Medical Dictionary Definitions A-Z List
Medical Dictionary Definitions A - Z - «O»:
Opioid: 1. A synthetic narcotic that resembles the naturally occurring opiates. 2. Any substance that binds to or otherwise affects the opiate receptors on the surface of the cell....
Opisthotonos: A great rigid spasm of the body with the back fully arched and the heels and head bent back. Opisthotonos occurs in meningitis, especially in infants, due to irritation of the membranes surrounding the brain and spinal cord. It also occurs with depressed brain function and injury to th...
Opium: An addictive narcotic drug derived from the unripe seedpods of the opium poppy. Preparations of opium were called laudanum. Derivatives of laudanum include paregoric (a drug to treat diarrhea), morphine and heroin.
For centuries, opium was used as a painkiller in the Middle and Far East. It...
Opportunistic condition: A condition that occurs especially
or exclusively in persons with a weak immune system due, for example,
to AIDS, cancer or immunosuppressive drugs, such as corticosteroids
or chemotherapy. An opportunistic condition may be an infection, such
as toxoplasmosis or cytomega...
Opportunistic infection: An
infection that occurs
because of a weakened immune system. Opportunistic
infections are a particular danger for people with AIDS.
The HIV virus itself does
not cause death, but the opportunistic infections that occur because of its
effect on the immune system can. S...
Opportunistic microorganism: A bacteria, virus,
or fungus that takes advantage of certain opportunities to
cause disease. Those opportunities are called opportunistic
conditions. These microorganisms are often ones that can
lie dormant in body tissues for many years, such as the
human herpes vi...
Optic: Having to do with vision. ...
Optic cup: The white, cup-like area in the center of the optic disc. The optic disk is the circular area at the back of the inside of the eye where the optic nerve connects to the retina....
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....
Optic glioma: A benign tumor on an optic nerve or the optic chiasm (the crossing of the two optic nerves). Optic gliomas are derived from glial cells and are classified as low-grade astrocytomas. They cause symptoms both by pressure and by destruction of normal optic nerve tissue. They may result i...
Optic nerve: The optic nerve connects the eye to the
brain. The optic nerve carries the impulses formed by the retina, the
nerve layer that lines the back of the eye and senses light and
creates impulses. These impulses are dispatched through the
optic nerve to the brain, which interprets them...
Optic nerve head
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
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 ...
Optic neuroma: A slow-growing benign tumor of the optic nerve. Optic gliomas are associated with neurofibromatosis type 1 (NF1), can lead to partial or complete loss of vision, painless bulging of the eye, and headache, and are treated by surgical excision (removal) or, if too extensive, by radiothe...
Optician: A specialist in fitting eyeglasses and
making lenses to correct vision problems. An optometrist
performs eye examinations and writes prescriptions for
corrective lenses; an optician fills that prescription....