Astigmatism: Causes, Symptoms, Prescription and Treatment Options

Astigmatism is one of the most common reasons people need glasses, yet it is also one of the most misunderstood. The word sounds like a disease, and many people worry when they first see it on their prescription. In reality, astigmatism is a very common focusing error, present to some degree in most people. Only larger amounts cause noticeable blur, and nearly all astigmatism can be corrected simply with glasses or contact lenses.

In astigmatism, the front surface of the eye or the lens inside it is not curved evenly in all directions. Instead of focusing light at a single point on the retina, the eye focuses it at two different points, creating blur and distortion at every distance. This guide explains the types and causes, how astigmatism feels, what the cylinder and axis numbers on your prescription mean, the correction options and the important link with keratoconus.

What is astigmatism?

In an ideal eye, the cornea is shaped like part of a perfectly round ball, curved equally in all directions. Light passing through it is focused to a single sharp point on the retina. In an eye with astigmatism, the cornea or lens is shaped more like the back of a spoon or a rugby ball: steeper in one direction and flatter in the direction at right angles to it. These two directions are called the principal meridians.

Because each meridian has a different focusing power, light is focused at two different points instead of one. A dot may appear smeared into a short line, and lines in one orientation may look sharper than lines in another. Astigmatism often occurs together with myopia or hyperopia. Our guide to refractive errors explains how these focusing problems relate to each other.

Types of astigmatism

Corneal versus lenticular astigmatism

• Corneal astigmatism is caused by an unevenly curved cornea. It is the most common type and accounts for most clinically significant astigmatism.
• Lenticular astigmatism comes from the crystalline lens inside the eye, due to its shape or tilt. It often partly offsets corneal astigmatism. Lenticular astigmatism can change with age and with cataract development.

The total astigmatism measured in a prescription is the combination of both. This is why your corneal measurements and your glasses prescription do not always match exactly, and why astigmatism can change after cataract surgery, when the natural lens is removed.

Regular versus irregular astigmatism

• Regular astigmatism: the two principal meridians are at right angles to each other. It can be fully corrected with standard cylindrical lenses. Most astigmatism is regular.
• Irregular astigmatism: the curvature is uneven and asymmetrical, so it cannot be fully corrected by glasses. It may be caused by keratoconus, corneal scarring after injury or infection, previous eye surgery or corneal degenerations. Rigid contact lenses often give much better vision in these cases.

With-the-rule and against-the-rule

Eye care professionals also describe regular astigmatism by orientation. In with-the-rule astigmatism, the vertical meridian is steeper; this is more common in children and young adults. In against-the-rule astigmatism, the horizontal meridian is steeper; this becomes more common with age. Oblique astigmatism has principal meridians that are neither horizontal nor vertical. The orientation affects how blur is experienced and which correction is easiest to adapt to.

What causes astigmatism?

• Genetics: the natural shape of the cornea is largely inherited. Most people are born with some astigmatism, and it often runs in families.
• Eyelid pressure: the eyelids press lightly on the cornea, which contributes to with-the-rule astigmatism. Lumps such as a large chalazion or a droopy eyelid can temporarily change corneal shape.
• Keratoconus and corneal thinning disorders.
• Corneal scarring from injury, infection or ulcers.
• Eye surgery, including cataract surgery, corneal transplant and some glaucoma or retinal operations.
• Ageing: the cornea tends to shift from with-the-rule to against-the-rule astigmatism over the decades.

Reading in dim light, sitting close to the television or using screens does not cause astigmatism. These activities can cause temporary eye strain, but they do not change the shape of the cornea.

Symptoms of astigmatism

Mild astigmatism often causes no symptoms at all. Larger amounts, or astigmatism combined with other refractive errors, can cause:

• Blurred or slightly distorted vision at both distance and near
• Ghosting or doubling of letters and edges in one eye
• Difficulty distinguishing similar letters, such as H and N or 8 and B
• Glare, starbursts and streaks around lights at night, which can make night driving uncomfortable
• Eye strain, tired eyes and frowning
• Headaches after reading, computer work or driving
• Squinting or tilting the head to see more clearly

Children with significant astigmatism may not complain at all, because they have never seen clearly. Uncorrected high astigmatism in early childhood can lead to a specific form of amblyopia called meridional amblyopia, in which vision for lines in certain orientations does not develop properly. This is one reason why children's eye examinations are so important.

How astigmatism is measured

Astigmatism is detected and measured during a routine eye examination with several complementary tests:

1. Visual acuity: reading letters on a chart shows whether vision is reduced.
2. Autorefraction and retinoscopy: objective measurements estimate the total refractive error, including astigmatism.
3. Subjective refraction: the examiner refines the cylinder power and axis, often using a special handheld lens called a Jackson cross cylinder and a clock-dial or fan chart.
4. Keratometry: measures the curvature of the central cornea in its two main meridians.
5. Corneal topography or tomography: creates a detailed colour map of the entire cornea's shape and thickness. This is essential for detecting irregular astigmatism and early keratoconus, and before contact lens fitting or refractive surgery.

