SMILE Pro for Astigmatism: What Patients Should Know

Astigmatism is often described as “the other thing on your prescription”, usually right after the numbers. It is responsible for a specific kind of blurriness (letters that smear in one direction, or a sense that nothing is ever quite crisp) and it behaves differently from simple short-sightedness in ways that affect how it is corrected.

What is astigmatism, in plain terms?

A perfect cornea curves evenly, like a sphere. An astigmatic cornea curves more in one direction than another, like the side of a rugby ball. Light entering it is focused differently depending on the direction it comes from, so there is no single point of focus.

That means an astigmatism correction has two components: an amount, and a direction: the axis. On a prescription, the axis is the number in degrees. Get the amount right and the axis wrong and the result is worse than doing nothing.

Can SMILE Pro correct astigmatism?

Yes, and typically at the same time as short-sightedness. The lenticule created by the laser is shaped so that removing it corrects both the spherical error and the cylinder, in the right orientation.

Because the correction is baked into the shape and orientation of the lenticule, the procedure depends on the eye being correctly aligned with the laser at the moment of treatment. That is a mechanical precision problem, and it is why newer platforms have introduced alignment-assistance features. The ZEISS VISUMAX 800 includes features aimed specifically at centration and at axis alignment for astigmatic correction, which the manufacturer describes on its product pages.

Why does the axis matter so much?

Consider the numbers. Being 10 degrees off on an axis has a much smaller effect than being 30 degrees off, and the effect scales with the amount of astigmatism being corrected. Someone with a large cylinder is therefore more sensitive to a small alignment error than someone with a small one.

This is one reason astigmatism correction is unforgiving of measurement error. If your pre-operative topography is unreliable (because your tear film was unstable on the day, for instance) the axis you are being treated on is itself in question.

It is also a reason to ask your clinic specific questions rather than general ones: how is the axis determined, what is used to verify it on the day, and what happens if the post-operative axis is off.

How much astigmatism can be treated?

Moderate astigmatism is commonly treated with SMILE. Very high astigmatism (and the numbers here depend on the platform and the surgeon’s preference) may be better addressed with a different technique, or may be partly corrected with the remainder managed with glasses.

The practical limits also interact with your corneal thickness, because correcting both a spherical error and a cylinder removes more tissue than correcting the sphere alone.

Does astigmatism make dry eye worse?

They are related, though not in the way people usually assume. A dry eye can produce inconsistent measurements, which means your astigmatism may be measured differently on different visits. That inconsistency is a signal that the surface needs treating before surgery, not that the astigmatism is changing.

There is a further wrinkle: a small amount of regular astigmatism can sometimes be a normal finding after refractive surgery and tends to settle as healing completes. Distinguishing between that and a genuine residual astigmatism is one of the reasons final measurements are not taken early.

What about irregular astigmatism?

There is a difference between regular astigmatism, where the two curvatures are at right angles to each other and predictable, and irregular astigmatism, where they are not. Regular astigmatism is what a glasses prescription corrects and what refractive surgery targets.

Irregular astigmatism (often associated with keratoconus or with scarring) is a different matter entirely, and it is generally a reason to be declined rather than treated. Topography is how the two are told apart, which is why topography is part of the assessment rather than an optional extra.

What this means for you

If you have astigmatism on your prescription, the useful questions are about axis accuracy rather than about the procedure’s general suitability. Ask how the axis is measured, what is used to confirm alignment during treatment, and what the clinic’s experience is with astigmatism of your magnitude.

And if your astigmatism reading has varied noticeably between visits, raise that. It is often a surface problem that can be fixed before surgery rather than a reason to accept a less predictable outcome.

Our eligibility page covers the measurements involved, and the comparison page shows how astigmatism correction differs between SMILE, LASIK and PRK.