Why We Always Fix Astigmatism With Cataract Surgery

When you go to an optometrist for glasses, they measure your astigmatism and they put the correction in the lenses. They don’t stop halfway through the test and say “you’ve got a bit of astigmatism, would you like that corrected, it’s extra.” It would be a strange question. A prescription that ignores your astigmatism is just an incomplete prescription.

Cataract surgery is a prescription too. The cloudy lens comes out and a new lens goes in, and the power of that new lens is chosen to correct your eye. So I’ve never understood why, in a lot of places, correcting the astigmatism at the same time gets presented as an optional upgrade with a price tag attached. In my practice we always fix it, unless there’s a very good reason not to. This post is about why.

What astigmatism actually is

A cornea with no astigmatism is curved the same amount in every direction, like the surface of a ball. An astigmatic cornea is curved more steeply in one direction than the other, more like the back of a spoon. Light coming through it gets focused at two different points instead of one, and the result is blur and smearing at every distance, not just far or near. We measure it in dioptres, and we also record its direction, which is called the axis.

It’s extremely common in the people I operate on. Around 40% of cataract patients have more than a dioptre of corneal astigmatism, about 20% have more than a dioptre and a half, and 8% have more than two. In one series of 4,540 eyes coming for cataract surgery, only 13% had no corneal astigmatism at all, and 22% had 1.5 dioptres or more. So this isn’t a niche problem for a few unusual eyes. It’s most of the people in my waiting room.

Why leaving it there makes no sense

A standard lens implant corrects the spherical part of your prescription, which is the long-sighted or short-sighted part. It does nothing for the astigmatism. If I put a plain lens into an eye with 1.5 dioptres of corneal astigmatism, you wake up with 1.5 dioptres of astigmatism, and you will either wear glasses for it or put up with blur.

The blur isn’t trivial. When researchers induced astigmatism in people with otherwise healthy eyes, distance and near acuity fell step by step as the astigmatism went up, and astigmatism at an oblique or against-the-rule axis, which is the type that becomes more common with age, was worse than the same amount at a vertical axis. An international consensus on astigmatism in cataract surgery concluded that residual astigmatism should be under 0.75 dioptres in anyone hoping to be less dependent on glasses.

It matters even more if you’re having a lens that gives you near vision as well as distance. Trifocal and extended depth of focus lenses split light between focal points, and they have little tolerance for blur added on top. In a study of eyes with a diffractive multifocal lens, vision at almost every distance fell in proportion to the amount of astigmatism added, and the lens only performed as intended when astigmatism was under a dioptre. If I’m putting in a trifocal for someone with astigmatism, it has to be the toric version of that trifocal, and that’s not a conversation about upgrades. It’s the only version that will work.

And even if you’re perfectly happy to wear glasses afterwards, correcting the astigmatism still gets you something. Your glasses are simpler and lighter, and the vision you have without them, first thing in the morning, at the beach, getting up at night, is far better than it would otherwise be.

What a toric lens is

A toric lens is a standard lens implant with the astigmatism correction built into it, and a pair of small marks on its surface that show me the axis. During surgery I rotate the lens so those marks line up with the steep meridian of your cornea. That’s it. It’s the same acrylic material as a plain lens, it goes in through the same small incision, the operation takes the same fifteen minutes, and the recovery and the drops are identical. There are toric versions of monofocal lenses, extended depth of focus lenses and trifocals, so the choice of toric or not sits underneath the choice of what kind of vision you want, rather than replacing it.

The evidence behind them is about as solid as anything in cataract surgery. A systematic review and meta-analysis in the journal Ophthalmology found high-quality evidence that toric lenses give better unaided distance vision than non-toric lenses, with greater independence from glasses, and no increase in complications. It also found that toric lenses left less residual astigmatism, by about 0.37 dioptres on average, than the older approach of a plain lens plus relaxing incisions in the cornea. A Cochrane review comparing the two methods came to a similar conclusion: a toric lens probably gives you a better chance of ending up with half a dioptre or less of astigmatism.

Standard of care, not an upgrade

So here’s how it works in my practice. Your cornea is measured before surgery. If it has enough regular astigmatism to affect your vision, you get a toric lens. I don’t put it in front of you as an optional extra, and I don’t ask whether you’d like your astigmatism corrected, for the same reason your optometrist doesn’t. Correcting the eye properly is the job. A lens that ignores a dioptre and a half of astigmatism isn’t a standard result with something left off. In my view it’s an incomplete result.

I’d add that this isn’t a new or exotic position. Toric lenses have been in routine use for well over a decade, the evidence I’ve quoted above is years old, and there is nothing experimental about aligning a lens to an axis. What has lagged behind is the way it’s sometimes sold.

The very good reasons not to

“Always, unless there’s a very good reason not to” does mean there are reasons, and I’d rather you knew what they are.

• Very small amounts. Below about half a dioptre, the lens correction available is coarser than the astigmatism itself, and where I place the incision can deal with it. Between half and three quarters of a dioptre I look at the axis and the rest of the eye before deciding.
• Irregular astigmatism. A toric lens corrects regular astigmatism, where the cornea is steeper in one direction in a predictable, symmetrical way. Keratoconus, corneal scarring, radial keratotomy and some corneal transplants produce irregular astigmatism, which a toric lens can only partly correct. In those eyes the plan is different, and sometimes the cornea needs treating before the cataract does.
• A cornea that’s still changing. If the measurements don’t agree with each other, or the surface is disrupted by dry eye, I treat the surface and measure again rather than guess. A toric lens set to the wrong axis is worse than no toric lens at all.
• An eye where the lens might not stay put. A toric lens depends on staying at the axis it was set to. In eyes with weak support for the lens, such as pseudoexfoliation or previous trauma, that isn’t guaranteed, and I’ll talk you through the trade-off.

Getting it right

The lens itself is the easy part. The result depends on the measurements and the alignment, and this is where the experience of the surgeon and the technology in the clinic matter.

The old way of measuring corneal astigmatism only looked at the front surface of the cornea. The back surface has astigmatism of its own, averaging about 0.30 dioptres, and because it usually runs the opposite way, ignoring it leads to overcorrecting people with with-the-rule astigmatism and undercorrecting people with against-the-rule astigmatism. In that study the error from front-surface measurement alone exceeded half a dioptre in one eye in twenty. I measure the cornea with tomography, which maps the back surface as well as the front, and I use modern calculators that account for it. If the ocular surface is dry, that gets treated first, because a poor tear film distorts every measurement of the cornea.

Alignment is the other half. A toric lens loses about 3% of its effect for every degree it sits away from the intended axis, so ten degrees off costs you a third of the correction, and at thirty degrees the correction is gone altogether. That’s why the axis is marked carefully and the lens is checked before I finish the operation.

Even with all of that, a small amount of astigmatism occasionally remains, because eyes heal and corneas settle. If it’s enough to matter, it can be dealt with: a laser touch-up on the cornea, rotating the lens, or in some eyes a toric add-on lens placed in front of the original implant. Those are the exceptions, though. The aim is to get it right the first time, and for the great majority of my patients that’s what happens.

The bottom line

If you have astigmatism and you’re having cataract surgery, it should be corrected during the operation, with a toric lens in most eyes. That’s not an upgrade, in the same way that putting your astigmatism into your glasses isn’t an upgrade. It’s what a complete correction looks like. If someone has offered you cataract surgery and the astigmatism came up as an optional extra, I’d ask them why.