Corneal ectasia is a sight-threatening complication of refractive surgery characterized by progressive steepening and thinning of the cornea. Understanding, recognizing, and accepting the risk factors for ectasia after refractive surgery are crucial steps toward a significant reduction in the occurrence of this adverse event. This article outlines the common warning signs of post-LASIK ectasia that can be detected preoperatively and presents a new metric to be considered in the detection of ectasia risk.
CONCEPT OF A RISK FACTOR
The main purpose of assessing risk is not to determine who will or will not develop ectasia, but rather, based on stringent scientific analysis, to determine what group or groups of people present a higher chance of the event happening.1 There is a significant difference between the definitions of the prevalence of a factor in a group, which is related to how many people in that group present with the factor, and the influence of the factor on that group, which is related to something that can occur in that group when exposed to the factor.
Analysis of prevalence is insufficient to investigate a risk factor. In order to investigate the association between a particular factor and an outcome, an appropriate tool is the odds ratio (OR) value. The OR represents the odds that an outcome will occur given a particular factor (risk factor, OR > 1; protective factor, OR < 1), compared with the odds of the outcome occurring in the absence of that factor.
MOST COMMON WARNING SIGNS
Ectasia is the result of a reduction in corneal biomechanical integrity below the threshold required to maintain physiologic curvature. This could theoretically occur when a preoperatively weak cornea undergoes surgery or when a relatively normal cornea is weakened below a safe threshold.
The most important warning sign for ectasia is abnormal corneal topography. Corneal topography showing an altered pattern represents a biomechanically fragile structure meaning a higher risk of ectasia if surgery is performed and tissue is removed. Although some diseases are related to recognizable abnormal topographic patterns, these are perhaps not the only ways an abnormal status may present. There are some obvious topographic signs of abnormality not classically related to keratoconus and some subtle signs of corneal alterations in the intrinsic structure.
Randleman et al2,3 provided a valuable contribution to the field of refractive surgery by identifying and determining the importance of risk factors through a stringent and validated scientific analysis that led to the creation of the ectasia risk score system (ERSS). Their studies corroborated the relationship between abnormal corneal topography and ectasia and identified abnormal topography as the most common and important high-risk sign, with a high OR value.
The studies conducted by Randleman et al2,3 also provided scientific evidence that thin residual stromal bed (RSB), low central corneal thickness (CCT), high myopia, and young age are significant risk factors for the development of post-LASIK ectasia. Although there has been some debate regarding the most sensitive and specific cut-off values, low RSB and CCT have been widely accepted as important risk factors for ectasia. Patients who develop ectasia present, on average, with significantly lower RSB thickness and thinner CCT than controls. Because of the variability of flap thickness, intraoperative pachymetry measurements should be obtained in all cases to determine an accurate RSB value.
A thin cornea can also be identified as a risk factor for ectasia because it can be a sign of disease, as keratoconic corneas tend to be thinner and also because a thinner cornea would experience more significant biomechanical instability than a thicker cornea given the same amount of treatment.4
The same reasoning can be applied to high myopia as a risk factor for post-LASIK ectasia, as it can be considered one of the signs of keratoconus; additionally, a high myopic treatment would involve a deeper ablation, and the anterior part of the cornea, where the keratocyte density is higher, would be removed.
Young age is probably the most controversial risk factor, and therefore is also the most overlooked. There are arguments against age as a risk factor, but case-control studies have clearly demonstrated its importance.2,3
Understanding the relative importance of different risk factors may be challenging. The unique feature of the ERSS—its capability of weighing multiple risk factors simultaneously—makes it a helpful tool in the screening strategy. In retrospect, although most patients who have developed post-LASIK ectasia have had identifiable risk factors that placed them at higher risk for this complication, ectasia cases in patients with normal preoperative topography still occur.5
ANOTHER METRIC TO CONSIDER
We recently proposed a metric that seems to be relevant, especially for detecting ectasia risk, in eyes that have normal preoperative topography or that are considered to be low-risk based on previously recognized risk factors.6
There is an integrated relationship among corneal thickness, ablation depth, and flap thickness that can be used in determining the relative amount of weakening that occurs after LASIK.6,7 We have investigated a metric, the percentage of anterior tissue altered (PTA), that describes this interaction6,7 during excimer laser refractive surgery. For LASIK, this relationship can be described as: PTA = (FT + AD)/CCT, where FT = flap thickness, AD = ablation depth, and CCT = preoperative CCT.
