Anterior segment OCT helps define variations in epithelial thickening

Take-Home

Anterior-segment optical coherence tomography was used to characterize changes in the corneal epithelial thickness patterns after myopic LASIK and PRK.

By Lynda Charters; Reviewed by Shaun Robinson, MD

Houston-Myopic LASIK and PRK are associated with greater thickening of the paracentral and central epithelium compared with changes in the peripheral epithelium. These central changes may to some extent counteract the myopic effect of the ablation.

“Previous studies have reported epithelial thickening after myopic ablations, with the greatest thickening occurring centrally and paracentrally,” said Shaun Robinson, MD, of Cullen Eye Institute, Baylor College of Medicine, Houston, at the time of this series, and currently with the Cabarrus Eye Center, Concord, NC. “However, no studies have investigated the regional variations in epithelial thickening after corneal refractive surgery.”

Epithelial thickness patterns

In light of this, using anterior-segment optical coherence tomography (AS-OCT) (RTVue-100, Optovue), Dr. Robinson and colleagues from the Cullen Eye Institute conducted a prospective, nonrandomized case series to characterize changes in the corneal epithelial thickness patterns after myopic LASIK and PRK. Eighteen eyes of nine patients who underwent myopic LASIK and 15 eyes of eight patients who underwent myopic PRK were included.

The changes in epithelial thickness were defined as the differences between the epithelial thickness measured preoperatively and postoperatively within each of 17 zones:

·      One central 2-mm diameter zone.

·      Eight paracentral zones-annulus from 2- to 5-mm diameter.

·      Eight peripheral zones-annulus from 5- to 6-mm diameter.

The measurements analyzed at each location were the averages of three scans obtained by AS-OCT centered on the pupil, according to Dr. Robinson.

Postoperatively, the mean preoperative spherical equivalent refractions in the LASIK and PRK groups decreased from –5.75 ± 2.09 D (range, –0.88 to –8.13 D) and –4.66 ± 2.24 D (range, –1.25 to –8.25 D), respectively, to –0.31 ± 0.41 D (range, –1.25 to +0.12 D) and –0.34 ± 0.66 D (range, –1.50 to +1.00 D) 3 months postoperatively.

The central corneal epithelium increased significantly from 53.52 µm at baseline to 57.43 µm 3 months after myopic LASIK and from 52.47 at baseline to 58.46 µm 3 months after myopic PRK (p < 0.05 for both comparisons). The same phenomenon occurred in the paracentral epithelial zones with increases from 53.04 at baseline to 57.65 µm and from 53.02 µm at baseline to 57.84 µm, respectively, at the same time point (p < 0.05 for both comparisons).

The peripheral corneal zones also increased in thickness from 52.46 at baseline to 53.91 µm and from 53.65 at baseline to 54.56 µm, respectively (p < 0.05 for both comparisons), at the same time point.

However, the increase in the peripheral zone was less than that seen centrally or paracentrally after both LASIK and PRK (p < 0.05 for each comparison). Of note, there was no difference between the central and paracentral zones after either LASIK or PRK and there was no difference between LASIK and PRK for any of these three regions (p > 0.05). Interestingly, the epithelium in the inferior peripheral zone became 2 µm thinner after myopic LASIK (p < 0.05).

"Thickening of the central and paracentral epithelium appears to be a response to thinning of the underlying stroma with myopic ablation, which in some small measure counteracts the myopic effect of the ablation," Dr. Robinson said.

The investigators concluded that the central and paracentral epithelium had greater thickening compared with the peripheral epithelium after both myopic LASIK and PRK, and this central thickening may to a small extent resist the myopic effect of the ablation. They described the thinning of the peripheral inferior epithelium after LASIK as “surprising and striking.”

Dr. Robinson commented that the inferior epithelial thinning after LASIK may be related to LASIK-associated dry eye, but ultimately believes this area needs further study to determine this conclusively.

Shaun Robinson, MD

E: shaunbrobinson@gmail.com

This article was adapted from Dr. Robinson’s presentation at the 2014 meeting of the American Society of Cataract and Refractive Surgery. Dr. Robinson has no financial interest in the subject matter.