Astigmatism

Astigmatism is a refraction error of the eye in which there is a difference in degree of refraction in different meridians. It is typically characterized by an aspherical cornea in which one axis of corneal steepness is greater than the perpendicular axis. Astigmatism causes difficulties in seeing fine detail, and can be often corrected by glasses with a cylindrical lens (i.e. a lens that has different radii of curvature in different planes), contact lenses, or refractive surgery.

Astigmatism occurs when either the cornea or the lens of the eye is not perfectly spherical. As a result, the eye has different focal points in different planes. For example, the image may be clearly focused on the retina in the horizontal plane, but in front of the retina in the vertical plane.

In some cases vertical lines i.e. walls, may appear to the patient to be leaning over like the tower of Pisa.

Types of Astigmatism

Types of astigmatism based on asymmetry of structure:

• Corneal astigmatism - astigmatism due to an irregularly shaped cornea
• Lenticular astigmatism - astigmatism due to an irregularly shaped lens
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Types of astigmatism based on axis of the principal meridians:

• Regular astigmatism
• Irregular astigmatism

Types of astigmatism based on focus of the principal meridians:

• Simple astigmatism
  • Simple hyperopic astigmatism
  • Simple myopic astigmatism
• Compound astigmatism
  • Compound hyperopic astigmatism
  • Compound myopic astigmatism
• Mixed astigmatism

Blur from astigmatic lens at different distances.

Compact Disc players use an astigmatic lens for focusing. When one axis is more in focus than the other, dot-like features on the disc project to oval shapes. The orientation of the oval indicates which axis is more in focus, and thus which direction the lens needs to move. A square arrangement of only four sensors can determine this bias without being fooled by long pits.