Astigmatism is derived from the Greek word stigma (point) and literally means lack of a focal point. The disorder is characterized by a curvature anomaly of the refractive media such that parallel incident light rays do not converge at a point but are drawn apart to form a line.
Forty-two per cent of all humans have astigmatism greaterthan or equal to 0.5 diopters. In approximately 20%, this astigmatism is greater than 1 diopter and requires optical correction.
The refractive media of the astigmatic eye are not spheri-cal but refract differently along one meridian than along the meridian perpen-dicular to it (Fig. 16.12). This produces two focal points. Therefore, a punc-tiform object is represented as a sharply defined line segment at the focal pointof the first meridian but also appears as a sharply defined line segment rotated 90 degrees at the focal point of the second meridian. Midway between these two focal points is what is known as the “circle of least confu-sion.” This refers to the location at which the image is equally distorted inevery direction, i.e., the location with the least loss of image definition.
The aggregate system lacks a focal point.
The combined astigmatic components of all of the refractive media com-prise the total astigmatism of the eye. These media include:
❖Anterior surface of the cornea.
❖ Posterior surface of the cornea.
❖Anterior surface of the lens.
❖ Posterior surface of the lens.
Rarely, nonspherical curvature of the retina may also contribute to astig-matism.
Astigmatism can be classified as follows:
❖ External astigmatism: astigmatism of the anterior surface of the cornea.
❖Internal astigmatism: the sum of the astigmatic components of the other media.
Astigmatism can also be classified according to the location of the meridian ofgreater refraction:
❖ With-the-rule astigmatism (most common form): The meridian with thegreater refractive power is vertical, i.e., between 70 and 110 degrees.
❖ Against-the-rule astigmatism: The meridian with the greater refractivepower is horizontal, i.e., between 160 and 20 degrees.
❖ Oblique astigmatism: The meridian with the greater refractive power isoblique, i.e., between 20 and 70 degrees or between 110 and 160 degrees.
The discussion up to this point has proceeded from the assumption that the anomaly is a regular astigmatism involving only two meridians approxi-mately perpendicular to each other (Fig. 16.12). This is presumably caused by excessive eyelid tension that leads to astigmatic changes in the surface of the cornea.
The condition above should be distinguished from irregular astigmatism. Here, the curvature and the refractive power of the refractive media are completely irregular (Fig. 16.13a). There are multiple focal points, which pro-duces a completely blurred image on the retina. This condition may be caused by the following diseases:
❖Corneal ulcerations with resulting scarring of the cornea.
❖ Penetrating corneal trauma.
❖ Advanced keratoconus.
Patients with astigmatism see everything distorted. Attempts tocompensate for the refractive error by accommodation can lead to asthenopic symptoms such as a burning sensation in the eyes or headache.
The keratoscope(Placido disk) permitsgrossestimation of astigmatism. The examiner evaluates the mirror images of therings on the patient’s cornea. In regular astigmatism, the rings are oval; in irregular astigmatism, they are irregularly distorted. Computerized cornealtopography (videokeratoscopy) can be used to obtain an image of the dis-tribution of refractive values over the entire cornea (see Fig. 5.3). A Helmholtzor Javal ophthalmometer can be used tomeasure the central corneal curva-ture, which determines the refractive power of the cornea (Fig. 16.14).
Early correction is crucial. Untreated astigmatism in children willeventually lead to uncorrectable refractive amblyopia because a sharp image is not projected on the retina.
Treatment of regular astigmatism: The purpose of the correction is to bringthe “focal lines” of two main meridians together at one focal point. This requires a lens that refracts in only one plane. Cylinder lenses are required for this application (Fig. 16.15a). Once the two “focal lines” have been converged into a focal point, additional spherical lenses can be used to shift this focal point on to the retina if necessary.
Treatment of irregular astigmatism: This formcannotbe corrected with eye-glasses. External astigmatism may be managed with a rigid contact lens (Fig. 16.13b), keratoplasty, or surgical correction of the refractive error. Irregular internal astigmatism is usually lens-related. In this case, removal of the lens with implantation of an intraocular lens is indicated.
Only regular astigmatism can be corrected with eyeglasses.