* * *Inability to see with one or both eyes.Transient blindness (blackout) can result from vertical acceleration causing high gravitational forces, glomerulonephritis (a kidney disease), or a clot in a blood vessel of the eye. Continuing blindness may arise from injuries or diseases of the eye (e.g., cataract, glaucoma), including the retina, the optic nerve, or the brain's visual centres. Many infectious, noninfectious, and parasitic systemic diseases can cause blindness. Sexually transmitted diseases and rubella in pregnant women can cause blindness in their infants. See also macular degeneration; visual-field defect.
* * *▪ medicinetransient or permanent inability to see any light at all (total blindness) or to retain any useful vision despite attempts at vision enhancement (functional blindness). Less-severe levels of vision impairment have been categorized, ranging from near-normal vision to various degrees of low vision to near-blindness, depending on the visual acuity and functional impact stemming from the vision loss. Legal blindness is a government-defined term that determines eligibility for various services or benefits as well as restrictions on certain activities such as driving.Specific causes of impaired vision are too numerous to list. In general, any process that causes malfunction of the retina, the optic nerve, or the visual centres and pathways of the brain can reduce vision. In severe cases, blindness may result. Broad categories of conditions that impair vision include infections (e.g., gonorrhea or congenital rubella infection), inflammations (inflammation) (e.g., uveitis), congenital or hereditary diseases (e.g., retinitis pigmentosa), tumours (tumour), cataracts (cataract), trauma or mechanical injury, metabolic (metabolic disease) and nutritional disorders (nutritional disease), glaucoma, vascular damage (e.g., diabetic eye disease or atherosclerosis), and refractive errors (e.g., nearsightedness (myopia) or farsightedness (hyperopia)). In addition, there are many vision-lowering conditions for which there is no well-understood cause (e.g., age-related macular degeneration (eye disease)).Many other potentially blinding disorders do not fit easily into general categories. Few of these conditions, however, lead to total blindness, and many of them have some form of available treatment. Even when the underlying problem cannot be corrected, multiple low-vision aids have been developed to optimize remaining vision. In cases of functional or total blindness, other senses and skills must be emphasized or developed. In addition, a strong psychosocial support system can greatly enhance a person's ability to cope with vision loss.
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