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Accommodation: The adjustment of the eye for seeing near objects, accomplished by increasing the power of the crystalline lens by changing its shape through the action of the ciliary muscle.

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 strabismus (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 grid of vertical and horizontal lines to test the central 20° square of visual field.

Angiography: Imaging of the vascular system. 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: Image seen by one eye differs in size from that seen by the other.

Anisocoria: Unequal pupillary size.

Anisometropia: Difference in refractive power of the two eyes.

Anophthalmos: Absence of the globe.

Anterior chamber: Space bounded anteriorly by the cornea and posteriorly by the iris that is filled with aqueous.

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: Different power of refraction in various meridians.

Axis: The meridian specifying the orientation of a cylindrical lens.

Binocular vision: Ability of both eyes to focus on an object and fuse the two images into one.

Biomicroscope: See Slitlamp.

Blepharitis: Inflammation of the lids.

Blepharoptosis (ptosis): Drooping of the upper lid.

Blepharospasm: Involuntary spasm of the lids.

Blind spot: “Blank” area in the visual field corresponding to the position of 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° diameter in the better eye.

Botulinum toxin: Neurotoxin A of the bacterium Clostridium botulinum injected into extraocular or facial muscles to produce temporary paralysis.

Buphthalmos: Enlarged globe in infantile glaucoma.

Canal of Schlemm: A circular modified venous structure in the ...

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