Despite its rarity, corneal ectasia is the most dreaded potential side effect in corneal refractive surgery. Ectasia is characterized by a progressive thinning and steepening of the central and inferior portions of the cornea.
Current theories suggest that the two main causes of post-LASIK ectasia are mechanical instability, due to weakening of the residual stromal bed from flap creation and laser ablation; and preexisting corneal pathology (eg, keratoconus or forme fruste keratoconus). Predisposition, irregular corneal thickness, different ablation rates and ultrasound pachymetry errors may also cause corneal ectasia.
How can refractive surgeons prevent this unfortunate event? There are several steps to ensure that the residual stromal bed is kept within the safety zone (ie, 250 µm to 300 µm) following LASIK.1 Because microkeratome ablation depth is not completely predictable, the created flap may result thicker than planned. Refractive LASIK may also leave the stromal bed thinner. As a result, estimate the residual stromal thickness prior to surgery by subtracting the predicted flap thickness and ablation depth from the preoperative central pachymetry measurements. Then, during surgery, perform intraoperative pachymetry immediately after lifting the flap. The surgeon may then estimate the residual stromal thickness by subtracting the flap thickness and ablation depth from the preoperative corneal thickness.
IDENTIFY AT-RISK CORNEAS
Additionally, proper patient selection may minimize the risk of corneal ectasia in patients undergoing refractive procedures. Be sure to identify corneas at particular risk, including those with forme fruste keratoconus, a major cause of post-LASIK corneal ectasia. Detecting this and keratoconus, in its early stages—before obvious clinical and topographic signs are present—is often a puzzling task. Scheimpflug systems and altitudinal maps are a valuable tool in keratoconus detection; they enable clinicians to identify warning signs such as posterior elevation of the cone.
Classifications, based on clinical examination and Placido disk-derived indices, were developed to aid in the topographic diagnosis of keratoconus:
• Lu et al2 reported a keratoconus scoring system by combining clinical history, examination and topographic indices for distinguishing normal patients from those with keratoconus and/or early and advanced keratoconus.
• Corneal topography plays a critical role in collecting information regarding corneal curvature, comparison with the fellow eye, irregular astigmatism detection, and corneal topographic changes after discontinuation of contact lens wear to ascertain the eye refractive stability before surgery. It is well known that direct mechanical pressure of the contact lens and corneal hypoxia may alter the shape of the cornea and induce corneal warping (Figure 1).
Even when these parameters are followed, refractive surgery may result with an unsatisfied patient developing corneal ectasia. So, what are we missing? Orbscan (Bausch & Lomb, Rochester, New York) and Pentacam (Oculus Optikgeräte GmbH, Wetzlar, Germany) printouts help summarize the most valuable information provided by keratometric, anterior and posterior elevation, and pachymetry maps, allowing for bidimensional visualization of the difference between anterior and posterior elevation maps.
IDENTIFYING PATTERNS
Analysis of pachymetry maps and its relationship to corneal curvature patterns are also useful. These identify patterns that provide additional data to alert the surgeon of an apparent normal cornea that is at risk for ectasia. To pick up every deviation from the normal three-dimensional shape of the cornea, we must consider that a normal cornea has a central thickness of 540 µm and a peripheral thickness of 700 µm; that it is usually thicker nasally and inferiorly; and it has a curvature gradient flatter nasally than temporally. The key is to locate the thinnest point of the cornea, because it corresponds perfectly to the most elevated area of the cone.
So, what aspects must we take into account?
• The location of the thinnest point and the correspondence between the thinnest point of the cornea and the steepest point on the anterior and posterior elevation maps.
• The thickness pattern of the entire cornea, identifying any asymmetry between the inferior and superior portion of the cornea and comparing the distribution of the patient's corneal thickness gradient with the reference normal one.
IRREGULAR PACHYMETRY MAP
Corneas with discrepancies (eg, an eccentric thinnest point and an irregular asymmetric pachymetry map pattern) are considered at risk for ectasia.
A normal astigmatic cornea usually presents with a bow-tie appearance on an axial map with dioptric values less than 47.00 D, a ridge shape on the anterior elevation map and a symmetric pachymetry map with the thinnest point located in the center (Figure 2). The classic appearance of keratoconus (Figure 3) on an axial map shows dioptric values more than 47.00 D, an island shape on the anterior elevation map, and an asymmetric pachymetry map with inferior thinning and a decentered thinnest point, corresponding to the steepest point of the cone.
Some corneas showing a normal topography with dioptric values less than 47.00 D and ridge-shaped altitudinal patterns may have a pachymetry map that deviates from the normal because it is asymmetric and the thinnest point is eccentric (Figures 4 and 5).
Sometimes, altitudinal maps show an incomplete ridge or a questionable island appearance with asymmetric pachymetry map and an eccentric thinnest point.
Refractive surgeons must, therefore, consider all the information given by preoperative pachymetry maps, as it appears to identify suspect corneas that otherwise would be missed during routine screening. These maps allow accurate and safe case selection through detection of borderline cases and provide important documentation of preoperative status, as well as useful information for improving surgical strategy, reducing the odds of performing LASIK on potentially pathological, and ectasia-prone corneas.
Elena Albe, MD, practices at the Istituto Clinico Humanitas Rozzano, Milano University, in Italy. Dr. Albe states that she does not have any financial interest in the products or companies mentioned.
Dan Epstein, MD, PhD, practices at Zurich University, in Switzerland. Dr. Epstein states that he does not have any financial interest in the products or companies mentioned.
Paolo Vinciguerra, MD, practices at the Istituto Clinico Humanitas Rozzano, Milano University, in Italy. Dr. Vinciguerra states that he is a paid consultant for Nidek (Gamagori, Japan). He may be reached at paolo.vinciguerra@humanitas.it. Dr. Vinciguerra is a member of the CRSToday Europe Editorial Board.
Up Front | Jan 2007
Prevention of Corneal Ectasia in Refractive Surgery
Use preoperative pachymetry maps to identify suspect corneas prone to this rare side effect.
Elena Albe, MD; Dan Epstein, MD, PhD; AND Paolo Vinciguerra, MD
