Glossary of Terms
Numbers – H | I – Q | R – Z
20/20 – An expression for normal eyesight. The numbers refer to feet (except in countries that use the metric system, where it’s expressed as 6/6, six meters being approximately equal to 20 feet). The first number refers to the distance between the eye chart and the eye being tested. The second number is a comparison of a normal eye and the tested eye. When the second number is 20, that indicates the tested eye can read the small letters on the chart from 20 feet away, as a normal eye can. When the second number is higher than 20, it indicates impaired vision, more impaired as the number is higher. For example, if a person’s vision is assessed at 20/100, it means that this person must be only 20 feet from the chart for it to look as clear as it does to a normal eye 100 feet away.
AAO – American Academy of Ophthalmology, an professional membership organization for ophthalmologists. Also the American Academy of Optometry, a professional membership organization for optometrists (also called American Optometric Association).
ABES – American Board of Eye Surgeons, a professional membership organization for ophthalmologists. It’s affiliated with the ACES, the American College of Eye Surgeons.
Ablation – Removal, especially surgically by cutting or abrading.
Ablation zone – The area of the cornea to be corrected in laser eye surgery.
ABO – American Board of Ophthalmology, a medical specialty board which offers education and examinations for ophthalmologists. Upon completing this 1 ½ year program, the ophthalmologist is Board Certified.
Abrade – To roughen by rubbing or scraping, thus removing small pieces of surface tissue.
Accommodation – The eye’s ability to switch focus from near objects to far objects. It’s done by tiny muscles attached to the eye’s lens, which pull on the lens to change its convexity. As we age, those muscles become weaker and the lens becomes stiffer, a condition known as presbyopia, where reading glasses become necessary to see close-up objects.
Acuity – (a-KEW-uh-tee) Clarity or sharpness of vision, commonly expressed as
20/20 vision in relation to the Snellen acuity chart. This is the eye chart seen at every eye doctor’s office, with the big E at the top.
Age-Related Macular Degeneration (ARMD) – Deterioration, as people age, of the macula lutea (also called the yellow spot), which is a small area on the retina which gives maximum vision. If left untreated, it results in blindness.
AK – Astigmatic Keratotomy, an ophthalmological procedure for correcting astigmatism.
Alcon – A manufacturer of medical equipment.
Algorithm – A mathematical formula such as is used when preparing for a LASIK, PRK, LASEK or Epi-LASIK surgery, to set the laser’s ablation area on that particular individual’s eyes.
ALK – Automated Lamellar Keratectomy, a procedure to correct opacities on the corneal surface.
Allegretto – A manufacturer of medical equipment.
Allergan – A company that makes ophthalmic equipment and drugs.
Amblyopia – (am-blee-OH-pee-uh) A condition where one eye is less used than the other, usually in childhood, without there being any obvious structural reason for it. It causes the less-used eye to deteriorate, becoming worse than 20/20 until, if no treatment is done, it will become useless. Also called “lazy eye”.
American Academy of Ophthalmology – See AAO.
American Academy of Optometry – See AAO.
American Board of Eye Surgeons – See ABES.
American Board of Ophthalmology – See ABO.
American College of Eye Surgeons – See ACES.
American College of Surgeons – A professional membership organization for surgeons. Its members are known as Fellows and often put FACS after their name for Fellow of American College of Surgeons.
American Optometric Association – A professional membership organization for optometrists. See AAO.
American Society of Cataract and Refractive Surgery – See ASCRS.
Ametropia – A generic name for eye conditions characterized by impaired refraction, e.g. myopia, hyperopia and astigmatism.
Amsler Grid – A test used to detect defects or distortions in the central visual field. It consists of a square subdivided into many hundreds of small squares by horizontal and vertical lines, with a small dot in the center.
Aniseikonia – (a-neece-eye-KON-ee-a) An impaired type of binocular vision where the left and right retinal images are of different sizes. It can occur naturally, and is sometimes induced by refractive surgery.
Anterior – An anatomical term meaning the front part of a structure as opposed to the posterior, the back part.
Anterior Basement Membrane Dystrophy – The most common corneal dystrophy. An eye condition where the membrane that lies beneath the epithelial cells (surface cells) of the cornea is uneven and traps cells below it which should normally rise above it. It impairs vision, is usually hereditary, and fluctuates in severity over the person’s lifetime. It’s also called Map-Dot-Fingerprint Dystrophy (a name based on how it looks microscopically), and Epithelial Basement Membrane Dystrophy.
