Refractive errors
To see clearly enough, it is necessary that the cornea and the crystalline (located behind the pupil), act together like lenses causing light rays to focus on the retina located at the rear of the ocular globe. The retina receives the shapes perceived, and transmits this information to the brain which then recognizes these objects and "processes" this data. When the eyes cannot correctly focus objects onto the retina, "refracting errors" occur, such as: myopia, hypermetropia, and astigmatism, caused by differences in the length or the form of the eye. Presbytia (or tired vision) occurs when the crystalline, usually because of age, loses its capacity to focus onto close objects. The main symptoms are blurred vision and a sensation of tension in the eyes and, occasionally, headaches caused by continued maximum effort. These symptoms usually disappear when the defect is corrected with glasses, surgery, or contact lenses.Myopia
Usual symptoms: facility to see close objects, but incapacity to focus onto distant objects. The myopic eye is usually longer than normal: the distance between the cornea and the retina is larger than usual. Hence, the "natural lenses" of the eye (cornea and crystalline) cannot focus onto objects and instead of focusing on the retina are focused in front of it. This refracting error is usually detected during childhood. If you see a child come much closer to objects when he carries out tasks, such as reading and writing on in school, if he approaches the chalkboard to see it and when he watches distant objects, he half closes his eyes, you might want to have him eye tested. Myopia might become worse until the patient reaches 20 years of age, sometimes so quickly that successive changes in glasses are needed. After 20, variations are usually smaller and much slower. Contrary to people beliefs, reading a lot, deficient illumination, nutritional factors, or use of contact lenses do not cause myopia nor influence its evolution. Myopias of more than 6 diopters are considered "high" and require periodic checks of the retina, since detachments are more frequent than in a normal eye.
Hypermetropia
Usual symptoms: blurring of vision for the close work or objects. In contrast to myopia, the eye is normally shorter than normal: the distance between the cornea and the retina is shorter than usual. This causes objects that are close to be focused not on the retina, but behind it. All children tend to be hypermetropes to some degree. When the eye is growing and becomes longer, hypermetropia is reduced or disappears. In spite of hypermetropia, children normally see distant as well as close objects with the same ease thanks to their young crystalline which has a great capacity to focus and compensates the shortness of the eye. Sometimes, hypermetropia in children goes together with a certain degree of eye deviation caused by a misalignment of the ocular muscles that must make constant adjustments to focus objects that are near. Sometimes it is associated with headaches, red eyes, and lack of interest in reading. It is advisable to correct hypermetropia in children the same way that any ametropia may cause a reduction in vision, small though it may be. A special case of hypermetropia are patients who have had cataract surgery and into which an intraocular lens has not been placed. These patients have very high hypermetropia (more than 15 diopters).
Astigmatism
Usual symptoms: blurred and unfocused vision (a point of light appears tailed of and a line, which consists of series of points, appears made of points, appears as a succession of strokes fused into a blurred image). For normal and non-distorted vision, the cornea must be regular and with a similar curvature in all directions. People with astigmatism have corneas that are curved more in one direction than in another. In general, astigmatism causes blurred and unfocused vision. Normally it is hereditary and appears at a very early age. Usually it remains unchanged throughout life. In general, astigmatisms are slight and do not always require correction. In other cases, corneal astigmatism is a consequence of previous surgery (refractive surgery, cataract surgery, cornea transplants...).
Presbytia (Tired Vision)
Usual symptoms: inability to accurately focus onto images that are close Usually appears over 40 because over the years the crystalline loses its elasticity. Tired vision is easily correctable with glasses, such as myopia, hypermetropia or astigmatism. Strangely enough, people with myopia improve their vision of near objects when presbytia appears. In a way it "compensates" myopia.
Prosthesis: Glasses and contact lenses
Glasses (near, far, bifocals, progressive, with solar filters...) usually do not imply any problem for people with slight refraction defects or who do not use them permanently. Contact lenses are not only useful from an aesthetic point of view, but also because they allow better visual quality than glasses and have good peripheral and central vision. Contact lenses can be hard, rigid permeable to gas, and soft. * Hard lenses are currently used very little because it is difficult to adapt to them. * The rigid permeable lenses to gas are most permeable to oxygen. They can correct astigmatisms better than soft lenses. * Soft lenses are more comfortable than the formers but can occasionally irritate the eye after a variable time of use.
Surgeries
Thanks to new technologies appeared several way to solve one’s sight issues with surgery. Several types exist, please consult a specialist in order to assess further your needs.
\n"; */