Astigmatism of the eye
Astigmatism of the eye (astigmatismus; a- - negative part + Greek stigmatos, stigma - point) is an anomaly of refraction, characterized by the fact that the refractive power of the optical system of the eye differs in different meridians, due to which the rays emanating from any point in space do not give point focus image on the retina.
The following types of eye astigmatism are distinguished:
- With oblique axes - the main meridians do not coincide with the horizontal and vertical meridians of the eye;
- Irregular (irregularis) - characterized by differences in refractive power not only in different meridians of the eye, but also in different segments within the same meridian;
- Reverse (reversus) - the refractive power in the main vertical meridian of the eye is less than in the horizontal one;
- Correct (regularis) - the refractive power in each of the eye meridians does not change within the optical zone;
- Simple (simplex) - characterized by emmetropia in one of the main meridians of the eye, in the other by ametropia;
- Simple hypermetropic (simplex hypermetropicus) - characterized by hyperopia in one of the main meridians of the eye;
- Simple myopic (simplex myopicus) - characterized by myopia in one of the main meridians of the eye;
- Direct (rectus) - the refractive power in the vertical main meridian of the eye is greater than in the horizontal;
- Corneal (cornealis) - due to the uneven curvature of the cornea;
- Composite (compositus) - characterized by ametropia of one type, but of varying degrees, in the two main meridians of the eye;
- Complex hyperopic (compositus hypermetropicus; synonym: hyperopic-hyperopic astigmatism) - characterized by varying degrees of hyperopia in the two main meridians of the eye;
- Complex myopic (compositus myopicus; synonym: myopic-myopic astigmatism) - characterized by myopia of varying degrees in the two main meridians of the eye;
- Mixed (mixtus; synonym: myopo-hyperopic astigmatism) - astigmatism in myopia in one of the main meridians of the eye, in the other - hyperopia;
- Physiological (physiologicus) - frequent correct direct astigmatism of a small degree (up to 0.5 diopters), which does not lead to the development of visual impairment and, as a rule, does not need correction;
- Lens (lenticularis) - due to the irregular shape of the lens.
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