What is astigmatism? Why it blurs, and what CYL and axis mean

Astigmatism is a common focusing error in which the clear front surface of the eye, the cornea, or sometimes the lens inside the eye, is curved more steeply in one direction than in the other. Light entering the eye then comes to a focus in two places instead of one, so things look slightly blurred or smeared at every distance, near and far. It is not an eye disease, most people have at least a little of it, and glasses or contact lenses usually correct it well.

The word often comes home from an eye exam with no explanation, written on the prescription as a cylinder and an axis. Here it is step by step, including the few situations where it needs more than new glasses.

A football, not a basketball

Think of a basketball: run a finger across it in any direction and the curve feels the same. In an eye without astigmatism, the cornea is curved fairly evenly like that, so light is bent by the same amount whichever way it passes through.

Now think of an American football, or a rugby ball, lying on its side. Along its length the curve is gentle; around its middle it is steep. A cornea with astigmatism is shaped a little more like that. The steeper direction bends light more strongly than the flatter one, so the eye has two focusing powers at once and no single point comes out sharp. Holding a page closer or farther away doesn't fix it, which is what sets astigmatism apart from simple nearsightedness or farsightedness.

The steepest and flattest directions are called the principal meridians. In most people they sit at right angles to each other. Eye doctors call this regular astigmatism, the everyday kind that ordinary glasses correct well. Irregular astigmatism, where the surface is uneven in a patchier way, is less common and can follow a corneal scar or injury, or come from keratoconus, described further down.

Many people are born with some astigmatism, it often runs in families, and it frequently comes together with nearsightedness or farsightedness. Reading in dim light and sitting close to the television do not cause it. Both can tire your eyes, but neither changes the shape of the cornea.

What astigmatism feels like

Small amounts often cause no symptoms at all. Larger amounts, or astigmatism that has never been corrected, can bring:

• blur or slight distortion at all distances, with letters or straight edges looking a little doubled or smudged
• tired eyes and eye strain after reading, close work or long stretches at a screen
• headaches, often later in the day
• squinting to bring things into focus
• poorer vision at night, with streaks, starbursts or halos around headlights and streetlights

The night trouble has a simple cause. In the dark your pupils open wider, so light passes through more of the unevenly curved cornea and the blur shows more. On a wet autumn evening in Kyoto, the lanterns along the river can stretch into long rays for an eye whose astigmatism has not been corrected.

How astigmatism shows on your prescription

A glasses prescription gives three main numbers for each eye. The sphere (SPH) is the power that is the same in every direction: minus for nearsighted, plus for farsighted. The cylinder (CYL) is the extra power for astigmatism, acting in one direction only, in steps of 0.25 diopters. A blank cylinder box, or "DS", means no astigmatism correction for that eye.

The axis is a number from 1 to 180 degrees that tells the lab which way to set the cylinder. Read it like a protractor laid over the lens: 90 is vertical and 180 is horizontal. It is a direction, not a strength: an axis of 170 is no stronger than an axis of 10. Strictly, the axis marks the direction in which the cylinder adds no power; its full extra power acts at right angles to that line.

Plus and minus cylinder

Some prescribers write the cylinder as a minus number and others as a plus number. Both describe exactly the same lens. To switch from one form to the other:

1. Add the cylinder to the sphere. That gives the new sphere.
2. Change the sign of the cylinder.
3. Turn the axis by 90 degrees, keeping it between 1 and 180.

So +0.50 -1.50 x 90 becomes -1.00 +1.50 x 180. A handy check is the spherical equivalent, the sphere plus half the cylinder, which stays the same in both forms: +0.50 - 0.75 = -0.25, and -1.00 + 0.75 = -0.25.

That example is mixed astigmatism: along one direction the lens has a power of +0.50, along the other -1.00, so one direction of the eye is farsighted and the other nearsighted. Averaged, it comes out close to zero, yet the lens corrects 1.50 diopters of astigmatism. The glasses prescription reader switches between the two forms for you and would describe this eye as close to zero overall with moderate astigmatism, a good reminder that one averaged number can hide a lot.

How much is a lot?

The prescription reader calls a cylinder under 1.00 mild, 1.00 to 2.00 moderate and over 2.00 marked, as plain descriptive labels rather than clinical grades. Small cylinders are very common. Whether a low amount is worth correcting depends on how clearly and comfortably you see, and you can decide that with the person examining your eyes.

Can astigmatism change?

