Purpose: To compare the intraocular pressure (IOP) measured by Non-Contact tonometer (NCT) and Goldmann Applanation tonometer (GAT) and their correlation with central corneal thickness (CCT) as well as the reliability of each tonometer.

Methods: 500 patients (1000 eyes) attending the ophthalmology OPD aged 18 years and above were taken up for the study. Patients with anterior and posterior segment pathologies like corneal ulcer, leukoma, staphyloma, corneal lacerations, ectatic corneal conditions, corneal dystrophies, oedema, perforations, acute angle closure glaucoma, retinal detachments, vitreous haemorrhage and unwilling patients were excluded from the study. IOP was recorded using NCT and GAT after assessing the patient’s visual acuity. Following IOP measurement, central corneal thickness (CCT) value of each patient was measured using pachymetry. All the data were collected and tabulated for statistical analysis to obtain results.

Results:  In the present study there were 256 males (51.2%) and 244 females (48.8%). The mean CCT in males was 0.5350 mm and in females 0.5340 mm respectively. The mean IOP measured by NCT is 16.43 mm hg whereas the mean IOP measured by GAT is 15.43 mm hg. IOP measured by NCT is significantly higher than the IOP measured by GAT (p<0.001). When correlated with CCT all the tonometers show significant correlation with GAT showing the strongest significant correlation. NCT overestimates IOP in normal, thin and thicker corneas when compared to GAT and are statistically significant.

Conclusion: From the present study we can conclude that IOP measured by NCT is higher than GAT. NCT values are significantly higher than GAT values in thin and normal corneas whereas it overestimates more in thicker corneas. All the tonometers show significant correlation with CCT with GAT showing the strongest significant correlation in this study. So, it is always advisable to measure the corrected IOP for each and every patient after taking into account the CCT of that particular person.

Keywords: Non-contact tonometer, Goldmann applanation tonometer, central corneal thickness, Intraocular pressure.

