Vision is our first sense. 90 % of the information processed by our brain is visual. But having a good vision in our times is increasingly difficult with so many hours in front of the TV or the computer. In the industrialized countries, 60 % of the persons need glasses. 25 % of the westerners suffer from myopia (the most common vision impairment), hypermetropia or astigmatism. To these cataract, glaucoma and macular degeneration can be added. These diseases, if untreated, can even end in blindness.
The eye's optical system includes the cornea, iris, crystalline lens and vitreous body. Cornea is a very effective optical machine. Its refraction index reaches a surprising value of 43 dioptres (corresponding to a focal distance of 22 mm) like a huge tele-objective for a small photo camera. This window to the external world of just 1.3 square centimetres is maintained wet, bright, clean and disinfected by tears through 10-15 blinks/minute. The system functions properly and the mucosas which surround the eye are not affected by an inflammation or allergenic irritation.
The lack of tears will leave the eye dry. Artificial tears can help in this case.
The deformation of cornea is called astigmatism and induces a blurry vision. Cylindrical lens glasses can fix this.
After passing through the cornea, the light rays will cross the iris and lens, when they are concentrated again. The anterior chamber, between cornea and iris-lens, is filled with the watery fluid, which serves for feeding the cornea and the lens. Each day 2-3 cubic centimeters of this liquid are produced. The excess is drained through the Schlemm channel. If the channel is obstructed, the pressure inside the eye grows.
A high pressure inside the eye can cause glaucoma, which is a lack of food and oxygen supply to the optical nerve and retina. This condition is very deceiving: the affected ones can live many years with it without perceiving any symptom, while during this time the destruction of optic nerve advances. In an advanced stage, the visual field is diminished and the image of the environment fills with shadows. If detected on time, the condition can be effectively treated.
There are eye drops that can reduce the intraocular pressure and the obstruction of the draining system can be eliminated through surgery. Glaucoma can affect 2 % of the population in some western countries, but only half of these people receive a treatment. 10 % of those over 40 develop glaucoma, that's why it is recommended an annual eye check after this age. Diabetics, those with myopia over 5 dioptres and those with family history should assess their intraocular pressure 4 times annually.
Today, the majority of those over 45 cannot read without glasses. Before it was thought that eye tiresome was caused just by the lens, which loses its elasticity with the age and cannot curb enough. New researchers showed that the deterioration of the tensor system of the fibers that sustain the lens also has its influence in the decrease of the short-range vision.
But the lack of clarity and the blurry vision can be also due to a cataract, an opacity of the lens produced by the accumulation of insoluble proteins. The lens of a teenager contains 3 % of those proteins, while in a person on his/her 80's can form 40 % of the lens' proteins. Still, in 10 minutes, the opaque lens can be substituted by an artificial implant.
Hypermetropia appears when the eye is too short. In this case, the lens cannot adapt to the distance and cannot form a clear image for the retina, as it is placed too close to do it well (the image forms beyond the layer of photosensitive cells). Like in the case of the tired vision, hypermetropia is corrected with convergent lens glasses.
Watching continuously from a close distance is a modern habit. Prehistoric people only had to accommodate their vision to short distances in very few situations, while the modern human forces his eyes for over 12 hours daily. Spending time in front of a computer or reading can lead to myopia. In this case, the eye is too long and the clear image of an object forms before the retina. Myopia is fixed with divergent lens.
95 % of the clarity of our vision is concentrated in a macula (fovea), a spot on the retina where light rays converge. It has a diameter of just 1.5 mm and is the place where receptor cells reach their maximum density. Compared to eagle sight of the macula, the other retina areas are blind like a mole. Because macula is so important, any disease on this area can result catastrophic. Macular degeneration, linked to aging, is the main cause of blindness for persons over 50.
Macular degeneration does not induce pain and develops slowly and without any strong evidence. The affected person usually notices first difficulties on reading, low clarity and a gray veil which looks like covering the environment. New techniques can treat this.
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