It can, usually slowly. In most adults it stays fairly steady, and small differences between exams are a normal part of measuring. Over the decades the amount or the axis often drifts a little, children's prescriptions change more as their eyes grow, and eye surgery, an injury or a corneal scar can also change it.

The cause worth knowing by name is keratoconus. In this condition the cornea gradually thins and bulges forward into a cone shape. It usually begins in the teenage years or early adulthood, and the astigmatism it produces keeps increasing and becomes irregular. Typical clues are a prescription that changes at almost every visit, new glasses that never seem quite sharp, and growing distortion or ghosting, often worse in one eye. Frequent, hard eye rubbing is thought to be one of the factors linked to it, so the habit is worth breaking.

Tests that map the shape and thickness of the cornea find it. Treatments exist that can slow or stop it from progressing, and special contact lenses can often restore good vision, so finding it early matters. Your eye doctor will decide which tests and treatment fit.

How it is corrected

Uncorrected astigmatism makes an adult's vision less sharp and can make the eyes work harder, but it does not damage them. Correction is about seeing clearly and comfortably. The usual options:

• Glasses with a cylinder in the lens. A first pair, or one with a larger cylinder or a changed axis, can feel odd for a few days, with floors that seem to tilt. This usually settles within a week or two; if it doesn't, go back and have the glasses checked.
• Soft toric contact lenses, shaped to correct astigmatism and designed to settle in the right orientation on the eye.
• Rigid gas permeable lenses, which keep their shape on the eye, so the layer of tears beneath them evens out much of the astigmatism that comes from the cornea.
• Laser refractive surgery, which reshapes the cornea and can correct many cases of regular astigmatism in adults with a stable prescription. An eye surgeon assesses the cornea's shape and thickness first, and a thin or cone-shaped cornea is a common reason to advise against it.
• At cataract surgery, an astigmatism-correcting lens implant may be an option. The column on cataract surgery timing compares the lens choices.

Contact lenses of any kind need a fitting and their own prescription, and the powers often differ from your glasses; the glasses to contacts calculator shows why. Which option suits you is something to work out with your eye doctor.

Astigmatism in children

For children the stakes are different. Vision develops most until about age 7 or 8. If an eye receives a persistently blurred image during those years, the brain may not learn to see sharply through it. This is amblyopia, or lazy eye, which affects about 2 to 3 of every 100 children. A large uncorrected astigmatism, especially when one eye has much more than the other, is one of its causes.

Children seldom mention blur, because they assume everyone sees the way they do. A child who squints, tilts their head or holds books very close needs an eye exam, and many countries offer vision screening for young children; ask your child's doctor or health visitor what is available where you live. Treatment for amblyopia works best in early childhood. The column on vision screening for young children explains what happens when a result says refer, and the guide to eye checks through life sets out which checks suit which ages.

If your child has glasses for astigmatism, follow the eye doctor's advice on wearing them; for a young child they are part of how vision develops, not only a matter of comfort.

When to see an eye doctor

Book a routine eye exam if things look blurred at any distance, if reading and screens bring eye strain or headaches, or if glare from lights at night makes driving harder. In many countries an optometrist can measure astigmatism and prescribe glasses or contact lenses; an ophthalmologist is a medical doctor who diagnoses and treats eye disease and performs surgery. The guide to your first eye exam describes a typical visit.

Go sooner if your prescription keeps changing, if new glasses never give you clear vision, or if distortion is growing in one eye.

Some symptoms are not astigmatism and should not wait:

• New flashes of light, a sudden shower of floaters, or a shadow or curtain across part of your vision need same-day eye care.
• Severe eye pain with blurred vision, halos around lights, headache or nausea can be acute angle-closure glaucoma. Painless streaks around headlights that you have had for years are a different matter.
• Sudden loss of vision in one eye, even if it returns within minutes, can be a warning sign of stroke.

For the last two, go to an emergency department or call your local emergency number (such as 911, 999, 112, or 119 in Japan). The guide to eye emergencies and first aid gathers these symptoms in one place.

Questions people ask

Can screens or reading make astigmatism worse?

There is no good evidence that screen time changes the shape of the cornea. Long screen sessions can make existing astigmatism more noticeable, though, because tired eyes that blink less make blur and strain harder to ignore.

Will not wearing my glasses make it worse?

In adults, no. Going without glasses may leave you with blur and tired eyes, but it does not change the astigmatism itself. Children are different: if glasses were prescribed to help vision develop, follow the eye doctor's instructions.

Sources

• National Eye Institute, patient information on refractive errors
• American Academy of Ophthalmology, eye health pages on astigmatism and keratoconus