Contact Lens Correction of Regular and Irregular Astigmatism

Abstract

An astigmatism is an ametropia in which light rays do not focus at a single point (American Academy Ophthalmology, 2005) but form two focal lines. This image of a point is called a conoid of Sturm with two main focal or two primary meridians (Michaels D, 1988). If the primary meridians are always 90o apart, then it is a regular astigmatism. An irregular astigmatism occurs when the primary meridians are not perpendicular. Astigmatisms can be classified as regular or irregular based on the contribution of the ocular component and by orientation (Benjamin W, 1998). Clinically, one of the most common criteria employed is with respect to the refractive error (i.e., myopia and hyperopia).  Compound myopic astigmatism: both focal lines lie in front of the retina.  Simple myopic astigmatism: one focal line is anterior to the retina, while the other one coincides with the retina (it is in focus).  Compound hyperopic astigmatism: both focal lines lie behind the retina.  Simple hyperopic astigmatism: one focal line is behind the retina, and the other one coincides with the retina (it is in focus).  Mixed astigmatism: one focal line lies in front of and the other behind the retina. In the correction of the refractive errors, spectacles should be considered before contact lenses or refractive surgery (American Academy Ophthalmology, 2005). However, in some astigmatism cases, contact lenses will be the choice method, especially in cases with irregular astigmatisms. The astigmatism compensation with spectacle lenses is possible if the primary meridians are perpendicular because ophthalmic astigmatic lenses can only correct orthogonal astigmatisms. An irregular astigmatism is difficult to correct with standard spectacles, and subjects often complain of blurring (due to the loss of the corrected visual acuity), monocular diplopia or poliopia. In these cases, obliquely crossed cylinders and other techniques have been proposed (Benjamin W, 1998), although visual acuity reached with this method could be inferior to the best possible treatment. This outcome represents an important problem in patients with induced irregular astigmatisms related to a primary eye disease or secondary to some eye surgical procedure or traumatism. Regular astigmatisms can be corrected with standard ophthalmic lenses, contact lenses and surgical procedures. However, irregular astigmatisms are more difficult to correct with glasses because the visual acuity could be lower than expected. Contact lens could be an