Ectasia

Corneal ectasia develops after excimer laser ablation when the corneal biomechanical integrity is reduced beyond its functional threshold; this complication results from performing surgery in patients who either are otherwise predisposed to developing corneal ectatic disorders or have a significantly reduced postablation residual stromal bed (RSB). The importance of an adequate RSB to prevent structural instability and postoperative corneal ectasia is discussed in Chapter 2. Ectasia has been reported far more frequently after LASIK than after surface ablation. Cumulative analysis of more than 200 eyes with postoperative ectasia found that ectasia is usually associated with LASIK performed in patients with preoperative topographic abnormalities. Other risk factors include younger patient age, thinner corneas, higher myopic corrections, and patients who have undergone several laser ablations. However, cases of ectasia without any demonstrable risk factors have also been reported.

For postoperative ectasia, corneal collagen crosslinking (CXL) is becoming the first-line treatment worldwide; in the United States, however, this treatment is under investigation but not yet approved by the Food and Drug Administration. Often, functional visual acuity can be restored with rigid gas-permeable or hybrid contact lens wear. The implantation of symmetric or asymmetric intrastromal ring segments to reduce the irregular astigmatism has been successful in select cases. In extreme cases, corneal transplantation may be required.

In 2005, a joint statement was issued by the American Academy of Ophthalmology, the International Society for Refractive Surgery, and the American Society of Cataract and Refractive Surgery summarizing current knowledge of corneal ectatic disorders and ectasia after LASIK. Their 8 conclusions at the time were

1.No specific test or measurement is diagnostic of a corneal ectatic disorder.

2.A decision to perform LASIK should take into account the entire clinical picture, not just the corneal topography.

3.Although some risk factors have been suggested for ectasia after LASIK, none is an absolute predictor of its occurrence.

4.Because keratoconus may develop in the absence of refractive surgery, the occurrence of ectasia after LASIK does not necessarily mean that LASIK was a causative or contributing factor for its development.

5.Risk factors for ectasia after LASIK may not also predict ectasia after surface ablation.

6.Ectasia is a known risk of laser vision correction.

7.Forme fruste keratoconus is a topographic diagnosis rather than a clinical one. It is not a variant of keratoconus. Rather, forme fruste implies subclinical disease with the potential for progression to clinically evident keratoconus.

8.Although to date no formal guidelines exist and good scientific data for future guidelines are presently lacking, in order to reduce some of the risks of ectasia after LASIK, the groups recommended that surgeons review topographic findings prior to surgery. Intraoperative pachymetry should be used to measure flap thickness and calculate the RSB after ablation to ascertain if the RSB is near the safe lower limits for the procedure, for that patient.

Current screening strategies that include a combination of these risk factors in a weighted fashion have been found to improve screening sensitivity and specificity.

Ambrósio R Jr, Randleman JB. Screening for ectasia risk: what are we screening for and how should we screen for it? J Refract Surg. 2013:29(4):230–232. Binder PS, Lindstrom RL, Stulting RD, et al. Keratoconus and corneal ectasia after LASIK. J Cataract Refract Surg. 2005;31(11):2035–2038.

Ou RJ, Shaw EL, Glasgow BJ. Keratectasia after laser in situ keratomileusis (LASIK): evaluation of the calculated residual stromal bed thickness. Am J