Anterior Chamber – The small space behind the cornea and in front of the iris, filled with the Aqueous Humor.
Anterior Ocular Segment – A name for the parts of the eye that are in front of the lens, namely, the cornea, anterior chamber, iris and ciliary body.
Antimetropia – A condition where one eye is nearsighted and the other one farsighted.
Antioxidants – Nutrients that neutralize or destroy free radicals.
AOA – See AAO.
Aphakia – (ah-FAY-kee-uh) The condition of having no crystalline lens in the eye, as happens when it’s been removed because of cataracts.
Aphakic Spectacles – Very thick and heavy glasses that in past years were the standard remedy after a cataract operation. They gave distorted peripheral vision. Modern ophthalmologists can instead implant an intraocular lens (IOL).
Aqueous Humor – (AY-kwee-us) The clear fluid that nourishes the lens and the cornea, flowing between them, secreted by the ciliary processes.
ASCRS – American Society of Cataract and Refractive Surgery, a professional membership organization for ophthalmologists who perform cataract and refractive surgeries.
Astigmatic Keratotomy – An eye surgery like Radial Keratotomy, where tiny incisions are made in the edges of the cornea to give it a rounder shape.
Astigmatism – (uh-STIG-muh-tiz-um) An eye condition where the cornea is oval-shaped rather than round. This oval shape has two curves: the steeper one and the flatter one. Each curve refracts light at a different angle, sending it to focus on the retina in two different places, and creating distorted vision. It can be mild, with just blurriness, or more severe, with ghosting and severe blurring. In a glasses prescription, the second number expresses the degree of astigmatism you have, and the next number is the number of degrees in the angle of refraction. For example, a prescription of -4.00 – 2.00x34o states that you have 2 diopters of astigmatism at an angle of 34 degrees.
Automated Lamellar Keratoplasty (ALK) – A refractive eye surgery that treats myopia. The surgeon uses a microkeratome to cut a very thin shaving or flap from the corneal surface. Then a tiny piece even thinner is removed from the underlying tissue, to flatten the cornea a little, and the flap is replaced. It heals up without any need of stitches.
B scan – An ultrasonic procedure that checks for abnormalities in the eye, and locates foreign bodies within it.
Basement Membrane – A microscopically thin layer of tissue below the epithelial cells (surface cells) of the cornea, connecting those cells to the stroma, which is the middle layer of cells in the cornea.
Bausch & Lomb – A company that makes ophthalmic equipment and products.
Best Corrected Visual Acuity – A measurement of your best vision, as corrected by your glasses or contact lenses.
Bifocals – The type of glasses we usually call “reading glasses”, where most of the lens corrects your distance vision, and a small area at the lower edge of the lens corrects your close-up vision. They’re used by people with
presbyopia. There are also trifocals.
Binocular – An adjective meaning with both eyes, e.g., binocular vision.
Binocular Vision – The use of both our eyes to see, such that the images from each eye blend to form a single image with dept perception.
Black Box Laser – A laser (usually imported to the U.S.) used by some eye surgeons that has been altered so it can do LASIK and other laser eye surgeries, but this alteration is not approved by the FDA. The safety consequences of using a black box laser have not been studied and are completely unknown.
Blepharitis – (blef-uh-RI-tus) Chronic inflammation of the eyelids. It can be caused by an allergic reaction to some product, excess oil excreted by eyelid glands, bacterial infection, or poor facial hygiene.
Blindness – See Legal Blindness.
Blind Spot – (a) The place on the retina where the optic nerve enters the eye. No visual cells are on the retina here, so no vision is possible at this place. In this sense, it’s a normal thing. (b) Also refers to any gap in a person’s visual field that corresponds to any area on the retina where visual cells are missing and in this sense, it’s associated with eye disease.
Board Certified – This is the phrase used for approval by the American Board of Ophthalmology (ABO), a non-profit and independent organization that was founded in 1916 to certify ophthalmologists.
Broadbeam – The term used for applying the excimer laser to the entire treatment area at one time. It’s the typical way laser treatment is done for myopia, hyperopia and astigmatism. See also Flying Spot and Variable Spot.
