| Brent Hanson - LasikFraud.com 2004-12-22, 7:11 pm |
| Concepcion, Chile
Santiago, Chile
21 December 2004.
When we saw the article by Binder,1 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
1. P.S. Binder, Ectasia after laser in situ keratomileusis, J Cataract
Refract Surg 29 (2003), pp. 2419-2429. SummaryPlus | Full Text + Links | PDF
(121 K)
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