What is Astigmatism?
Astigmatism is a refractive error in which the eye does not focus light evenly on the retina due to an irregular curvature of the cornea or, less commonly, the crystalline lens. In a normal eye, the cornea is spherical, like a basketball, with equal curvature in all meridians. In astigmatism, the cornea is shaped more like a rugby ball (or American football), with one meridian being steeper than the other. This means light rays entering the eye are focused at two different points rather than a single focal point on the retina, resulting in blurred or distorted vision at both near and far distances. Astigmatism is classified as regular (when the two principal meridians are perpendicular, the most common type) or irregular (when the meridians are not perpendicular, often caused by corneal scarring, keratoconus, or surgery). It is measured in diopters of cylinder and often coexists with myopia or hyperopia. Most people have some degree of astigmatism; it becomes clinically significant when it causes symptoms or requires correction.
Symptoms
- Blurred or distorted vision at all distances
- Eye strain and discomfort
- Headaches, particularly after prolonged visual tasks
- Difficulty with night vision and glare from lights
- Squinting to see more clearly
- Difficulty distinguishing similar shapes (e.g., M and H, or 8 and 0)
- Tilting the head to find a clearer angle of vision
Mild astigmatism may be asymptomatic. In children, uncorrected astigmatism can contribute to amblyopia (lazy eye) if not detected and treated early.
Causes and Risk Factors
Astigmatism is usually present from birth and is believed to have a strong hereditary component. The cornea develops with an asymmetric curvature that results in two different refractive powers along the principal meridians. Factors that may cause or worsen astigmatism include:
- Genetics — the most common cause; often runs in families
- Keratoconus — a progressive thinning and steepening of the cornea that causes increasing irregular astigmatism
- Corneal scarring from infection, injury, or disease
- Previous eye surgery (e.g., cataract surgery, corneal transplantation)
- Pterygium — a growth on the cornea that distorts its surface
- Chalazion or eyelid masses pressing on the cornea
- Habitual eye rubbing, which has been linked to corneal changes
Diagnosis
Astigmatism is diagnosed during a comprehensive eye examination. Refraction testing (subjective and objective) quantifies the degree of cylindrical error and its axis. Keratometry measures the curvature of the anterior corneal surface. Corneal topography provides a detailed color-coded map of the entire corneal surface, essential for detecting irregular astigmatism and screening for keratoconus. Corneal tomography (e.g., Pentacam) evaluates both the anterior and posterior corneal surfaces and provides pachymetry data. Wavefront aberrometry can measure higher-order aberrations that contribute to visual quality beyond what standard refraction captures. These tests are particularly important when planning contact lens fitting or refractive surgery.
Treatment
Treatment for astigmatism aims to provide clear, comfortable vision by correcting the unequal refractive power of the eye:
- Eyeglasses with cylindrical (toric) lenses — the most common and straightforward correction method
- Toric contact lenses — available in soft, rigid gas permeable (RGP), and hybrid designs; RGP lenses can also mask mild corneal irregularity
- Orthokeratology — overnight rigid lenses that temporarily reshape the cornea
- Refractive surgery — LASIK, PRK, or SMILE can permanently reshape the cornea to correct astigmatism; toric ICL implantation is an option for higher levels
- Toric intraocular lenses — used during cataract surgery to simultaneously correct pre-existing astigmatism
- For irregular astigmatism (e.g., keratoconus) — scleral lenses, corneal cross-linking (CXL) to halt progression, or intracorneal ring segments (ICRS)
Prevention and Recommendations
- Schedule regular eye examinations, especially for children — early detection prevents amblyopia
- Avoid habitual eye rubbing, which can worsen corneal irregularity
- Wear prescribed corrective lenses consistently for optimal visual comfort
- If you wear contact lenses, follow proper hygiene and replacement schedules
- Discuss refractive surgery options with your ophthalmologist if you wish to reduce dependence on glasses or contacts
- Seek evaluation if you notice progressive distortion of vision, which may indicate keratoconus or other corneal conditions
- Protect your eyes from injury with appropriate safety eyewear during sports and hazardous activities