Blindness and Visual Impairments
Blindness is usually considered as an inability to see or a complete loss of vision, although legally, a blind person may retain some vision. In contrast, visual impairment indicates a loss of vision such that there is an impact on daily living, which usually implies partial loss of vision.
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 affecting both eyes), accidental (mechanical injury to the eyeball), inflammation of the eye tissues (uveitis), acute or extended exposure to harmful chemicals or radiation (acids, alkali, tobacco smoke, UV radiation), dietary imbalance (lack of vitamin A), medication (corticosteroids), systemic diseases (diabetes, renal failure), or simply an aging process.
The majority of visual impairments do not lead to blindness and are related to the refractive power of the lens and cornea. However, they are often troublesome and possibly restrictive in one's choice of job. A large number of people 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 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), and a small proportion of cataracts are congenital, resulting from the over-proliferation of lens epithelial cells. Most cataracts can be removed by surgery, although in rare cases, post-operative bacterial infection (endophthalmitis) develops, which can compromise newly restored vision.
The eye tissues are all interconnected and a problem with one can cause a problem with another. The best example is the vitreous, which in addition to the accumulation of calcium and cholesterol leading to decreased transparency and subsequent impairment of vision, can shrink, leading to vitreal or retinal detachment. If the macular region is affected, some loss of visual acuity can follow and in any case floaters or flushes' appear in the visual field.
Disorders and changes affecting the retina are the leading cause of blindness in developed countries. The 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. The central vision can also be destroyed by hemorrhages of the neovascular vessels developing in the retina as a result of the aging process or diabetic retinopathy.
Irreversible loss of vision occurs due to optic nerve damage resulting from glaucoma. Glaucoma is caused by an increase in the intraocular pressure (IOP), which develops in the aqueous and is transmitted to the back of the eye, damaging the optic nerve and consequently causing severe reduction of visual field and loss of peripheral vision.
In the older population, complete or partial blindness is caused mainly by the aging process. Changes that lead to the destruction of vision are the non-enzymatic 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 to 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, AMD) both resulting in blindness or severe visual impairment. Most of the age-related changes are non-reversible, with the exception of the cataracts that can be surgically treated.
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.
Agnieszka M. Lichanska
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