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AbstractSO.15.09 CCT independent methods of measuring IOP Hager A., Loge K., Füllhas M. O., Schroeder B., Wiegand W.
Department of Ophthalmology, Asklepios Klinik Nord/Heidberg, Hamburg Objective: Dynamic Contour tonometry (DCT) has been introduced recently to measure IOP independently of CCT. The handling of DCT requires some experience and is costly so that alternatives are looked for. IOP measurement using the Ocular Response Analyzer (ORA) offers a non contact tonometry (NCT) value and corneal compensated IOP (IOPcc) that is based on the measurement of Corneal hysteresis (CH) and supposed to be independent of CCT.
Methods: The ORA allows measurement of biomechanical corneal properties using an air-puff triggered, dynamic, bi-directional corneal applanation process. The difference of the pressure values at the point of applanation during in- (AP1) and outward- (AP2) movement is called CH. Using the ORA instrument, in 66 normal eyes and 203 glaucoma eyes NCT and IOPcc were determined. Subsequently, measurement by Goldmann applanation tonometry (GAT) and DCT were performed. IOP-values were analyzed in respect of CCT. AP1 and AP2 were calculated and related to the deviations of IOPcc and DCT-values determined.
Results: CCT did not differ in these two groups. Average values were: IOPcc 18.43±6.2 mmHg, NCT 17.76±5.6 mmHg, GAT 14.63±4.4 mmHg, DCT 15.12±3.9 mmHg, CH 10.1±2.8 mmHg and CCT 554±43µm. There is a positive correlation between GAT and CCT that does not exist for IOPcc and DCT values. However, IOPcc and DCT differ significantly in Bland-Altmann analysis (p<0.01). On the basis of regression analysis we calculate a correction factor for normal and glaucoma eyes that allows us to transfer ORA-IOPcc to DCT values.
Conclusions: By means of this correction factor it is possible to measure with the ORA a CCT-independent IOP that is consistent with DCT. The advantage is the easy to handle and non contact method of IOP measurement and the specification of CH.
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