As mentioned in my last column, I attended the excellent Aspen Invitational Refractive Symposium in March. During one of my presentations, I asked audience members to vote on whether they considered a 3-mm epithelial defect during LASIK to be a complication. Essentially, the group agreed that it was an occurrence but not a complication. Then, I asked how they felt about performing LASIK on a cornea that had a central pachymetry of 500µm. Approximately 60% of the group voted that they would perform LASIK on such a patient, and the remainder preferred surface ablation.
I found the vote concerning LASIK and a thin cornea interesting and puzzling. The subsequent exploration of this topic is simply my opinion, based on my experience.
NORMAL OR ABNORMAL?
The crux of the matter is to decide whether a 500-µm thick cornea without overt corneal irregular astigmatism is normal or abnormal. Experience reveals that many high myopes have a normal cornea of 510µm. I would submit, however, that a corneal thickness of 500µm is more than three standard deviations from the norm. By definition, it is an abnormal cornea. The standard deviation to the thick side is much greater than to the thin side. For example, if the average cornea is 520µm, the standard deviation to the thick side might be 20µm but only 7µm to the thin side (virtually all corneas that are 490µm thick have keratoconus, but those that are 600µm thick are normal). I believe that my clinical experience supports this concept.
Clinically, there can be no doubt that a cornea with a central thickness of 500µm has a much greater chance of developing irregular astigmatism (mild ectasia) after LASIK compared to a cornea with a preoperative central thickness of at least 515µm. The problem occurs because the cornea is not of normal strength preoperatively and cannot resist the IOP and retain the proper shape after LASIK. If the cornea is 500µm thick and of normal strength, then LASIK will be successful in the long term. Unfortunately, it is not possible to distinguish preoperatively between the strong (normal) or weak (keratoconic) cornea.
About 20 years ago during the keratomileusis era, I discussed this issue with Richard Troutman, MD, and we referred to it as 20/20 keratoconus. The patient has 20/20 vision without irregular astigmatism but has hidden collagen weakness. The lamellar procedure weakens the cornea enough to allow for ectasia. We made several attempts to judge the strength of patients' collagen by methods such as a skin biopsy and skin laxity over the proximal knuckle of the third finger. None of these attempts produced any worthwhile information.
If mild irregular astigmatism is present preoperatively, then a diagnosis of keratoconus is easy. Very often, mild irregular astigmatism is more easily observed with a manual keratometer than with an automated topographer.
HOW TO PROCEED
In my opinion, it is prudent to perform a surface laser procedure on “normal” corneas that have a central corneal thickness of 505µm or less. This strategy will probably avoid unexpected ectasia. Whether the percentage of thin corneas that have a hidden collagen defect is 5% or 50%, LASIK is too risky.
This question of LASIK and the thin cornea points out that a true refractive surgeon must perform the procedure that is most appropriate for a given case. An ophthalmologist who performs only LASIK is not a comprehensive refractive surgeon but merely a LASIK surgeon. Quality refractive surgery demands that the surgeon's ego or quest for the wow factor not be the dominant force in selecting the procedure. Rather, ophthalmologists should make a prudent decision as to the refractive procedure of choice, which entails the least risk and the likelihood of an excellent result.
Lee T. Nordan, MD, is a technology consultant for Vision Membrane Technologies, Inc., in Carlsbad, California.
Dr. Nordan may be reached at (760) 431-1846; firstname.lastname@example.org.