Reliability of Intraocular Pressure Measurement by Goldmann Applanation Tonometry After Refractive Surgery: A Review of Different Correction Formulas
Received 28 May 2020
Accepted for publication 27 July 2020
Published 24 September 2020 Volume 2020:14 Pages 2783—2788
Checked for plagiarism Yes
Review by Single anonymous peer review
Peer reviewer comments 2
Editor who approved publication: Dr Scott Fraser
Maddalena De Bernardo, Giovanni Cembalo, Nicola Rosa
Department of Medicine, Surgery and Dentistry, “Scuola Medica Salernitana”, University of Salerno, Baronissi, Salerno, Italy
Correspondence: Maddalena De Bernardo
Department of Medicine, Surgery and Dentistry, “Scuola Medica Salernitana”, University of Salerno, Via Salvador Allende, Baronissi 84081, SA, Italy
Tel +39089 965063
Fax +39 089672407
Email [email protected]
Abstract: Myopia is one of the main risk factors for the onset of open-angle glaucoma. One of the first steps to assess glaucoma occurrence is the measurement of the intraocular pressure (IOP) by the Goldmann applanation tonometry (GAT). Even if this device is considered to be the gold standard for such measurements, it is affected by several sources of errors. Among these, there are the corneal thickness and curvature, both modified by corneal refractive surgery (CRS), that nowadays has become a very popular method to treat refractive errors. Indeed, CRS, by modifying the corneal shape and structure, causes an underestimation of the IOP measurements. In the literature, several IOP correction formulas to utilize with different devices have been proposed to overcome this problem. This paper aims to review the various correction formulas applied to the GAT in the attempt to improve the reliability of this measurement.
Keywords: IOP measurement, corneal refractive surgery, GAT, laser in situ keratomileusis, photorefractive keratectomy
A Letter to the Editor has been published for this article.
A Response to Letter has been published for this article.
An accurate evaluation of intraocular pressure (IOP) is very important both for an early diagnosis of glaucoma and to follow the evolution of the disease over the years.1
It has been demonstrated that even a difference of 1 mmHg can be relevant in the progression of this disease, especially in myopic patients, as the refractive error is one of the established risk factors for open angle glaucoma.2
Goldmann applanation tonometry (GAT) is currently considered the gold standard in measuring the IOP.3 Several other instruments have been developed for such purpose and, to be validated, they all have been compared to GAT. Unfortunately, GAT is not very precise; in fact, it underestimates the IOP value, mainly when corneas are thinner, but also to a lesser extend when they are flatter or edematous.4–9
Corneal refractive surgery (CRS) techniques, such as Photorefractive Keratectomy (PRK), laser in situ keratomileusis (LASIK) or laser epithelial keratomileusis (LASEK), modifying corneal biomechanics10 making corneas thinner and flatter.11–14 Probably the most important effect of CRS on the cornea being its change in the buckling stress and stress redistribution under applanation.15 For this reason, even using the GAT, a value lower than real will be obtained, and if one of these patients will develop glaucoma, the diagnosis can be delayed.16,17
These methods can be divided in:
Methods based on knowledge of the patients clinical history
No history-based methods
Methods Based on Knowledge of the Patients’ Clinical History
Knowledge of Preoperative IOP
Chihara et al18 evaluated 93 right eyes of 93 patients that underwent LASIK. IOP was measured preoperatively and 3 months after surgery with GAT and air puff tonometer (Topcon CT90A); analyzing their data, authors elaborated a formula to obtain a correct IOP (IOPc) based on preoperative IOP (IOPpre):
Change in IOP (ΔIOP) = −6.455 + (0.596 x IOPpre).
The final formula is:
IOPc = postoperative IOP (IOPpost) + ΔIOP.
