Astigmatism

What astigmatism is

A healthy cornea is curved evenly in all directions, like a ball, so light focuses to a single point on the retina. In astigmatism, the cornea or lens has two different curves at right angles, like the back of a spoon. Light entering along the steep curve focuses at a different point than light along the flatter curve. The result is blur and distortion that affects near and far vision.

Astigmatism often occurs together with myopia (nearsightedness) or hyperopia (farsightedness). It is one of the refractive errors, not an eye disease.

Types

• Corneal astigmatism: from an unevenly shaped cornea. The most common type.
• Lenticular astigmatism: from an unevenly shaped lens inside the eye.
• Regular astigmatism: the two curves are at right angles. Easily corrected with glasses.
• Irregular astigmatism: the cornea is unevenly shaped in a patchy way, from keratoconus, scarring or injury. It is harder to correct with glasses and often needs rigid contact lenses.

Symptoms

• blurred or distorted vision at all distances
• eye strain and tired eyes, especially after reading, screens or driving
• headaches, often across the forehead
• squinting to see clearly
• glare, halos or starbursts and streaks around lights at night
• difficulty reading small print
• in children: rubbing eyes, head tilting, poor attention in class

Mild astigmatism may cause no symptoms. Children often do not complain because they think their vision is normal, which is why eye screening matters.

Causes and risk factors

• Genetics: most astigmatism is present from birth and runs in families.
• Keratoconus: the cornea gradually thins and bulges into a cone shape, typically starting in the teens or twenties, causing increasing irregular astigmatism. Heavy eye rubbing and allergies are linked.
• Eye surgery or injury: corneal scars and some operations change the corneal shape.
• Eyelid pressure: a large chalazion or droopy lid can press on the cornea and cause temporary astigmatism.
• Pellucid marginal degeneration: a rare thinning of the lower cornea.

Reading in dim light or sitting close to a screen does not cause astigmatism, though it can cause temporary eye strain.

Diagnosis

An optometrist or ophthalmologist diagnoses astigmatism during a routine eye exam:

• Visual acuity: reading letters on a chart.
• Refraction: trying lenses to find the clearest prescription, often starting with an automated measurement.
• Keratometry and corneal topography: measure the curvature and map the shape of the cornea, important for spotting keratoconus and fitting contact lenses.

Reading an astigmatism prescription

Your prescription includes three numbers for astigmatism-related correction:

• Sphere (SPH): the nearsighted (minus) or farsighted (plus) correction.
• Cylinder (CYL): the amount of astigmatism, in diopters. Up to about 0.75 is mild, 1.00 to 2.00 moderate, and over 2.00 more significant.
• Axis: a number from 1 to 180 degrees showing the direction of the astigmatism.

Treatment

Glasses

Glasses with a cylindrical lens correct regular astigmatism. Some people need a few days to adjust to new or stronger astigmatism correction, which can make floors look tilted at first.

Contact lenses

• Toric soft lenses: designed with weighting or other features so they stay in the right orientation.
• Rigid gas permeable (RGP) lenses: create a smooth new front surface, correcting both regular and irregular astigmatism. Often best for keratoconus.
• Scleral lenses: large rigid lenses resting on the white of the eye, for more severe irregular astigmatism.
• Orthokeratology: rigid lenses worn overnight to temporarily reshape the cornea, for mild cases.

Refractive surgery

• LASIK and PRK: a laser reshapes the cornea to correct astigmatism along with nearsightedness or farsightedness.
• SMILE: a newer procedure that removes a small disc of corneal tissue through a tiny incision.
• Toric intraocular lenses: correct astigmatism during cataract surgery.
• Corneal cross-linking: strengthens the cornea to stop keratoconus from progressing.

Surgery is generally offered when the prescription has been stable for at least a year, and is not suitable for keratoconus or a thin cornea.

Astigmatism in children

Many infants have astigmatism that lessens in the first years of life. Significant astigmatism that persists can cause amblyopia (lazy eye) if not corrected, because the brain does not learn to see clearly. Children should have vision screening at regular well-child visits, and a full eye exam if screening is failed or there is a family history.

Frequently asked questions

What does vision with astigmatism look like?

Vision looks blurred or slightly stretched in one direction at all distances. Letters may look shadowed or smeared, and lights at night may have streaks, starbursts or halos. Many people squint to sharpen their vision, and mild cases may be barely noticeable.

Can astigmatism go away on its own?

Astigmatism present from birth usually stays stable through adulthood, though some infant astigmatism decreases as the eye grows. Astigmatism caused by lid pressure from a chalazion can resolve once the lump goes. Keratoconus-related astigmatism tends to worsen until it stabilizes.

Why do lights look streaky at night with astigmatism?

At night the pupil widens, letting light pass through more of the unevenly curved cornea. This spreads light along one direction, causing streaks and starbursts around headlights and streetlights. Glasses or contact lenses that correct the astigmatism usually reduce this.

Is astigmatism serious?

Regular astigmatism is not a disease and is easily corrected. It becomes a concern when it is causing headaches, poor school performance or lazy eye in children, or when it is rapidly worsening, which can be a sign of keratoconus and needs specialist assessment.

Can LASIK fix astigmatism?

Yes. LASIK, PRK and SMILE can correct most regular astigmatism along with nearsightedness or farsightedness. They are not suitable for everyone, especially people with thin corneas or keratoconus. A detailed eye assessment determines whether you are a candidate.