Buttonhole Flap – A LASIK problem where the microkeratome cuts through the top surface of the cornea while creating the corneal flap, while lifting it, or while folding it back. It can be caused by loss of the suction which holds the microkeratome against the cornea, or inadequate suction, or by poor matching of microkeratome to patient. If possible, the flap is replaced, and no further surgery is attempted for several months, after it has healed. Sometimes it can cause scarring which impairs vision, but usually there are no permanent ill effects.
Cataract – An opaque or cloudy area that forms in the eye’s lens, impeding vision. They tend to occur with aging, but can also be caused by trauma. Cataracts are treated by the lens being removed and an artificial lens implanted. See intraocular lens.
Central Ablation Zone – The ablation zone is the area of the eye treated by the laser in LASIK surgery. Around it is the transition zone, where the treated area gradually merges with the original corneal surface that lies outside the ablation zone.
Central Island – An area of the laser-treated part of the cornea which is erroneously not treated, so that its level remains microscopically higher than the surrounding treated surface. The term island describes its appearance. It causes diplopia (double vision).
Ciliary Body – Part of the eye which is in front of the lens, and behind the iris and cornea. It consists of (a) the ciliary muscle which controls the shape of the lens, making it flatter for far distant objects and more convex for closer things; and (b) the ciliary processes, tiny projects which secrete aqueous humor.
CK – Acronym for Conductive Keratoplasty.
CLE – Acronym for Clear Lens Exchange.
Clear Lens Exchange – See Refractive Lens Exchange.
Co-management – Collaboration between two or more doctors in caring for a patient. For refractive surgery, usually an optometrist co-manages with an ophthalmologist. The optometrist provides the pre-operative testing and post-operative care, while the ophthalmologist does the surgery itself.
Coma – A higher order aberration which makes points of light look like comets with blurry tail-like smudges. It can be diagnosed and treated with wavefront-guided LASIK procedures.
Complex Wavefront Retreatment – An off-label use of the excimer laser to do corrective secondary surgery after the original wavefront-guided surgery has left the patient with higher-order aberrations.
Concave Lens – The type of lens used to correct nearsightedness (myopia). Concavity is the hollow type of curve that recedes in the center and raises up at the edges, opposite to convex.
Conductive Keratoplasty – A type of refractive surgery which uses radio waves through a tiny probe to create planned shrinkage of the cornea, such that hyperopia or astigmatism are reduced, or eliminated.
Conjunctivitis – (kun-junk-tih-VI-tis) Inflammation of the conjunctiva, which is a thin mucus membrane covering the inside surfaces of the eyelids and the white outer surface of the eye. It looks reddened and itches. It’s a contagious conditions and can be treated with eyedrops.
Contact Lens – A way of correcting myopia and hyperopia, or other refractive error of the eyesight, which consists of curved plastic lenses placed in front of the iris. They’re designed so that they retain good contact with the eye’s surface and remain in place.
Contact Lens Assisted Pharmacologically Induced Kerato Steepening – A way to undo overcorrection in laser eye surgery (LASIK, PRK and RK). A tightly-fitting contact lens is placed on the eye and anti-inflammatory eyedrops are used. The goal is to make the cornea more steep, after it’s been made too flat.
Contact Lens Disinfectant – A solution for removing bacteria and micro-organisms from contact lenses.
Contact Lens, Daily Wear – Contact lenses designed to be worn only during the daytime.
Contact Lens, Disposable – Contact lenses designed to be worn once and then thrown out, as opposed to the kind that’s removed, cleaned, and reinserted. Depending on the eye doctor’s prescription, disposable contact lenses can be worn for one day, or for up to a week.
Contact Lens, Extended Wear – Contact lenses designed to be worn continuously for up to a week, not being removed for sleep.
Contact Lens, Therapeutic – Special contact lenses designed to help heal the eye and protect it while it heals. They’re often used along with eyedrops specially-prescribed to promote healing.
Contact Lens, Toric – Contact lenses designed to correct astigmatism, with two curvatures at different angles, one for astigmatism, and the other for hyperopia or myopia. They have the ability to remain in place despite blinking and eye movements, so that they give you clear vision.
Convex Lens – Lenses with the opposite curvature to concave lenses. They bulge outwards like a ball and are used to correct hyperopia and presbyopia.