Knowledge of IOPpre, Ablation Depth and Age
Rashad & Bahnassy19 performed a LASIK in 166 eyes of 93 patients with a mean age of 30.6 ± 8.0 years. Preoperatively and 12 months after surgery, IOP and Central Corneal Thickness (CCT) were measured, respectively, with GAT and ultrasound pachymeter (Teknar Ophthasonic pachymeter, Teknar Corporation, St. Louis, MO); they found a correlation between preoperative IOP, change in CCT (ΔCCT) and patient age in years and developed a multiple linear regression analysis:
IOPpost = 0.987 + 0.627 x IOPpre + 0.0143 x ΔCCT + 0.03044 x age
Knowledge of Ablation Depth
Munger et al20 evaluated 481 eyes of 381 patients with a mean preoperative refractive error of –6,50 that underwent PRK. Twenty-four months after surgery, IOP was measured with GAT and CCT with Mentor Advent ultrasound pachymeter; they found a correlation between change in IOP and ΔCCT and proposed the following correction formula:
IOPc = IOPpost + 2.1 x ΔCCT/100
Emara et al21 studied 85 eyes of 50 myopic patients evaluating IOP with GAT and CCT with Sonogage Corneo-Gage Pulse 2 pachymeter before undergoing LASIK 3 months after. They found a correlation between decrease in IOP and ablation depth. The post-LASIK slope was 0.027 mmHg/µm or a decrease of 1 mmHg per 37.8 µm reduction in CCT:
IOPc = IOPpost + ΔCCT/37.8
Duch et al22 evaluated 118 eyes of 60 patients that underwent LASIK. Before and 3 months after surgery, they measured IOP with GAT and contact Pneumotonometer (Mentor Modular One Pneumotonometer, Mentor O & O), CCT with ultrasound pachymeter (DGH 2000, DGH Technology, Frazer, PA). They found a correlation between ΔIOP and ΔCCT with a mean decrease of 2.9 mmHg per 70µm of reduction in CCT; they exposed also a correlation between ΔIOP and ΔCCT with the following formula:
ΔIOP = 1.59 + 0.019 x Δ CCT;
Then the final formula developed is:
IOPc = IOP post + Δ IOP
Knowledge of Ablation Depth and Effective Treatment
Rosa et al.23 Studied 87 eyes of 87 patients, with a mean dioptric error of −7.6 ± 4.1, that underwent PRK and the fellow eyes were used as controls; they measured IOP with GAT 12 months after PRK. They developed a correction formula for IOP based on IOPpost, ablated cornea (A) and effective treatment in diopters (D):
IOPc = IOPpost + (0.025 x A) + (0.34 x D)
Knowledge of Ablation Depth and Preoperative K Readings
Svedberg et al24 evaluated 40 eyes in 40 patients divided into 2 groups of 20 eyes, 1 with 20 eyes that underwent PRK/LASEK and 1 with 20 eyes who underwent LASIK. IOP was assessed with GAT, while CCT and K values with Orbscan. The authors did not specify when the postoperative measurement was performed. They developed two correction formulas, taking into account their measurement and the correlation between ΔIOP, ΔCCT and differences in corneal power (ΔK):
Δ IOP = 2.765–0.001 x ΔCCT - 0.424 x Δ K (PRK/LASEK)
Δ IOP = 4.340 + 0.018 x ΔCCT - 0.440 x Δ K (LASIK)
In the end the final formula is:
IOPc = IOPpost + Δ IOP
No History-Based Methods
Kohlhaas et al25 examined 101 eyes of 59 patients before and 6 months after LASIK; IOP was measured with GAT, CCT with ultrasound pachymeter (Ultrasonic Pachymeter DGH-500 Pachette; TechnoMed Inc, Wallace, NC) and K values with topography (TechnoMed); evaluating their data, authors found a correlation between IOP, CCT and postoperative K values (k post) and derived a correction formula:
IOPc = IOP post + (540 – CCT post)/71 + (43 – K post)/2.7 + 0.75 mmHg
The IOP is routinely measured during the ophthalmological examination; however, the finding of low IOP values could be misleading and could delay the diagnosis of glaucoma, mainly in myopic eyes where the optic disc examination and the visual field can be difficult to evaluate.
