Blindness and visual impairments

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Blindness and visual impairments

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Blindness is a complete loss of vision, although legally blind people may retain some vision. Visual impairment is partial loss of vision that adversely affects daily living.

There are many causes of visual impairment or blindness, and all parts of the eye (cornea, retina, lens, optic nerve) can be affected. The causes can be genetic (inherited eye diseases), accidental (mechanical injury), inflammation of the eye tissues (uveitis), acute or extended exposure to harmful chemicals (acids, alkalis, or tobacco smoke) radiation (UV radiation), dietary imbalance (lack of vitamin A), medications (corticosteroids), systemic diseases (diabetes, renal failure), or simply age.

Most impairments do not lead to blindness and are related to the refractive power of the lens and cornea. They are, however, often troublesome and may restrict ones choice of job. Many people, for example, have problems focusing due to a variety of conditions. These can include near-sightedness (myopia), far-sightedness (hyperopia), astigmatism (inability to obtain a sharp focus), presbyopia (difficulty in accommodation), animetropia (unequal vision in each eye), and finally, aniseikonia, which can develop as a result of surgery, resulting in images that are perceived by the eyes as different sizes and shapes.

Keratoconus, which arises from the thinning of the central stromal layer of the cornea, possibly due to abnormalities in collagen metabolism, affects the cornea and usually causes some impairment of vision, but can be treated.

Cataracts are the leading cause of blindness in developing countries and result from increased opacity of the lens, which interferes with vision. In developed countries, cataracts are mainly age-related or arise as a diabetic complication. They can also result from an environmental trauma (toxic substance exposure, radiation, mechanical or electrical injury); a small proportion are congenital, resulting from the over-proliferation of lens epithelial cells. Most cataracts can be removed by surgery, although in rare cases, postoperative bacterial infection (endophthalmitis) develops, which can compromise newly restored vision.

Because all eye tissues are interconnected, a problem with one can cause a problem with the other. The best example is the vitreous humor, which in addition to accumulating calcium and cholesterol, which decreases transparency and impairs vision, can shrink, leading to vitreal or retinal detachment. If the macular region is affected, some loss of visual acuity can follow, and floaters or flushes appear in the visual field.

Retinal disorders and changes are the leading cause of blindness in developed countries. Abnormalities in the central retina can affect retinal pigment epithelium, leading to blurry vision, or can affect the macular region (photoreceptors) leading to color misperception. Color blindness can also originate from the lack of one or more type of cones. Total color blindness (monochromatic vision) is very rare; most commonly various levels of single color deficits are found. Central vision can also be destroyed by neovascular hemorrhages in the retina as a result of the aging process or diabetic retinopathy.

Irreversible vision loss occurs if glaucoma damages the optic nerve. Glaucoma is an increase in the intraocular pressure (IOP), which develops in the aqueous and is transmitted to the back of the eye, eventually damaging the optic nerve and consequently causing severe visual field reduction and loss of peripheral vision.

In older people, complete or partial blindness is most often caused by the aging process, including nonenzymatic modifications in proteins, lipids, and DNA, which affect their structure, composition, and function. Glycosylation, carbamylation and deamination of the proteins, oxidation of proteins and lipids, UV-induced damage to proteins and DNA are the main culprits. An accumulation of these changes leads to decreased transparency of the lens (cataracts) and retinal degeneration (age-related macular degeneration or AMD); both result in blindness or severe visual impairment. Most age-related changes are irreversible, with the exception of cataracts, which can be treated surgically.

Research into the causes of blindness, especially glaucoma and AMD, is being undertaken by many groups in order to develop preventative measures and new treatment methods.

See also Nerve impulses and conduction of impulses; Nervous system; Neuron; Neuroscience; Neurosurgery.

Resources

BOOKS

Berman, E.R. Biochemistry of the eye. Perspectives in Vision Research. New York: Plenum Press, 1991.

Guyton, Arthur C., and Hall, John E. Textbook of Medical Physiology. 10th ed. Philadelphia: W.B. Saunders Co., 2000.

Kandel, E.R., J.H. Schwartz, and T.M. Jessell, eds. Principles of Neural Science. 4th ed. New York: Elsevier, 2000.

OTHER

Karolinska Institutet. Eye diseases <http://www.mic.ki.se/Diseases/c11.html> (accessed January 28, 2002).

Foundation Fighting Blindness. Vision Disorders: Causes of Blindness <http://www.blindness.org/visiondisorders/causes.asp> (accessed November 6, 2006).

World Health Organization. Magnitude and Causes of Visual Impairment <http://www.who.int/mediacentre/factsheets/fs282/en> (accessed November 6, 2006).

Agnieszka M. Lichanska