The intraocular pressure in the eye is maintained by the aqueous fluid, which brings nutrients to the cornea, iris and lens, and helps the eye maintain its shape. About 4cc of aqueous fluid are produced each day; too much or too little produced or drained from the eye changes the pressure inside the eye and can damage vision, according to VisionRX.
Normal IOP
Normal intraocular pressure, or IOP, is between 10 and 20mm Hg. According to the Merck Manual, an estimated one-third of people with glaucoma (compression of the nerve fibers of the optic nerve), have normal IOP. Normal tension glaucoma is thought to be caused by poor blood supply to the optic nerve, according to AllAboutVision. Normal tension glaucoma causes vision loss in areas of vision corresponding to nerve damage on parts of the optic nerve. vision loss in certain areas is known as visual field defects.
Elevated IOP
Elevated IOP, or ocular hypertension, is a pressure greater than 21mm Hg, and can result in severe vision loss if not treated. High IOP that compresses the optic nerve fibers can permanently damage the eye and is called glaucoma. Glaucoma is not a single disease, but a symptom of elevated IOP, and is classified as narrow angle or open angle, with most cases being open angle.
Glaucoma is the leading cause of blindness in adults, according to Vision RX. Early open angle glaucoma may have no symptoms except for a slow loss of peripheral vision and visual field defects, according to AllAboutVision.com. However, it can cause pain in the eye, blurry vision, vision changes such as halos around lights and dilated pupils, if narrow angle glaucoma occurs. High IOP that doesn't affect the optic nerve is not glaucoma.
Low IOP
IOP is considered abnormally low if it's less than 10mm Hg. A transient drop in IOP may occur after glaucoma surgery or from certain disease processes, such as Hansen's disease (leprosy) or giant cell arteritis, a disease that affects blood vessels. A severe drop in IOP is called optic hypotony, and refers to IOP of less than 6mm Hg. Hypotony can be the result of trauma that causes loss of aqueous fluid from the eye, or as an aftereffect of concussion, according to the Dictionary of Optometry and Visual Sciences. Ocular hypotony can result in vision loss if choroidal or retinal detachment occur as a consequence.
