Astigmatism is a refractive error or ametropia in which there is a different ocular refraction along the various meridians (example 180 ° and 90 °). It is characterized by a corneal profile in which a meridian has a greater power than its orthogonal. Optically astigmatism due to two different lines of focus on the retina, which causes blurred vision (or ghosting) at all distances. Astigmatism can be associated with myopia, hyperopia and presbyopia. The compensation of astigmatism involves the use of astigmatic or toric lenses, contact lenses or refractive surgery. The term is not to be confused with the astigmatism of oblique bundles, optical aberration.
Types of astigmatism
- Corneal astigmatism, due to an abnormal curvature of the cornea
- Internal or lenticular astigmatism, due to alterations of the dioptric media inside the eye.
Astigmatism is classified according to the orientation of the meridian of the cornea more curved, considering a range of ± 30 ° to the horizontal or vertical meridian:
- Astigmatism second rule, the most common, in which the meridian curve is vertical
- Astigmatism against the rule, in which the meridian curve is horizontal
- oblique astigmatism
Astigmatism can be classified according to the position of the two foci compared to the retina:
- Simple astigmatism, in which you only have astigmatism. A meridian focuses on the retina, while the other depending on the position can be divided into:
- Simple myopic astigmatism
- Simple hyperopic astigmatism
- Astigmatism compound, in which the astigmatism is associated with a ametropia:
- Compound myopic astigmatism
- Compound hypermetropic astigmatism
- Mixed astigmatism, in which a focalina is myopic and the other hyperopic
The degree of astigmatism is expressed in diopters:
- Astigmatism weak / mild: 0 to 1 diopter
- Astigmatism medium: 1 to 2 diopters
- Astigmatism strong / high: greater than 2 diopters
There are several studies that examine the prevalence of this ametropia. The data are highly variable in the population under consideration. In some samples, such as students, it was estimated that about a third (33%) of the population has astigmatism. It should be considered, however, that the average degree of astigmatism that is detected is about 0.50-0.75, and usually occurs in combination with myopia.
Depending on the type of astigmatism symptoms may be very different. Mild astigmatism may be asymptomatic or present with symptoms resulting from the continuous change of focus (accommodation) in an attempt to get an image as sharp as possible. These symptoms consist of pain in the eyeballs, eyelashes arch pain, headache, burning eyes, excessive tearing. Average high degree of astigmatism usually have blurred vision, both in distance vision than for near.
There are several tests used by ophthalmologists and otometristi during a visual examination to diagnose and determine precisely astigmatism. The Snellen chart is used to quantify the visual acuity reduced dall'astigmatismo. The keratometer or ophtalmometer to measure the anterior curvature of the cornea in its central area. This test is important because most of the astigmatism, especially medium and high grade, are due to an astigmatic cornea. The corneal topography allows to obtain a mapping point to point on the curvature of the cornea. This test is important in contact lenses and refractive surgery. The autorefrattometro or schiascopio allow to obtain an objective measure (no cooperation of the patient) of the astigmatic component. Finally, the test subjective refraction performed by a professional vision allows for collaboration with the patient the correct measure astigmatism.