Reduced visual acuity that is uncorrectable by lenses in the absence of detectable anatomic defects in the eye or visual pathways.

Astigmatism (from the Greek 'a' meaning absence and 'stigma' meaning point) is a condition in which the parallel rays of light entering the eye through the refractive media are not focused on a single point. Both corneal and noncorneal factors contribute to refractive astigmatism. Corneal astigmatism is mainly the result of an aspheric anterior surface of the cornea, which can be measured readily by means of a keratometer; in a small fraction of cases (approximately 1 in 10) the effect is neutralized by the back surface. The curvature of the back surface of the cornea is not considered in most studies, because it is more difficult to measure; moreover, in the case of severe corneal astigmatism, there is evidence that both surfaces have the same configuration. Noncorneal factors are errors in the curvature of the 2 surfaces of the crystalline lens, irregularity in the refractive index of the lens, and an eccentric lens position. Since the cornea is the dominant component of the eye's refracting system, a highly astigmatic cornea is likely to result in a similarly astigmatic ocular refraction (summary by Clementi et al., 1998).

A severe form of myopia with greater than -6.00 diopters.

Underdevelopment of the optic nerve.

Decreased amplitude of eletrical response upon electroretinography.

Complete lack of the M photopigment, which is replaced with the L photopigment. Affected individuals tend to confuse red and green.

Blue and green cones only; no functional red cones.