News » Eye refractive defects: nature, causes and classifications
The eye is a sensory organ that allows us to see all that surrounds us.
We give so obvious that operation, most of us, not even know the meaning of "refractive error." The observation of all the things around us is the result of a, for us natural mechanism, which takes place precisely because of the refractive phenomenon.
When light is present, the objects that are hit reflect the light rays, the same that, when we observe them, reach our eyes, and, in particular, the retina after passing through the cornea, the pupil and the crystalline lens.
However, the eye is extremely sophisticated and are sufficient small anatomical imperfections borne of the cornea, the lens and the eyeball to generate these refractive defects known as nearsightedness, farsightedness and astigmatism.
Causes of refractive defects
The refractive defect, or refractive error, is that defect due to the inability of the eye in focusing, so perfect, the image of what we watch on the retina.
It therefore depends on an altered power of focus by the natural lens of the eye, ie the cornea and the crystalline lens. One speaks in this case of "altered geometry" of the eyeball or more causes combined.
We can compare the cornea to a fixed focus lens, the lens, instead of a biconvex lens with variable curvature regulated by structures (ciliary muscles) which allow to vary its thickness to the crystal for a perfect focusing on the subject into consideration the distance (accommodation).
What can alter the correct view?
Our ability to see things is the product of a series of steps and very articulate and complex activities. So that everything has properly place without refractive defects, certain conditions must be met:
• Let the optical path of the light rays is not hindered in any way. Therefore the cornea, aqueous humor, lens and vitreous humor (dioptric media) must be perfectly transparent.
• Let the optical path of the light rays is not deflected. Therefore assumes particular importance, in this case, the homogeneity of the medium to which the index of refraction and the geometry of the natural lens of the eye (cornea and crystalline lens).
• That the retinal position is correct so that the focus of the image not to be defective.
Presbyopia: this really is a refractive error?
On closer inspection, presbyopia can not be technically classified as a refractive defect.
As we age, usually after fifty years, the physiological elasticity of the lens tends to decrease in a natural way, presenting problems of accommodation.
From this it derives a greater difficulty to read and, in general, to focus on small objects and neighbors. Presbyopia, therefore, does not depend on an error of refraction, but the loss of accommodation, physiological ability of the crystalline lens, which degenerates with age.
The enormous progress made by the refractive surgery in recent times now enable to correct, permanently, these types of problems plaguing more and more people.
Thanks to interventions that tend to change the refractive power and to restore the spherical symmetry of the cornea, many are those who are seen reduced, or even eliminated, dependence on lenses and eyeglasses.
A great step in the care and treatment of refractive defects.
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