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Diurnal variation in biomechanical corneal properties and IOP with CorVis ST and ORA
Session Title: Corneal biomechanics
Session Date/Time: Tuesday 08/10/2013 | 08:00-10:30
Paper Time: 08:40
Venue: Elicium 1 (First Floor)
First Author: : J.Gonzalez-Meijome (VAT:234024003) PORTUGAL
Co Author(s): : H. Neves N. Garcķa-Porta S. Peixoto-de-Matos J. Jorge L. Dias J.
To evaluate the changes in intra-ocular pressure (IOP) and the biomechanical properties of the cornea measured with Corvis ST and Ocular Response Analyzer in a population of young healthy patients in the morning and in the afternoon.
CEORLab - Center of Physics - University of Minho - Braga - Portugal
Eighty eyes from 80 young and healthy patients (60 females, 20 males) were evaluated in random order with Ocular Response Analyzer (ORA, Reichert, Depew, NY) and with Corvis ST (Oculus, Wetzlar, Germany). Measurements were performed in the morning between 9:00 and 11:00 AM (minimum of 2 hours after awaking) and repeated in the afternoon between 16:00 and 18:00 after 7 to 9 hours. Instruments were applied in random order to minimize the potential effect of ocular massage in aqueous outflow and changes in intra-ocular pressure and biomechanical properties of the cornea. IOP, central corneal thickness (CCT) and 10 other parameters potentially related with the biomechanical corneal response to a controlled air puff were obtained from Corvis ST. Goldmann equivalent IOP (IOPg), corneal compensated IOP (IOPcc), corneal resistance factor (CRF) and corneal hysteresis (CH) were obtained with ORA. Differences were computed subtracting Afternoon minus Morning values for a given parameter. 95% confidence intervals (CI) were also calculated. Considering the normal distribution of data, comparisons were performed using paired sample T-test.
IOP and CCT obtained with Corvis ST undergone a significant decrease of 1.5 mmHg and 20 microns, respectively over the period of the study (p<0.001 and p=0.034). IOPg measured with ORA also decreased significantly while ORAs IOPcc remained stable as well as CRF (p=0.051) and CH (p=0.602). Several biomechanical parameters measured with Corvis ST changed significantly over the period of study; those included time for applanation 1 (diff:+0.069ms; p<0.001), time for applanation 2 (diff:-0.129ms; p<0.001), velocity of applanation 2 (diff:+0.018mm/s; p=0.028), highest concavity peak distance (diff:-0.053mm; p=0.023), highest concavity radius (diff:+0.168mm; p=0.008) and highest concavity deformation amplitude (diff:-0.043mm; p<0.001). CCT changes had a weak effect on the change in highest concavity radius (r2=0.088; p>0.05) or highest concavity deformation amplitude (r2=-0.223; p=0.047).
Present results show that Corvis ST is able to differentiate small changes in the biomechanical behavior of the cornea as a result of normal diurnal variations. Changes observed are consistent with a trend towards a higher radius of the deformation concavity (flatter) in the morning and a slower recovery in the afternoon with longer times to achieve applanation 2. Physiologic changes in CCT upon awakening have a weak effect on changes in the biomechanical parameters measured with Corvis ST so other changes in the corneal properties might be involved.