Abstract
Abstract: :
Introduction: Intraocular pressure (IOP) measurement is influenced by corneal rigidity. The corneal elasticity, thickness and hydration and some yet unidentified factors have been suggested/identified to affect corneal rigidity and hence the IOP measurement. One possible measure of ocular rigidity is ocular hysteresis. Hysteresis, as measured by the Reichart ORA, has been described as the difference between inward (movement) applanation and outward applanation of the central corneal tissue after the air puff from the tonometer. Purpose: To analyse the relationship between CCT and hysteresis in normal eyes. Materials and Methods: 182 eyes of normal patients were randomly recruited from relatives or normal patients’. Full informed consent was obtained from all patients. Exclusion criteria included any history of glaucoma or previous ocular surgery or eye infections and anyone on topical eye medication. Firstly intraocular pressure (IOP) of each patient was measured by Goldmann applanation tonometer. Then the IOP, hysteresis and excess ocular pressure (EOP) were measured by the Ocular Response Analyzer non–contact tonometer (ORA) (Reichert Ophthalmic Instruments, Buffalo, NY) and then the IOP was again measured by Goldmann tonometer to note any difference before and after NCT measurements. Finally CCT was measured using a hand held ultrasonic pachymeter (DGH– 550, DGH Technology Inc, and Exton, PA) in the mid–pupillary axis. Three readings were taken and the lowest value was used as the CCT. Results: The mean CCT was 546.10 + 37.28µm, the range being 475 – 685µm and the mean hysteresis was 5.03 + 1.81mmHg, the range being 2 to 12.8 mmHg. The regression equation was hysteresis = –2.15 + 0.01 CCT. The F ratio was 12.66 and P < 0.001 using analysis of variance. Conclusion: The mean central corneal thickness was positively correlated to hysteresis. However, hysteresis gives a different measure to CCT for ocular rigidity and is a useful additional tool, when trying to assess the accuracy of IOP measurement.