Astigmatism Explained: Symptoms, Causes and Correction

What astigmatism is

A healthy eye focuses light to a single point on the retina. The clear front surface of the eye, the cornea, does most of that focusing, helped by the lens inside. If the cornea is evenly curved, like a football, it bends light the same amount in every direction.

In astigmatism the cornea (or sometimes the lens) is curved more steeply in one direction than the other, more like the back of a spoon or a rugby ball. Light in one direction focuses at a different distance from light in the other, so no single point is sharp. Vertical lines might look crisp while horizontal ones are fuzzy, or the reverse, or the blur may sit at a diagonal.

The steep and flat directions are usually at right angles to each other. That is called regular astigmatism, and glasses correct it well. Irregular astigmatism, from an injury or a condition such as keratoconus, has no simple pattern and needs special lenses.

Symptoms

• Blurry or slightly stretched vision at both near and far distances.
• Letters with shadows or double edges, especially fine print.
• Streaks, starbursts or halos around headlights and street lights at night.
• Squinting to see clearly, and tired eyes or headaches after reading or screen work.
• Some lines on a pattern (tiles, a clock dial, window blinds) looking darker or sharper than others.

Mild astigmatism often causes no obvious symptoms at all, because the eye partly compensates by adjusting focus. That effort is what leads to eye strain. Children rarely complain, which is why regular eye exams matter for them: uncorrected astigmatism in early childhood can affect how the visual system develops.

Causes and how common it is

Most astigmatism is simply the shape you were born with and is often inherited. It may change a little over the years, and it commonly gets slightly stronger with age as the eyelids press on the cornea differently. It is not caused by reading in poor light, sitting close to screens or using a phone, although those can make the symptoms more noticeable.

Some degree of astigmatism is present in a large share of people. Most of it is small, under 1 dioptre, and many people never need correction for it. It often comes together with short sight (myopia) or long sight (hyperopia).

Less common causes include eye injury or surgery that scars the cornea, and keratoconus, a condition where the cornea thins and bulges into a cone. Keratoconus usually shows up in the teens or twenties and gets worse over time, so astigmatism that keeps increasing in a young person needs an eye specialist.

Reading the numbers on a prescription

A glasses prescription has three numbers for each eye:

As a rough guide, a cylinder up to about 0.75 is mild, 1.00 to 2.00 moderate, and above 2.00 high. Some opticians write cylinders as minus numbers and some as plus; they describe the same eye and convert into each other.

The axis is only meaningful together with a cylinder. Around 180 or 90 is called "with the rule" or "against the rule" astigmatism; in between is oblique. Your optician uses it to orient the lens in the frame.

The clock dial test

The astigmatism test on this site uses a clock dial: lines radiate from the centre like the numbers on a clock. Cover one eye, look at the centre from a comfortable distance without your glasses, and check whether all lines look equally dark and sharp.

If some lines look darker, thicker or sharper than others, the eye focuses that direction differently, which is a sign of astigmatism. The test shows an approximate axis from the lines you pick. If all lines look the same, you may have little or no astigmatism, though very small amounts are easy to miss.

Repeat with the other eye, and then with glasses on. If your glasses are correct, the lines should look even. Pair it with the eye test for a rough check of sharpness.

How astigmatism is corrected

• Glasses with a cylindrical (toric) lens bend light more in one direction to balance the eye. They are the simplest and safest option.
• Toric soft contact lenses are weighted to stay in the right orientation. Rigid gas permeable lenses correct astigmatism well because they give the eye a new, round front surface.
• Laser surgery (LASIK, PRK, SMILE) can reshape the cornea for many people with regular astigmatism. It is not suitable for everyone, especially with thin corneas or keratoconus.
• Toric lens implants can correct astigmatism during cataract surgery.

New glasses for astigmatism can make floors look tilted or straight lines look curved for a few days. That usually settles within a week or two. If it does not, ask your optician to check the axis.