“A” selections in the Glossary of Terms for
Anatomy, Physiology & Pathology of the Human Eye

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A, vitamin
see vitamin A

the automatic adjustment of the eye for seeing at different distances, effected chiefly by changes in the convexity of the crystalline lens

achromatopia; a very rare color deficiency in which the colors of the spectrum are seen only as shades of white, gray, and black; can be either incomplete color blindness (blue-cone monochromatism, an X-linked recessive disorder where only the blue cones and the rods are funtioning properly; characterized by reduced visual acuity and sometimes also photophobia and nystagmus) or complete color blindness (rod monochromatism or total color blindness, an autosomal recessive disorder where no cones are functioning and only rods are functioning properly; characterized by reduced visual acuity, photophobia, and nystagmus)

acquired immune deficiency syndrome (AIDS)
also known as acquired immunodeficiency syndrome; a disease of the human immune system that is caused by infection with HIV, that is characterized cytologically especially by reduction in the numbers of CD4-bearing helper T cells to 20 percent or less of normal, that in modern industrialized nations occurs especially in homosexual and bisexual men and in intravenous users of illicit drugs, that is commonly transmitted in blood and bodily secretions (as semen), and that renders the subject highly vulnerable to life-threatening conditions (as Pneumocystis carinii pneumonia) and to some that become life-threatening (as Kaposi's sarcoma)

acuity, visual
see visual acuity

Adie’s tonic pupil
an abnormally dilated pupil that reacts poorly to light and somewhat sluggishly to accommodation, but will constrict with a dilute solution of pilocarpine; associated with Adie’s syndrome, a neurologic syndrome (due to an inflammatory process, such as from a virus, in the third nerve cilliary ganglion) that affects especially women between 20 and 40 and also is characterized by a lack of ankle-jerk and knee-jerk reflexes

age-related macular degeneration (ARMD)
a loss of central vision in both eyes produced by pathological changes in the macula lutea and characterized by spots of pigmentation, leakage of fluid, or other abnormalities

“lazy eye”; inability of an eye to achieve its best expected visual acuity with no observable pathology or reason for the decrease in acuity; most often due to eccentric fixation in an eye with strabismus

a refractive error (such as myopia, hyperopia, or astigmatism) in which images fail to focus upon the retina, requiring spectacle lenses or contact lenses to refocus the incoming light onto the retina

Amsler grid
a grid of 400 small squares (white separated by black lines or black separated by white lines), with a fixation dot in the middle, used to detect macular degeneration in an eye by the presence of “metamorphopsia”—curved, distorted, or broken lines—in the grid

anemia, pernicious
see pernicious anemia

angle, irido-corneal
see irido-corneal angle

congenital or traumatically induced absence or defect of the iris

a difference in size between the images projected on the retinas of both eyes, due to anisometropia or lens corrections of unequal powers; can cause binocular vision difficulties, since the brain attempts to combine two images of different sizes into one image

inequality in the size of the pupils of the eyes, often from injury or disease

a difference in refractive error between the two eyes

ankylosing spondylitis
Marie-Strumpell disease or rheumatoid spondylitis; rheumatoid arthritis of the spine

annulus of Zinn
a cone-shaped structure, behind the eyeball, composed of five extraocular muscles (medial rectus, lateral rectus, superior rectus, inferior rectus, and superior oblique), within which runs the optic nerve (cranial nerve II), the ophthalmic artery, and the ophthalmic vein

anomalous dichromatism
a color deficiency in which one type of cone receptor is missing or does not work at all and only two of three primary colors (red, green, and blue) can be discerned well

anomalous trichromatism
a color deficiency in which all three types of cone receptors work but one type does not work as well as the other two

anterior chamber
the space in the eye bounded in front by the cornea and in back by the iris and middle part of the lens; contains the aqueous humor

anterior compartment
the space in the eye, located anteriorly to the crystalline lens, which is the combination of the anterior and posterior chambers

antimycotic; a substance produced by, or a semisynthetic substance derived from, a microorganism and able in dilute solution to inhibit or kill another microorganism

a substance (such as amphotericin B, clotrimazole, ketoconazole, and tolnaftate) used to treat fungal infections

a substance (such as AZT, ganciclovir, interferon, and trifluridine) used to treat viral infections

aqueous humor
the transparent fluid occupying the anterior compartment (the space between the cornea and the crystalline lens) of the eye; produced by ciliary epithelium and circulates into the posterior chamber (between the iris and the crystalline lens), through the pupil, into the anterior chamber (between the cornea and the iris), and out of the eye through the trabecular meshwork and canal of Schlemm; nourishes the lens and epithelial cells

aqueous layer
see lacrimal layer

a harmless opaque, cloudy ring or arc in the corneal periphery, most often seen in aged persons (arcus senilis) but not uncommonly in middle aged and younger persons (arcus juvenalis); due to the deposition of fat granules in the cornea or to hyaline degeneration; may indicate elevated blood cholesterol levels

argon laser
an ion laser, using argon as the active medium, with two dominant wavelengths—514 nm “green” and 488 nm “blue”—used in argon laser trabeculoplasty (ALT) in which the fluid channels of the trabecular meshwork of the eye are opened, helping aqueous fluid to drain out of the eye more easily and reducing intraocular pressure to help control glaucoma; also used to treat neovascularization in diabetic retinopathy, where the hemoglobin in blood cells absorbs the laser light, causing heat damage and sealing of the blood vessels and controlling further bleeding into the vitreous

Argyll Robertson pupil
a small, irregular pupil that does not react to light but does react to accommodation; associated with the effects of neurosyphilis on the midbrain

a chronic disease characterized by abnormal thickening and hardening of the arterial walls with a resultant loss of elasticity

artery, central retinal
see central retinal artery

artery, hyaloid
see hyaloid artery

inflammation of joints due to infectious, metabolic, or constitutional causes; chronic autoimmune disease (rheumatoid arthritis) of unknown cause that is characterized especially by pain, stiffness, inflammation, swelling, and sometimes deformity or destruction of joints

artificial tears
over-the-counter eyedrops, often formulated to match the pH of human tears, used to soothe the eyes, moisturize dry spots on the eyes, supplement the eye’s own tears, and protect the eye from irritation

asteroid hyalosis
tiny spherical or disc-shaped, “soapy” globs located in the vitreous of one eye or occasionally both eyes; calcium-containing lipid complexes usually suspended throughout the vitreous

a defect of an ocular structure (most commonly the cornea or the crystalline lens) causing rays from a point to fail to meet in a single focal point, resulting in an imperfect blurred or smeared image; an unequal refractive error in the meridians of one eye

an arteriosclerosis characterized by atheromatous deposits in and fibrosis of the innermost layers of arteries

atrophy, optic
see optic atrophy

axis, visual
see visual axis

a usually long and single nerve-cell process (for instance, located in the optic nerve of the eye) that conducts impulses away from the cell body (for instance, located in the retina)

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