Among these, there is an IOP underestimation. In the attempt to find a reliable method in measuring the IOP after CRS, several devices have been tested, such as pneumotonometer, Tono Pen (TP), non-contact tonometer (NCT), such as TonoPachymeter, dynamic contour tonometer (DCT), Ocular Response Analyzer (ORA), Corvis ST and CATS Tonometer and only the last four has been proven to be as reliable as GAT.42–52
Schipper,26 to rehabilitate the GAT, in 2000 suggested to measure the IOP with GAT and TP in the corneal periphery because they found a higher IOP of 1.8–2.3 mmHg in the corneal periphery compared to the values measured in the central corneal region.
This proposal was supported by Abbasoglu et al43 that, utilizing GAT and Pneumotonometer, noticed an underestimation by GAT of a mean of 2.40 mmHg in central corneal region; this underestimation was not present with evaluations by Pneumotonometer neither in central corneal nor in the peripheral cornea.
Nevertheless, these findings were not confirmed by other studies:
Zadok et al53 found no statistically significant difference between pre and post-operative evaluations in central IOP versus peripheral IOP measured by pneumotonometry.
Rashad & Bahnassy19 in their study concluded that there was no difference between central corneal IOP measurements and peripheral corneal measurements both before and after surgery.
Park et al54 found that the IOP measurements in the nasal cornea region were statistically lower than at baseline, although the drop in nasal cornea was not so high as the IOP difference in central cornea between preoperative and postoperative evaluations.
The problem, that was born in the years and has made Schipper’s proposal difficult to apply, is that corneal ablations were initially made with a diameter of 3–4 mm but currently, to get less regression and less night vision problems, the ablation can reach the 7–9 mm diameter.
Therefore, it is almost impossible to find, in the periphery, a non-treated corneal area, considering that the GAT tip is about 3 mm in diameter.
Most of the authors tried to overcome the problem, describing correcting factors to apply to IOP readings. These correcting factors are related to different data such as: ΔCCT alone;20–22 or to the IOPpre, the ΔCCT and the age of the patient;19 or to the A and D;23 or only to the IOP value measured before surgery;18 or to the ΔCCT and the ΔK24 or to the CCT and the Kpost.25
Mardelli et al tried to explain the IOP reduction post CRS and found no correlation between ΔIOP and ΔCCT or keratometry.55 They related this loss of correlation with the removal of the Bowman’s membrane that causes the production of new collagen tissue, affecting corneal resistance to applanation. Their study, however, is based on small ablated areas in which the pachymetric difference before and after surgery does not exceed the average value of 23 μ.
Montes-Micò et al56 disagree with the findings of Mardelli et al because they found the same difference post PRK and post LASIK (where the Bowmann’s layer is not ablated) concluding that Bowman’s membrane does not play a significant role in corneal rigidity. They agreed with a study by Patel and Aslanides57 that explains the reduction in IOP after PRK based on a general softening of the cornea during healing process.
De Bernardo et al tested several of these formulas in 121 eyes of patients that underwent PRK and concluded that the best results were obtained by using Rashad, Chihara and Rosa formulas (R2 = 0.8593, <0.001; R2 = 0.5389, 0.128; R2 =0.2489, <0.001, respectively).58
The reliability of these studies is influenced by several factors:
1. The real IOP is unknown, because the direct measurement, for ethical reasons, can be realized only during cataract surgery.
2. It had been shown that if single tonometry readings are used by the same observer, they must differ by at least 4.5 mmHg before it can be said that the IOP is different; for variations of smaller magnitude, there is only a 10% chance that this difference is due to a change alone.59
3. Whitacre and Stein recommended not to ascribe any clinical significance to changes in the IOP measured across two different measurement sessions that were smaller than 2–3 mmHg.4
4. In most of these studies, the modified IOP was compared with preoperative values and it is not sure that no change in real IOP is present after 6 months.
