Astigmatism is a vision defect in which the patient sees badly from far, intermediate, and near distances. Therefore, there is no point where he can see in focus. Normally, the cornea and lens are smooth and curved equally in all directions. This helps to focus light rays sharply onto the retina at the back of your eye. If your cornea or lens isn’t smooth and evenly curved, light rays aren’t refracted (bent) properly. In this case, the eyes are more of a shape of a rugby football.
This type of defect is more difficult to correct because the correction must be only on one of the two axes. On one axis the correction must be 0 while on the defective axis, it must be corrected according to the entity of the defect.
The lenses that correct astigmatism are called cylindrical, astigmatic, or toric lenses. When you hear about “toric” or “cylinder”, we are sure that we are referring to a lens that corrects astigmatism.
Astigmatism can be corrected with toric lenses and they can be put on glasses, contact lenses, or inside the eye in place of the crystalline lens. The crystalline lens can be removed in cases of cataract or other needs and be replaced with an artificial lens that corrects the eye defect. Moreover, such action also can correct astigmatism.
Whenever we hear of astigmatic or toric lenses, we know that they are lenses for correcting astigmatism.
One of the characteristics of this kind of lenses is that they must be oriented on a certain axis. If we take a myopic or hyperopic lens and we rotate it, the patient always sees the same way. On the other hand, if we rotate an astigmatic or toric lens, the patient immediately sees worse because the lens has a function that is determined by the orientation of the axis.
Astigmatism is a visual defect, so the image of a point appears more or less elongated. It is perhaps one of the visual defects less understandable to patients. When having it, the light rays from an object endlessly diverge: one or both principal meridians of the eye are nearsighted (myopic astigmatism) and one or both principal meridians are farsighted (hyperopic astigmatism).
Figure: normal eye is to the left and eye with astigmatism is to the right
When the light rays relative to the two meridians focus respectively one in front and one behind the retina, we speak of mixed astigmatism. It can be combined in various ways with myopia and hyperopia producing complex defects.
The defect can be diagnosed by a routine eye exam, together with other visual defects such as myopia and hyperopia. It can be corrected with glasses, contact lenses or refractive surgery. The astigmatic subject uses cylindrical lenses (toric) that correct only the ametropic axis and have neutral power on the emmetropic axis.
Modern laser refractive surgery with LASIK technique can correct this defect thereby reducing or eliminating the need to use glasses or contact lenses.
Where it is not possible to perform an operation with a laser, an alternative is the use of premium intraocular lenses. Solutions with contact lenses or glasses may have drawbacks especially for those doing sports.