In my practice, Epithelial Thickness Mapping (ETM) with the Avanti Widefield OCT system (Optovue) serves several purposes. I use it to aid in keratoconus diagnosis and assess cross-linking patients pre- and post-operatively.

I also use ETM for dry eye diagnosis and management before refractive surgery. Acquiring an epithelial thickness map doesn’t disrupt patient flow — it’s a noncontact test, so technicians can do it — and it provides a great deal of useful data.

Normal ETM versus Keratoconus

Once data is obtained from the OCT system, I get a map of the right eye and left eye that shows the total corneal thickness and the epithelial thickness. In a cross-sectional view, the software clearly identifies the epithelial layer, showing the posterior of the epithelium as well as the posterior corneal surface, which can be important when trying to locate landmarks.

On a normal map, we’ll see a corneal thickness of approximately 550 microns and a corneal epithelium of about 50 microns pretty much all the way through. In an eye with untreated keratoconus where the diagnosis is obvious on the topographic maps, the ETM will show a little blue thinning of the epithelium over the apex of the cornea where the cone protrudes. When the ETM shows thinning of the epithelium on only one eye, I might observe the fellow eye for a little while as opposed to aggressively recommending treatment.

Evaluation Before LASIK

When we see a patient for LASIK evaluation and there is inferior steepening or a borderline cornea, we need to determine if the person is at an above-average risk for ectasia. Most patients prefer LASIK over PRK when given a choice, but LASIK may not be the best choice if the risk of ectasia is heightened. ETM offers another tool to help us feel more confident about whether to recommend LASIK, because epithelial thinning over the central cornea could indicate increased risk of ectasia.

ETM also helps us evaluate and address ocular surface issues, such as dry eye, before surgery. We might see a patient who has relatively normal topography, a cornea that looks pretty good clinically, and perhaps a little fluorescein staining on the cornea. But when we look at the ETM, the epithelium has a very irregular pattern caused by dryness. OCT is very useful for detecting this irregularity, which indicates that the surface is not pristine.

Post-LASIK Follow Up

When we perform LASIK surgery and outcomes are less than perfect, ETM can help us decide if and when to perform an enhancement. For example, I performed LASIK on a high myope. After surgery, the patient’s topography looked pretty good, showing some flattening of the cornea.

The patient was happy with his overall vision, but still hoped for 20/20 or better without glasses, and his vision in the uncorrected eye was 20/30-20/40. We thought maybe we would enhance him when he was stable, but when we followed him for 6 months, the ETM showed some interesting changes.

As we commonly see with a myopic ablation, there is a flattening of the cornea with a relatively normal ETM after surgery. Over time, the epithelium in the central cornea where the ablation was flattened demonstrated epithelial thickening (hyperplasia). This thickening occurs very often in patients who need enhancements. Before I enhance the patient, I want to see not only a stable refraction, but also a stable ETM. When it is unstable, I delay enhancement — sometimes as long as 9 months or 1 year — which was the case with this patient.

When we use ETM to follow patients after LASIK, we can also see how the epithelium fills in after surgery, which helps us understand cases where the post-op refractive error changes. Changes in refraction often correlate to changes in the corneal epithelium, which can alter as it heals.

ETM is a great diagnostic tool to help us identify keratoconus (clear-cut cases or forme fruste), evaluate patients before and after cross-linking, and diagnose and manage dry eye disease. ❖