Regrettably, to date, it is impossible to measure the real IOP, but, on the other hand, several authors who published a study on this topic utilized the preoperative values to test the reliability of the proposed formulas.18–24
It is said that when there are too many ways to solve a problem it means that none of them is reliable. We think the real problem is that so far, as it is evident from the above, only a few methods have been tested in a sufficient number of patients, while most of them are just theoretical and have been verified in few patients. The other problem is that several methods are clinical history based and, unfortunately, in most of the patients the preoperative keratometry values and the exact refractive treatment are not available, so we can conclude that nowadays there are not reliable and immediate methods to apply to assess IOP post corneal refractive surgery.
Pending further studies comparing the IOP measurements with real IOP, the suggestion of the present study is that further studies are necessary to establish a method that could make GAT measurements reliable in patients that underwent CRS.
The authors report no conflicts of interest for this work.
1. Goldmann H, Schmidt T. Further contribution to applanation tonometry. Ophthalmologica. 1961;141(6):441–456. doi:10.1159/000304099
2. Chihara E, Liu X, Dong J, et al. Severe myopia as a risk factor for progressive visual field loss in primary open glaucoma. Ophthalmologica. 1997;211(2):66–71. doi:10.1159/000310760
3. Goldmann H, Schmidt T. Applanation tonometry. Ophthalmologica. 1957;134(4):221–242. doi:10.1159/000303213
4. Whitacre MM, Stein R. Sources of error with use of Goldmann type tonometers. Surv Ophthalmol. 1993;38(1):1–30. doi:10.1016/0039-6257(93)90053-a
5. Rosa N, De Bernardo M. Central corneal thickness determination in corneal edema. Graefes Arch Clin Exp Ophthalmol. 2017;255(6):1251–1252. doi:10.1007/s00417-017-3634-9
6. De Bernardo M, Rosa N. Central corneal thickness measurement with different devices in keratoconic patients. J Curr Ophthalmol. 2017;30(1):97. doi:10.1016/j.joco.2017.11.005
7. De Bernardo M, Rosa N. Evaluation of Goldmann applanation tonometry, rebound tonometry and dynamic contour tonometry in keratoconus. J Optom. 2018;11(2):130–131. doi:10.1016/j.optom.2017.10.001
8. Yu AY,Duan SF, Zhao YE, et al. Correlation between corneal biomechanical properties, applanation tonometry and direct intracameral tonometry. Br J Ophthalmol. 2012;96(5):640–644. doi:10.1136/bjophthalmol-2011-300124
9. McCafferty S, Levine J, Schwiegerling J, Enikov ET. Goldmann applanation tonometry error relative to true intracameral intraocular pressure in vitro and in vivo. BMC Ophthalmol. 2017;17(1):215. doi:10.1186/s12886-017-0608-y
10. De Bernardo M, Cornetta P, Marotta G, Rosa N. A prospective longitudinal study to investigate corneal hysteresis as a risk factor for predicting development of glaucoma. Am J Ophthalmol. 2018;195:243. doi:10.1016/j.ajo.2018.04.030
11. De Bernardo M, Rosa N. Intraocular pressure after myopic photorefractive keratectomy. J Ophthalmic Vis Res. 2018;13(4):520. doi:10.4103/jovr.jovr_209_17
12. De Bernardo M, Rosa N. Corneal thickness measurement after hyperopic photorefractive keratectomy. J Ophthalmic Vis Res. 2018;13(3):365. doi:10.4103/jovr.jovr_217_17
13. De Bernardo M, Rosa N. Intraocular pressure after LASEK. Graefes Arch Clin Exp Ophthalmol. 2018;256(10):2009–2010. doi:10.1007/s00417-018-4047-0
14. De Bernardo M, Rosa N. Intraocular pressure after refractive surgery. J Glaucoma. 2017;26(6):e196. doi:10.1097/IJG.0000000000000676
15. McCafferty SJ, Schwiegerling JT, Enikov ET. Corneal surface asphericity, roughness, and transverse contraction after uniform scanning excimer laser ablation. Invest Ophthalmol Vis Sci. 2012;53(3):1296–1305. doi:10.1167/iovs.11-9267
16. Faucher A, Grégoire J, Blondeau P. Accuracy of Goldmann tonometry after refractive surgery. J Cataract Refract Surg. 1997;23(6):832–838. doi:10.1016/s0886-3350(97)80239-8
17. De Bernardo M, Salzano FA, Rosa N. Steroid-induced ocular hypertension after photorefractive keratectomy. J Cataract Refract Surg. 2018;44:118. doi:10.1016/j.jcrs.2017.10.048
18. Chihara E, Takahashi H, Okazaki K, Park M, Tanito M. The preoperative intraocular pressure level predicts the amount of underestimated intraocular pressure after LASIK for myopia. Br J Ophthalmol. 2005;89(2):160–164. doi:10.1136/bjo.2004.048074
19. Rashad KM, Bahnassy KM. Changes in intraocular pressure after laser in situ keratomileusis. Refract Surg. 2001;17:420–427.
