1120-6721/798-06$25.00/0 © Wichtig Editore, 2009 European Journal of Ophthalmology / Vol. 19 no. 5, 2009 / pp. 798-803 Diurnal variation of corneal biomechanics and intraocular pressure in normal subjects BANU ONCEL, UMUT ASLI DINC, EBRU GORGUN, BELKIS ILGAZ YALVAÇ Yeditepe University Eye Hospital, Istanbul - Turkey PURPOSE. To assess and investigate the relationship between diurnal variations in intraocular pressure (IOP) and biomechanical properties of cornea. METHODS. Sixty-two healthy volunteers were recruited. Corneal compensated IOP (IOPcc), Goldmann- correlated IOP (IOPg), corneal resistance factor (CRF), and corneal hysteresis (CH) parameters were recorded by the Ocular Response Analyzer (ORA). Central corneal thickness (CCT) was measured using an ultrasonic pachymeter. Three consecutive measurements of IOPcc, IOPg, CRF, CH, and CCT were recorded at 8:00 AM, 11:00 AM, 2:00 PM, and 5:00 PM. One-way analysis of variance and Pearson cor- relation tests were used for statistical analysis. Intra device repeatability was evaluated using Scheffe multiple comparison. RESULTS. IOPcc, IOPg, CRF, CH, and CCT measurements displayed a stable profle during daytime acquisitions and no statistically signifcant variation was noted (p>0.05). All measurements of IOPcc, IOPg, CRF, and CH recorded by ORA at 8:00 AM, 11:00 AM, 2:00 PM, and 5:00 PM were all found to be highly repeatable (p>0.05). There was a statistically signifcant and inverse correlation between IOPcc and CH (p=0.001), and a positive correlation was detected between IOPcc and CRF (p=0.001). CRF and CH were both found to be positively correlated to CCT (p=0.001). CONCLUSIONS. IOPcc readings and recordings of corneal biomechanics were constant throughout the day in healthy eyes. CRF and CH seem to exert different effects on IOPcc as diverse correlations were detected between CRF, CH, and IOPcc. Assessment of IOP and corneal biomechanics using ORA was found to be a highly repeatable. (Eur J Ophthalmol 2009; 19: 798-803) KEY WORDS. Corneal biomechanics, Corneal hysteresis, Diurnal variation, Intraocular pressure Accepted: January 11, 2009 INTRODUCTION Intraocular pressure (IOP) exhibits remarkable diurnal vari- ation which may affect the diagnosis, follow-up, and treat- ment strategy of patients with glaucoma and ocular hy- pertension (1-4). Corneal factors may influence the diurnal variation of IOP. Biomechanical properties of the cornea became a focus of interest since novel techniques have made in vivo measurements possible. Ocular Response Analyzer (ORA) is a recently introduced noncontact tonom- eter that was developed to investigate the biomechanical properties of the cornea in vivo. Assessment of corneal biomechanics is related far more than pachymetric evalu- ation alone, including the evaluation of corneal viscosity, elasticity, and hydration. Using ORA, corneal compensated IOP (IOPcc), Goldmann- correlated corneal noncompensated IOP (IOPg), corneal resistance factor (CRF), and corneal hysteresis (CH) pa- rameters are measured. Two IOP readings are recorded us- ing ORA, termed as inward and outward applanation pres- sure (Fig. 1). CH is defined as the difference between the inward and outward applanation pressures on the tonom- eter and is suggested to indicate the corneal resistance to changes in corneal shape. It would be expected that the cornea would applanate at the same pressure on the way in and out; on the other hand, two different applanation EJO_798-803_ONCEL.indd 798 27-08-2009 16:23:32