Thin cornea and high myopia: TransPRK or ICL implant?

“Doctor, I was told my cornea was too thin, and that with my myopia the only option was an implant.” I hear this very often, from patients whose myopia lies between -6 and -10. That is not always the case. A thin cornea frequently rules out LASIK, but not necessarily laser surgery: it often remains possible thanks to TransPRK, a technique that uses less tissue. I have operated on several hundred eyes at these levels of correction, with excellent results.

The ICL implant keeps its full place: it is an excellent technique, the solution when laser surgery is no longer possible, often beyond -10. Here is how I tell the difference.

Why a thin cornea is a problem with high myopia

To correct myopia, the laser removes a thickness of about 15 microns of cornea per diopter. At -9, that is around 135 microns. The higher the myopia, the more tissue is removed.

The first safety criterion is what remains underneath: the residual stromal bed. Classic recommendations place it above 280 microns; I set myself 300 microns.

Take a cornea of 500 microns, slightly thin, and a myopia of -9. This is only an order of magnitude:

• With LASIK, a flap of about 110 microns is cut first, then the laser works underneath. What remains is 500 - 110 - 135, about 255 microns. That is too little.
• With TransPRK, there is no flap. The laser removes the surface layer of the cornea, the epithelium, about 50 microns thick, which grows back afterwards, then works directly on the surface. What remains is 500 - 50 - 135, about 315 microns.

There is an intermediate option: LASIK with an ultra-thin flap, of 90 or even 80 microns, which leaves a little more thickness under the treatment. I have performed it and it gives good results, but cutting such a thin flap is more delicate. I therefore reserve it for specific cases.

Beyond 300 microns: the other safeguards

The residual bed alone is not enough to judge safety. Other criteria matter just as much:

• The proportion of cornea altered. This is the ratio between what is altered, flap included, and the total thickness of the cornea. Above about 40%, the risk of ectasia, a progressive deformation of the cornea, rises sharply after LASIK: it was the most frequent factor in patients who developed it [1].
• The volume removed. Two treatments of the same depth do not remove the same volume of cornea: it all depends on the diameter of the treated zone. That is why the percentage of volume altered is also considered, not only the thickness [2]. This is where the pupil comes in. In a patient whose pupil dilates little in the dark, a smaller treatment diameter can be used, which removes less tissue and is therefore safer. When the pupil is large, I warn that this choice may cause a few more halos at night. Some patients readily accept it: it is part of the discussion and of the choice.
• The shape of the cornea, age, starting thickness and prescription. Ectasia risk scores combine them, and an abnormal topography remains the most important warning sign [3].

These criteria explain the value of TransPRK on a thin cornea. With LASIK, the flap is cut in the strongest part of the cornea, its most anterior layer [4], and it hardly contributes to its strength any more. With TransPRK, everything the laser does not remove keeps contributing to the strength of the cornea. This is one of the reasons why ectasia is extremely rare after PRK [5].

Myth: “My cornea is thin, so I’m not a candidate.”

Thickness alone is not enough to decide. I also look at the regularity of the cornea, its curvature, both of its surfaces, the thickness of its epithelium. A thin but perfectly regular cornea is often suitable. Conversely, a cornea of normal thickness but deformed is not. Surgeons do not all give the same weight to each of these criteria, which explains why two opinions can differ. That is what the pre-operative assessment is all about.

TransPRK for high myopia: what are the results?

In my practice, the results of TransPRK at -9 or -10 on a relatively thin cornea are excellent, and the literature points the same way:

• On the same Schwind platform as mine, a French team followed 69 highly myopic eyes, averaging -7.4: at six months, the remaining correction was almost zero [6]. Another series, averaging -7.9, found 91% of eyes within half a diopter of the target at one year, with results comparable to LASIK [7].
• Compared with LASIK, of 85 highly myopic eyes treated with TransPRK, 87% were within half a diopter of the target at one year, versus 73% after LASIK [8].
• Other recent series, from -5 to -10, find 92 to 98% of eyes at 20/25 or better without glasses at six months [9, 10], with no loss of corrected vision and good quality of vision [11]. The results are as good as with conventional PRK, with a more comfortable procedure [12].
• In the situation I am describing, 187 highly myopic eyes in which LASIK would have taken too much of the cornea were treated with PRK, with good visual results and no ectasia over the follow-up period [13].

