If the cornea is unphysiologically curved in its meridians, image distortion occurs. Regular astigmatism is a refractive error defined by such an altered corneal curvature. The normal-sighted eye has a cornea that is not spherically curved, but has precisely defined curvatures in the vertical and horizontal directions.
These curvatures are essential to be able to refract light coming from the side, from below or from above, as well as light coming from the front. If, however, the cornea is altered in one of the directions (meridian) compared to the physiologically correct curvature and is therefore bent more or less strongly, the light that passes through the pupil is refracted differently and then resolved differently in the individual meridians. A round object, such as a sphere, therefore appears as a line or rod (astigmatism).
Irregular corneal curvature
Irregularities on the corneal surface are called irregular astigmatism. This form of astigmatism is caused by an irregular curvature of the cornea, resulting in a loss of focus. The cause of the irregularity of the cornea can be, for example, a corneal disease with scarring.
Not only are the axes changed, but also shifted against each other. In the case of regular astigmatism, the correction is carried out with the help of cylindrically ground glasses. These so-called cylindrical lenses refract the light in one direction only, which is why one axis direction is always given in degrees when prescribing appropriate lenses.
The concave and convex cylinders are ground so that the two axes are perpendicular to each other and only their refractive power is changed. Often the first fitting of cylindrical lenses in adulthood is problematic because the long habituation to seeing without compensation has led to an adaptation of the processing of seen images. The sudden change to normal vision due to the strong lenses may result in sharper vision, but at the same time it often causes headaches.
For this reason, adolescents and also adults who need glasses with cylindrical lenses for the first time usually start with a weaker lens than would be required according to the measurement. Then the strength is gradually adjusted. A treatment with the so-called excimer laser makes a direct correction on the eye possible, whereby the refractive power is changed in such a way that sharp vision is possible without any aids.
The zone of increased refraction is then ablated with the cold light laser in case of astigmatism. This laser penetrates the cornea only slightly, thus making it possible to gently change the cornea without damaging adjacent tissue. However, due to individual characteristics of the cornea, such as corneal thickness, not every affected person can be treated with laser therapy.
