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- occlusion
- The transient approximation of the edges of a natural opening, i.e., occlusion of the eyelids.
- ocellus
- A patch of light-sensitive receptor cells in the skin, present in some species.
- ocular
- Of, pertaining to, or affecting the eye.
- ocular hypertension
- Condition in which the intraocular pressure of the eye is elevated above normal without
any obvious optic nerve damage or visual field defects. Over time, ocular hypertension
may develop into glaucoma.
See Free Eye Tests.
- ocular motility
- The movement of the eye.
- onchocerciasis
- River blindness, a disease caused by a parasitic worm (Onchocerca volvulus) transmitted by biting blackflies (buffalo gnats) that breed in fast-flowing rivers. The adult worms can live for up to 15 years in nodules beneath the skin and in the muscles of infected persons, where they produce millions of worm embryos (microfilariae) that invade the skin and other tissues including the eyes. About 18 million persons are affected, mostly in Africa and also in Yemen and Latin America. Both living and dead microfilariae cause severe itching in the skin and sometimes blindness after many years.
Since 1987, the drug ivermectin (brand name: Stromectol) has been provided by the manufacturer (Merck) free of charge. A single oral dose administered once a year prevents the accumulation of microfilariae in persons at risk. No drug suitable for mass treatment can kill the adult worms in the body, and therefore, onchocerciasis cannot be wiped out. The blindness, however, can be eliminated.
- open-angle glaucoma
- The less serious of two types of glaucoma, a condition caused by excessive buildup of
fluid inside the eye putting pressure on the retina. Sometimes called chronic glaucoma, it is
caused by a gradual blocking of aqueous outflow from the eye. It can develop slowly with
no noticeable symptoms. If untreated, open-angle glaucoma results in a gradual, painless,
irreversible loss of vision.
See Free Eye Tests.
- ophthalmologist
- A medical doctor (M.D.) with education, training and experience in medical and surgical
treatment of eye diseases and disorders.
- ophthalmoscope
- An instrument for viewing the interior of the eye. It comprises a concave mirror with a hole in the center through which the observer examines the eye, a source of light that is reflected into the eye by the mirror, and lenses in the mirror that can be rotated into the opening in the mirror to neutralize the refracting power of the eye being examined. It makes the image of the retina clear.
- ophthalmology
- Branch of medical science that deals with the structure, functions and diseases of the
human eye.
- opsin
- Protein portion of the visual pigment of rod cells.
- optic disc
- The circular area in the back of the inside of the eye where the optic nerve connects to the retina. Also called the optic nerve head. Contains no photoreceptors and therefore creates a blind spot in the visual field.
- optic nerve
- Connects the eye to the brain. It carries the impulses formed by the retina, the nerve layer that lines the back of the eye and senses light and creates impulses. The brain interprets the images. Using an ophthalmoscope, the head of the optic nerve can be seen. It can be viewed as the only visible part of the brain (or extension of it).
- optic nerve head
- The circular area in the back of the inside of the eye where the optic nerve connects to the retina.
- optic nerve pathways
- The left and right branches of the optic nerves join behind the eyes, just in front of the pituitary gland, to form a cross-shaped structure called the optic chiasma. Within the optic chiasma, some of the nerve fibers cross. The fibers from the nasal (inside) half of each retina cross over, but those from the temporal (outside) half do not. Specifically, the fibers from the nasal half of the left eye and the temporal half of the right eye form the right optic tract; and the fibers from the nasal half of the right eye and the temporal half of the left form the left optic tract. The nerve fibers then continue along in the optic tracts. Just before they reach the thalamus of the brain, a few of the nerve fibers leave to enter nerve nuclei that function in visual reflexes. Most of the nerve fibers enter the thalamus, forming a junction (synapse) in the back of the thalamus. From there the visual impulses enter nerve pathways called the optic radiations, which lead to the visual (sight) cortex of the occipital (back) lobes of the brain.
- optic neuritis
- Inflammation of the optic nerve. Characterized by rapid onset of decreased vision and
usually accompanied by discomfort upon eye movement and central visual field defect.
See Free Eye Tests.
- optic neuroma
- A benign tumor of the optic nerve.
- optic tract
- The portion of each optic nerve between the optic chiasma and the diencephalon proper.
- optician
- Technicians trained and, in some states, certified to fill lens prescriptions and contacts and
to fit eyeglasses. Some specialize in contact lenses and work in optometric or medical
offices, working with an optometrist or ophthalmologist to fit contact lenses.
- optometrist
- Doctors of Optometry (O.D.) are primary health care providers who examine, diagnose,
treat and manage diseases and disorders of the eye and associated structures. In
accordance with state law, optometrists prescribe, fit and dispense ocular medications,
glasses and contact lenses. An optometrist cannot perform surgery but often works with
an ophthalmologist on pre-and post-surgical care.
- optometry
- The profession of examining the eyes, measuring vision and prescribing corrective lenses,
diagnosing diseases of the eye and treating certain conditions that do not require license as
a medical doctor.
- orbit
- Pyramid-shaped cavity in the skull containing the eyeball, its muscles, blood supply, nerve
supply and fat.
- orbital tumor
- Process by which a living organism assimilates food and uses it for growth and
tissue replacement. In the prevention of ocular disease, the role of vitamins and
minerals has taken on a major role. To maintain the health of the eye, a high-fiber,
high-carbohydrate, high-antioxidant, low-fat, low-protein diet is best.
See Free Eye Tests.
- orthokeratology
- Controversial nonsurgical contact lens procedure designed to eliminate nearsightedness
and astigmatism. The procedure involves the programmed use of a series of rigid oxygen
permeable contact lenses that are designed to gradually change the contour of the front of
the cornea. The lenses are changed every two to eight weeks until the maximum desired
result is achieved. Then, retainer contact lenses are worn as necessary to stabilize the
shape.
See Free Eye Tests.
- overconvergence
- Condition in which the eyes come too far inward when focusing on a near object, resulting
in blurring.
- oversize lens
- A lens that exceeds 61 millimeters in diameter
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