December 21, 2004

Ghostbusters Look for Cause of Ectasia

Ectasia after LASIK 

Mario Oyarzún MD and Luis Bravo MD 
Letter to Journal of Cataract & Refractive Surgery 

When we saw the article by Binder, we expected to find the answer to many questions about how to predict the patients at higher risk for developing corneal ectasia after laser in situ keratomileusis (LASIK). Perhaps a magical formula would be described.

After reading the excellent review by Binder, we see that, unfortunately, the answer to the predictive question still eludes us. In the large experience with LASIK reported in the article, we did not see 4 aspects that might be important in predicting corneal ectasia after LASIK:

  1. Matching the number of LASIK cases performed during a year with each brand of laser versus the number of cases of corneal ectasia reported with each brand would give some figures related to the safety of the equipment. Interpretation of this matching might be difficult unless the date of LASIK surgery is taken in account to compare with the number of cases performed with the brand of laser the same year.
     
  2. The intraocular pressure (IOP) is not measured in the reported cases. Because many myopes, even young ones, may have high IOP, this measurement should be included in the evaluation of patients at risk for corneal ectasia. Some mention of this is made related to the corneal thickness (“…the posterior cornea is unable to withstand normal [IOP] if it is too thin”), thinking that the ocular wall is the key factor and missing that the eventually abnormal IOP itself is the triggering factor.
     
  3. The age of the patients is not mentioned and is not commented on by the author as a risk factor. Although to our knowledge though there is no evidence that the collagen of the young cornea is weaker and/or more elastic than the collagen of the older cornea, some consideration of the age factor should be taken. It makes sense to us that young corneas might be more complacent to deformation due to interaction between the IOP (normal or higher) and the corneal thickness. In fact, young myopic eyes that do not have stable refractive errors do have axial length changes over time. Since the increase in axial length stops by the third decade of life, a kind of “maturity” of the ocular wall might decrease the risk for corneal ectasia after this age. Whether the change in the ocular size in young myopic patients happens because the sclera is very elastic or the cornea is stretched and/or deformed at the same time is unknown. Regarding the size of the eye, the different evolution of the eye of a 5-year-old child with congenital glaucoma and the eye of a 60-year-old patient gives an idea of the tissue difference due to age. The zero rate of corneal ectasia after hyperopic LASIK might be related to the thicker and harder corneas found in older eyes (hyperopic LASIK tends to be performed in older patients than myopic LASIK) and/or to the fact that the central corneal thickness is not modified by the laser (beyond the creation of the corneal flap). The stability of the hyperopic eye relative to corneal ectasia might be related to a different histological or histochemical condition of scleral and corneal tissue that share the same embryological origin; ie, the hyperopic ocular wall is able to bear normal or even high IOP without the risk for developing corneal ectasia.
     
  4. Since different tissue behavior in men and women is an empiric observation in other areas of our physiology, the sex of patients with corneal ectasia after LASIK should be taken into account. Any study should match the number of LASIK cases performed in a given universe with the number of cases of corneal ectasia after LASIK that occurred in this universe.
     
  5. In the literature, we have not found a numeric threshold to validate the differences in pachymetry between the superior and inferior areas of the cornea. There is also no mention of this pachymetric asymmetry in the review by Binder. We lack information about the significance of this finding in some patients, and we do not know whether it is significant when the asymmetry is larger than some value. Theoretically, a big difference (how much?) between the superior and inferior pachymetry could be a pre-ectatic stage.

In conclusion, the formula for identifying patients at risk for developing corneal ectasia after LASIK is probably multifactorial and includes residual stromal thickness, IOP, age, sex, a collagen-factor (unknown so far), and retreatment or LASIK enhancement. The difference between superior and inferior pachymetry may be another variable in this multifactorial formula.

The expression “collagen-factor” used above is a way to assume our ignorance of what is going on in the corneal tissues after LASIK. Identifying the risk factors and disclosing the specific role of each factor will help us reduce this complication of refractive surgery.

References

P.S. Binder, Ectasia after laser in situ keratomileusis, J Cataract Refract Surg 29 (2003), pp. 2419–2429.

Posted by Admin at December 21, 2004 10:57 AM