The article by Amoils and coauthors calls attention to the inducement of iatrogenic keratoconus in low myopia, including cases respecting the 250 μm cushion, 50% rule, and with normal preoperative topography. The conclusion that “[f]orme fruste keratoconus is a definite contraindication to LASIK” applied unfortunately at best to 8 of the 13 cases described. There remains the important question of what might be of similar predictive value in the other 5 cases that developed ectasia after LASIK without forme fruste signs preoperatively.
Based on prescreening evaluation of well over 1000 patients using an Orbscan tomographer with posterior elevation analysis, I suggest that posterior corneal elevation outside a statistically derived norm be considered, as there is a group of patients with posterior corneal elevations far beyond the norm without other topographic or acuity abnormality. Just as normal acuity with early topographic signs of inferior steepening led to the identification of forme fruste keratoconus (FFK), more exacting measure of the posterior cornea should, I believe, allow the identification of posterior ectasia to be extended further.
The norm for posterior corneal elevations appears to be below 50 μm, certainly below 75 μm. It is scientifically appealing to believe FFK would have as its earliest manifestation a weakening of the posterior cornea associated with elevation outside the norm before topography or acuity is affected. These eyes are suspicious either as representing an earlier degree of measurable ectasia or having within them a subgroup of such corneas and therefore representing a higher risk. A higher incidence of iatrogenic ectasia would likely result from LASIK being performed on these corneas, and this might provide an etiology for the non-forme-fruste cases described.
Jerry Horn MD
aChicago, Illinois, USA© 2001 by Lippincott Williams & Wilkins, Inc.