20. Munger R, Hodge WG, Mintsioulis G, Agatipos PJ, Jackson WB, Damji KF. Correction of intraocular pressure for changes in central corneal thickness following photorefractive keratectomy. Can J Ophthalmol. 1998;33:159–165.
21. Emara B, Probst LE, Tingey DP, Kennedy DW, Willms LJ, Machat J. Correlation of intraocular pressure and central corneal thickness in normal myopic eyes and after laser in situ keratomileusis. J Cataract Refract Surg. 1998;24(10):1320–1325. doi:10.1016/s0886-3350(98)80222-8
22. Duch S, Serra A, Castanera J, Abos R, Quintana M. Tonometry after laser in situ keratomileusis treatment. J Glaucoma. 2001;10(4):261–265. doi:10.1097/00061198-200108000-00003
23. Rosa N, Cennamo G, Breve MA, La Rana A. Goldmann applanation tonometry after myopic photorefractive keratectomy. Acta Ophthalmol Scand. 1998;76:550–554. doi:10.1034/j.1600-0420.1998.760508.x
24. Svedberg H, Chen E, Hamberg-Nyström H. Changes in corneal thickness and curvature after different excimer laser photorefractive procedures and their impact on intraocular pressure measurements. Graefes Arch Clin Exp Ophthalmol. 2005;243(12):1218–1220. doi:10.1007/s00417-005-0072-x
25. Kohlhaas M, Spoerl M, Boehm AG, Pollack K. A correction formula for the real intraocular pressure after LASIK for the correction of myopic astigmatism. J Refract Surg. 2006;22(3):263–267. doi:10.3928/1081-597X-20060301-11
26. Schipper I, Senn P, Oyo-Szerenyi K, Peter R. Central and peripheral pressure measurements with the Goldmann tonometer and tono-pen after photorefractive keratectomy for myopia. J Cataract Refract Surg. 2000;26(6):929–933. doi:10.1016/s0886-3350(99)00461-7
27. Schallhorn JM, Schallhorn SC, Ou Y. Factors that influence intraocular pressure changes after myopic and hyperopic LASIK and photorefractive keratectomy: a large population study. Ophthalmology. 2015;122(3):471–479. doi:10.1016/j.ophtha.2014.09.033
28. Recep OF, Cağil N, Hasiripi H. Correlation between intraocular pressure and corneal stromal thickness after laser in situ keratomileusis. J Cataract Refract Surg. 2000;26(10):1480–1483. doi:10.1016/s0886-3350(00)00546-0
29. Doughty MJ, Zaman ML. Human corneal thickness and its impact on intraocular pressure measures: a review and meta-analysis approach. Surv Ophthalmol. 2000;44:367–408. doi:10.1016/s0039-6257(00)00110-7
30. Siganos DS, Papastergiou GI, Moedas C. Assessment of the pascal dynamic contour tonometer in monitoring intraocular pressure in unoperated eyes and eyes after LASIK. J Cataract Refract Surg. 2004;30(4):746–751. doi:10.1016/j.jcrs.2003.12.033
