The Effect of Corneal Anatomical Indices on Measurements of Intraocular Pressure by Applanation Tonometry, Pneumotonometer and TonoPen in Patients with Open-Angle Glaucoma

Abstract

Abstract: : Purpose: This study evaluates the effect of corneal anatomical parameters on IOP as measured by the Goldmann applanation tonometer, pneumotonometer and Tono Pen, in a sample of glaucoma patients with normal tension glaucoma(NTG), open angle glaucoma(OAG) and ocular hypertension(OHT). Methods: 100 eyes with OHT(n=18), OAG(n=58) or NTG(n=24) with no other ocular or systemic condition known to affect the corneal anatomy were enrolled. IOP was measured using Goldmann applanation tonometer, pneumotonometer and TonoPen in random order. Central and peripheral corneal thickness was measured by ultrasonic pachymetry (25 megahertz trasducer) and the radius of curvature was measured by autokeratometry. Central corneal thickness(CCT) was correlated with the type of glaucoma and measurement technique. Results: CCT ranged from 481µm to 651µm. Using a partial correlation analysis, controlling for topical and systemic IOP lowering drugs, IOP measurements on Goldman applanation tonometry (r²=0.20), Tono Pen (r²=0.22) and pneumotonometer (r²=0.11) correlated significantly with the central corneal thickness (p<0.01). Peripheral corneal thickness also correlated significantly with IOP measurements on Goldmann applanation tonometer (r²=0.29), Tono Pen (r²=0.33) and pneumotonometer (r²=0.19)(p<0.01). There was no significant correlation between IOP measurements with any of the methods and the corneal radius of curvature. In a subgroup analysis based on CCT, all three measurement techniques showed a significant (p<0.05) linear correlation between CCT and IOP in cases in the 1^st(481-538µm) and 4^th quartile (575-651µm). However, IOP as measured by any technique showed no correlation with CCT in the 2^nd(538-557µm) and 3^rd quartiles(557-575 µm) of CCT in the sample population. Also, the mean CCT in NTG patients (532+36µm) was significantly (p<0.01) lower than that in OAG(558+29µm) or OHT patients (577+34µm) . The difference in CCT between OAG and OHT was also statistically significant (p=0.04). Conclusions: IOP measurement was minimally affected by CCT in range of 538-575 um. However, at the extremes of corneal thickness (1^st & 4^th quartiles), measurement of IOP using was significantly affected by CCT using any of the techniques. Over the entire population, measurement of IOP by pneumotonometer was the least affected by CCT. NTG eyes tend to have thinner corneas and OHT eyes tend to have thicker corneas.

Keywords: intraocular pressure