Glossary of Ophthalmology Terms
A
Acanthamoeba keratitis
Acanthamoeba is a microscopic, free-living amoeba that is relatively common in the environment. Most people will be exposed to Acanthamoeba during their lifetime and will not get sick. However, Acanthamoeba is capable of causing several infections in humans including Acanthamoeba keratitis, a local infection of the eye that typically occurs in healthy persons and can result in permanent visual impairment or blindness.
Accommodation
Adjustment of focal power of the eye from distance to near vision achieved by contraction of the ciliary muscle which causes a thickening of the crystalline lens and a slight forward shift in its position, both of which increase its refractive power.
Age-Related Macular Degeneration (AMD)
A disease associated with aging that gradually destroys sharp, central vision. Central vision is needed for seeing objects clearly and for common daily tasks such as reading and driving. AMD affects the macula, the part of the eye that allows you to see fine detail. AMD causes no pain. In some cases, AMD advances so slowly that people notice little change in their vision. In others, the disease progresses faster and may lead to a loss of vision in both eyes. AMD is a leading cause of vision loss in Americans 60 years of age and older. AMD occurs in two forms: wet and dry.
Amsler Grid
Visual field testing grid consisting of evenly spaced horizontal and vertical lines, typically white lines on a dark background with a central dot to mark the point of fixation; used as a simple test for detecting defects or distortions in the central 20 degrees of the visual field.
Anisocoria
Uneven size of pupils in the two eyes, usually reserved to describe more than a 1 mm difference in diameter.
Astigmatism
Visual defect attributable to the presence of an elliptical rather than spherical shape in the refracting surfaces of the eye, resulting in the diffusion of light rays along a particular line.
Autofluorescence
Autofluorescence is the self emission of light. It is distinguished from other forms of fluorescence which require the accumulation of a drug or dye in order to emit light. Autofluorescence has been used to identify retinal pathology by illuminating the tissue with blue light. Elements within the cell selectively absorb the light and re-emit this light typically at a longer wavelength, which can then be captured by a sensor. Several studies demonstrate that autofluorescence signals are reliable markers of lipofuscin in retinal pigment epithelium (RPE) cells. There is considerable interest in the effect of lipofuscin on RPE function and its role in retinal diseases.
B
Bjerrum’s Area
Area of retinal nerve fibers corresponding to the area between 12 and 20 degrees of the visual field; this area is the most vulnerable area to damage by glaucoma.
Blind Spot
Area where the retina is joined to the optic nerve such that it forms a “funnel” of nerve cells that is not sensitive to light at its center; not usually noticed subjectively but readily detected even with the most simple visual field test. Also called the physiologic scotoma.
Blindness
Partial or total lack of the visual sense, more properly referred to as amaurosis.
C
Central Retinal Artery Occlusion (CRAO)
Blockage of the central retinal artery resulting in sudden, permanent loss of vision across a wide area of the visual field.
Central Retinal Vein Occlusion (CRVO)
Blockage of the central retinal vein resulting in retinal hemorrhage and sudden loss of vision, usually involving the central visual field.
Central Serous Chorioretinopathy (CSCR)
Condition in which there is swelling and elevation of retinal tissues in the area of the macula, sometimes progressing to the point of detachment that causes a perceptible but usually not permanent visual field loss.
Choroidal Neovascularization (CNV)
Abnormal growth of new blood vessels in the choroid. Commonly associated with AMD, but can also occur as a result of other eye conditions.
Chorioretinitis
An inflammation of the back of the eye involving the choroid and retina. May be caused by a number of different diseases which affect the body, such as toxoplasmosis, histoplasmosis, sarcoidosis, tuberculosis and syphilis.
Choroid
Highly vascular tissue layer lying under the retina, merging at the angle of the anterior chamber with the ciliary body and the iris.
Choroiditis
Inflammatory disease of the choroid; type of posterior uveitis.
Computerized Tomography Scan (CT Scan)
Imaging technique using ionizing radiation to visualize inner structures of the body; in ophthalmology used to evaluate fractures, inflammation or tumors.
Cone Cells or Retinal Cones
One of the two light-receiving retinal cells (the other is the rod) that is responsible for daylight vision, color vision, high visual acuity and bright light vision. The area of the retina that provides central vision, known as the fovea, contains only cones.
Cone Dystrophy
Hereditary degenerative disorder of the cone cells with progressive loss of central and color vision.
