night blindness
night blindness
night blindness
night′ blind`ness
n.
Noun | 1. | night blindness - inability to see clearly in dim light; due to a deficiency of vitamin A or to a retinal disorder |
单词 | night blindness | |||
释义 | night blindnessnight blindnessnight blindnessnight′ blind`nessn.
night blindnessnight blindness,inability to see normally in subdued light. It is usually a result of vitamin A deficiency. The rod cells, one of two light-sensitive areas of the retina of the eyeeye,organ of vision and light perception. In humans the eye is of the camera type, with an iris diaphragm and variable focusing, or accommodation. Other types of eye are the simple eye, found in many invertebrates, and the compound eye, found in insects and many other ..... Click the link for more information. , are impaired in their capacity to produce a chemical compound called rhodopsin, or visual purple, that is necessary for the perception of objects in dim light. Consequently, the visual threshold, or the minimum intensity of light necessary for sight, is greatly increased. Folk medicine has long recognized the role of the ingestion of liver in alleviating the condition, but it was not until the first quarter of the 20th cent. that vitamin A was identified as the crucial element. Treatment of night blindness consists of the oral or intravenous administration of vitamin A. night blindness[′nīt ‚blīnd·nəs]night blindnessnight blindnessDepending on its brightness, light is perceived by either of two sets of visual cells located in the retina of the eye. One set, the cones, perceive bright light primarily; the other set, the rods, perceive dim light primarily. Dim light produces a change in a pigment called rhodopsin in the rods. This change causes nerve impulses to travel to the brain, where they register as visual impressions. Night blindness occurs when the rods lack rhodopsin. One cause of night blindness is a deficiency of vitamin A—the primary source of rhodopsin. The defect in vision usually can be cured by proper diet plus therapeutic doses of the deficient vitamin. In the elderly, there is sometimes a diminution of rhodopsin, with resulting night blindness. Other losses in vision may follow. Diminished blood supply to the eyes is thought to be a cause of this form of the condition. Treatment generally is only of limited effectiveness. Night blindness sometimes accompanies glaucoma. nyc·ta·lo·pi·a(nik-tă-lō'pē-ă),night blindnessnight blindnessnyc·ta·lo·pi·a(nik'tă-lō'pē-ă)Synonym(s): night blindness. blindness(blind'nes)Blindness may be caused by diseases of the lens, retina, or other eye structures; diseases of the optic nerve; or lesions of the visual cortex or pathways of the brain. A small number of infants are born blind, but far more people become blind during life. In the U.S., blindness due to infection is rare, but worldwide diseases like trachoma and onchocerciasis are relatively common causes of severe visual impairment. In malnourished people, vitamin A deficiency is an important cause of blindness. A variety of free services are available for the blind and physically handicapped. Talking Books Topics, published bimonthly in large-print, cassette, and disc formats, is distributed free to the blind and physically handicapped who participate in the Library of Congress free reading program. It lists recorded books and magazines available through a national network of cooperating libraries and provides news of developments and activities in library services. Subscription requests may be sent to Talking Books Topics, CMLS, P.O. Box 9150, Melbourne, FL 32902-9150. amnesic color blindnesscolor blindnesscortical blindnessday blindnessHemeralopia.eclipse blindnessgreen blindnessAglaucopsia.hysterical blindnesslegal blindnessletter blindnessnight blindnessNyctalopia (1).note blindnessobject blindnesspsychic blindnessred-green blindnessRed-green color blindnessred-green color blindnessriver blindnessSee: onchocerciasissnow blindnesssolar blindnessEclipse blindness.taste blindnesstransient monocular blindnessEtiologyIn older adults, causes of carotid atherosclerosis include smoking, diabetes mellitus, hypertension, obesity, and hypercholesterolemia. When atherosclerotic plaques form within the carotid artery, they may ulcerate. The exposed endothelium within the artery becomes a focus of inflammation and blood clotting. Blindness occurs when tiny clots from the carotid arteries embolize to the ophthalmic arteries. SymptomsPatients often describe a dark shade descending into the field of vision. At the same time they may have other stroke symptoms, e.g., difficulty with speech or weakness of the hand on the side opposite the affected eye. TreatmentA patient who may have carotid atherosclerosis should begin taking aspirin or other antiplatelet drugs if these are tolerated. Blood pressure and lipid levels should be controlled. The patient should be referred for noninvasive evaluation of blood flow through the carotid arteries, e.g., ultrasonography. If the carotid arteries are significantly blocked, the patient and physician should consider the risks and benefits of carotid endarterectomy. violet blindnessword blindnessAlexia.night blindnessModerately reduced to severely defective vision in dim light. Night blindness (nyctalopia) occurs in many people with no objectively discernible eye disorder, but is common in short-sighted people, in those with vitamin A deficiency and in the early stages of degenerative diseases of the RETINA including RETINITIS PIGMENTOSA.night blindnesssee VITAMIN A.hemeralopiaSee XN night blindness
Synonyms for night blindness
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