Astigmatism: What It Is, How It Affects Vision, and Correction Options

What Is Astigmatism?

Astigmatism is a refractive error caused by an irregular curvature of the cornea or lens, resulting in two different focal points rather than one. A normal cornea is spherical — like the surface of a ball — with equal curvature in all meridians. An astigmatic cornea is toric — like the surface of a football or egg — with greater curvature in one meridian than the perpendicular meridian. This unequal curvature means that light entering the eye focuses at different points, causing blurry or distorted vision at all distances.

The degree of astigmatism is measured in diopters (cylinder) and the orientation of the steeper meridian is described by the axis (0–180 degrees). A prescription of -1.00 -1.50 x 90 means -1.00 D of myopia with 1.50 D of astigmatism oriented at the 90-degree meridian (vertical steepening).

Regular vs Irregular Astigmatism

Regular astigmatism is the most common form. The two principal meridians are perpendicular to each other, and the curvature within each meridian is uniform. Regular astigmatism is correctable with standard cylindrical lenses in glasses, toric contact lenses, and LASIK. It accounts for the vast majority of clinically significant astigmatism.

Irregular astigmatism occurs when the corneal surface has non-uniform curvature that cannot be described by a simple cylinder and axis. Causes include keratoconus, corneal scarring, prior surgery, and trauma. Irregular astigmatism cannot be fully corrected with glasses or standard contact lenses. Topography-guided LASIK is specifically designed to address irregular astigmatism in appropriate candidates, and scleral lenses can optically neutralize irregular surface by creating a smooth liquid interface.

Symptoms of Astigmatism

Astigmatism symptoms differ from pure myopia or hyperopia because the distortion affects all distances:

• Blurry or distorted vision at all distances (not just near or far)
• Letters or objects appearing stretched, doubled, or shadowed
• Eye strain and headaches from the continuous accommodative effort to compensate for the blur
• Difficulty with reading — letters may appear to run together
• Halos or starbursts around lights at night (particularly prominent with significant astigmatism)
• Squinting to try to improve focus

How Common Is Astigmatism?

Astigmatism of at least 0.25 D occurs in approximately 33% of people; clinically significant astigmatism (0.75 D or more) affects roughly 15–30% of the population. Most astigmatism is corneal (caused by corneal shape) rather than lenticular (caused by lens shape). Astigmatism is very commonly found in combination with myopia or hyperopia — pure astigmatism without any spherical component is relatively uncommon.

Correction Options for Astigmatism

Correction Method	Regular Astigmatism	Irregular Astigmatism	Permanent?
Spherocylindrical glasses	Yes	Partially	No
Toric soft contact lenses	Yes	No	No
Rigid/scleral contact lenses	Yes	Yes (optically)	No
Standard LASIK	Yes (up to 6 D)	No	Yes
Topography-guided LASIK	Yes	Yes (eligible cases)	Yes
PRK	Yes	Partially	Yes
Toric ICL	Yes	Partially	Reversible

LASIK for Astigmatism

LASIK effectively corrects regular astigmatism up to 6 diopters by using the excimer laser in a non-circular ablation pattern to selectively flatten the steeper corneal meridian. Modern laser platforms with cyclotorsion tracking (which compensates for rotational eye movement during treatment) provide excellent astigmatism correction accuracy. See LASIK prescription limits for astigmatism. The key concern for astigmatism treatment is accurate axis alignment — small cyclotorsional errors of even 5–10 degrees during surgery can reduce the effective astigmatism correction by 15–35%.

Astigmatism Combined with Myopia or Hyperopia

Most LASIK patients have astigmatism combined with myopia (most common) or hyperopia. This combination, called compound myopic astigmatism or compound hyperopic astigmatism, is treated simultaneously in a single LASIK treatment that addresses both the spherical and cylindrical components. The presence of astigmatism does not meaningfully complicate LASIK candidacy assessment; the key factors remain corneal thickness relative to total tissue removal, residual stromal bed requirements, and prescription stability. See refractive error types for a comprehensive overview.