Comparison of the intraocular pressure measurements with the tono-pen and the goldmann applanation tonometer and the effect of central corneal thickness on measurements

Abstract

Purpose: Incorrect values of applanation tonometers according to varying corneal thickness were investigated many times. In this study, we aimed to compare intraocular pressure (IOP) measurements of the Tono-Pen and Goldmann applanation tonometer (GAT) in eyes with normal corneas of various thicknesses. Material and Methods: IOP was measured with Tono-Pen and GAT respectively in 255 patients’ eyes with normal corneas. Only the right eyes were analyzed for statistical purposes. Central corneal thickness (CCT) was measured using an ultrasound pachymeter after all IOP determinations had been made. Results: With both instruments, IOP varied with CCT. Readings with Tono-Pen showed a mean increase in IOP with increasing CCT of 0,18 mmHg/10μm and an increase of 0,16 mmHg/10 μm with the GAT. The Tono-Pen consistently recorded comparatively higher IOPs than the GAT (p<0,05). Conclusion: In our study, the difference between the Tono-Pen and GAT measurements is statistically insignifi cant and 60% of this difference is within the 1 mmHg range. Although it is thought that Tono-Pen may be less affected by corneal thickness due to the measurement from a smaller area we did not obtain data that Tono-Pen is less affected by corneal thickness as a result of linear regression analysis.The Tono-Pen is more affected by CCT when used to measure IOP in eyes with normal corneas. This is contrary to expectations, based on the theory that TonoPen is least affected by the CCT because it needs smaller applanation area.