Astigmatism is a relatively common ophthalmic disease, a type of refractive error, often combined with myopia and hyperopia, and related to the curvature of the cornea and lens. Astigmatism is mainly manifested by blurred and double vision, easy visual fatigue, squinting, or head tilted vision. Clinically, there are two types of astigmatism: regular astigmatism and irregular astigmatism. Regular astigmatism is caused by the difference in refractive power of the two main meridians of the cornea or lens, where light rays from different directions through the refractive system of the eye do not fall on the same focal point and form two focal lines. Most people have regular astigmatism in their eyes. An example of how an astigmatic eye sees: Draw a cross on a white sheet of paper with a pen. A nearsighted person would need to wear a 300 degree myopic lens to see the horizontal line clearly, and a 400 degree myopic lens to see the vertical line clearly. This condition of only being able to see in one direction is a sign of astigmatic vision. Irregular astigmatism is a condition in which the refractive power is inconsistent across the meridians, or in different parts of the same meridian, the refractive power manifests itself differently. Irregular astigmatism is often caused by pathological causes, and the performance varies, such as caused by corneal scarring, cone corneas or after certain internal and external eye surgeries. In general, astigmatism is caused by irregularities in the cornea or lens sphere. Regular astigmatism is caused by inconsistent curvature of mutually perpendicular meridians, while irregular astigmatism has no such pattern and is usually caused by pathological causes.