Astigmatism is an irregular curvature of the cornea in one or more of its meridians. In addition to simple myopia or hyperopia and the variations of astigmatism, and because the human organism has two eyes which must have coordinated visual reception for good vision to occur, a multitude of refractive variations are possible. Anisometropia refers to different refractive errors in each eye, and aniseikonia denotes a difference in the image size in the two eyes.
Refractive errors tend to be inherited, but there is no pattern of inheritance.
Size of the eyeball, shape of the cornea, shape of the lens, and depth of the anterior chamber are all variables in refractive errors. These variables increase the possible ocular combinations for refractive errors. Astigmatism may cause visual fatigue, headaches, frowning, and squinting. Progressive myopia is genetically determined as a recessive trait. Myopia and hyperopia are treated by the use of spherical concave and convex lenses, respectively.