Astigmatism

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 Italy, by custom, it is said that the eye has a visual acuity (or visual acuity) of 10/10 (Snellen fraction). Unlike the myopia and 'hypermetropia, refractive vices that are secondary to the length of the eyeball, theastigmatism, etymologically "no point" (focus) is a defect related to abnormal curvature of the cornea (excluding the rare astigmatism of the lens). In fact, if we imagine the cornea as a hemisphere and in many scomponessimo “slices” (meridiani) and for example, we took the one at 90 ° and the 180 ° would be perfectly identical, but in reality the cornea is not a hemisphere, indeed resembles half of a rugby ball. In this case if we took the meridian at 90 ° and the 180 °, (the red and blue curved line of the diagram above) we note that the difference in curvature of these would result in, instead of only one focal point (as in myopia and in 'hypermetropia), the formation of two “focal lines” placed at a greater distance between them the higher l 'astigmatism and at different distances from the the macula. Such refractive defect results in an unsatisfactory vision for both distance that for near. The two meridians used as reference are steeper and flatter that are responsible for the 'asse dell'astigmatismo. Physiologically we have the meridian at 90 ° little more curved (of 0.5-0.75 diopter and that requires no correction) of the 180 ° (astigmatism physiological). Much more rarely the curvatures of these meridians are reversed flatter the vertical, more curved the horizontal (astigmatism controregola). Astigmatism is familiar, is present at birth and does not tend to change over the life (with the exception of Keratoconus) increasing slightly in a "controregola" in old age. A astigmatismo irregolare may be secondary to trauma, corneal, surgery or inflammation / infection that alter the geometry of the cornea. Astigmatism is still divided into simple (when not associated with other refractive defects) in which a meridian focuses on the retina, while the other front of the retina (simple myopic astigmatism) or behind this (simple hyperopic astigmatism ) the composed, when associated with myopia (compound myopic astigmatism) or hyperopia (compound hypermetropic astigmatism). When a meridian focuses in front of the retina and the other behind this one speaks of misto astigmatism. Astigmatism is' an error of refraction very frequently being present in approximately 44% of the population (greater than 0.50 diopter), the 10% of the population has more than 1 diopter and l ' 8% of the population has more than 1.50 diopters or more. E 'can correct astigmatism by the use of cylindrical lenses (positioned along the axis), but also with toric contact lenses or by refractive surgery.