Charles W McMonnies1, Gavin C Boneham. 1. School of Optometry and Vision Science, University of New South Wales, Kensington, Australia. c.mcmonnies@unsw.edu.au
Abstract
PURPOSE: To examine corneal responses to elevated intraocular pressure (IOP). METHODS: For a sample of 10 normal subjects, noncontact tonometry was used to measure IOP elevations in response to scleral indentation from a standardized ophthalmodynamometer (ODM) force. Using the same ODM force, corneal topography was assessed for the same controls and a sample of 10 subjects with keratoconus (KC). It was assumed that the mean and range of IOP elevations were similar for both samples. RESULTS: The ODM induced a mean IOP elevation for the control eyes of 99.4%. IOP elevation during topography was 15-20 seconds for both samples. With elevated IOP, there were no significant topographical changes for control subjects, but the mean values for steepest point of curvature and flat and steep simulated keratometry were significantly increased in subjects with KC [+1.84 (P < 0.029), +0.64 (P = 0.046), and +1.31 diopters (D) (P = 0.03), respectively]. The changes were significantly greater in subjects less than 30 years (P < 0.05). There were no significant topography changes from baseline after IOP elevation, for either control or KC samples. CONCLUSIONS: Abnormal elastic (reversible) increased distensibility in some KC corneas is consistent with reduced corneal rigidity (lower elastic modulus and/or thickness). Abnormal distending responses may be increased when IOP elevations are higher and/or longer and/or more frequent. The results suggest that abnormal distending responses to elevated IOP in KC may reduce with age.
PURPOSE: To examine corneal responses to elevated intraocular pressure (IOP). METHODS: For a sample of 10 normal subjects, noncontact tonometry was used to measure IOP elevations in response to scleral indentation from a standardized ophthalmodynamometer (ODM) force. Using the same ODM force, corneal topography was assessed for the same controls and a sample of 10 subjects with keratoconus (KC). It was assumed that the mean and range of IOP elevations were similar for both samples. RESULTS: The ODM induced a mean IOP elevation for the control eyes of 99.4%. IOP elevation during topography was 15-20 seconds for both samples. With elevated IOP, there were no significant topographical changes for control subjects, but the mean values for steepest point of curvature and flat and steep simulated keratometry were significantly increased in subjects with KC [+1.84 (P < 0.029), +0.64 (P = 0.046), and +1.31 diopters (D) (P = 0.03), respectively]. The changes were significantly greater in subjects less than 30 years (P < 0.05). There were no significant topography changes from baseline after IOP elevation, for either control or KC samples. CONCLUSIONS: Abnormal elastic (reversible) increased distensibility in some KC corneas is consistent with reduced corneal rigidity (lower elastic modulus and/or thickness). Abnormal distending responses may be increased when IOP elevations are higher and/or longer and/or more frequent. The results suggest that abnormal distending responses to elevated IOP in KC may reduce with age.
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