Is My Cornea Too Thin for Cross-Linking? Almost Never

Every few weeks someone comes to see me who has been told, somewhere else, that their cornea is too thin to cross-link. Usually they’ve been told there’s nothing to be done except wait for a transplant. They’ve often lived with that for a year or two before they got a second opinion.

I want to say this as plainly as I can. There is almost no such thing as a cornea that’s too thin to cross-link. Every case is different, and I’ll come to the exceptions, but the 400-micron cut-off that some doctors still quote is so out of date that it’s close to comical. We’ve been safely cross-linking corneas under 400 microns for years and years, and there’s ample published evidence that it’s both safe and effective. If you’ve been told to give up on cross-linking because of a number, this post is for you.

Where the 400-micron number came from

Cross-linking was first described in 2003, in Dresden, and the original protocol used one fixed dose of ultraviolet light for every cornea. Ultraviolet light is absorbed by riboflavin as it passes through the cornea, and the concern was the endothelium, the single layer of cells on the back surface that keeps the cornea clear. If the cornea was thin enough, that fixed dose might reach the endothelium at a level that could harm it. So a safety margin was set: don’t treat a cornea with less than 400 microns of stroma. It was a sensible rule for a fixed dose, and it protected patients in the early years.

The trouble is that it was a rule about one particular way of doing the treatment, and it’s been repeated ever since as if it were a fact about corneas.

The rule has been out of date for a long time

Surgeons started working around it almost immediately. Within a few years of the Dresden protocol, thin corneas were being swollen with a hypo-osmolar riboflavin solution to get them over the line, or treated with a contact lens laid over the surface to add thickness, or with an island of epithelium left in place over the thinnest point. Those were workarounds, and they worked, but they were still bending a thin cornea to fit a fixed dose.

The proper answer was to do it the other way round and fit the dose to the cornea. Farhad Hafezi’s group in Zurich built a model of how deep the cross-linking effect reaches for a given dose, and used it to calculate, for any measured thickness, how much light could be delivered while still leaving an untreated cushion of stroma above the endothelium. That became the sub400 protocol. It was published in the American Journal of Ophthalmology at the end of 2020, so it is now a long way from new. In that first series, 39 eyes with stromal thickness between 214 and 398 microns were treated, and progression was halted in 90% of them at twelve months. A separate study has since looked specifically at the endothelium in thin corneas treated this way and found it undamaged.

Then came the second generation. The updated ELZA-sub400 protocol sets the intensity of the light to the thickness of the cornea as well as the total dose, which brings the treatment down to about eighteen minutes, and it still leaves a margin of roughly 70 microns of untreated stroma above the endothelium. It treats corneas down to the same 214-micron floor, so in practical terms corneas around the 200-micron mark can now be cross-linked. In its published series, progression was halted in 76% of these very thin corneas at a year, with no endothelial problems.

Put that together and the picture is clear. The threshold moved from 400 microns to roughly 200 more than five years ago, the safety data has been checked, and the technique has been refined since. Someone quoting 400 microns to you today is quoting a rule from 2003 about a treatment that a thin cornea shouldn’t be having anyway.

What I actually do with a thin cornea

I’ve been treating corneas under 400 microns for many years now, and it’s a routine part of my keratoconus practice rather than something exotic. What it involves is care rather than courage.

The cornea is measured properly, at its thinnest point, and the dose of light is set to that measurement so that the cross-linking effect stops well short of the endothelium. Where the cornea is thin enough to need the sub400 approach, that’s what it gets. Where it isn’t, it gets the topography-guided epi-on treatment I use for most of my patients. Either way the aim is the same, which is to stop the disease in a cornea that would otherwise keep getting thinner.

I’ll be honest about the exceptions, because there are some. A cornea with significant scarring at the apex, a cornea that has already had hydrops, or a cornea genuinely below the 200-micron region needs a different conversation. But those are a small minority of the people who arrive having been told they’re untreatable. Most of them can be treated, and most of them should have been treated a year or two earlier.

Does it work as well in a thin cornea?

The honest answer is that it works, and that the goal is slightly different. A thin cornea is usually a more advanced cornea, and the job of cross-linking there is to stop it getting worse and to keep it out of the transplant queue. The figures above, 90% and 76% stable at a year, are for exactly those corneas. Improvement in the shape and in vision is a bonus when it happens, and it does happen, but stability is what you’re buying. If the vision is poor because the cone is advanced, that’s a separate problem with separate solutions, including CAIRS, once the cornea is stable.

Will it need doing again?

Very rarely. This is another point where patients arrive with an idea that doesn’t match reality.

Every cornea changes in the first twelve months after cross-linking. It swells, it compacts as the collagen tightens, the maps steepen a little and then flatten, and the numbers move around before they settle. We call this pseudo-progression, and it’s well described in the literature: a temporary worsening in the first months, followed by steady improvement from around six months onwards. So I don’t count anything inside the first year as progression, and I’d ask you not to either. It isn’t impossible for a cornea to genuinely progress in that window, but it is exceedingly rare, and the right response to a wobbly six-month scan is patience.

Beyond the first year, the large majority of corneas stay stable for good. When one does progress again, which in the published series happens on average several years after the first treatment, a second cross-linking is safe and works well. It’s uncommon enough that I’ve written separately about what happens if cross-linking doesn’t work. A third treatment is almost unheard of.

One more option for the very thinnest corneas

I’ll finish with a small point, because it belongs here. For the rare cornea that is very thin, very steep, and hasn’t stabilised with cross-linking, there’s an option short of a full corneal transplant. Bowman layer transplantation places a thin sheet of donor tissue within the cornea, and the published results show it flattens the cornea by around eight dioptres in the first month and then holds that shape, with an 84% success rate at five years. It was developed for exactly the corneas that were once called untreatable, and it may be a possibility for some of them. It’s not for everyone, but it’s another reason not to accept that nothing can be done.

The short version

If you’ve been told your cornea is too thin to cross-link, get another opinion from someone who treats thin corneas regularly. The 400-micron rule is a 2003 safety margin for a fixed-dose treatment nobody uses on thin corneas any more. Individualised protocols have been treating corneas down to about 200 microns for years, with published safety and efficacy data behind them. It rarely needs repeating, the first year of changes on your maps is expected, and even at the very thin end there are options. I’ve been doing this for a long time, and I’d be glad to look at your scans.