Not everything is perfect, though: in a series of high myopes averaging -8.9, a few patients needed an enhancement, and two eyes developed haze [14]. These are the two points to watch with such corrections. Against haze, the most effective protection remains avoiding sun exposure.

A somewhat longer recovery

TransPRK requires more patience than LASIK, and even more so in high myopia. The first two or three days are the same as after any TransPRK: it hurts quite a lot, while the surface of the cornea closes (see recovery). Then vision recovers with fluctuations for a good month, and full recovery takes two to three months. I always warn: at one week, you already see better than before without glasses, but you have not yet reached the final result.

Haze: a healing response to watch

After high corrections, surface healing can be slightly inflammatory and leave a faint opacity in the cornea: this is haze. It was common with older PRK. It has become very rare. During the procedure, I systematically apply mitomycin C, a product that slows this excessive healing; at this dose, it does not alter the cornea in the long term [15]. Lasers have also improved: on the same platform, a study found no haze at six months in high myopes, even without this product [6].

The best prevention remains simple: protect your eyes from the sun for the six months that follow. And, more broadly, keep the habit of wearing sunglasses, on holiday as in daily life.

Can myopia come back a little?

In high myopia, a slight return is more frequent than in moderate myopia. A Danish study re-examined patients 13 to 19 years after PRK performed in the 1990s: among high myopes, about half of the eyes remained within one diopter of the target, versus nearly three quarters among low myopes [16]. Part of this return came from the myopia itself, which had continued to progress. Above all, lasers in the 1990s were much less precise and caused more healing reactions. These results, already respectable at the time, are much better today, even if a slight return remains a little more frequent in high myopia.

There are also cases where the cornea does not allow the full correction to be treated. I say so from the consultation: I treat as much as the cornea safely allows, and a slight residual myopia will remain, for example -0.50 to -1 in the stronger eye. A -10 patient who can only be corrected by -9 keeps a small myopia, but regains real independence.

Laser or implant: independence or perfect vision

This is often where the choice is made. In very high myopia, the ICL implant generally gives the best quality of vision, and this must be acknowledged: it corrects over a large diameter, without touching the cornea. In myopia beyond -8, its quality of vision is often better than that of PRK [17].

But many patients do not want an implant. Some are afraid of having a lens in their eye, which is understandable: it is a more invasive surgery, which enters the inside of the eye. It is also about twice as expensive: around €5,500, versus €2,800 for TransPRK (see prices). When laser surgery remains possible, even with a slight compromise, many choose surface surgery. The goal of laser surgery is independence and everyday comfort, not necessarily the perfect vision an implant can offer.

The implant becomes the best option:

• beyond -10, sometimes -12 with a thick cornea;
• when the cornea does not allow the correction to be treated safely, based on all the criteria above;
• when the cornea is irregular or suspected of keratoconus;
• when visual expectations are very high: patients used to seeing very well with contact lenses, sometimes rigid lenses, who would not accept a slight residual myopia.

There must also be room in the eye to place the implant, which is measured during the assessment. I do not place these implants: when it is the right option for you, I refer you to a colleague who does (see the alternatives to laser).

Myth: “Beyond -8, the implant is the only option.”

No. Between -8 and -10, TransPRK is often possible, including on a relatively thin cornea, and LASIK is too on a thick cornea. The number alone is not enough: it is its combination with the thickness and regularity of the cornea that decides.

After surgery, the retina is still that of a high myope

Laser surgery corrects vision, not the structure of the eye. A highly myopic eye is longer, and its retina thinner and more stretched. It deserves a fundus examination every two to three years, even when you no longer wear glasses.

In summary

A thin cornea with high myopia is not an automatic “no”. Up to about -10, TransPRK is often possible, with excellent results, at the cost of a somewhat longer recovery or a slight visual compromise. Beyond that, or when the cornea does not allow it, the implant takes over, and it gives very good vision.

Everything is decided case by case, according to your eyes and what you expect from your vision. Because we do not operate on eyes, we operate on people. To find out where you stand, start with the Am I a candidate? test, then come for a consultation.