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S. N. Mirza, D. Schonlau, A. M. Mahmoud, P. A. Weber, E. J. Rouse, C. J. Roberts; The Measurement of Biomechanical Parameters as a Function of Peak Applied Pressure in the Reichert Ocular Response Analyzer, in a Glaucomatous Population. Invest. Ophthalmol. Vis. Sci. 2008;49(13):704.
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© ARVO (1962-2015); The Authors (2016-present)
To determine if the biomechanical parameters measured by the Ocular Response Analyzer (ORA) are a function of the characteristics of the applied air pressure curve in a glaucomatous population.
16 eyes of 8 subjects previously diagnosed with primary open angle glaucoma (POAG) were prospectively recruited. Subjects were measured with a commercial ORA with custom software to allow the applied air pressure to be manipulated through a graphical user interface created in Matlab. Each trial involved 3 replicate measurements at each of the programmed pressure values. One trial was taken at each pressure setting of 500, 400, 300, 250, 200 and 100 arbitrary units. The infrared applanation signal data and air pressure data were stored each corresponding to a specific time value, as well as the output parameters of corneal hysteresis (CH), corneal resistance factor (CRF), and corneal compensated intraocular pressure (IOPcc). Linear regression analysis was performed on the data from each of the 16 eyes to determine the nature of the relationship between peak air pressure and biomechanical parameter measurement. Response in POAG was compared to a similar previous study using a young normal population.
Unlike the young normal population which showed a decrease in IOPcc as a function of applied air pressure, there was no significant relationship between applied air pressure and IOPcc in the POAG population. In addition, the POAG response in terms of linear regression of both CH and CRF to applied air pressure, although statistically significant (p=0.0227 and p=0.0067 respectively), had a much lower magnitude than in the normal population. The range of mean CH values as a function of applied air pressure in POAG was 9.3mm Hg to 10.8mm Hg, compared to normals over the same applied air pressure levels, which ranged from 6.1mm Hg to 12.1mm Hg.
Corneal viscoelastic response is a function of the rate of force application. However, the response in POAG has much less variation as a function of applied air pressure than in a young normal population. Therefore, the biomechanical response to an applied air pressure pulse is distinct in POAG and young normal populations.
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