Abstract
Introduction: Intraocular pressure (IOP) is one of the basic and most important investigations. Central corneal thickness influences IOP measured by various devices.
 Objective: In this study, we attempt to determine the agreement and influence of the central corneal thickness in the measurement of IOP obtained by Goldman applanation tonometer, Airpuff tonometer and tonopen.
 Methodology: A cross-sectional analytical study of Central corneal thickness (CCT) was done using Ultrasonic pachymetry. IOP was adjusted using Ehler's formula. Mean and the standard deviation was measured using the observed and predicted values for each instrument for its accuracy irrespective of the CCT.
 Results: 200 eyes of 100 patients were included in the study. Mean IOP measured was 16mmHg (SD 4). Tonopen was found to have closer observed values when compared with the predicted values to IOP obtained by Goldmann's applanation tonometer after CCT adjustments with Mean difference of 0.0134 mmHg with SD of 0.814. Air Puff tonometer was found to be the least accurate with Mean difference -2.08mmHg and SD of 4.704. Linear regression analysis also predicted that while the tonopen tend to underestimate the IOP levels by 5 %, Airpuff tonometer had a tendency to overestimate the IOP by 13%.(p<0.05).
 Conclusion: Tonopen had the greatest agreement and significant correlation with the GAT over a range of IOP and CCT and replicate measurements that are closest to the values obtained by using GAT after CCT adjustments. CCT adjustments may not even be required or has very little influence on IOP when using Tonopen.
Highlights
Glaucoma is a chronic and o en progressive op c neuropathy with typical structural and func onal changes in the op c nerve head
Tonopen was found to have closer observed values when compared with the predicted values to Intraocular pressure (IOP) obtained by Goldmann's applana on tonometer a er Central corneal thickness (CCT) adjustments with Mean difference of 0.0134mmHg with SD of 0.814
Tonopen had the greatest agreement and significant correla on with the GAT over a range of IOP and CCT and replicate measurements that are closest to the values obtained by using GAT a er CCT adjustments
Summary
Glaucoma is a chronic and o en progressive op c neuropathy with typical structural and func onal changes in the op c nerve head. Intraocular pressure (IOP) remains the one and an only adjustable risk factor for glaucoma .1-3. A prolonged eleva on in IOP o en results in irreversible damage to the re nal ganglion cells and postganglionic nerve fibers.[4,5] Elevated IOP is not the cause of all damage in POAG-but a major risk factor.[6,7,8] The issue of IOP, has been made controversial by corneal thickness- which is both a parameter that may cause inaccurate readings with applana on tonometry and an independent factor that may be predic ve of the risk of developing open-angle glaucoma.[9] The mechanism by which elevated IOP damages the op c nerve is not clear, but ischemia of the op c disc or nerve fiber layer, direct mechanical compression of axons, local toxicity, or some combina on of these has been implicated
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