Astigmatism Definition

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.

Epidemiology: Forty-two per cent of all humans have astigmatism greater than or equal to 0.5 diopters. In approximately 20%, this astigmatism is greater than 1 diopter and requires optical correction.

Pathophysiology: The refractive media of the astigmatic eye are not spherical but refract differently along one meridian than along the meridian perpendicular 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 point of 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 confusion." This refers to the location at which the image is equally distorted in every 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 comprise 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.

— Image formation with an astigmatic cornea.

— Image formation with an astigmatic cornea.

Fig. 16.12 The two main meridians (I and II) are perpendicular to each other. A punctiform object (o) is represented as a line segment Y'II and Y'I at the focal points of the two meridians. Midway between these two focal points is the "circle of least confusion" (Kr), the location with the least loss of image definition.

Fig. 16.12 The two main meridians (I and II) are perpendicular to each other. A punctiform object (o) is represented as a line segment Y'II and Y'I at the focal points of the two meridians. Midway between these two focal points is the "circle of least confusion" (Kr), the location with the least loss of image definition.

Rarely, nonspherical curvature of the retina may also contribute to astigmatism.

Classification and causes: 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 of greater refraction:

❖ With-the-rule astigmatism (most common form): The meridian with the greater refractive power is vertical, i.e., between 70 and 110 degrees.

❖ Against-the-rule astigmatism: The meridian with the greater refractive power is horizontal, i.e., between 160 and 20 degrees.

❖ Oblique astigmatism: The meridian with the greater refractive power is oblique, 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 approximately 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 produces 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.

442 16 Optics and Refractive Errors Irregular corneal astigmatism. -

442 16 Optics and Refractive Errors Irregular corneal astigmatism. -

Geodesic Curve Focal Points

Fig. 16.13 a Curvature and refractive power of the refractive media are totally irregular, resulting in multiple focal points. b Correction of irregular corneal astigmatism with a rigid contact lens.

❖ Advanced keratoconus.

Symptoms: Patients with astigmatism see everything distorted. Attempts to compensate for the refractive error by accommodation can lead to asthenopic symptoms such as a burning sensation in the eyes or headache.

Diagnostic considerations: The keratoscope (Placido disk) permits gross estimation of astigmatism. The examiner evaluates the mirror images of the rings on the patient's cornea. In regular astigmatism, the rings are oval; in irregular astigmatism, they are irregularly distorted. Computerized corneal topography (videokeratoscopy) can be used to obtain an image of the distribution of refractive values over the entire cornea (see Fig. 5.3). A Helmholtz or Javal ophthalmometer can be used to measure the central corneal curvature, which determines the refractive power of the cornea (Fig. 16.14).

Treatment: Early correction is crucial. Untreated astigmatism in children will eventually 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 bring the "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. 16.15 a). 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.

— Diagnosis of corneal astigmatism with an ophthalmometer.

— Diagnosis of corneal astigmatism with an ophthalmometer.

Fig. 16.14 The diagram shows the corneal reflex images (outline cross [1] and solid cross [2]) of the Zeiss ophthalmometer. These images are projected on to the cornea; the distance between them will vary depending on the curvature of the cornea. The examiner must align the images by changing their angle of projection. After aligning them, the examiner reads the axis of the main meridian, the corneal curvature in millimeters, and the appropriate refractive power in diopters on a scale in the device. This measurement is performed in both main meridians. The difference yields the astigmatism. In irregular astigmatism, the images are distorted, and often a measurement cannot be obtained.

— Correction of regular astigmatism with cylinder lenses.

Irregular Distances Mesuring

Fig. 16.15 a Cylinder lenses refract light only in the plane perpendicular to the axis of the cylinder. The axis of the cylinder defines the nonre-fracting plane. b-d Cylinder lenses can be manufactured as plus cylinders (c) or minus cylinders (d).

Fig. 16.15 a Cylinder lenses refract light only in the plane perpendicular to the axis of the cylinder. The axis of the cylinder defines the nonre-fracting plane. b-d Cylinder lenses can be manufactured as plus cylinders (c) or minus cylinders (d).

Treatment of irregular astigmatism: This form cannot be corrected with eyeglasses. 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.

H Only regular astigmatism can be corrected with eyeglasses.

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