Cornea – (KOR-nee-uh) The front clear part of the eye in front of the pupil and iris. It acts as a lens, refracting light rays as they enter the eye. The eye’s crystalline lens refracts them further to focus them on the retina at the back of the eye. The cornea also allows light to pass out through it, making the iris visible, the eye’s color.
Corneal Abrasion – A scrape or scratch on the cornea, the front surface of the eye.
Corneal Ectasia – A complication of LASIK similar to the inherited condition of
keratoconus. It can happen when the eye was over-treated by a LASIK procedure, so that not enough thickness is left in the cornea to contain the eye’s internal pressure. That pressure pushes against the cornea and causes it to bulge outward. Vision then becomes progressively worse.
Corneal Flap – A small circular piece of the cornea’s surface layer (epithelium) which is cut, all but one section like a hinge, and folded back before the laser treats the stroma. After treatment, it’s folded back into position and heals by itself in a few days. Very occasionally there are complications with this flap. For example, it may have been sized wrongly, or cut too deeply, it might be completely cut instead of retaining a hinge, or it might heal in the wrong position, with a slight wrinkle or with swelling. Many of these complications can be dealt with successfully.
Corneal Haze – An after-effect of Excimer laser surgery, where the cornea develops opaque white cells which cloud the vision to some extent. It can cause glare from bright lights and a vague fogginess of vision. It usually clears up after 6 or 8 months. If it persists, there’s an enhancement procedure which can reduce it. As vision correction techniques improve there’s less incidence of corneal haze.
Corneal Refractive Therapy – A reshaping of the eye with contact lenses, also called Orthokeratology. These lenses are rigid and worn while you sleep, so they gently persuade the eye to change its shape by the time you wake in the morning. The effect lasts only a day or two, so you need to wear these lenses every night. It was approved by the FDA in June, 2002 and is a non-surgical way of temporarily creating the effect created permanently by LASIK surgery.
Corneal Topographical Map – A map of the cornea that shows its surface profile.
Corneal Topographer – A specialized camera-computer system which photographs and prints out a map of the surface details of a cornea. This is done before any LASIK procedure.
Corneal Transplant – Surgery to replace the cornea, the clear front area of the eye. Corneal tissue comes from a donor and since the cornea has a small blood supply, there is little risk of rejection, and the new cornea can function very well for years. A corneal transplant can be done to treat Keratoconus,
Fuch’s Dystrophy, or damage from a severe infection or injury. It’s a painless outpatient procedure.
CRSQA – Pronounced “Surs-kah”. See Council for Refractive Surgery Quality Assurance.
CrystaLens – A type of intraocular lens which changes focus between close and distant objects. More information.
Crystalline lens – The eye’s natural lens, which is behind the iris and in a healthy eye is completely clear. Light passes through it and is refracted by the lens to focus on the retina. Tiny muscles attached to the lens change its convexity according to where the eye is focused. As we age, it can start to become cloudy (cataracts), impairing our vision.
Custom Cornea – The trade name for a wavefront-guided LASIK that uses the LADARVision excimer laser, made by Alcon.
CustomVue – The trade name for a wavefront-guided LASIK that uses the VISX S4 Excimer laser.
Decentration – A complication of eye surgery. When centration is perfect, the corneal ablation, or the position of the intraocular lens being implanted, is in the center of the eye, lined up with the pupil. Then vision is perfectly centered through the ablated area and pupil, so that we see clearly and fully, like looking through the center of your glasses. When decentration happens, the ablation has been done off-center, or the lens’ position is shifted, so that vision is partially corrected and partially as it was originally.
Degenerative Myopia – Nearsightedness thought to be hereditary, which may start at birth, or in later childhood. It’s a more severe form of myopia and can lead to blindness. It’s associated with to cataract formation and with retinal changes and can lead to retinal detachment.
Depth Perception – Our ability to judge the relative distances of multiple objects. Each eye receives a slightly different image. The dominant eye looks directly at it and the non-dominant eye looks from a slight angle. The brain compares these two images and arrives at an estimate of their relative distance. See also Strabismus.
Diabetes Type 1 – Insulin-dependent diabetes, also known as juvenile diabetes because it’s often diagnosed in young people. The pancreas, which normally would produce insulin, is unable to do so, so insulin must be injected so the patient can convert sugars and starch into energy. A person with Diabetes, Type 1 or 2, is susceptible to vision problems because the tiny blood vessels in the eyes weaken and start to leak, damaging the retina. See Diabetic Retinopathy.