Cone-Rod Degeneration
Also called cone-rod dystrophy. Leads to a loss of visual acuity between 20/200 to 20/400 – legal blindness. First there is a loss of cone photoreceptors followed by a loss of rod photoreceptors. Other symptoms include visual fields loss, abnormal color vision and photophobia. Currently, no treatment is available.
Confocal Laser Scanning Microscopy (CLSM)
A technique for obtaining high-resolution optical images. The key feature of confocal microscopy is its ability to produce in-focus images of thick specimens, a process known as optical sectioning. Images are acquired point-by-point and reconstructed with a computer, allowing three-dimensional reconstructions of topologically-complex objects. The principle of confocal microscopy was originally patented by Marvin Minsky in 1957, but it took another thirty years and the development of lasers for CLSM to become a standard imaging technique.
Cornea
Clear structure at the front of the eye overlying the iris. It imparts the greatest focusing power of all the ocular media. Composed of five distinguishable layers: epithelium, Bowman’s membrane, the stroma, Descemet’s membrane and the endothelium.
Corneal Abrasion
Injury in which tissues are scraped from an area on the surface of the cornea, usually involving the cornea epithelium but possibly extending more deeply into the corneal tissue.
Corneal Dystrophy
General term for a hereditary condition in which there is defective development or degeneration of corneal tissue.
Corneal Edema
Condition in which the cornea swells with water and becomes cloudy; almost always a result of damage to the corneal endothelium.
Corneal Erosion
Loss of the corneal epithelium over some or all of the area of the cornea.
Corneal Ulcer
Loss of tissue from the surface of the cornea due to a disease process.
Cornea in vivo Histology
The spatial arrangement of the epithelium, nerves and keratocytes that can be visualized by 3D-CLSM. 3D-CLSM enables analysis of the spatial arrangement of the anterior corneal nerves and visualization of the epithelium and keratocytes in the living human cornea. The developed method provides a basis for further studies of alterations of the cellular arrangement and epithelial innervation in corneal disease. This may help to clarify alterations of nerve fiber patterns under various clinical and experimental conditions.
Cupping of the Optic Disc
A sign of glaucomatous damage in which the optic disc is affected by an area of increasing concavity, representing non-functioning retinal ganglion cells.
Cycloplegia
Paralysis of the ciliary muscle in which the eye does not accommodate in response to the usual stimuli.
Cystoid Macular Edema (CME)
Swelling of the central focusing area of the retina, typically as a result of trauma or as a complication of ophthalmic surgery.
D
Detached Retina
A condition in which the retina separates from another layer of cells in the back of the eye, resulting in a decrease in nutrition and visual function. It may be due to a hemorrhage, trauma, tumor, vascular malformation or from traction of the vitreous to which it is attached. Sometimes people with high myopia will develop a retinal detachment, which requires emergency surgery.
Diabetic Retinopathy
Ocular effects of diabetes mellitus, characterized by edema, bleeding, and neovascularization of the retina with progressive loss of vision if left untreated; laser therapy is currently used in treatment.
Dilation
General term for widening of an opening. In ophthalmology, the pupil widens in dim light or as a result of pharmaceuticals; dilation must be induced in order to perform certain intraocular examinations and surgical procedures.
Diopter
Measure of focusing power of a lens, defined as the reciprocal of the focal length of the lens or of the eye’s optical system in meters.
Diplopia
Perception of two images where there is only one object (colloquially known as double vision).
Dominant Progressive Foveal Dystrophy
Dominant Stargardt’s disease. A degeneration of the RPE and photoreceptors of the eye. Slowly progressive, leading to legal blindness later in life. Usually starts in the 20s to 40s. Results in decreased visual acuity, central scotoma and defective color vision. There is no treatment.
Drusen
Circular, yellowish bodies that appear on the choroid as a consequence of aging or in some retinal degenerations; vision is rarely affected.
E
Endothelial Cell Counting
The determination of the number of endothelial cells present on the posterior surface of the cornea may be an essential aspect of the evaluation for patients who are being considered for various intraocular procedures, such as cataract surgery. This diagnostic procedure may be included as part of complete preoperative evaluation, especially if the cornea appears abnormal.
Endophthalmitis
Inflammation of the internal ocular tissues, occasionally an infection after surgery or penetrating injury that can lead to a loss of vision and the eye itself if not controlled.
