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Astigmatism, together with nearsightedness and farsightedness, is a type of refractive error. In 2010, astigmatism was the cause of worsened sight for over 100 million people globally, leading to the total blindness of close to 7 million people. 

In cylindrical eye defects, deformations in the cornea or lens shape (irregularities) occur; these deformations cause inaccurate focusing of light on the retina. The exact cause of astigmatism formation has not been fully documented, but the development of this eye defect may involve various inflammations or injuries when the eyes are affected. The main risk factor is the genetic predisposition of an individual. 

Discovering the presence of potential disease-causing genes involved in the development of astigmatism can be helpful for designing an effective screening program and for a more precise identification of eye defects. Astigmatism is often accompanied by nearsightedness, or farsightedness. Astigmatism may even develop during childhood.
Astigmatism is caused by an irregular curvature of the cornea , which in this case is not perfectly symmetrical and circular but has a slightly ellipsoidal shape - the refractive surface of the eye is more curved in one plane than in the plane perpendicular to it. Instead of joining the light rays from all directions into one focus on the retina, they pass each other, and they appear on the retina as differently large and curved surfaces. Irregular curvature results in uneven focusing, and the perceived image may be blurry in one plane, while sharp in the second plane.
There are several treatment options. The patient may wear either toric lenses, which compensate for the irregularity of the cornea, or use the spectacle correction by cylindric glasses. Laser surgery is another solution for eliminating irregular curvature.