Diabetes Type 2 – The more common of the two types of Diabetes, where the pancreas can produce insulin, but not enough. Insulin may or may not have to be injected for the person to convert sugars into energy. Sometimes Type 2 can be well-managed by sticking to the right diet and exercise routines.
Diabetic Retinopathy – (ret-in-AHP-uh-thee) Damage done to the small blood vessels that feed the retina. In the early stage it’s known as background diabetic retinopathy. The small arteries in the retina weaken and leak, which often causes swelling and impaired vision.
The later stage is called proliferative diabetic retinopathy. Since the arteries are not functioning well to bring oxygen to the eye’s cells, retinal cells become ischemic (lacking in oxygen). The eye develops new blood vessels (a response known as neovascularization) but these are weak and also start leaking, so the problem becomes worse.
Eventually, as the body continues to grow extra blood vessels, which continue to leak, scars can form, and the retina can detach from its membrane at the back of the eye, causing blindness.
Diffuse Lamellar Keratitis (DLK) – Also known as Sands of Sahara. A rare complication of LASIK surgery, appearing between 2 and 5 days after surgery, where inflammation develops between the corneal flap and the corneal tissue beneath it. The cause isn’t certain. It causes blurriness of vision when it’s severe enough.
Various measures are usually taken to prevent it, such as use of sterile tape to keep the eyelashes away from the treatment area, rinsing the cornea before making the flap, and rinsing again before replacing the flap. After surgery, medicated eyedrops are typically used to prevent inflammation. DLK needs to be detected promptly so that it won’t impair vision. Usually it responds well to use of corticosteroid eyedrops and sometimes the surgeon will lift the flap again to rinse beneath it.
Dilation – Enlargement of the eye’s pupil. The pupil changes size constantly, allowing more or less light into the eye according to how bright the surroundings are. For some eye procedures it’s dilated with special eye drops.
Diopter – (di-AHP-tur) The unit of measurement for a lens. Positive diopter numbers indicate the lens is convex (curving outwards) and negative numbers indicate it’s concave. A one-diopter lens will bend straight light rays to focus them one meter away from itself. A two-diopter lens will bend them to focus only a half-meter from itself. A lens with any minus number doesn’t focus the light rays, but scatters them.
Diplopia – The term for double vision. Binocular diplopia can occur when both eyes are open, and monocular diplopia when only one is open.
Dominant Eye – The eye that looks directly at an object. The non-dominant eye looks at it from a slight angle, and this combination creates depth perception.
Double vision – Seeing objects in duplicate, with the second image being lighter and less distinct. Also known as ghosting.
Dry eye Syndrome – Insufficient moisture in the eye, which gives a feeling of grittiness, burning, stinging, or uncomfortable dryness, and extra sensitivity to light. It can be a symptom or complication of another condition, or caused by certain medications. It is also a common temporary result of a LASIK procedure. It can be treated with moisturizing eyedrops, eyedrops which stimulate more tear production, or punctal plugs which block the drainage into the nasal passages and sinuses.
DSAEK – DSAEK (Descemet’s Stripping Automated Endothelial Keratoplasty) is a new procedure devised in 2007 for improving a corneal transplant. Descemet’s Membrane (named after French physician Jean Descemet (1732-1810), is the basement layer, the innermost of the cornea’s five layers. DSAEK transplants only this very thin layer rather than the entire cornea. The replacement layer from a donor stays in place without the need for any sutures. Recovery from DSAEK is one to three months, as compared to the one to two years’ recovery required by a full corneal transplant.
Dystrophy – Weakening or wasting of body tissue, such as happens in Epithelial Dystrophies and Stromal Dystrophies, where abnormalities appear in different corneal membranes, causing loss of function and often impairing vision.
Endothelial Dystrophy – See Fuch’s Dystrophy.
Endothelium – A lining of flat cells inside the cornea, facing on to the anterior chamber.
Epikeratome – A surgical tool similar to a microkeratome, used to make the corneal flap in an Epi-LASIK procedure. It has a blunt separator where the microkeratome, used in LASIK procedures, has a very thin and sharp blade. It’s another way of making the corneal flap and does not involve the use of alcohol, as LASEK procedures do. Each way of making the corneal flap has its advantages and disadvantages and consultation with an experienced eye surgeon will determine which would be best for you.
