ASTIGMATISM, a defect of vision in which the rays of light do not converge to a single point on the retina. It is usually due to irregularities in the curvature of either the cornea or the crystalline lens, or of both, so that rays refracted in one meridian are brought to a focus at a point in advance of or behind those refracted in the meridian at right angles to the first, or it may be due to functional de fective control of the crystalline lens. A slight degree of astigmatism is present in all eyes, be cause neither the cornea nor the anterior or posterior lens surface is part of a perfect sphere; but normally the defect is so slight as not to be noticeable. The conventional star shaped appearance assumed by very distant points of light, as for example, the stars them selves, is due to this universal defect. If, how ever, the radii of curvature are markedly un equal in the different meridians, a more or less serious blurring of the images formed on the retina by all objects results. Two main va rieties of astigmatism are recognized. The commoner is regular astigmatism, in which there is a difference of curvature in two me ridians, usually at right angles to each other, but the refraction is the same throughout any given meridian. In irregular astigmatism, how ever, there are variations of curvature along the length of the different meridians them selves; that is, each one does not, as in regular astigmatism, represent a segment of a circle. This form is usually the result of injury or disease of the cornea or lens, and is difficult to correct by glasses. Regular astigmatism is
nearly always congenital and is frequently pres ent in several members of the same family. The symptoms produced astigmatism may be purely local and comprise blurred vision and fatigue after protracted near work, but even slight amounts may in some cases cause serious disturbances of the general health by the con stant headache and reflex symptoms, such as nausea and vomiting, dizziness, etc., that result. Many of these symptoms may be purely psycho genic and indicate an unwillingness on the part of the unconscious to overcome slight defects. The difficulties complained of are usually, at least partly, relieved by resting the eyes. Numer ous methods of examination are employed by oculists to detect astigmatism, one of the sim plest being the use of test diagrams represent mg radiating lines or sectors, rows of dots, etc., which, to the astigmatic eye, appear more or less blurred in certain meridians. The treat ment consists in the use of glasses which are ground so as to compensate for the variations in refracting power of the optic media in the different meridians. This correction is accom plished by using cylindrical lenses, which are segments of cylinders and refract only at right angles to their axis, combined, if necessary, with the appropriate spherical lenses. Psycho genic symptoms may be relieved by a proper psychotherapy. See EYE; VISION, DEFECTS OF.