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Emilio Almeida Torres Netto, Alain Saad, Imene Salah-Mabed, Damien Gatinel; Effect of the corneal epithelium on the topographical properties of thin and irregular corneas. Invest. Ophthalmol. Vis. Sci. 2017;58(8):2632.
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The presence of a thin or irregular cornea in refractive surgery candidates may preclude them from LASIK surgery, and in these eyes, PRK may be safer. It has been suggested that the corneal epithelium remodeling may mask some alterations in Bowman’s layer contour that could, otherwise, indicate an early stage of keratoconus. This prospective study investigated in vivo topographic properties of the corneal epithelium in suspicious corneas, presenting irregular astigmatism and/or reduced thickness.
Sixty-five eyes selected for PRK were prospectively analyzed. The reasons for choosing PRK were (1) the presence of thin cornea (<500 microns) and/or (2) irregularities raising keratoconus suspicion. Two indices were used to objectively raise the suspicion of keratoconus: Corneal Score from the OPDscan III specular topograph and SCORE Analyzer from the Orbscan IIz slit scanning topograph. Corneal topography was assessed preoperatively and during PRK after epithelial removal. Corneal keratometry, Fourier and Zernike terms, corneal asymmetry indices and asphericity were analyzed.
Following epithelial removal mean preoperative astigmatism and also the flat, steep meridian and apex keratometry increased. The average epithelial induced astigmatism magnitude was 0.37 ± 0.38 D (figure 1). The shift of the astigmatism axis was computed in polar notation, and remained with the rule. Fourier and Zernike analysis revealed a reduction of the minimum radius of anterior corneal curvature and an increase in the coma, trefoil and irregularity. There were significant differences in some corneal indices used for keratoconus screening (ISV, IHA, IHD and ABR), which respective values remained within the normal range. Significant change in asphericity was observed only in the nasal hemi-meridian. For all variables, a calculated p-value <0.05 was considered statistically significant.
The epithelium affected the topographical properties of the cornea by significantly reducing the magnitude of corneal topographic astigmatism and high-order aberrations. Moreover, these data suggest that the modifications of the corneal topography after epithelial removal in thin and irregular corneas have similar magnitude to that of normal myopic eyes. This knowledge may be relevant not only for preoperative evaluations, but also surgical planning of techniques such as transepithelial or topography customized PRK.
This is an abstract that was submitted for the 2017 ARVO Annual Meeting, held in Baltimore, MD, May 7-11, 2017.
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