Identical excimer laser PTK treatments in rabbits result in two distinct haze responses

Abstract

Purpose: To obtain objective light-scattering measurements to test a hypothesis that identical PTK treatments cause distinct low- and high-level light-scattering responses in rabbit corneas.

Methods: An excimer laser was used to produce identical 6-mm diameter phototherapeutic keratectomy treatments (PTK) in 32 pigmented rabbits. Eyes were treated by performing a 40-microm epithelial ablation, followed by a 100-mum stromal PTK. Objective scattering measurements were made before treatment, weekly up to 5 weeks, and then biweekly to 9 weeks. Confocal microscopy was performed on several corneas at 4 and 7 weeks.

Results: Mean scattering levels split into distinct low- and high-scattering groups 2 weeks after treatment and remained distinct until week 7 (P < 0.003). Scattering in the low group reached a broad peak that lasted from weeks 2 to 4 at approximately 3 times the pretreatment level. Scattering in the high group peaked at 3 weeks at approximately 12 times the pretreatment level. Scattering levels diminished after reaching their peaks. Confocal images showed a band of highly reflective material in the anterior stroma that extended much deeper in corneas from the high group. The reflective band in the highly scattering corneas obscured the posterior stroma from view for up to 5 weeks.

Conclusions: Quantitative scattering data obtained with the scatterometer suggest that identical PTK treatments indeed result in distinct low- and high-level light-scattering responses in rabbits.

FIGURE 1

Schematic diagram of the scatterometer. The fiber optic is located at the image plane of the slit lamp objective and therefore acts as a field stop, defining the 1.7-mm diameter region of the cornea from which scattered light is detected. The slit is adjusted so that its width is 2.2 mm. A 550-nm, 50-nm bandwidth interference filter was used for wavelength selection. The optical arrangement assures that specularly reflected light is not detected.

FIGURE 2

Relative scattering measured from the center of the treated area at various times after identical PTK treatments. The points are the scattering measurements from individual eyes of the eight rabbits in the first two treatment groups, normalized by the baseline measurements made before treatment in the same eyes. In this method of plotting, each rabbit eye serves as its own control. It was these data that suggested the hypothesis that identical treatments cause distinct low- and high-level light-scattering responses in rabbit corneas.

FIGURE 3

A histogram of the numbers of corneas as a function of relative scattering at 3 weeks after the identical PTK treatments. The histogram clearly shows that the data separate into distinct low- and high-scattering groups, with the high-scattering group showing greater variability.

FIGURE 4

Mean relative scattering levels and standard deviations at various times after identical PTK treatments. After 2 weeks, the mean scattering levels split into distinct low and high-scattering groups. The groups remain statistically distinct up to 7 weeks (P < 0.005; cf., Table 1).

FIGURE 5

Laser scanning confocal microscope z-scan images of corneas at different times after identical PTK treatments. The vertical lines in the images, which are very bright in (b), were caused by a reflection artifact in the microscope optics. The extremely high reflectivity of this particular cornea exacerbated the artifact. Relative intensities should not be compared between the different images. (a) Cornea from the low-scattering group 4 weeks after treatment. Relative scattering for this cornea was 2.6. Small arrow: scattering centers inside the epithelium. The epithelial thickness was 40 μm. There was a narrow reflective band beneath the epithelium and a more diffuse band that extended nearly halfway through the cornea (large arrows). (b) Cornea from the high-scattering group 4 weeks after treatment. Relative scattering for this cornea was 13.8. The basement membrane was irregular, and the epithelial thickness varied between 34 and 43 μm. The wide reflective band beneath the epithelium obscured the underlying stroma and endothelium from view. (c) Cornea from the low-scattering group 7 weeks after treatment. Relative scattering for this cornea was 1.4. The epithelial thickness averaged ∼46 μm. The very narrow subepithelial reflective band (large arrow) has nearly resolved in this particular cornea. (d) Cornea from the high-scattering group 7 weeks after treatment. Relative scattering for this cornea was 4.2. The epithelial thickness averaged ∼46 μm. A broad band of reflectivity persisted in the anterior stroma, but it did not obscure the underlying stroma and endothelium.