Myopia

The parallel rays coming from infinity are diverted (refracted) from the system cornea/crystalline on a specific portion of the retina that allows the sharp vision: the macula. So an eye that can perfectly converge parallel rays coming from infinity on the macula is an eye emmetropic. In this eye has gone a harmonious and balanced development of its size and its "Refractive system" (Cornea / lens). In this eye has gone a harmonious and balanced development between its anteroposterior and its refractive system that is, the cornea and lens. In Italy, by custom, an emmetropic eye has a’visual acuity (or visual acuity) of 10/10 (ten / tenths) (Snellen fraction). They are often confused the tenths of this fraction, then how many lines that express the law on patient’ottotipo (the table using the ophthalmologists) out of a total of 10 (then the patient will read; 1/10; 2/10 and so on up to 10/10 for the smallest letters) with diopters (improperly degrees) that are, the unit of measure of the power of the lenses (positive, negative or cylindrical) necessary to correct a particular defect of refraction . (For example, a slight myopic needs an eyewear - 2 diopters to see the 10/10 otherwise your eyesight without glasses would be below 1/10). We are all born with an eyeball shorter than the standard (farsighted), which, unlike its front part (cornea and crystalline reaching already 2 years, the normal size), in its posterior portion grows rapidly (probably influenced not only by genetic factors as well as environmental factors and neurological) taking the size of the adult (about 23,5 mm in a normal subject), adolescence. If during this development, the eye becomes longer than expected realized an imbalance between the cornea-lens and axial length of the eye and is called myopic (refractive defect opposite of ' hypermetropia). In this eye parallel rays (coming from infinity), are focused by the system cornea / lens, front of the retina instead of this, namely the macula, are formed of the images even more blurred the greater is the imbalance between the two systems (that is, the more likely it is myopia). Therefore needed to correct the diverging lenses (the negative) Power (expressed in diopters) higher, the higher the myopia, that are able to "deviate" the rays so as to bring the focus exactly on the macula. The correction of this refractive defect is actualized through the use of glasses, contact lenses or the refractive surgery, which are able to bring the focus exactly on the macula. It is estimated that myopic are among the 10% and the 20% of the population in Europe and the U.S., while in Japan this percentage reaches 30% and in China even 70%. Myopia is a defect on a hereditary basis and presents, in most cases, during adolescent development (9-10 age) to stop usually after developing (20-23 age), however, being able to present also in other age. Separate mention the pathological myopia.