Epiretinal Membrane (ERM)
Detachment of the internal limiting membrane from the retina and vitreous body. Localized epiretinal membranes may occur at the posterior pole of the eye without clinical signs, or may cause marked loss of vision as a result of distorting or detaching the fovea centralis.
F
Floaters
Small condensations of cells in the vitreous body, the fluid in the eye, which cast shadows on the back of the eye (retina). This is normally associated with aging. Floaters may indicate a more serious problem such as a retinal detachment.
Fluorescein Angiography
Imaging technique in which fluorescein dye is injected into the arterial system. The dye fluoresces, revealing the circulatory system of the retina and choroids in vivo.
Fovea
Central pit in the macula that produces sharpest vision. Contains a high concentration of cones and no rods or retinal blood vessels.
Fundus
The bottom or base of any hollow organ. The fundus of the eye is the interior posterior surface that is usually viewied by a doctor with an ophthalmoscope.
Fuch’s Dystrophy
A slowly progressing disease that usually affects both eyes. Fuchs’ dystrophy occurs when endothelial cells gradually deteriorate without any apparent reason. As more endothelial cells are lost over the years, the endothelium becomes less efficient at pumping water out of the stroma. This causes the cornea to swell and distort vision.
Fusarium keratitis
Fusarium is a fungus commonly found in organic matter such as soil and plants. Infection cannot be transmitted from person to person. People who have trauma to the eye, certain eye diseases and problems with their immune system may be at increased risk for this type of infection. An inflammation of the cornea (keratitis) due to Fusarium infection is called Fusarium keratitis.
G
Glaucoma
Glaucoma is a general term for any disease that causes damage to the eye’s optic nerve and result in vision loss and blindness. Primary Open-Angle Glaucoma is a leading cause of blindness throughout the world. Primary Open-Angle Glaucoma causes irreversible vision loss. In its early stages, there are no symptoms.
Glaucoma Probability Score (GPS)
The Glaucoma Probability Score, or GPS, is an automated analysis performed by the HRT Glaucoma Module Premium Edition software that combines 3-D modeling of the entire topographical image with an advanced neural network classification technique. The analysis is based on the 3-D shape of the optic disc and peripapillary retinal nerve fiber layer (RNFL). The GPS utilizes large, ethnic-selectable databases and employs a new form of artificial intelligence: a Relevance Vector Machine (RVM). The RVM derives the probability of damage consistent with glaucoma.
Gonioscopy
The examination of the anterior chamber angle between the iris and cornea. This is accomplished by placing a special type of contact lens (goniolens) over the cornea.
H
Hemianopia
Partial or total loss of vision in half the visual field in one or both eyes; the upper or lower portions of the visual field, as well as the right and left sides, can be affected.
Hyperopia
A condition in which visual images come to a focus behind the retina of the eye and vision is better for distant, rather than for near objects (farsightedness). Hyperopia may be due to the surface(s) of the cornea and/or crystalline lens having reduced (too flat) curvature, an eyeball which is too short, and/or an index of refraction of one of the ocular media that is too low.
I
Indocyanine Green (ICG)
A special dye used in imaging to evaluate the circulatory system of the choroid, the layer just behind the retina. ICG reacts to light with a longer wavelength than fluorescein dye, allowing the clinician to pinpoint the location of leaking vessels deeper within the eye that may not be apparent with fluorescein angiography.
Intraocular Pressure
Pressure within the eye caused by the dynamics of the formation and drainage of aqueous humor; measured like atmospheric pressure as the height of a column of mercury that the pressure can support, thus the unit of measure is in millimeters of mercury.
Iris
Mobile, vascular, ring-shaped structure that lies behind the cornea and in front of the crystalline lens; its movements control the size of the pupil and, thus, the amount of light passing through to the retina; attached at its outer edge to the ciliary body and covered with a highly pigmented epithelial layer.
Iritis
Inflammation of the iris characterized by pain, photophobia, and redness; type of anterior uveitis.
K
K-Reading
Radius of corneal curvature (measured with a keratometer) and expressed in millimeters or diopters.
Keratitis
Keratitis is an inflammation of the cornea, the front part of the eye. Keratitis has many causes including bacteria, viruses and fungi. Keratitis is frequently associated with long term contact lens wear. It can be difficult to determine the cause of the keratitis. If left untreated or misdiagnosed, scarring and/or loss of the cornea may result.
