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  • 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 ...

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