31. Pepose JS, Feigenbaum SK, Qazi MA, Sanderson JP, Roberts CJ. Changes in corneal biomechanics and intraocular pressure following LASIK using static, dynamic, and noncontact tonometry. Am J Ophthalmol. 2007;143(1):39–47. doi:10.1016/j.ajo.2006.09.036
32. Chihara E. Assessment of true intraocular pressure: the gap between theory and practical data. Surv Ophthalmol. 2008;53(3):203–218. doi:10.1016/j.survophthal.2008.02.005
33. Li H, Wang Y, Dou R, et al. Intraocular pressure changes and relationship with corneal biomechanics after SMILE and FS-LASIK. Invest Ophthalmol Vis Sci. 2016;57(10):4180–4186. doi:10.1167/iovs.16-19615
34. Lin MY, Chang DC, Shen YD, Lin YK, Lin CP, Wang IJ. Factors influencing intraocular pressure changes after laser in situ keratomileusis with flaps created by femtosecond laser or mechanical microkeratome. PLoS One. 2016;11(1):e0147699. doi:10.1371/journal.pone.0147699
35. Jethani J, Dave P, Jethani M, Desai Y, Patel P. The applicability of correction factor for corneal thickness on non-contact tonometer measured intraocular pressure in LASIK treated eyes. Saudi J Ophthalmol. 2016;30(1):25–28. doi:10.1016/j.sjopt.2015.11.001
36. Bahadir Kilavuzoglu AE, Bozkurt TK, Cosar CB, Sener AB. A sample predictive model for intraocular pressure following laser in situ keratomileusis for myopia and an “intraocular pressure constant”. Int Ophthalmol. 2018;38(4):1541–1547. doi:10.1007/s10792-017-0617-0
37. Rosa N, Cennamo G, Del Prete A, Pastena B, Sebastiani A. Effects on the corneal endothelium six months following photorefractive keratectomy. Ophthalmologica. 1995;209:17–20. doi:10.1159/000310568
38. Cennamo G, Rosa N, Del Prete A, Breve MA, Sebastiani A. The corneal endothelium 12 months after photorefractive keratectomy in high myopia. Acta Ophthalmol Scand. 1997;75(2):128–130. doi:10.1111/j.1600-0420.1997.tb00106.x
39. Rosa N, Cennamo G, Rinaldi M. Correlation between refractive and corneal topographic changes after photorefractive keratectomy for myopia. J Refract Surg. 2001;17:129–133.
40. De Bernardo M, Rosa N. Diehl-Miller nomogram for IOL power calculation. J Cataract Refract Surg. 2013;39(11):1791. doi:10.1016/j.jcrs.2013.08.040
41. Rosa N, Iura A, Romano M, Verolino G, Romano A. Correlation between automated and subjective refraction before and after photorefractive keratectomy. J Refract Surg. 2002;18:449–453.
