Catch Mild Keratoconus Before Laser Vision Correction

Some of my happiest patients are those on whom I have performed laser vision correction (LVC) who no longer need glasses or contact lenses. I may not have cured a pathologic condition, but those patients often tell me that it has changed their lives.

Conversely, some of the most unhappy patients in my practice are those who developed corneal ectasia after LVC. They went from seeing well with correction to needing hard contact lenses, scleral lenses, or even corneal transplant surgery. Most of these patients had mild keratoconus prior to their LVC.

Are We Still Progressing in Detecting Mild Keratoconus?

One of the "holy grails" in refractive surgery over the past 15-20 years has been the ability to distinguish eyes with normal corneas that are likely to do well with LVC from eyes with mild keratoconus that are likely to respond poorly. Moderate to severe keratoconus is easy to identify and has long been considered a contraindication for LVC.

Soon after the US Food and Drug Administration approvals of photorefractive keratectomy and laser in situ keratomileusis in the mid to late 1990s, performing an analysis of the anterior corneal curvature with corneal topography became fairly routine during the preoperative evaluation of patients for LVC. This technology does a good job of evaluating the anterior corneal curvature and can demonstrate mild irregularities suggestive of mild keratoconus.

Corneal topography is a big reason why we don't see more cases of post-LVC corneal ectasia. Yet, why do such cases persist today?

We have learned that keratoconus often (or possibly always) begins with a change in the posterior corneal curvature. Early on, the anterior corneal curvature (and corneal topography) remains normal, as does the corrected visual acuity. Patients with very mild keratoconus may not be identified with "routine" screening measures. Slit-beam imaging (eg, Orbscan) and Scheimpflug imaging (eg, Pentacam, Galilei) can identify abnormalities that occur in the posterior corneal curvature prior to developing on the anterior corneal curvature. Such technology is now widely used in preoperative assessments to identify eyes with early keratoconus at risk for post-LVC corneal ectasia.

But even combining screening modalities has not eliminated post-LVC corneal ectasia. Patients who rub their eyes and "bring on" ectasia after LVC may be one of the reasons for that.

Diagnosing Keratoconus Before Abnormalities Are Found

Abnormal corneal biomechanics have been proposed as an early indicator of keratoconus. The Ocular Response Analyzer is a device that has been around for years. Although it measures corneal biomechanics and can distinguish normal corneas from those with obvious keratoconus, it does not do a good job with mildly abnormal corneas.

The corneal epithelial thickness pattern is another parameter that is known to be abnormal in keratoconus, but it has been technically difficult to measure.

In a recent study, investigators used a swept-source polarization-sensitive optical coherence tomography (PS-OCT) machine to analyze the corneal epithelium and the Bowman's layer in an attempt to distinguish keratoconic corneas from normal corneas. Using PS-OCT, they were able to identify the epithelial "doughnut pattern" and "moth-like damage profile" in the keratoconic eyes but not in the normal eyes. The authors acknowledge numerous limitations to their study, including that they only compared normal eyes with known keratoconic eyes.

PS-OCT has great potential to identify patients with very early keratoconus who are at risk of developing ectasia after LVC. It may also allow us to monitor and objectively measure keratoconus, and consider corneal crosslinking in those patients with progression.

Although it may not be the holy grail moment quite yet, this study shows that investigators remain committed to improving our ability to identify keratoconus in order to better the outcomes for our patients undergoing LVC.

Christopher J. Rapuano, MD, is a nationally and internationally recognized expert in corneal diseases, chief of the Wills Eye Hospital Cornea Service, and professor of ophthalmology at Sidney Kimmel Medical College at Thomas Jefferson University.

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