BLOG: Rethinking progression
We know that cross-linking is indicated for progressive keratoconus, which raises the question of how exactly one should identify progression.
There are several ways to think about this topic: 1) Is there clear evidence of progression? 2) Is the patient in a risk category in which progression is highly likely? 3) Do you have what is needed to document progression in order to secure third-party reimbursement? These are all very different questions.
A 2015 consensus paper in Cornea provides a nice, clear overview. The authors note that progression is defined by a change in two of the following parameters:
1. Steepening of the anterior corneal surface.
2. Steepening of the posterior corneal surface.
3. Thinning and/or an increase in the rate of corneal thickness change from the periphery to the thinnest point.
Importantly, they note that the magnitude of the change must be above the normal variability of the measurement system. In other words, it is not just a particular Kmax level, and not simply an increase in Kmax (which may be noise), but a meaningful increase in Kmax that matters.
Visual acuity and age are not part of this consensus definition of progression, but they are important for our clinical decision-making about progression. In a patient younger than 30 years, I am concerned about a rapid increase in myopia or astigmatism. If that patient is a child or teen and I do not have access to prior records, or if the patient has never needed vision correction before, I would certainly not want to wait a year for more information; high-risk patients should be seen again within 3 months to avoid the risk of rapid progression. Additionally, tomography is useful because it provides more information about what is happening to the posterior cornea, where we are most likely to see early changes. By the time change can be detected on the anterior corneal surface, the patient may already have progressed beyond the mild stage of keratoconus. In repeating Pentacam (Oculus) tomography imaging, I try to bring patients back to the same office so they can be tested again with the same machine, eliminating even that minor potential source of test variation.
We also must take into account patients’ subjective perception of their vision. As we learned in cataract and refractive surgery, it is possible for patients to have 20/20 vision with poor visual quality due to higher-order aberrations or a poor quality tear film. Keratoconus can also affect visual quality in subtle ways that are difficult to quantify, so I take seriously any patient with keratoconus or high suspicion of keratoconus who tells me his or her vision is getting worse.
Finally, the last consideration in thinking about progression is whether your clinical findings meet the sometimes arbitrary definitions established by payers. This is not inconsequential, as it can affect access to treatment. Be aware of what those requirements are for your major payers, educate referring clinicians about what you need to meet them, and advocate for your patients to be treated when necessary.
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