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Refraction defects: myopia

Myopia

What is it?

Myopia is a visual defect due to which you see blurred from afar, while near vision is good. In medical parlance it is called a defect of refraction.

Myopia can be distinguished, on the basis of the type of defect, in small (up to 4 diopters), medium (4 to 9 diopters), high (more than 9 diopters).

Symptoms

Myopic patients usually squirt half-closing the eyelids trying to focus on distant objects. The greater the visual defect is, the less the distance at which one can see.

Causes

The main causes of myopia are three:

  • Eyeball longer than normal: it is the most common cause;
  • Curvature of the cornea or bigger lens than the norm;
  • Increase of the refractive power of the lens (the lens inside the eye is more powerful than normal).

How you correct it

Traditionally myopia requires corrective lenses (glasses or contact lenses).
Glasses are the most popular and easy to use, but inevitably involve visual problems that increase the stronger the myopia is and has to be corrected.
The resulting image perceived by the myopic subject, in fact, is made smaller more the lens is strong and it’s clear only in the central portion due to distortions (arising from the periphery of the lens itself). The visual quality improves with the application of contact lenses they don’t cause shrinking or distortion of the image, in addition, they allow a clear view in all those situations in which the glasses are not appropriate (eg during exercise) .

Laser techniques provide very good results in mild and medium defects, but less in strong ones. The laser works by changing the curvature of the cornea and, consequently, its refractive power (enlargement capacity), allowing you to focus on the image on the retinal plane (picture in focus).
The excimer laser can be used in two ways: on the front surface of the cornea and, in this case the procedure is called PRK ("corneal photoablation of surface”), or on a deeper layer of the cornea (after having made a microscopic semicircular incision which allows to lift a thin layer of tissue called strip), in this case the procedure is called LASIK or keratomileusis with excimer laser.
The indication and choice of the type of surgery is taken by the surgeon based on variables such as the type and degree of myopia, corneal thickness and the needs of the patient.

Correction of myopia by surgery is represented by the installation of an artificial lens inside the eye. The lens can be added to the natural lens (phakic IOL) into the anterior chamber (in front of the iris) or in the posterior chamber (in front of the lens) or it can be implanted directly in place of the lens.
The lens implant techniques are, therefore, more invasive and expose the eye to some additional risks (the same as other intraocular surgeries such as, for example, that of cataracts); they allow to correct the strongest types of myopia, (in which the laser procedures are not possible) and provide, the same as for treated myopia, better visual quality.