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M. Wyble, J. Wang, S. MacRae, W. Fischer, K.R. Huxlin; Quantification of Stromal Light Scatter Following Photorefractive Keratectomy in the Cat . Invest. Ophthalmol. Vis. Sci. 2006;47(13):2745.
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© ARVO (1962-2015); The Authors (2016-present)
To assess the magnitude, location and time course of changes in stromal light scatter following photorefractive keratectomy (PRK) using optical coherence tomography (OCT) and to correlate these findings with slit lamp imaging and corneal histology.
Eight normal adult cats (felis cattus) underwent –6 or –10D PRK (6mm optical zone, 217Z Technolas laser, Bausch& Lomb). Slit lamp and OCT imaging were conducted pre–op, every 2 weeks up to 12 weeks post–op. Five of the cats were sacrificed at 1, 2, 4, 5 and 12 weeks post–op. In OCT images, the intensity of stromal light scatter was calculated from a region 63µm wide spanning the entire depth of the central stroma. The mean intensity of 25µm thick horizontal slices through this region was then normalized to the mean intensity of the entire stroma. Immunohistochemistry was performed on corneal sections to detect Vimentin (keratocyte marker) and α–smooth muscle actin (αSMA – an indicator of keratocyte differentiation into myofibroblasts).
The central corneal stroma of all pre–op cat eyes exhibited a flat OCT scatter profile across its thickness and a relatively uniform appearance on thin slits. After PRK, a band of increased light scattering appeared in the anterior stroma on both slit lamp and OCT images, lasting for ∼4 weeks, after which the intensity profile progressively returned to baseline levels. On OCT images, the band of increased scattering was only 100µm deep, with a mean peak intensity 2.5X brighter than the rest of the stroma. Its appearance coincided with the appearance of a band of αSMA expression under a thin, acellular layer beneath the ablation zone on corneal sections. Between 2 and 4 weeks post–op, the band of peak light scattering intensity moved closer to the epithelium. This corresponded with the disappearance of the acellular layer and shifting of the band of αSMA staining to just under the epithelium on corneal sections. Beyond 4 weeks post–op, αSMA expression decreased markedly.
We used OCT as a precise, non–contact method to measure the intensity, location and time–course of increases in corneal light scatter following PRK, a likely correlate of post–operative haze. There was good qualitative correlation of OCT appearance with slit lamp images, but the higher magnification of corneal layers by the OCT allowed better spatial resolution and quantification of changes in light scattering. Increases in light scatter measured with the OCT were also well correlated with the appearance and localization of αSMA positive cells in the corneal stroma of the same animals, confirming the documented role of these cells in post–operative haze.
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