Keratoconus
Progressive malformation of the cornea such that it is thin and cone-like in shape rather than rounded; causes painless visual distortion and loss; also called ecstatic corneal dystrophy.
Keratoplasty
General term for a corneal grafting procedure (commonly called a corneal transplant) in which a patient’s cornea is removed with a keratome, and replaced with a donor tissue.
L
Laser
A device generating a concentrated beam of light having the characteristics of a single wavelength (single color), coherence and collimated. Various materials can be used to generate the laser emission, resulting in different wavelengths of light. When applied in medicine, lasers have variable effects on tissue and can be used for heating, cutting, photochemical and non-invasive imaging. Laser is an acronym for Light Amplification through the Simulated Emission of Radiation.
M
Macula
Small central area of the retina surrounding the fovea containing mostly cones; it is the area of acute central vision.
Moorfields Regression Analysis (MRA)
The Moorfields Regression Analysis (MRA) is the regression of the logarithm of the rim area to the disc area compared to the selected ethnic-specific normative database. It provides a predicted value for the rim size depending on the given disc size and age of the patient.
Mydriatic
Excessive or prolonged dilation of the pupil of the eye.
Myopia
Commonly known as nearsightedness. A condition in which visual images come to a focus in front of the retina of the eye and vision is better for near than for far objects; may be due to the surface(s) of the cornea and/or crystalline lens having excessive (too steep) curvature, an eyeball which is too long, and/or an index of refraction of one of the ocular media that is too high.
O
The Ocular Hypertension Treatment Study (OHTS)
A multi-centered longitudinal study to determine the safety and efficacy of topical ocular hypertensive medication in delaying or preventing the onset of Primary Open Angle Glaucoma (POAG).
Optic Atrophy
A disease of the optic nerve in which the nerve fibers carrying the electrical impulses from the eye to the brain start to die off. In such cases, the optic nerve has a pale or whitish appearance instead of the normal pink color. Optic atrophy is associated with poor reading vision and often is the cause of legal blindness. May be associated with a serious medical condition and requires further medical examination to determine the cause of the atrophy. This condition is irreversible.
Optic Disc
The ocular end of the optic nerve head. Denotes the exit of retinal nerve fibers from the eye and entrance of blood vessels to the eye.
Optic Nerve
The largest sensory nerve of the eye; transmits electrical impulses for sight from the retina to the brain.
Optic Nerve Hypoplasia
Incomplete development of the optic nerve.
Optic Neuritis
An inflammation of the optic nerve usually with some loss of sight (may be temporary). It may signify a more serious neurological condition. A leading cause of optic neuritis is multiple sclerosis (MS).
P
Pachymetry
A test for measuring the thickness of the cornea or anterior chamber depth, using the optical principle of split images.
Perimetry
Method of charting the extent of a stationary eye’s field of vision with test objects of various sizes and light intensities. Aids in detection of damage to sensory visual pathways.
Peripheral Vision
Side vision; vision elicited by stimuli falling on retinal areas distant from the macula.
Photophobia
Abnormal sensitivity to, and discomfort from, light. May be associated with excessive tearing. Often due to inflammation of the iris or cornea.
Photo Refractive Keratectomy (PRK)
Use of high intensity laser light (e.g., an excimer laser) to reshape the corneal curvature to correct for refractive errors.
Ptosis
Drooping of upper eyelid. May be congenital or caused by paralysis or weakness of the third cranial nerve or sympathetic nerves, or by excessive weight of the upper lids.
Pupil
Variable-sized black circular opening in the center of the iris that regulates the amount of light that enters the eye.
R
Refraction
Test to determine an eye’s refractive error and the best corrective lenses to be prescribed. Series of lenses in graded powers are presented to determine which provide the sharpest, clearest vision.
Refractive Error
Optical defect in an unaccommodating eye; parallel light rays are not brought to a sharp focus precisely on the retina, producing a blurred retinal image. Can be corrected by eyeglasses, contact lenses, or refractive surgery.
Retina
Light sensitive nerve tissue in the eye that converts light into electrical impulses that are sent along the optic nerve to the brain. Forms a thin membranous lining of the rear two-thirds of the globe.
Retinal Detachment
Separation of the retina from the underlying pigment epithelium. Disrupts visual cell structure and thus markedly disturbs vision. Almost always caused by a retinal tear; often requires immediate surgical repair.
