The advent of corneal topography and its increasing application in practice has led to an accumulation of our knowledge about the shape of the cornea.1 It is vitally important that we determine the ability of the cornea to undergo laser refractive surgery and avoid corneal ectasia and visual impairment in otherwise healthy eyes. Corneal ectasia, first reported by Prof. Theo Seiler,2 is characterized by progressive protuberance and steepening, increasing myopia and/or astigmatism with distorted and decreased best-corrected vision in the involved eye. Some of the reported cases can be traced back to the preoperative evaluations and lack of recognition for risk factors.
The most important presurgery diagnosis in potential LASIK patients is that of forme fruste keratoconus. Data collected over several years of retrospective analyses of post-LASIK ectasias underscore the importance of this presurgery diagnosis. Although this issue has been repeatedly addressed, it has not been solved.
Through the years, the Orbscan II and the Orbscan IIZ3 (Bausch & Lomb, Rochester, NY) have more become the standard for preoperative screening among refractive surgeons. It is an important diagnostic tool to help us identify preclinical cases of corneal instability and forme fruste keratoconus and determine appropriate candidates for surgery.
Much data have been published on the selection criteria for LASIK patients. Most parameters have been established empirically by studying postoperative ectasia as an unexpected result. Some surgeons were able to find correlations with preoperative corneal maps and then quantify abnormal preoperative corneal map findings.
In addition to topographical parameters, it is imperative that surgeons consider clinical signs, including fluctuation of subjective refraction, younger patients, history of keratoconus and steep/distorted keratometry readings. Corneal topography maps must be analyzed on the system, not on the printout.
The decision to proceed with refractive laser surgery is not based solely on one clue, but on a combination of criteria. For example, the Orbscan has a quad map that provides red flags or yellow flags. A red flag signals a no-go situation and a yellow flag indicates a suspicious condition and will focus the surgeon?s attention more closely to the other corneal maps (Figure 1).
PACHYMETRY
An absolute contraindication for lamellar corneal laser surgery is a thinnest point of <470 µm.4-5 When pathological, this point is often displaced inferotemporal (Figure 2). A difference of <30 (yellow flag) or <20 µm (red flag) between the central pachymetry and the peripheral thickness indicators can be seen in abnormal corneas. A difference of >100 µm from the thinnest point to the values at the 7-mm optical zone implies a steep gradient of thinning from the midperiphery toward the thinnest point (yellow flag) (Figure 3).
POSTERIOR ELEVATION MAP
When viewing an elevation map, the most common reference surface is the best fit sphere (BFS). For example, a posterior high point located >50 µm above BFS might indicate early posterior ectasia. If the cornea happens to be cylindrical with >2.50 D astigmatism, the elevation can be induced by astigmatism and therefore needs to be compared with other corneal maps. A posterior high point of >35 µm above the BFS that has corresponding thinning on the pachymetry map is a contraindication for LASIK, but not for PRK, LASEK or epi-LASIK.
In a normal cornea, the power of the posterior BFS is about 51.00 D. If the BFS is >55.00 D on the posterior profile,6 early keratoconus must be considered. This criterion alone is not always a sign of early ectasia, however, as it can be seen in small corneas, steep corneas or in Asian eyes. A power between 53.00 and 54.00 D can be suspicious and must be correlated with other signs and symptoms.
Early keratoconus is indicated by a relative difference >100 µm between the highest and lowest point on the posterior elevation map (Figure 4). If the relative difference is >70 µm, the surgeon must proceed with caution ? except when the cornea is very symmetrical and when it is caused by a regular astigmatism (Figure 5).
POWER MAP
Be suspicious of steep corneal curvatures. A keratometric mean power map is >46.00 D or a total mean power map >45.00 D are both definite red flags (Figure 6). Surgeons should also be suspicious of bow-tie/broken bow-tie patterns. A so-called lazy C on the axial power map is especially suspicious when the astigmatism shifts >20º from a straight line (Figure 7).
Pay close attention to central corneal asymmetry on the power map. A change within the central 3-mm optical zone of the cornea >3.00 D from superior to inferior should raise a yellow flag and can be correlated with the presence of vertical coma (Figure 7). This can be merely a sign of asymmetric astigmatism and not necessarily indicative of pathology, so it must be correlated with other signs.
COMPOSITE INFORMATION
Ultimately, all of the information must be integrated in order to determine whether or not to perform surgery. Perhaps the strongest topographic sign7 of early keratoconus is the correlation of signs with the highest point in the posterior elevation. If the highest point on the posterior elevation coincides with the highest point on the anterior elevation, the thinnest point on pachymetry, and the point of steepest curvature on the power map, never perform laser refractive surgery (Figure 4).
Be sure to analyze efkarpides criteria: This is the ratio of the radius in millimeters of the anterior BFS divided by the radius in millimeters of the posterior BFS. Surprisingly, in normal corneas, this ratio will be around 1.21. Between 1.23 and 1.27 is suspicious, and corneas with a ratio of ≥1.27 should never be treated with a laser (Figure 8).
When the anterior and posterior profiles are similar, this implies a forward bending of those areas above BFS. This sign needs to be evaluated within the context of other parameters,8 as it could relate to structural weakness (Figure 9). Inferotemporal displacement of the highest point on the anterior and posterior elevation profiles can be indicative of early keratoconus (Figure 10). When a difference of >1.00 D of astigmatism between two eyes is seen, a high risk of postoperative ectasia exists.
The information in the center of the quad maps is important to the overall decision to operate. As Richard L. Lindstrom, MD, has said, an irregularity of >1.5 D in the 3-mm central zone and of >2.00 D in the 5-mm central zone should be an alert (Figure 11).
A normal band scale should be present ? this means an elevation within ±0.25 µm of the BFS for the anterior and posterior elevation maps. The normal band for the total corneal power map is 40.00 to 48.00 D and 500 to 600 µm for corneal thickness (Figures 12 to 14).
It is imperative to remember that, if one eye fails on the indices mentioned but the other one does not, neither eye should be treated. These indices add to the overall armamentarium of preoperative evaluations for potential LASIK patients and help guide surgeons toward safe and effective outcomes.
Erik L. Mertens, MD, FEBO, is a cataract and refractive surgery specialist. Dr. Mertens is medical director of the Antwerp Eye Center, Antwerp, Belgium. Dr. Mertens is a consultant for Bausch & Lomb and STAAR Surgical. He does not have a financial interest in any product or company mentioned. He may be reached at e.mertens@zien.be or +32 3 828 29 49.
