Most, but not all, forms of glaucoma are characterized by high eye (intraocular) pressure. Intraocular pressure remains normal when some of the fluid (aqueous humor) produced by the eye's ciliary body flows out freely (follow blue arrow). Aqueous humor flows from the ciliary body into the anterior chamber, out through a spongy tissue at the front of the eye called the trabecular meshwork and into a drainage canal (dark blue region next to the trabecular meshwork). In open-angle glaucoma, fluid does not flow freely through the trabecular meshwork, causing an increase in intraocular pressure, damage to the optic nerve and vision loss.
Glossary of Terms
Anterior chamber: The region of the eye between the cornea and the lens that contains aqueous humor.
Aqueous humor: The fluid produced in the eye.
Bruch's membrane: Located in the retina between the choroid and the retinal pigmented epithelium (RPE) layer; provides support to the retina and functions as the 'basement' membrane of the RPE layer.
Ciliary body: Part of the eye, above the lens, that produces the aqueous humor.
Choroid: Layer of the eye behind the retina, contains blood vessels that nourish the retina.
Cones: The photoreceptor nerve cells present in the macula and concentrated in the fovea (the very center of the macula); enable people to see fine detail and color.
Cornea: The outer, transparent structure at the front of the eye that covers the iris, pupil and anterior chamber.
Drusen: deposits of yellowish extra cellular waste products that accumulate within and beneath the retinal pigmented epithelium (RPE) layer.
Fovea: The pit or depression at the center of the macula that provides the greatest visual acuity.
Iris: The colored ring of tissue behind the cornea that regulates the amount of light entering the eye by adjusting the size of the pupil.
Lens: The transparent structure suspended behind the iris that helps to focus light on the retina.
Macula: The portion of the eye at the center of the retina that processes sharp, clear straight-ahead vision.
Optic nerve: The bundle of nerve fibers at the back of the eye that carry visual messages from the retina to the brain.
Photoreceptors: The light sensing nerve cells (rods and cones) located in the retina.
Pupil: The adjustable opening at the center of the iris through which light enters the eye.
Retina: The light sensitive layer of tissue that lines the back of the eye.
Retinal Pigmented Epithelium (RPE): A layer of cells that protects and nourishes the retina, removes waste products, prevents new blood vessel growth into the retinal layer and absorbs light not absorbed by the photoreceptor cells; these actions prevent the scattering of the light and enhance clarity of vision.
Rods: Photoreceptor nerve cells in the eyes that are sensitive to low light levels and are present in the retina, but outside the macula.
Sclera: The tough outer coat that protects the entire eyeball.
Trabecular meshwork: Spongy tissue located near the cornea through which aqueous humor flows out of the eye.
Vitreous: Clear jelly-like substance that fills the eye from the lens to the back of the eye.
Illustration by Bob Morreale, provided courtesy of the BrightFocus Foundation.