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The retina is a cuplike structure. It contains millions of specialized nerve cells that are sensitive to light. Some cells are sensitive to color, making color vision possible. Other cells detect shades of light and darkness. These cells enable you to see in dim light. These cells do not distinguish colors and vision in dim light is nearly always colorless.
When exposed to light, cells on the retina send impulses to other nerve cells. These nerve cells join at the back of the eye to form the optic nerve. Impulses travel along the optic nerve to the areas of the brain that control vision. Here the impulses interpret as images. No image can register on the retina at the point where the optic nerve leaves the eyeball. Thus, everyone has a blind spot in each eye.
When there is damage to the retina vision a well-focused image will not occur if some or all the light receptors do not function properly. There are eye disorders that affect the retina and below you will find the detached retina is the separation of the retina from the choroids, the nourishing middle layer of the eye. The early signs of a detached retina include flashes of light and loss of peripheral vision. Part of the retina falls away from the tissue supporting it. Causes for this condition will include, a sudden blow to the head as in a sports injury, normal aging, or eye tumors. When the eye moves, there will be a sudden appearance of floating spots that may decrease over a period of weeks. Total blindness in the affected eye may occur if this condition remains untreated as the retina may detach completely. Laser therapy has been very successful in this eye disorder. A tight collar around the eyeball to increase pressure within the eye puts pressure upon the vitreous humor holds the retina in place against the rear of the eyeball. Without proper treatment, eye damage and loss of vision will continue. Early surgery will correct this condition.
Diabetic retinopathy results from a disorder involving hormone insulin called diabetes mellitus. Diabetes causes small hemorrhages in retinal blood vessels that disrupt the oxygen supply to the photoreceptors. The eye will respond to the disruption of the oxygen supply to the photoreceptors by building new but abnormal vessels that block vision and may cause detachment of the retina. The recognizable lesions on fundoscopy are dilation of veins and "microaneurysms," which consist of small punctuate hemorrhages located near the macula. They can occur anywhere but within the macula, impaired vision occurs. Relatively early lesions are waxy exudates, probably due to lipid leakage from the vessels. This stage of the retinopathy can remain for many years. When a person develops juvenile diabetes it may progress to a more malignant stage, that of neovascularization and proliferate retinopathy. The new blood vessels usually emanate from the disk and grow toward the vitreous. Fibrous and collagenous tissue forms if a pre-retinal hemorrhage occurs. The shrinkage of the scare tissue produces retinal detachment. This condition shows in varying degrees in more than 90 per cent of diabetic patients after 20 years of clinical diabetes. Diabetes is the third most frequent cause of blindness in the United States. Treatment has helped such as the form of a type of laser therapy in which laser beams will seal off hemorrhaging retinal vessels and has been successful in many cases.
Glaucoma occurs when internal fluid pressure in the eye increases to the point of damaging the optic nerve. It is one of the most common causes of blindness in the United States. This is a group of diseases having the common characteristic of a hardened eyeball caused by the internal pressure of the fluids of the eye. When fluid pressure against the retina increases above normal, blood flow through the retina slows. The reduced blood flow causes degeneration of the retina and thus a loss of vision. In some way not yet understood the canals that drain away the fluid of the eye stop up. Sharp pain, and cloudy visions are acute symptoms of Glaucoma in chronic form may not produce any symptoms for years in the earliest stages. As chronic glaucoma progresses damage first appears at the edges of the retina causing a gradual loss of peripheral vision commonly known as tunnel vision. Blurred vision and headaches may also occur. "Halos appear around bright lights as the damage becomes more extensive. This chronic stage may be painless. Blurred vision or foggy problems can occur with adjusting to darkened rooms. The disease rarely appears before 35 to 40 years of age and is usually progressive. Many of the symptoms are due to pressure upon the retina and optic nerve.
Nyctalopia is the degeneration of the retina and can cause difficulty with seeing at night or in dim light. This is night blindness and is due to a deficiency of vitamin A. Photopigment is a light-sensitive chemical that triggers stimulation of the visual nerve pathway. When there is not enough vitamin A then the result is a lack of photopigment in rods, a condition that impairs dim-light vision. Some forms of night blindness follow a sex-linked hereditary pattern. Since rods are not in the fovea centralis (region of acute vision) you can find looking out of the corners of your eyes very useful when you enter a dark room. Pilots often use this type of night vision.
Age-related macular degeneration is the leading cause of permanent blindness in the elderly as it is a progressive degeneration of the central part of the retina. The part of the retina that is essential to good vision is the central region called the macula. After age 50 the risk for developing this condition increases if you have known risk factors such as cigarette smoking and a family history of this disorder.
Color blindness is the inability to distinguish between certain colors and about one person of every twenty-five in the United States will have this condition. Color blindness is an abnormality and is a clinical disease. Often this retinal disorder is hereditary. Colorblind individuals see colors, but they cannot distinguish between them normally. Because color blindness is an X-linked genetic trait, more men than women have this condition. Color blindness results from disease and injury of the retina and optic nerve. There is no medical cure for color blindness as those who have it through practice can detect the difference between stop and go lights because of the change in brilliancy.
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