Current guidelines for preventing post-LASIK ectasia may not suffice for all eyes, as preoperative pachymetry measurements can be misleading, and microkeratomes sometimes cut flaps that are thicker than the surgeon intended. In fact, it is scientifically recognized that ectasia occurs during deep ablation and that it develops 1 to 2 years after LASIK. Furthermore, some surgeons suggest that the stromal bed should be at least 250 µm after LASIK. This article reviews my experience with a patient who developed post-LASIK ectasia even though her treatment met the aformentioned criterion.
AN UNUSUAL EPISODE
A 35-year-old white female presented for a refractive surgery evaluation. Her preoperative refraction was -4.0 D, and she had undergone five measurements with two different contact pachymeters, both of which showed 520 µm of central thickness and did not reveal any corneal irregularities. She underwent LASIK with a 180-µm flap cut by the Hansatome microkeratome (Bausch & Lomb Surgical, San Dimas, CA). The ablation depth was 74 µm, which left a stromal bed thickness of 266 µm. At 18 months, she developed ectasia (the only patient to do so out of a study of 1,500 eyes; Figure 1). All 1,500 eyes were followed for 36 months after the LASIK procedure, and all patients underwent the same preoperative examinations. The patient did not have any preoperative clinical characteristics that I believed would indicate a predisposition to post-LASIK ectasia. I concluded that contact pachymetry measurements are insufficient and unreliable in some eyes, because, unlike noncontact pachymeters such as the Orbscan (Bausch & Lomb Surgical), contact pachymetry cannot detect internal corneal irregularities. The Orbscan can show the internal shape of the cornea (the endothelial portion). These parameters cannot be detected by a contact pachymeter or by standard corneal topography. I believe that we must consider contact pachymetry and the internal corneal shape to have enough information regarding the cornea's predisposition to LASIK surgery. We need a way to measure not only the thickness, but also the tensile strength and elasticity of the cornea. Unfortunately, we cannot measure the elasticity of the cornea in vivo. Although some of the newer microkeratomes may be able to gauge flap thickness more accurately than others, the thickness of the flap is not always predictable. It is also important to remember that the flap does not contribute to postoperative corneal stability, because the collagen fibers have been cut. As a result, stromal bed thickness is the most important factor in corneal stability.
RETREATMENT REQUIRES SPECIAL CONSIDERATION
Take particular care when performing retreatments because of changes in the cornea that occur after primary LASIK. Before retreatment, epithelial thickening can occur, especially in eyes undergoing correction of a high refractive error; this can make the corneal thickness appear to be sufficient when it is not. Therefore, it is important to determine the thickness of the stromal bed and base the calculation on the initial treatment. If you were not in charge of the initial treatment, contact the responsible surgeon. Consider a patient with a preoperative pachymetry measurement of 530 µm, a flap of 160 µm, and an ablation depth of 90 µm at the time of the initial surgery. In this example, the maximum ablation depth for retreatment would be 30 µm. When the patient's cornea is too thin at the time of retreatment to ablate the necessary depth for optimal correction,
I perform intraepithelial PRK (PRK on the epithelium without removing it before laser ablation). At my institute, where these patients are still under observation, we have been using this method, which has yielded satisfactory results for small amounts of regression (approximately 10 to 15 µm of ablation). Using the intraepithelial approach, we do not ablate the stromal bed. My preferred strategy is to perform intraepithelial PRK, which involves ablating the epithelium at the back of the flap, and I have had satisfactory results with this approach.
Be scrupulous during patient selection in order to avoid ectasia. There are several types of patients who should not have LASIK performed under any circumstances. These patients include those with irregular astigmatism, subclinical keratoconus, or pellucid marginal degeneration and those with an abnormal internal corneal shape as detected by Orbscan (Figure 2). Although we have limited data concerning ectasia, the best treatment plan appears to involve leaving a stromal bed of 250 µm or more and using safe microkeratomes with a flap thickness of 160 or 130 µm. After lamellar corneal surgery, increased stress on the cornea due to the microkeratome pass is a long-term condition, and individual corneas may react differently. Appropriate patient selection for LASIK translates into choosing patients with safe corneas, as far as pachymetry measurements are concerned, and performing the necessary examinations in order to retain a postoperative stromal thickness sufficient to provide corneal stability.
GUIDELINES FOR ECTASIA PREVENTION
In order to prevent ectasia, plan LASIK procedures based on stringent rules using accurate measurements. These principles of prevention include obtaining the mean of several contact pachymetry measurements taken at different times of the day in order to account for changes in pachymetry resulting from differing degrees of hydration. In addition, obtain Orbscan measurements so that you can analyze the internal shape of the cornea.
If you suspect that the flap thickness is greater than planned and if retreatment is necessary, leave a stromal bed thicker than the recommended minimum of 250 µm. For example, if the cornea's central thickness were 530 µm and the flap thickness were 160 µm, the maximum ablation would be 120 µm, which would leave a stromal bed of 250 µm. However, if the patient is a likely candidate for an enhancement, the ablation should leave more than 250 µm in the stromal bed. In the future, the incidence of ectasia may be decreased through the use of new microkeratomes that are able to create flaps of appropriate thickness with a less aggressive cut and no-blade LASIK using the femtosecond laser. In addition, ectasia may be reduced by studying the possibility of creating a type of pachymeter-elastometer that would be able to show pachymetry measurements and the elasticity of the cornea simultaneously.Roberto Pinelli, MD, is Scientific Director of Istituto Laser Microchirurgia Oculare in Brescia, Italy. He does not hold a financial interest in the product mentioned herein. Dr. Pinelli may be reached at +0039 030 2428343; firstname.lastname@example.org.