How to Optimize Corneal Imaging In Refractive Surgery Screening

Maximizing the breadth and depth of the data Scheimpflug photography and OCT can provide.

BY MICHAEL W BELIN, MD

The last decade has seen rapid changes and improvements in the preoperative assessment of the refractive surgery patient. As with other subspecialty areas in ophthalmology, imaging techniques have played a large part in these advances. Corneal imaging modalities today, including Scheimpflug photography and OCT, offer significantly more information than those available in the early years of refractive surgery. This article explores how to maximize the capabilities of these modalities to gather accurate data and improve results.

Rapidity Essential in Screening

Every patient who presents for a refractive surgery evaluation must undergo some imaging evaluation. This is an important point to consider because a “screening” test is only effective if it is relatively rapid and most of your technicians can perform it, and it does not disrupt normal patient flow. A test that requires an inordinate amount of time or specialized technicians, or both (eg, UBM) will not function as an effective screening tool because it will not be performed.

The two most commonly used “screening” tools had been ultrasonic pachymetry and placido-based computerized videokeratoscopy (topography). They were easy to perform, rapid and did not require special skills beyond the typical ophthalmic technician. Corneal topography and central corneal thickness quickly became the standard of care for screening refractive surgery patients.

Over the years, however, clinicians found that these two techniques, while meeting the requirements of a screening test, failed to provide the detail and depth of data that newer corneal imaging devices could. Ultrasonic pachymetry only conveyed information on one point of the cornea, which at times neither was the thinnest point or representative of the overall corneal thickness. Videokeratoscopes only provided curvature information from the anterior corneal surface and only covered about 50% of the corneal surface.

Figure 1: Both corneas have the same value for the thinnest point, but the cornea on the left has a normal progression from the thinnest point to the periphery, while the cornea on the right shows a highly abnormal progression with a much higher rate of change.

Figure 2: Sagittal curvature map is unremarkable and fairly spherical. The central ultrasound pachymetry was 520 μm, but the cornea thinned to 502 μm inferotemporally, easily detecting a prominent and abnormal posterior ectasia (lower right map).

Advantages of Scheimpflug Photography

Meanwhile, newer imaging techniques, including optical cross-sectioning, Scheimpflug photography and OCT, continued to evolve. These modalities allowed measurements of both the anterior and posterior cornea and also provided greater corneal coverage than videokeratoscopes.

Figure 3: In this example of the Belin/Ambrosio III display, detected abnormalities on the posterior cornea, pachymetric progression index and Ambrosio relational thickness would place this eye 4 SD from the norm in spite of a normal anterior cornea.

Of these, rotating Scheimpflug photography provides the most information relative to the speed of the exam and expertise required for the technician to perform it. It functions as an excellent and effective screening tool.

The other benefit of a device that measures both anterior and posterior corneal surfaces is that it can produce full pachymetric maps (corneal thickness maps) rather than a single central thickness. Full corneal thickness maps provide a wealth of additional information that ultrasonic pachymetry cannot. Here, a single test, rotating Scheimpflug photography, not only provides more information, but a single device can replace two — the videokeratoscope and ultrasonic pachymeter.

The value of a full pachymetric map is obvious in a number of examples. First, one customarily performs residual bed computations before ablative refractive surgery. Traditionally, the surgeon subtracted the ablation depth plus the estimated flap thickness from the central corneal thickness determined by ultrasound. The resultant number was the estimated residual bed.

More Accurate Read on Ectasia

Clinicians thought flap thickness had the greatest variability, and they often used errors in residual bed estimates to explain post-LASIK ectasia after what they viewed as a normal preoperative exam. We published in 2008 a study that compared the thinnest corneal-thickness readings and apical readings (presumably at the point where ultrasound reads) in more than 1,400 refractive patients who had been screened as normal by placido topography and ultrasound pachymetry.¹ Our findings showed that ultrasound apical readings could overestimate the true thinnest point by nearly 100 μm (Table).