Epi-LASIK – A variation on the basic LASIK treatment, where instead of using a microkeratome to cut the corneal flap before applying the laser, an epikeratome is used. This instrument gently separates the epithelial cells. Epi-LASIK is done for people whose corneas are too flat for traditional LASIK.
Epithelial Dystrophies – A group of inherited dystrophies where the surface layer of the cornea atrophies and vision is impaired. See Anterior Basement Membrane Dystrophy, Reis-Buckler’s Dystrophy, and Meesmann’s Dystrophy. (Click here to read more about Epithelial Dystrophies…)
Epithelium – A layer of cells which protects the tissue below. In the eye, it’s the outside layer of cells on the cornea.
Esotropia – Inward turning of the eyes; crossed eyes. Usually one eye looks straight ahead and the other turns inward. There are several types: Congenital (evident at birth and may continue into Infantile Esotropia); Infantile (usually detected at about 2 months of age and may be accompanied by strabismus and poor gross motor development; Accommodative (usually detected at about two years of age and is related to difficulty with focusing the eyes); and Partially Accommodative (a mix of Basic Esotropia, i.e., non-accommodative, and accommodative esotropia).
There is also Pseudoesotropia, which looks like esotropia but is a temporary condition where a young child’s eyes haven’t yet grown the white part next to the nose. The bridge of the nose appears wide and the eyes appear to be both turned inward. This resolves itself as the child’s face develops.
Misalignment of the eyes in childhood should be corrected as soon as possible, so that binocular vision (the brain’s ability to use both eyes together) can develop and amblyopia can be avoided. Surgery may be needed; or glasses, an eye patch, or other types of treatment.
Excimer laser – (EKS-ih-mur) An ultraviolet laser used in eye surgery and other surgeries. Its name comes from the terms excited and dimer, where excited refers to a molecule which has been stimulated, and is in an excited state, and dimer is the term for a molecule with two identical components. This all refers to the way the excimer laser is created. It’s made from argon and fluoride and gives off pulses of light with a wavelength of 193 nm.
Excimer lasers are “cool” lasers, meaning that they’re relatively cool, because all lasers give off some heat.
These are very finely-tuned or subtle lasers. Each pulse removes only 1/4000 of a millimeter of corneal tissue, and to get a better idea of how small that is, think of the width of a human hair: it would take 200 pulses of the excimer laser to cut that hair in half.
It cuts tissue by breaking bonds within collagen molecules. Its wavelength is such that the ultraviolet light is instantly absorbed by the water within the corneal tissue. The cornea has a high water content, and this absorption of the laser light by the corneal surface prevents it from penetrating any further into the eye. This makes it a good tool for precise sculpting of the corneal surface.
Excimer lasers have been used since 1987 for vision correction and were approved by the FDA in 1995 for correcting nearsightedness. Since then, they’ve been also approved for treating farsightedness and astigmatism.
FACS – Fellow of the American College of Surgeons.
Farsightedness – The popular term for hyperopia.
FDA – U.S. Food and Drug Administration.
Floaters – Tiny specks or strands that float in the field of vision. They move when the eyes move so they can’t be directly focused upon. Those shapes are the shadows cast on the retina by small clumps of cells in the vitreous humor. Often they’re more visible against a blank background like the sky or a wall. They become more common with age, as the vitreous starts to thicken and clump together. Mostly floaters are harmless, but if flashes of light accompany them, it could indicate a potential retinal detachment.
Flying Spot – A method of applying the excimer laser light used in refractive surgery. A flying spot laser is used, which has a 1mm – 2mm diameter light beam. The computer that controls the laser is programmed to send pulses to changing spots on the cornea, with no spots overlapping. Part of the ophthalmic laser system is an eye tracker, which responds to all eye movement during surgery. So even if the eye moves while it’s being treated, the entire targeted area will be treated, because the treatment plan has determined where the laser should shine, in the series of flying spots that will cover it entirely, and the whole system moves to correspond with any eye movements during surgery.
Flying spot lasers can be used in both traditional LASIK procedures and those using wavefront technology.
Focusing power of the eye – The combined action of the cornea and the lens to refract light on to the retina. As light enters the eye, it’s refracted (bent) by the cornea, because of the cornea’s curvature. As the light continues further into the eye, passing through the pupil to reach the lens, it’s refracted more, to a variable degree according to how far away from the eye the object is which is reflecting this light. If it’s down the road a ways, like a distant traffic light, it needs to be refracted less in order to land in focus on the retina. If it’s close up, like the clock on the dashboard, it needs to be refracted more.