Retinal Nerve Fiber Layer (RNFL)
Innermost retinal layer (closest to the vitreous). Contains axons and ganglion cells that follow a characteristic pattern toward the optic disc: Where all fibers exit the eye as the optic nerve.
Retrobulbar Neuritis
An inflammation of the optic nerve immediately behind the eyeball. It causes a loss in vision and is sometimes indicative of a neurological disease.
Rod
Light-sensitive, specialized retinal receptor cell that works at low light levels (night vision). A normal retina contains 150 million rods.
S
Schlemm’s Canal
Circular channel deep in limbus that carries aqueous fluid from the anterior chamber of the eye to the bloodstream.
Sclera
Opaque, fibrous, protective outer layer of the eye (”white of the eye”) that is continuous with the cornea in front and with the sheath covering the optic nerve behind.
Scotoma
An absence of vision in part of the visual field. It is present in such conditions as glaucoma, or in more serious diseases within the brain. Often detected by a visual field test.
Slit Lamp
An instrument consisting of a high-intensity light source that can be focused to shine as a slit lamp. It is used in conjunction with a microscope to facilitate examining the eye; allows cornea, lens and otherwise clear fluids and membranes to be seen in layer-by-layer detail.
Snellen Chart
The familiar eye chart with larger letters at the top and smaller ones at the bottom. Used for measuring central vision.
Stereopsis
Also known as depth perception. The separation between the eyes provides for slightly different views of an object by each eye. The brain, for the purpose of determining the location of an object in 3-D space, uses this difference in views between the eyes or for disparity between the eyes.
Stargardt’s Disease
Also called juvenile macular degeneration. Early in the course of the disease the retina may look normal to the eye doctor. Later in the disease process, there is a total loss of the RPE and photoreceptors in the macula. Disease progression is rapid, leading to a central scotoma, reduced central vision, legal blindness and some loss of color vision by the age of 20 years. Patients sometimes become photophobic. No treatment is available. Also see dominant progressive foveal dystrophy.
T
Tonometer
An instrument used to measure the pressure within the eye. This is one of several factors used in diagnosis of glaucoma. The results may also be used to follow the response of treatment to this disease.
Tomography
Imaging by sections or sectioning; any of several techniques for making detailed three-dimensional images of a plane section of a solid object in the body, such as the optic nerve head.
Trabecular Meshwork
Mesh-like structure inside the eye at the iris-scleral junction of the anterior chamber angle. Filters aqueous fluid and controls its flow into the canal of Schlemm, prior to its leaving the anterior chamber.
U
Uvea, Uveal Tract
Pigmented layers of the eye (iris, ciliary body, choroid) that contain most of the intraocular blood vessels.
Uveitis
Inflammation of the uveal tract. It may be anterior involving the iris and ciliary body (iridocyclitis) or posterior involving the choroid (choroiditis).
V
Visual Acuity
Assessment of the eye’s ability to distinguish object details and shape, using the smallest identifiable object that can be seen at a specified distance (usually 20 ft. or 16 in.).
Visual Field
Full extent of the area visible to an eye that is fixating straight ahead.
Vitreous
The transparent gelatinous material which fills the interior of the eye behind the lens. It allows the eye to maintain its shape.
Vortex Veins
Four veins (2 superior, 2 inferior) that provide the main blood outflow from the eyes, exiting near the equator on each side of the superior and inferior recti and draining into the superior and inferior ophthalmic veins.
W
Window Defect
Retinal pigment epithelial defect due to cellular loss. With fluorescein angiography, choroidal vessels that are usually unseen are visible.
X
Xerophthalmia
Drying of eye surfaces. Characterized by loss of corneal and conjunctival luster, Bowman’s membrane degeneration and infiltration of the corneal stroma with cells and fluid. Associated with vitamin A deficiency.
Y
Y axis of Fick
Imaginary line through an eye’s center of rotation, connecting the geometric center of the cornea (anterior pole) with geometric center of the back of the eye (posterior pole). Torsional (tilting) eye rotations occur around the axis.
Z
Z axis of Fick
Imaginary line that passes vertically through the center of each eye; associated with horizontal eye rotations.
Zero Vergence
Refers to light rays that are parallel and have no converging or diverging optical power.
Zonules
Radial fibers that suspend the lens from the ciliary body and hold it in position. (See also accommodation)