like interior begins to shrink and pull against the retina. Photopsia can be accompanied by a shower of spots and floaters.
photoreceptor A light-sensitive cell found in the retina. Photoreceptors in the human retina are classified as cones and rods. Cones are located in the central retina (the fovea) and control color vision. Rods are located outside the fovea and control black/white vision in low-light conditions.
phytochemicals Chemicals found in plants that help protect against disease.
pigment dispersion syndrome An eye condition where pigment granules that normally adhere to the colored part of the eye (iris) flake off and adhere to the posterior surface of the cornea and other structures in the anterior chamber of the eye. If the pigment accumulates in the drainage angle of the anterior chamber, it can reduce the drainage of aqueous from the eye, causing ocular hypertension and (potentially) pigmentary glaucoma.
pinguecula A yellowish, thickened lesion on the conjunctiva near the cornea. Pingueculae represent a benign degenerative change in the conjunctiva caused by the leakage and deposition of certain blood proteins through the permeable capillaries near the limbus. Read more about pterygium and pinguecula.
placebo A "false" but harmless treatment that has no proven medical value. Placebos usually are incorporated into clinical trials to measure how patients respond to an authentic therapy that is being tested. Some patients are given placebos, and others are given the actual therapy, to provide objective comparisons and assessments.
plano A term eye care professionals use to describe lenses with no corrective power. The term is most often applied to nonprescription sunglasses or contact lenses that are worn for cosmetic purposes only.
PMMA (polymethyl methacrylate) Old-fashioned hard contacts were made of PMMA, which is not oxygen-permeable; today's rigid lenses contain other polymers that allow oxygen to reach your eye. Some intraocular lenses are made of PMMA because the material is well tolerated inside the eye.
polarized lenses Lenses that block light reflected from horizontal surfaces such as water, to reduce glare.
porphyria Disorder in which the body produces too much of a compound called porphyrin and releases it in the urine, causing a reddish color. Other symptoms include light sensitivity, skin that swells or is sensitive to sunlight, abdominal pain, blisters and muscle weakness.
polycarbonate Plastic that is very impact-resistant, and is thus sometimes used for spectacle lenses and frames.
posterior chamber Part of the eye behind the iris and in front of the lens.
posterior capsular opacification Haziness that develops behind the artificial intraocular lens inserted during cataract surgery. A common complication following cataract removal, a PCO can be removed with a YAG laser capsulotomy.
presbyope Person who has difficulty reading print and seeing near objects.
presbyopia Condition in which the aging eye beginning at around age 40 is unable to focus at all distances, often noticed when print begins to blur. Additional symptoms include eye strain, headaches, and squinting. Read our article about presbyopia.
prescription lenses Lenses that provide vision correction as prescribed by an eye care practitioner.
prism In optics, a lens that can have precise geometric configurations enabling light to be bent or reflected in certain ways. A prism also can split white light into different wave lengths and colors.
PRK (Photorefractive Keratectomy) Surgical procedure in which an excimer laser is used to remove corneal tissue to correct vision problems.
progressive lenses Also called progressive addition lenses or PALs. Multifocal lenses whose corrective powers change progressively throughout the lens. A wearer looks through one portion of the lens for distance vision, another for intermediate vision, and a third portion for reading or close work. Each area is blended invisibly into the next, without the lines that traditional bifocals or trifocals have.
proliferative vitreoretinopathy (PVR) A common complication of surgery for retinal detachment, which causes scarring of the retina. PVR may require a vitrectomy and intricate surgical removal of scar tissue.
propionate A soft, flexible material that is sometimes used in goggles.
prosthetic Refers to a prosthesis, which is an artificial replacement for a part of the body. Read more about prosthetic contact lenses.
protective eyewear Eyewear made with impact-resistant lenses, usually polycarbonate, that protects the eyes, especially in working situations or sports.
protein Large, complex organic molecules found in all living cells. These molecules contain enzymes, antibodies, hormones and other elements that help organisms function. Proteins are present in human tears and can collect on contact lenses, resulting in discomfort and cloudy vision.
pseudoexfoliation syndrome A condition of unknown cause in which light gray, dandruff-like material forms on the pupil margin and anterior lens capsule in the eye. This material also can lodge in the trabecular meshwork, causing an increase in internal eye pressure and pseudoexfoliative open-angle glaucoma.
pseudotumor cerebri A condition whose symptoms mirror those of a brain tumor: increased intracranial pressure, headache, nausea, brief periods of vision loss (graying or blurring) and double vision. The cause is unknown, but patients are often obese women.
pterygium Triangular fold of tissue on the white of the eye that can eventually grow over part of the cornea; the cause may be irritation from sun (i.e., UV rays), dust and wind. Some people have no symptoms, while others may have redness or blurred vision. Pterygia that are chronically inflamed can become itchy. Read more about pterygium and pinguecula.
ptosis Drooping eyelid. Congenital ptosis is caused by a problem with the levator muscle (which lifts the eyelid). In adults, ptosis is commonly caused by the aging of the levator's connective tissue.
puncta Tiny openings through which tears drain away from the eyes. Four puncta are in the nasal corner of the eye two in the upper inner eyelid and two in the lower inner eyelid. Punctal plugs sometimes are used to block these openings so that more tears are retained as a treatment for dry eye syndrome.
punctal cautery A procedure that uses heat or laser energy to permanently close channels from which tears drain. Punctal cautery increases the accumulation of moisture as a treatment for dry eye. For more information, please see our dry eye syndrome article.
punctal plugs Tiny inserts often made of plastic that are placed in channels or ducts of the eye where moisture drainage occurs. Punctal plugs can help stop excessive drainage to keep the eye moistened in conditions such as dry eye syndrome. For more information, please see our dry eye syndrome article.
pupil The round, dark center of the eye, which opens and closes to regulate the amount of light the retina receives.
pupillary distance This is the distance between the center of each pupil. Opticians use a special ruler to measure your pupillary distance before ordering your eyeglasses. It is an essential measurement because the optical center of each eyeglass lens must be positioned directly over the center of each pupil. An incorrect measurement means you would have difficulty focusing when wearing the glasses.
[Page updated November 2012]
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