Many people have corneas which refract light either too much (causing it to focus in front of the retina and thus creating nearsightedness), or too little (causing it to focus behind the retina, causing farsightedness). Refractive surgery can correct this by reshaping the corneal surface to make it more or less steeply curved.
Food and Drug Administration – The U.S. federal agency in charge of evaluating and approving medical devices (and other things). It’s not responsible for new medical procedures, just for how the new devices are used. For example, it has approved of excimer lasers for treating nearsightedness.
Fovea – The central area of the macula, which is the most sensitive area of the retina. The fovea is less than one percent of the retina, but uses over 50% of the visual cortex in the brain (that part of the brain which receives visual information). The fovea is highly sensitive to details of vision but not to dim light, which is why we can see dim objects, such as distant stars, better by not looking at them directly.
Fuch’s Dystrophy – A progressive, inherited eye disease (dystrophy) in which the cornea loses cells from the endothelium which normally remove fluids and impurities from the eye. Without those cells, the eye retains too much fluid and begins to swell. By changing the cornea’s curvature, this makes vision blurry, especially first thing in the morning, since while the eyes are closed in sleep, no moisture can evaporate from them. It causes other symptoms, e.g. blisters, light sensitivity, pain, and decreased depth perception. There is no cure, but there are some ways to minimize symptoms. A corneal transplant eventually becomes necessary. Also called Endothelial Dystrophy. (Click here to read more about Fuch’s Dystrophy…)
Ghosting – A name for double vision. The eye sees objects in duplicate, with the second image fainter than the main one.
Glaucoma – (glaw-KOH-muh) An eye disease where pressure builds up inside the eye. If it isn’t diagnosed and treated, it can damage the optic nerve, reducing the field of vision gradually, until blindness results. It’s treated with special eye drops which lower the pressure.
There are several types of glaucoma, the most frequent one being Open-Angle Glaucoma, which has no obvious symptoms at first. About 15% of glaucoma cases are Closed-Angle Glaucoma, and there are noticeable symptoms, such as nausea, eye pain, blurred vision, and headaches. There’s also Normal-Tension Glaucoma, where the intraocular pressure doesn’t build up, and no cause has yet been established for this yet, although there are theories.
Congenital Glaucoma begins at or near the time of birth. Secondary Glaucoma is a result of some other illness. Each form of Glaucoma has its own causes and symptoms.
Granular Dystrophy – A hereditary eye condition where pale gray granules appear in the stromal layer of the cornea, like little crumbs. It’s usually detected by the time a person is about 20. By age 40 or so, vision will be increasingly impaired as those granules, or lesions, expand, increase in number, coalesce, and penetrate more deeply into the stroma. It can be treated in earlier stages with an excimer laser, or in other ways that remove the granules, and later on by a corneal transplant.
Halos – A visual condition where a light source appears with a blurry circle of light around it, rather than having visible edges. It can be a complication of refractive surgery, and can also occur naturally. It makes night vision difficult.
Haze – Clouding of the cornea. It can be caused by inflammation, too much moisture, scar tissue, or some kind of foreign substance as from a medication.
Higher Order Aberration – The lower order aberrations are familiar names to most of us: myopia, hyperopia, and astigmatism, and they’re refractive errors. The higher order aberrations are irregularities of the eye that are not refractive. Sixty-four have been detected and measured so far, using wavefront technology. They’re not treatable with traditional LASIK procedures, glasses or contact lenses, but can be diagnosed and treated by wavefront-guided LASIK. Any given eye usually has more than one, which interact, producing symptoms. That means the aberrations can’t necessarily be established based on symptoms. They come into play at night, or in low lighting conditions. Some examples are:
Spherical Aberration (irregularity in the eyeball’s shape), which creates halos around points of light
Coma, which makes points of light look like comets with blurry tail-like smudges
Loss of contrast
Hyperopia – (hi-pur-OH-pee-uh) Farsightedness. It’s one of the lower order aberrations (along with myopia and astigmatism), and is correctible by glasses, contact lenses, and LASIK procedures. The higher order aberrations are more subtle and are non-refractive errors of the eye.
Numbers – H | I – Q | R – Z