Accommodation: The adjustment of the eye for seeing at near distances, accomplished by changing the shape of the lens through action of the ciliary muscle, thus focusing a clear image on the retina.
Acquired: Contracted after birth.
Agnosia: Inability to recognize common objects despite an intact visual apparatus.
Albinism: A hereditary deficiency of melanin pigment in the retinal pigment epithelium, iris, and choroid.
Alternate cover test: Determination of the full extent of heterotropia and heterophoria by alternately covering one eye and then the other with an opaque object, thus eliminating fusion.
Amaurosis fugax: Transient loss of vision. Usually reserved for transient loss of vision due to retinal embolus.
Amblyopia: Reduced visual acuity in the absence of sufficient eye or visual pathway disease to explain the level of vision.
Ametropia: See Refractive error.
Amsler grid: A chart with vertical and horizontal lines used for testing the central visual field.
Angiography: A diagnostic test in which the vascular system is examined. The ocular circulation can be highlighted by intravenous injection of either fluorescein, which particularly demonstrates the retinal circulation, or indocyanine green, to demonstrate the choroidal circulation.
Aniridia: Congenital absence of the iris.
Aniseikonia: A condition in which the image seen by one eye differs in size or shape from that seen by the other.
Anisocoria: Unequal pupillary size.
Anisometropia: Difference in refractive error of the eyes.
Anophthalmos: Absence of a true eyeball.
Anterior chamber: Space filled with aqueous bounded anteriorly by the cornea and posteriorly by the iris.
Aphakia: Absence of the crystalline lens.
Aqueous: Clear, watery fluid that fills the anterior and posterior chambers.
Asthenopia: Eye fatigue from muscular, environmental, or psychological causes.
Astigmatism: Refractive error that prevents the light rays from coming to a point focus on the retina because of different degrees of refraction in the various meridians of the cornea or crystalline lens.
Axis: The meridian specifying the orientation of a cylindric lens.
Binocular vision: Ability of the eyes to focus on one object and then to fuse two images into one.
Biomicroscope: See Slitlamp.
Bitot’s spots: Keratinization of the bulbar conjunctiva near the limbus, resulting in a raised spot—a feature of vitamin A deficiency.
Blepharitis: Inflammation of the eyelids.
Blepharoptosis (ptosis): Drooping of the eyelid.
Blepharospasm: Involuntary spasm of the lids.
Blind spot: “Blank” area in the visual field, corresponding to the light rays that come to a focus on the optic nerve.
Blindness: In the United States, the usual definition of blindness is corrected visual acuity of 20/200 or less in the better eye, or a visual field of no more than 20° in the better eye.
Botulinum toxin: Neurotoxin A of the bacterium Clostridium botulinum used in very small doses to produce temporary paralysis of the extraocular or facial muscles.
Buphthalmos: Large eyeball in infantile glaucoma.
Canal of Schlemm: A circular modified venous structure in the anterior chamber angle that drains aqueous to the aqueous veins.
Canaliculus: Small tear drainage tube in inner aspect of upper and lower lids leading from the punctum to the common canaliculus and then to the tear sac.
Canthotomy: Usually implies lateral canthotomy—cutting of the lateral canthal tendon for the purpose of widening the palpebral fissure.
Canthus: The angle at either end of the eyelid aperture; specified as outer (lateral) or inner (medial).
Cataract: An opacity of the crystalline lens.
Chalazion: Granulomatous inflammation of a meibomian gland.
Chemosis: Conjunctival edema.
Choroid: The vascular middle coat between the retina and sclera.
Ciliary body: Portion of the uveal tract between the iris and the choroid. It consists of ciliary processes and the ciliary muscle.
Coloboma: Congenital cleft due to the failure of some portion of the eye or ocular adnexa to complete growth.
Color blindness (deficiency): Diminished ability to perceive differences in color.
Concave lens: Lens having the power to diverge rays of light; also known as diverging, reducing, negative, or minus lens, denoted by the sign (−), and used to correct myopia.
Cones and rods: Two kinds of retinal photo receptor cells. Cones are primarily involved in fine visual discrimination (optimal visual acuity) and color vision; rods with peripheral vision and vision in decreased illumination.
Congenital: Existing at or before birth, not necessarily inherited (hereditary).
Conjunctiva: Mucous membrane that lines the posterior aspect of the eyelids and covers the anterior sclera.
Contact lenses: Thin lenses that fit directly on the eye, usually on the cornea but sometimes on the sclera.
Convergence: The process of directing the visual axes of the eyes to a near point.
Convex lens: Lens having power to converge rays of light and to bring them to a focus; also known as converging, magnifying, or plus lens, denoted by the sign (+), and used to correct hyperopia or presbyopia.
Cornea: Transparent portion of the outer coat of the eyeball forming the anterior wall of the anterior chamber.
Corneal graft (keratoplasty): Operation to replace a portion of the cornea, either involving the full thickness (penetrating keratoplasty), only a superficial layer (lamellar keratoplasty), or only the endothelium (endothelial keratoplasty), with donor cornea from the same human (autograft), or another human (homograft).
Cover test: Determination of the presence and degree of heterotropia by covering one eye with an opaque object and examining for any movement of the uncovered eye to fixate a target.
Cross cylinder: A specialized spherocylindrical lens used to measure astigmatism.
Crystalline lens: A transparent biconvex structure suspended in the eyeball between the aqueous and the vitreous. Its function is to bring rays of light to a focus on the retina. Accommodation is produced by variations in the magnitude of this effect. (Usually called simply the lens.)
Cyclodestructive procedures: Surgical techniques to reduce aqueous production by destroying portions of the ciliary body in the treatment of intractable glaucoma, using cryotherapy (cyclocryotherapy), lasers (cyclophotocoagulation), or diathermy.
Cycloplegic: A drug that relaxes the ciliary muscle, paralyzing accommodation.
Cylindrical lens: A segment of a cylinder, the refractive power of which varies in different meridians, used to correct astigmatism.
Dacryocystitis: Infection of the lacrimal sac.
Dacryocystorhinostomy: A procedure by which a communication is made between the nasolacrimal duct and the nasal cavity to relieve an obstruction in the nasolacrimal duct, or sac.
Dark adaptation: The ability to adjust to decreased illumination.
Diopter: Unit of measurement of refractive power of lenses.
Diplopia (double vision): Seeing one object as two.
“E” test: A system of testing visual acuity in illiterates, particularly preschool children.
Ectropion: Turning out of the eyelid.
Emmetropia: Absence of refractive error.
Endolaser: Application of laser from a probe inserted into the globe.
Endophthalmitis: Extensive intraocular infection.
Enophthalmos: Abnormal retrodisplacement of the eyeball.
Entropion: A turning inward of the eyelid.
Enucleation: Complete surgical removal of the eyeball.
Epicanthus:
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