In a normal (emmetrope) eye, the light coming from an infinite distance focuses exactly on the surface of the retina, generating a clear image. In the shortsighted eye, conversely, light focuses at a point in front of the retina and the greater is the visual defect, the shorter is the distance at which the patient is able to see well.
Based on its extent, shortsightedness can be defined mild (up to -3 D), moderate (from -3 to -6 D) or high (more than -6 D).
The factors that can cause shortsightedness are:
– An excessive length of the eyeball: shortsightedness, in particular if it is high, often derives from an imperfect geometry of the eyeball, which appears to be too long. This defect is referred to as axial myopia. In the presence of axial myopia, the light that crossed the cornea and the lens, even if there are no deviations by these lenses, will focus in front of the retina and also in this case there will be a difficulty in focusing on distant objects.
– The cornea and the surface of the lens are too steep: if the cornea and/or the lens present a focusing power higher than normal, the eye will focus the image of an object in front of the retina; in this situation the eye cannot focus distinctly on distant objects and there will be a refractive error referred to as refractive myopia.
– The refraction index of the lens nucleus is higher than normal (index myopia).
A blurred vision when looking at distant objects and a clear vision when looking at close objects. Headache when trying to focus.