We believe that this metric may more accurately represent the risk of ectasia than any of the individual components that comprise it. As compared with specific RSB or CCT values, PTA likely provides a more individualized measure of biomechanical alteration because it considers the relationships among thickness, tissue altered through ablation and flap creation, and ultimate RSB thickness.
Our recent case-control studies have determined the role of PTA in post-LASIK ectasia and have established cut-off values for eyes with normal topography that were considered low-risk based on previously recognized risk factors. We have shown PTA’s prevalence and that it has a high OR value, demonstrating the relationship between the risk factor (PTA) and the event (ectasia).6
New technologies including corneal tomography8 and biomechanical analysis7 are significant additions in recognizing risk factors and, more important, in determining more subtle signs of subclinical keratoconus.
The corneal weakening predicted by both PTA and topographic irregularities does not mean ectasia will occur in all, or even most, high-risk eyes; it merely suggests that, if measures are validated, these eyes carry an increased risk for ectasia. Rather than representing a specific disease entity, postoperative ectasia most likely represents an end-stage manifestation of biomechanical failure that arises from a variety of causes, including weak corneas before surgery, a surgical change in the structural integrity, and patients otherwise destined to develop keratoconus regardless of surgical intervention.
Fortunately, there appears to be a declining trend in the incidence of ectasia after LASIK that is likely related to greater knowledge of risk factors among refractive surgeons and improved methods for risk assessment.
Marcony R. Santhiago, MD, PhD, is Head of the Cataract and Refractive Surgery Department and a Professor of Ophthalmology at Federal University of Rio de Janeiro and an Associate Professor at the University of Sao Paulo in Brazil. Dr. Santhiago states that he has no financial interest in the products or companies mentioned. He may be reached at e-mail: firstname.lastname@example.org.
- Randleman JB. Evaluating risk factors for ectasia: what is the goal of assessing risk? J Refract Surg. 2010;26:236-237.
- Randleman JB, Woodward M, Lynn MJ, Stulting RD. Risk assessment for ectasia after corneal refractive surgery. Ophthalmology. 2008;115:37-50.
- Randleman JB, Trattler WB, Stulting RD. Validation of the Ectasia Risk Score System for preoperative laser in situ keratomileusis screening. Am J Ophthalmol. 2008;145:813-818.
- Randleman JB, Dawson DG, Grossniklaus HE, et al. Depth-dependent cohesive tensile strength in human donor corneas: implications for refractive surgery. J Refract Surg. 2008;24:85-89.
- Klein SR, Epstein RJ, Randleman JB, Stulting RD. Corneal ectasia after laser in situ keratomileusis in patients without apparent preoperative risk factors. Cornea. 2006;25:388-403.
- Santhiago MR, Smadja D, Mello G, Wilson SE, Randleman JB. Role of percentage of tissue altered as risk factor for ectasia after LASIK in eyes with normal preoperative topography. Paper presented at: the 2014 ASCRS meeting; April 25-29, 2014; Boston.
- Santhiago MR, Wilson SE, Hallahan KH, et al. Changes in custom biomechanical variables after femtosecond LASIK and PRK for myopia. J Cataract Refract Surg. In Press.
- Smadja D, Touboul D, Cohen A, et al. Detection of subclinical keratoconus using an automated decision tree classification. Am J Ophthalmol. 2013;156:237-246.