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. Astigmatic corrections are cylindrical and are added to any prescription for myopia or hyperopia.
Presbyopia necessitates the use of bifocals or trifocals.
In the absence of disease or other ocular abnormalities, glasses or contact lenses are the only treatment needed for refractive errors. Eye exercises are of no benefit in improving refractive errors, since they cannot alter the size of the eyeball or the refractive power of the lens. Long-term effects have yet to be observed, analyzed, and documented. Contact lenses have revolutionized the corrective lens industry, however, both hard and soft contact lenses continue to be somewhat controversial. As with surgical solutions to refractive errors, there are precautions and problems yet to be solved.