astig'matism

A state of irregular refraction of an eye, usually congenital, in which the rays of light diverging from a single point can not be brought to a focus at a point on the retina; an asymmetrical condition of refraction of the different meridians of the eye. acquired a. A. dependent on flattening of the cornea from inflammatory changes, on irregular apposition of the flaps after a cataract extraction, or on dislocation of the crystalline lens, compound hypermetropic a. That state of refraction in which an unequal hypermetropia exists in the two principal meridians, compound myopic a. That state of refraction in which an unequal myopia exists in the two principal meridians, corneal a. A. due to causes residing in the cornea., hypermetropic a. See simple and compound hypermetropic^ a. irregular a. A. in which there is an irregularity in curvature along some one meridian or group of meridians of the refractive surfaces, lenticular a. A. due to causes residing in the crystalline lens, mixed a. A form of a. in which myopia exists in one principal meridian and hypermetropia in the other. myopic a. See simple and compound myopic^ a. regular a. That error of refraction in which the meridians vary regularly from a maximum curvature in one principal meridian to a minimum curvature in the second principal meridian at right angles to the first. Usually the defect is in the cornea, simple hypermetropic a. A. with emmetropia one of the principal meridians of the eye and hypermetropia in the other, simple myopic a. A. with emmetropia in one of the principal meridians of the eye and myopia in the other. [Lat., astigmatic us, from Gr., a, priv., + stigma, a spot.]