Most astigmatism is caused by the shape of the cornea. The cornea is the outer most surface of the eye. It is the clear structure that first bends light (refracts). Instead of being spherical with a single curvature, it has two curvatures that result in the light focusing in two different locations within the eye. Ideally, the light focuses at one point on the retina. (emmetropia). If the eye is myopic (nearsighted) the light focuses in front of the retina. If the eye is hyperopic, far-sighted, the light focuses behind the retina. If the eye is astigmatic, the light focuses in two different places, one focus can be on the retina and one in front of or behind the retina.
Astigmatism is not a disease, it is a refractive condition. A common example optometrists use to describe astigmatism is the difference between a basketball and a football. A basketball is round, and has only one curve for the entire ball. You can think of that curve as a lens power. A football on the other hand has two curves. This is like an eye that has two powers or an astigmatism.
Like other refractive errors, astigmatism can be corrected with glasses, contact lenses or refractive surgery.
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