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D. M. Grzybowski, M. D. Twa, A. M. Mahmoud, C. E. Morin, J. Ou, S. Yeates, D. Castellano, C. J. Roberts; Time-Course of the Biomechanical and Corneal Swelling Response to Nitrogen in Normal and Post LASIK Eyes. Invest. Ophthalmol. Vis. Sci. 2008;49(13):657.
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© ARVO (1962-2015); The Authors (2016-present)
To evaluate changes in pachymetry and biomechanical parameters of normal and post-LASIK corneas in response to induced swelling.
Both eyes of 18 subjects were exposed to humidified nitrogen (N2) gas for 2 hours via modified swim goggles. Subjects were comprised of 7 bilateral LASIK patients (mean 39±9.9yrs) and 11 non-surgical patients(mean 29±6.3yrs). All subjects were evaluated immediately before treatment, after treatment and every 30 mins. thereafter for 150 min. Response variables were analyzed using a repeated measures analysis of variance model. Changes in central, paracentral, and peripheral corneal thickness were measured with Orbscan II. The corneal response to treatment was also assessed using several indices from the Ocular Response Analyzer: Corneal hysteresis(CH), Corneal Resistance Factor(CRF), Goldmann-corrleated IOP(IOPg), and corneal compensated IOP(IOPcc). Peak amplitude and width from ORA’s raw IR signal were evaluated.
Corneal thickness increased significantly (p<0.05) in all regions in both groups after exposure to N2. At 150 min, central thickness did not recover to baseline in either group (p<0.05). Paracentral and peripheral thickness was not significantly different from baseline after 120 min in Post-LASIK eyes, but remained different from baseline at 150 min in the control group. IOPcc was significantly increased relative to baseline (p<0.05) immediately after swelling in the post-LASIK group and later decreased, remaining significantly less than baseline at 150 min. IOPg was significantly reduced in post-LASIK eyes compared to baseline and remained decreased at 150 min. CH was significantly reduced compared to baseline at the first post-swelling time point in post-LASIK eyes, and recovered by 30 min. There were no significant changes in IOPcc, IOPg, or CH at any time point in the control group. CRF was initially reduced in LASIK eyes (p<0.05 for first 2 post-edema measurements), and recovered by 60 min. CRF had no initial change in control eyes, but showed a significant reduction at 120 and 150 min post-swelling. Both groups had a significant increase in height of the first IR peak(Peak 1) of the ORA signal after swelling, indicating corneal stiffening, with faster recovery in the post-LASIK group by 60 min. Peak 1 remained significantly increased in controls at 150 min.
The spatial and temporal pattern of swelling and the biomechanical response in LASIK corneas was distinct from the response in normals.
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