Our online astigmatism test uses a clock-dial chart to show whether some lines look darker or sharper than others. It can be a useful prompt to book an appointment, but it cannot measure your prescription or check your eye health, and it does not replace a professional examination.

Understanding cylinder and axis

A glasses prescription with astigmatism has three numbers for each eye: sphere, cylinder and axis.

So a prescription written as -2.00 / -1.25 x 90 means 2 dioptres of myopia plus 1.25 dioptres of astigmatism with its axis at 90 degrees (vertical). The axis is not a measure of severity; it simply describes the direction. Some countries write cylinder in plus form and others in minus form; both describe the same correction, just expressed differently.

As a rough guide, astigmatism under about 0.75 D is considered low and often needs no correction unless it causes symptoms; 1.00 to 2.00 D is moderate; and above about 2.00 to 3.00 D is high. Our guide on how to read your glasses prescription covers every term in detail.

Correction options

Glasses

Spectacle lenses with a cylindrical component (sometimes called toric or sphero-cylindrical lenses) correct regular astigmatism precisely by adding the right power in the right meridian. They are safe, simple and effective. When people get glasses with a new or significantly changed astigmatism correction, the world can look tilted or curved, and floors may seem to slope for a few days. This adaptation usually settles within one to two weeks. If it does not, the prescription or fitting should be rechecked.

Toric soft contact lenses

Toric soft contact lenses have different powers in different meridians and a design that keeps them in the correct orientation on the eye, using weighting or thicker zones. Modern toric lenses are available for a wide range of prescriptions, including daily disposables, and can correct most regular astigmatism comfortably. Vision may fluctuate slightly if the lens rotates, especially with blinking or looking sideways.

Rigid gas-permeable lenses

Rigid gas-permeable (RGP) lenses keep their shape on the eye, and the tear layer behind them fills in the irregularities of the cornea. This makes them highly effective for both regular astigmatism and irregular astigmatism caused by keratoconus or scarring. Larger scleral lenses, which vault over the cornea and rest on the white of the eye, are often used for more advanced irregular corneas.

Refractive surgery

Laser procedures such as LASIK, PRK and SMILE can treat many cases of regular astigmatism in adults with a stable prescription and a healthy, sufficiently thick cornea. Thorough corneal mapping before surgery is essential, because laser surgery on an eye with undiagnosed keratoconus can lead to progressive corneal weakening (ectasia). Implantable toric lenses are an option for some people who are not laser candidates.

Toric intraocular lenses and cataract surgery

During cataract surgery or refractive lens exchange, the cloudy or natural lens is replaced with an artificial intraocular lens. A toric intraocular lens has built-in astigmatism correction and is carefully aligned in the eye to cancel corneal astigmatism. Small incisions placed at the steep meridian of the cornea (limbal relaxing incisions) are another option for smaller amounts. Many people with significant astigmatism can achieve good distance vision without glasses after cataract surgery thanks to these techniques. Read more in our guide to cataract.

Astigmatism and keratoconus

Keratoconus is a condition in which the cornea gradually thins and bulges forward into a cone shape. It usually begins in the teens or twenties and can progress over years. One of its earliest signs is astigmatism that increases quickly, changes axis or becomes irregular, so that glasses no longer give clear vision.

Warning signs that astigmatism might be due to keratoconus include:

• Frequent changes in glasses prescription, especially rising cylinder
• Vision that cannot be corrected well with glasses
• Ghosting or multiple images in one eye
• Increased sensitivity to light and glare
• A family history of keratoconus
• Habitual vigorous eye rubbing, eye allergies or atopic conditions such as eczema and asthma

Early detection matters because a treatment called corneal cross-linking can strengthen the cornea and slow or halt progression in many patients. It does not reverse existing changes, but it can help preserve vision and reduce the chance of needing a corneal transplant. Avoiding eye rubbing is also important. Learn more in our full guide to keratoconus.

Can astigmatism be cured with exercises?

No. Astigmatism is caused by the physical shape of the cornea or lens, and no eye exercise, eye yoga or special diet can change that shape. Claims that exercises can cure astigmatism are not supported by scientific evidence. Relaxation exercises and regular breaks may reduce eye strain during screen work, but they do not correct the blur. Glasses, contact lenses and surgery remain the only effective corrections.

When to see a doctor

Book an eye examination if you notice blurred or distorted vision at any distance, ghosting, glare around lights at night, frequent headaches or eye strain, or if you have not had an eye test for two years or more. Ask for corneal mapping if your astigmatism keeps changing, if glasses do not give clear vision, or if you have a family history of keratoconus. Children should have regular vision checks, as untreated astigmatism can affect visual development.