1. Gunvant P, Baskaran M, Vijaya L, Joseph IS, Watkins RJ, Nallapothula M, Broadway DC, O'Leary DJ. Effect of corneal parameters on measurements using the pulsatile ocular blood flow tonograph and Goldmann applanation tonometer. Br J Ophthalmol 2004; 88(4):518-522.
2. Mark HH. Corneal curvature in applanation tonometry. Am J Ophthalmol 1973; 76(2):223-224.
3. Chakrabarty L. Goldmann applanation tonometry versus non-contact tonometry: a comparative study. Int J Res Med Sci 2016; 4:4683-7.
4. Shih CY, Graff-Zivin JS, Trokel SL, Tsai JC: Clinical significance of central corneal thickness in the management of glaucoma. Arch Ophthalmol 2004, 122:1270-1275.
5. Ehlers N, Bramsen T, Sperling S: Applanation tonometry and central corneal thickness. Acta Ophthalmol (Copen) 1975; 53:34-43.
6. Ismail AR, Lamont M, Perera S. Comparison of IOP measurement using GAT and DCT in patients with penetrating keratoplasties. Br J Ophthalmol. 2007; 91:980-981.
7. Kirwan C, O’Keefe M. Measurement of intraocular pressure LASIK and LASEK patients using the Reichert Ocular Response Analyzer and Goldmann applanation tonometry. J Refract Surg. 2008; 24:366-70.
8. Milla E, Duch S, Buchacra O, Masuet C. Poor agreement between Goldmann and Pascal tonometry in eyes with extreme pachymetry. Eye 2008; 28.
9. Francis BA, Hsieh A, Lai MY, Chopra V, Pena F, Azen S, et al., Los Angeles Latino Eye Study Group. Effects of corneal thickness, corneal curvature, and intraocular pressure level on Goldmann applanation tonometry and dynamic contour tonometry. Ophthalmology 2007; 114: 20–26.
10. Medeiros FA, Sample PA, Weinreb RN. Comparison of dynamic contour tonometry and Goldmann applanation tonometry in African-American subjects. Ophthalmology 2007; 114: 658–665.
11. Medeiros FA, Weinreb RN. Evaluation of the influence of corneal biomechanical properties on intraocular pressure measurements using the ocular response analyzer. J Glaucoma 2006; 15: 364–370.
12. Swathi Nagarajan; Veerabahu Velayutham; G Ezhumalai. Comparative evaluation of applanation and indentation tonometers in a community ophthalmology setting in southern India. Saudi Journal of Ophthalmology (2016) 30:83-87.
13. Jorge J, Diaz-Rey JA, Gonzalez-Meijome JM, Almeida JB, Parafita MA. Clinical performance of the Reichert AT 550: new noncontact tonometer. Ophthalmic Physiol Opt. 2002; 22:560-4.
14. Domke N, HagerA & Wiegand W (2006): Intraocular pressure and corneal thickness. A comparison between non-contact tonometry and applanation tonometry Oph-thalmologe103: 583–587.
15. Tonnu PA, Ho T, Newson T, El Sheikh A, Sharma K, White E et al. The influence of central corneal thickness and age on intraocular pressure measured by pneumotonometry, non-contact tonometry, the Tono-Pen XL, and Goldmann applanation tonometry. Br J Ophthalmol 2005; 89: 851–854.
16. Parker VA, Herrtage J, Sarkies NJ. Clinical comparison of Keeler Pulsair 3000 with Goldmann applanation tonometry. Br J Ophthalmol. 2001; 85:1301-4.
17. Farhood QK. Comparative evaluation of intraocular pressure with an air-puff tonometer versus a Goldmann applanation tonometer. Clin Ophthalmol. 2013; 7:23–27.
18. Yilmaz I, Altan C, Aygit ED, Alagoz C, Baz O, Ahmet S, Urvasizoglu S, Yasa D, Demirok A. Comparison of three methods of tonometry in normal subjects: Goldmann applanation tonometer, non-contact airpuff tonometer, and Tono-pen XL. Clin Ophthalmol. 2014; 8:1069–74.
19. Loewen NA, Liu JH, Weinreb RN. Increased 24-hour variation of human intraocular pressure with short axial length. Invest Ophthalmol Vis Sci. 2010; 51(2):933±7.
20. Lee SY, Bae HW, Kwon HJ, Seong GJ, Kim CY (2018) Utility of Goldmann applanation tonometry for monitoring intraocular pressure in glaucoma patients with a history of laser refractory surgery. PLoS ONE 13(2): e0192344.
21. S-Y Hsu, M-M Sheu, A-H Hsu, K-Y Wu, J-I Yeh, J-N Tien and R-K Tsai. Comparisons of intraocular pressure measurements: Goldmann applanation tonometry, noncontact tonometry, Tono-Pen tonometry, and dynamic contour tonometry. Eye (2009) 23, 1582–1588.
22. Feng, Chi Shian. Comparison of Intraocular Pressure Measurements Obtained by Rebound, Noncontact, and Goldmann Applanation Tonometry in Children. American Journal of Ophthalmology, Volume 160; Issue 5: 937 - 943.
23. Cook JA, Botello AP, Elders A, Fathi Ali A, Azuara-Blanco A, Fraser C, et al. Systematic review of the agreement of tonometers with Goldmann applanation tonometry. Ophthalmology. 2012; 119:1552–7.
24. Vernon SA. Reproducibility of Keeler Pulsair 2000 non-contact tonometer. Br J Ophthalmol.1995; 79: 554–557.
25. Lafaut AS, Van Malderen L & Zeyen T. Is pulse synchronized pneumotonometry more reproducible than routine pneumotonometry and more in agreement with Goldmann applanation tonometry? Eur J Ophthalmol.2007; 17: 178–182.
26. Ogbuehi KC & Almubrad TM. Accuracy and reliability of the Keeler Pulsair EasyEye non-contact tonometer. Optom Vis Sci.2008; 85: 61–66.
27. Ogbuehi KC. Assessment of the accuracy and reliability of the Topcon CT80 non-contact tonometer. Clin Exp Optom.2008; 89: 310–314.
28. Grieshaber MC, Schoetzau A, Zawinka C. Effect of central corneal thickness on dynamic contour tonometry and Goldmann applanation tonometry in primary open-angle glaucoma. Arch Ophthalmol 2007; 125:740–744.
29. Kamppeter BA, Jonas JB. Dynamic contour tonometry for intraocular pressure measurement. Am J Ophthalmol 2005; 140:318–320.
30. Kaufmann C, Bachmann LM, Thiel MA. Comparison of dynamic contour tonometry with Goldmann applanation tonometry. Invest Ophthalmol Vis Sci 2004; 45:3118–3121.
31. Schneider E, Grehn F. Intraocular pressure measurement-comparison of dynamic contour tonometry and Goldmann applanation tonometry. J Glaucoma 2006; 15:2
32. Kniestedt C, Lin S, Choe J. Clinical comparison of contour and applanation tonometry and their relationship to pachymetry. Arch Ophthalmol 2005; 123:1532–1537.
33. Ku JY, Danesh-Meyer HV, Craig JP. Comparison of intraocular pressure measured by Pascal dynamic contour tonometry and Goldmann applanation tonometry. Eye 2006; 20:191–198.
34. Harada, N. Hirose, T. Kubota, A. Tawara. The Influence of Central Corneal Thickness and Corneal Curvature on Intraocular Pressure Measured by Different Tonometers; Non–Contact and Goldmann Applanation Tonometers. Invest. Ophthalmol. Vis. Sci. 2006;47(13):4457.
35. Punit Singh, Raghunandan Kothari, Himadri Patel. The influence of central corneal thickness on intraocular pressure, measured by different tonometers: Noncontact and Goldmann applanation tonometers. National journal of medical research. Volume 7(1): Jan – Mar 2017.
36. Babalola OE, Kehinde AV, Iloegbunam AC, Akinbinu T, Moghalu C, Onuoha I. A comparison of the Goldmann applanation and non-contact (Keeler Pulsair EasyEye) tonometers and the effect of central corneal thickness in indigenous African eyes. Ophthalmic Physiol Opt. 2009;29(2):182-188.
37. Behrooz Kouchaki, Hassan Hashemi, Abbasali Yekta, Mehdi khabazkhoob. Comparison of current tonometry techniques in measurement of intraocular pressure. Journal of Current Ophthalmology.2017; 29:92-97.

Dr Santanu Das

MS Ophthalmology, Department of Ophthalmology, Kempegowda Institute of Medical Sciences, Bangalore