The optician says I have astigmatism. Is it serious?

The word does a lot of unnecessary frightening. People often hear “astigmatism” at the optical shop and wonder whether something is wrong inside their eye. So, plainly: astigmatism is not a disease. It is a description of the shape of your eye, and for the great majority of people it is nothing more than a detail in their spectacle prescription.

What astigmatism is

Think of the clear front window of the eye, the cornea. In a perfectly regular eye, it is curved like the surface of a football, equally round in every direction, so light focuses to a single clean point.

In an eye with astigmatism, the cornea is curved a little more steeply in one direction than the other, more like the side of a rugby ball. Light entering the eye then focuses at two slightly different points instead of one, and the result is a mild smearing or shadowing of the image, at all distances, not just far away.

That is the whole story. It is a shape, present usually from birth, often running in families, and very common. Most people have at least a small amount. It does not mean the eye is weak, diseased, or wearing out, and ordinary reading, screen use and “eye strain” do not cause it or worsen it.

How it is corrected

Glasses correct regular astigmatism well. The lens is ground with a compensating curve, called a cylinder, which is the “cyl” number you see on your prescription. When you first wear a cylinder correction, or when the axis changes, the world can look subtly tilted for a few days. That is your brain adjusting, and it settles.

Contact lenses work well too. Regular soft lenses correct small amounts, and toric contact lenses, which are shaped to sit in a fixed orientation on the eye, correct larger amounts. Astigmatism can also be treated with laser surgery in suitable eyes.

Why astigmatism matters at cataract surgery

Here is the part many people only discover in their sixties. When a cataract is removed, the cloudy natural lens is replaced with an artificial lens, an IOL. A standard IOL corrects the main power of the eye but not the astigmatism, so a person with significant corneal astigmatism would still need glasses for clear distance vision afterwards.

For those eyes a toric IOL can be used, a lens with the astigmatism correction built into it, aligned inside the eye during cataract surgery. A toric lens can reduce astigmatism at the time of cataract surgery; some residual astigmatism can remain and glasses may still be needed. This is why the cornea is measured carefully before a lens is chosen.

So if you have astigmatism and a cataract is developing, mention the astigmatism when surgery is planned. It can change the choice of lens.

When astigmatism deserves closer attention

There is one situation that deserves prompt attention. Ordinary astigmatism is stable, the number stays roughly the same for decades. If astigmatism is increasing steadily, especially in a teenager or young adult, or if the axis keeps shifting and no pair of glasses ever seems quite right, the cornea should be examined more closely.

Occasionally that pattern points to keratoconus, a condition in which the cornea gradually thins and steepens into a cone shape. It is uncommon, it is not caused by the astigmatism itself, and found early, its progression can often be halted with treatment. The test is a non-contact corneal map called topography, done in a few minutes. Frequent eye rubbing, strong allergies and a family history raise suspicion.

That is the scope of concern: not astigmatism itself, but astigmatism that keeps changing.

The short answer

A stable cylinder number on your prescription is a shape to be corrected, not a condition to be feared. Wear the correction, and have it re-checked at your routine eye examination. If your number has been climbing year after year, have the cornea examined by an eye doctor.