BLOG: How thin is too thin for cross-linking?
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Ideally, corneal cross-linking is offered as an early intervention for keratoconus as soon as there is evidence of progression and before significant vision loss or ectasia.
But what if you see a patient with a cornea that is already quite thin?
It’s a common misconception that eyes with corneas thinner than 400 µm cannot be treated with cross-linking. In fact, they usually can and should be treated.
Treatment options
Thin corneas are not a contraindication when using iLink (Glaukos), the only cross-linking system approved in the U.S. (pregnancy and active ocular infection are the only contraindications).
But the confusion about thin corneas likely arises because in clinical trials for the system, and in subsequent guidance from the manufacturer, it is recommended that practitioners initially use Photrexa Viscous (riboflavin 5’-phosphate in 20% dextran ophthalmic solution, Glaukos), measure the corneal pachymetry just prior to UV light application and, if it is less than 400 µm, apply the hypotonic riboflavin solution Photrexa (riboflavin 5’-phosphate ophthalmic solution, Glaukos), without dextran, for about 10 minutes to swell the cornea. This is the protocol we have always followed in our practice, although some practices have switched to using Photrexa throughout or alternating the two solutions in some or all eyes.
There is no guidance related to preoperative pachymetry. Patients with corneal pachymetry of at least 300 µm were eligible for enrollment in the trial.
Either threshold — 300 µm or 400 µm — is somewhat arbitrary. The goal of avoiding treatment of thin corneas is to ensure that the corneal endothelium and other ocular structures are protected from potential photochemical damage during cross-linking. My colleagues and I have become increasingly convinced, however, that even very thin corneas could benefit from cross-linking to stabilize the cornea.
In general, a thinner cornea is one that has already progressed to more advanced stages of keratoconus and likely has a high risk for continued progression. If the patient does not undergo cross-linking, they are likely to need penetrating keratoplasty (PK).
While PK has a high rate of success, recovery is difficult. Patients require long-term steroids, may be difficult to fit in contact lenses and will always have the risk of graft failure. On the other hand, even if some patients eventually need transplants after corneal cross-linking, they would still have had the benefits of a more stable peripheral cornea for the graft and improved quality of life during the period when PK was deferred.
Retrospective study
We evaluated the safety of cross-linking in 37 eyes of 34 patients with baseline pachymetry less than 400 µm and late-stage ectasia who would otherwise undergo PK. This was a single-center, retrospective study conducted at Vance Thompson Vision by an experienced surgeon according to the FDA-approved epi-off protocol. Patients were followed at 3 months and at the latest available follow-up, which was 10.5 months, on average (Ibach M, et al).
The average age of the patients in the study was 43.6 years (range 16-55) and 62% were men. Mean preoperative Kmax was 69.45 D (range 49.9-97.3), and the mean thinnest pachymetry was 327 µm (range 136-399).
After cross-linking, mean Kmax decreased, or improved, by 1.1 D at 3 months and 3.4 D at the longest follow-up, at which time 77% of the eyes had less than 1 D of steepening. Corneal pachymetry remained stable after treatment. Most importantly, there were no cases of endothelial failure, and no eyes required PK. Unfortunately, we did not have preoperative endothelial cell density (ECD) measurements to be able to show pre- and posttreatment impact on ECD.
While pachymetry does not always correlate to visual acuity, it is typically the case that thinner corneas have more advanced keratoconus and, because of that, may have worse visual acuity to start with and worse visual acuity outcomes than a patient with earlier-stage keratoconus. In our study, more than half of those with visual acuity data available (17/29, 58.6%) showed an improvement in visual acuity of at least one line. The majority of eyes (22/37, 59.4%) required scleral contact lenses after cross-linking, which, again, is not surprising in this later-stage population.
Although it was retrospective, this study demonstrates that cross-linking is effective in slowing disease progression without significant adverse events in eyes with very thin corneas. I encourage my colleagues to refer patients with progressive keratoconus for a cross-linking evaluation, regardless of the stage of the disease or degree of corneal thinning.
Reference:
- Ibach M, et al. Severe ectasia with ultrathin pachymetry: Is it too late for corneal cross-linking? American Academy of Optometry Annual Meeting; Oct. 7-22, 2020; virtual meeting.
For more information:
Mitch Ibach, OD, FAAO, is a cornea, glaucoma, cataract and refractive surgery specialist at Vance Thompson Vision in Sioux Falls, South Dakota. He can be reached at mitch.ibach@vancethompsonvision.com.
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