A full-thickness map not only allows one to measure and locate the true thinnest point, but to also analyze the pachymetric progression from the thinnest point to the periphery. A single thickness reading limits the surgeon in determining what is normal in the same way it would be nearly impossible to determine whether an 80-kg individual is obese without knowing the individual's height. A 4-ft 10-in, 80-kg individual would be obese, while a 6-ft, 4-in individual would be gaunt.

Similarly, corneas having the same thinness value may share dramatically different pachymetric progressions. Abnormal corneas (ie, corneas showing ectatic change or tendency) have a more rapid thinning from the corneal periphery to the thinnest point. The work of Renato Ambrosio Jr. from Rio de Janeiro has standardized this rate of change.² As illustrated (Figure 1), both corneas have the same value for the thinnest point, but the cornea on the left has a normal progression from the thinnest point to the periphery (ie, cornea is normal), while the cornea on the right shows a highly abnormal progression with a much higher rate of change. This rapid rate of pachymetric progression in a preoperative cornea is highly suggestive of ectatic change.

Measure of Posterior Corneal Surface

The other major benefit of newer imaging techniques is their ability to measure the posterior corneal surface itself. Videokeratoscopes, as a reflective technology, could not measure anything below the anterior corneal surface. The posterior cornea, however, is an earlier indicator of ectatic change or ectasia susceptibility. Combined with full pachymetric data, this measure serves as a more sensitive screening tool then anterior topography and ultrasound pachymetry combined.

Abnormalities on the posterior cornea can appear before any topographic change on the anterior surface. Full pachymetric data can help detect highly abnormal posterior corneas in patients with otherwise normal placido and ultrasound screening. In Figure 2, the sagittal curvature map is unremarkable and fairly spherical. The central ultrasound pachymetry was 520 μm, but the cornea thinned to 502 μm inferotemporally, easily detecting a prominent and abnormal posterior ectasia (lower right map).

Combining Elevation Data With Pachymetry

A comprehensive refractive screening display (Belin/Ambrosio Enhanced Ectasia Display – version 3) is currently offered on the Pentacam (Oculus GmbH, Wetzlar, Germany) that combines both anterior and posterior elevation data with pachymetric data in a unified screening tool. The display uses the following parameters in a regression analysis to assist the refractive surgeon:

• Anterior elevation at the thinnest point.

• Posterior elevation at the thinnest point.

• Change in anterior elevation.

• Change in posterior elevation.

• Corneal thickness at thinnest point.

• Location of thinnest point.

• Pachymetric progression.

• Ambrosio relational thickness.

• Kmax.

The display (Belin/Ambrosio III) (Figure 3, page 46) individually reports each parameter as standard deviations, and then reports a final overall reading based on a regression analysis of all the previously mentioned parameters. One can see abnormalities on the posterior cornea, pachymetric progression index and Ambrosio relational thickness, with an overall reading that would place this eye 4 SD from the norm, in spite of a normal anterior cornea.

Conclusion

Comprehensive corneal analysis used for preoperative refractive screening includes information from the posterior cornea and full pachymetric data. This added information improves the refractive surgeon's ability to screen patients for occult ectatic disease or to identify patients potentially at higher risk for post-LASIK ectasia. Rotating Scheimpflug cross-sectional analysis meets the criterion for a successful screening tool because it not only provides the necessary data, but also does so in amanner that does not interrupt patient flow nor require skills beyond those of most ophthalmic technicians. OM

References

1. Khachikian SS, Belin MW, Ciiolino JB. Intrasubject Pachymetric Asymmetry Analysis. J Refract Surg. 2008; 24:606-609.

2. Ambrosio R Jr, Dawson DG, Salomao M, Guerra FP, Caiado ALC, Belin MW. Corneal Ectasia after LASIK despite low risk: Evidence of Enhanced Sensitivity based on Tomographic and Biomechanical Findings on the unoperated stable fellow eye. J Refract Surg. 2010; 26:906-911.

Michael Belin, MD, is professor of ophthalmology and vision science, University of Arizona, Southern Arizona VA Healthcare System, Tucson. He serves as a consultant to Oculus GmbH. He received no royalties for the Belin/Ambrosio software