Astigmatism blurs a person's vision because of an abnormality in eye shape. The eye is oval-shaped, causing light to focus on two points on the retina, rather than on one. Astigmatism glasses correct for this by using an irregularly-shaped lens to even out the abnormality on the eye. Different eyesight conditions allow for different types of astigmatism glasses. These include glasses for astigmatism alone or for compounded vision problems, such as astigmatism with myopia, hyperopia, or presbyopia.
When an individual receives his spherocylinder prescription from an optometrist, he receives three pairs of numbers: sphere, cylinder, and axis. "Sphere" refers to the degree to which he is nearsighted or farsighted, while "cylinder" indicates astigmatism. Patients with abnormal cylinder readings have an irregularly-shaped lens or cornea. "Axis" refers to the location of the irregularity that causes astigmatism. Based on these figures, the optometrist can determine exactly what sort of astigmatism glasses the patient needs.
People who suffer from astigmatism without any other eye problem can benefit greatly from glasses with toric lenses. The lenses in astigmatism glasses curve more in one area than in another, relative to the location and degree of the irregularity in the eye. This offsets the eye's curve and allows light to focus on a single point in the retina, restoring normal vision.
If the patient suffers from myopia, or nearsightedness, the toric lenses can be compounded with concave lenses. This allows for both issues to be resolved through one pair of astigmatism glasses. The concave lenses adjust for the myopia, while the toric lenses address the issue of astigmatism. Similarly, individuals with both hyperopia, or farsightedness, and astigmatism can benefit from convex lenses compounded with toric ones.
Astigmatism glasses can also be made for those who suffer from presbyopia, or blurred vision in near objects. Bifocal glasses, which adjust vision for both near and far objects, can be combined with toric lenses. Reading glasses can also be adjusted the same way.
Although astigmatism glasses do provide clearer vision, they do not cure a person of astigmatism. They are also less effective in managing severe cases of mixed astigmatism, in which the two irregular curves on the eyeball aren't perpendicular to each other. The only known treatment for astigmatism to date is laser eye surgery, which has less predictable results. The added risk makes most individuals who have astigmatism opt for managing the condition with astigmatism glasses or contact lenses.