ETM can ease diagnosis burden for screening refractive surgery candidates

Widefield epithelial thickness mapping (ETM) with Optovue Avanti optical coherence tomography (OCT) has been recognized as an important addition to topography and tomography for screening candidates for refractive surgery.

Investigators recently concluded that the corneal ETM patterns in eyes with one mild topographic or tomographic abnormality were similar to the patterns observed in normal eyes in patients with myopia.

This is an important observation when considering the number of candidate patients seeking LASIK or PRK. According to Ella Faktorovich, MD, topography and tomography are the mainstays of screening patients for corneal refractive surgery.

“Diagnostic decisions are easy to make in patients with clearly discernible abnormalities, such as significant inferior or central steepening, skewed astigmatism axes, very thin corneas, and significant posterior float,” she said. “The presence of these problems typically prevents patients from undergoing LASIK and possibly even PRK depending on the severity of the abnormality.”

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No simple decision

However, the decision is not always so simple in other patients with more subtle findings. Surgeons must grapple with some key issues, including whether inferior corneal steepening of 1.5 D would rule out LASIK o ig a 490-μm-thick cornea and a mild prescription would require PRK?

“When faced with such dilemmas, I often have wished for another reasonably sensitive and specific diagnostic method to assess the cornea to ensure that I am safely recommending a procedure to a patient,” said Dr. Faktorovich, who is in private practice in San Francisco. Her evaluation of published studies on ETM has revealed a discernible pattern of epithelial thickness distributions in normal corneas compared with corneas with severe to mild keratoconus.

These two types of ETM patterns (normal corneas vs. corneas with keratoconus) were highly reproducible when different staff conducted the measurements and also reproducible between different studies, she pointed out.

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A retrospective evaluation

In light of this, she and her colleagues retrospectively analyzed the ETMs, topographies (Atlas 9000, Zeiss), and tomographies (OCULUS Pentacam) in normal eyes compared with eyes with mild topographic and tomographic abnormalities.

Two hundred ninety-eight eyes of 149 consecutive patients were included who were myopic and stopped wearing soft contacts a week before the scans, which were part of their preoperative workup. Only patients with normal-appearing corneas and normal tear film on slit-lamp examinations were included in the evaluation. Patients with corneal scars, epithelial basement membrane dystrophy, superficial punctate keratitis, and decreased tear break-up time were excluded.

The analysis showed that 190 eyes (95 patients) (group 1) had normal topography and tomography scans; 89 eyes (49 patients) (group 2) had one of the following: pachymetry 475 to 510 μm (10 eyes/five patients), 1.50 D or less of inferior steepening (35 eyes/22 patients), 1.50 D or less of superior steepening (16 eyes/eight patients), central steepening (eight eyes/four patients), claw shape (14 eyes/seven patients), and posterior float (six eyes/three patients).

The minimal pachymetry thickness, central epithelial thickness, ratio of inferior epithelial thickness to the superior epithelial thickness, minimal epithelial thickness, the difference between the maximal and minimal epithelial thickness were compared between the two groups.

When the investigators compared the ETM patterns in the two groups, they found no differences in any parameters between the normal eyes and those with one mild topographic or tomographic abnormality. The retrospective chart review also identified 10 eyes (five patients) with a thin cornea (475 to 500 μm) and an additional mild abnormality, i.e., central steepening, a corneal thickness less than 475 μm, or a slightly skewed astigmatic axis.

Dr. Faktorovich recounted that the 10 eyes had epithelial thinning over the thinnest corneal spot, consistent with one of the possible ETM findings in patients with forme fruste keratoconus.

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Recommendations

Dr. Faktorovich provided the following pearls:

1. If the ETM is normal in patients with one mild topographic or tomographic abnormality, they may be good LASIK candidates. Therefore, the ETM is a useful additional screening tool for patients with topographic or tomographic abnormalities that pose a diagnostic dilemma regarding whether to recommend LASIK, PRK, or any corneal surgery. “I often find this technology helpful as a tie breaker when patients present to me for a third or fourth opinion after receiving different recommendations from different surgeons based on their topographies and tomographies. If the ETM is normal, I recommend LASIK if the residual stromal bed is sufficiently thick. If the ETM is abnormal, I recommend PRK or even no surgery, depending on the severity of the abnormality,” she stated.

2. Patients with several abnormalities on topography and/or tomography, even if very mild, should be approached cautiously. “We found that ETMs are consistent with forme fruste keratoconus in these patients. PRK or no corneal surgery, rather than LASIK, may be best for them,” she advised. She recounted the case of a 39-year-old trauma surgeon with corneal pachymetry of 498 μm and very slight skewing of the astigmatic axis. If the ETM had been normal, LASIK may have been recommended. Instead, the ETM pattern of epithelial thinning overlying the thinnest corneal spot that was displaced slightly inferotemporally led to a recommendation for PRK.

3. Mild inferior corneal steepening and areas of epithelial thickening on ETMs could be signs of epithelial basement membrane dystrophy even when patient’s cornea appears normal on slit lamp exam. PRK is recommended over LASIK to avoid epithelial loosening and sloughing during flap lift often seen in patients with epithelial basement membrane dystrophy.

The technology has proven so valuable that Dr. Faktorovich has incorporated it into the screening protocol of all refractive surgery candidates.