42. Garzozi HJ, Chung HS, Lang Y, Kagemann L, Harris A. Intraocular pressure and photo-refractive keratectomy: a comparison of three different tonometers. Cornea. 2001;20(1):33–36. doi:10.1097/00003226-200101000-00006
43. Abbasoglu OE, Bowman RW, Cavanagh HD, McCulley JP. Reliability of intraocular pressure measurements after myopic excimer photorefractive keratectomy. Ophthalmology. 1998;105(12):2193–2196. doi:10.1016/S0161-6420(98)91215-5
44. Cennamo G, Rosa N, La Rana A, Bianco S, Sebastiani A. Non-contact tonometry in patients that underwent photorefractive keratectomy. Ophthalmologica. 1997;211(6):341–1343. doi:10.1159/000310825
45. Gimeno JA, Muñoz LA, Valenzuela LA, Moltò FJ, Rahhal MS. Influence of refraction on tonometric readings after photorefractive keratectomy and laser assisted in situ keratomileusis. Cornea. 2000;19(4):512–516. doi:10.1097/00003226-200007000-00022
46. Levy Y, Zadok D, Glovinsky Y, Krakowski D, Nemet P. Tono-pen versus Goldmann tonometry after excimer laser photorefractive keratectomy. J Cataract Refract Surg. 1999;25(4):486–491. doi:10.1016/s0886-3350(99)80044-3
47. Rosa N, De Bernardo M, Iaccarino S, Cennamo M. IOL power calculation; a challenging case. Optom Vis Sci. 2014;91:e29–31. doi:10.1097/OPX.0000000000000127
48. Chatterjee A, Shah S, Bessant DA, Naroo SA, Doyle SJ. Reduction in intraocular pressure after excimer laser photorefractive keratectomy. Correlation with pretreatment myopia. Ophthalmology. 1997;104(3):355–359. doi:10.1016/s0161-6420(97)30308-x
49. Yang CC, Wang IJ, Chang YC, Lin LL, Chen TH. A predictive model for postoperative intraocular pressure among patients undergoing laser in situ keratomileusis (LASIK). Am J Ophthalmol. 2006;141(3):530–536. doi:10.1016/j.ajo.2005.10.022
50. Riva I, Quaranta L, Russo A, Katsanos A, Rulli E, Floriani I. Dynamic contour tonometry and goldman applanation tonometry: correlation with intracameral assessment of intraocular pressure. Eur J Ophthalmol. 2012;22(1):55–62. doi:10.5301/ejo.5000067
51. Kotecha A, Elsheikh A, Roberts CR, Zhu H, Garway-Heath DF. Corneal thickness- and age-related biomechanical properties of the cornea measured with the ocular response analyzer. Invest Ophthalmol Vis Sci. 2006;4(12):5337–5347. doi:10.1167/iovs.06-0557
52. McCafferty SJ, Tetrault K, McColgin A, Chue W, Levine J, Muller M. Modified Goldmann prism intraocular pressure measurement accuracy and correlation to corneal biomechanical metrics: multicentre randomised clinical trial. Br J Ophthalmol. 2019;103:1840–1844. doi:10.1136/bjophthalmol-2018-313470
53. Zadok D, Tran DB, Twa M, Carpenter M, Schanzlin DJ. Pneumotonometry versus Goldmann tonometry after laser in situ keratomileusis for myopia. J Cataract Refract Surg. 1999;25(10):1344–1348. doi:10.1016/s0886-3350(99)00202-3
54. Park HJ, Uhm KB, Hong C. Reduction in intraocular pressure after laser in situ keratomileusis. J Cataract Refract Surg. 2001;27(2):303–309. doi:10.1016/s0886-3350(00)00782-3
55. Mardelli PG, Piebenga LW, Whitacre MM, Siegmund KD. The effect of excimer laser photorefractive keratectomy on intraocular pressure measurements using the Goldmann applanation tonometer. Ophthalmology. 1997;104(6):945–948. doi:10.1016/s0161-6420(97)30202-4
56. Montés-Micó R, Charman WN. Intraocular pressure after excimer laser myopic refractive surgery. Ophthalmic Physiol Opt. 2001;21(3):228–235. doi:10.1046/j.1475-1313.2001.00581.x
57. Patel S, Aslanides IM. Main causes of reduced intraocular pressure after excimer laser photorefractive keratectomy. J Refract Surg. 1996;12:673–674.
58. De Bernardo M, Capasso L, Caliendo L, Vosa Y, Rosa N. Intraocular pressure evaluation after myopic refractive surgery: a comparison of methods in 121 eyes. Semin Ophthalmol. 2016;31:233–242. doi:10.3109/08820538.2014.962156
59. Motolko MA, Feldman F, Hyde M, Hudy D. Sources of variability in the results of applanation tonometry. Can J Ophthalmol. 1982;17:93–95.
© 2020 The Author(s). This work is published and licensed by Dove Medical Press Limited. The full terms of this license are available at https://www.dovepress.com/terms.php and incorporate the Creative Commons Attribution - Non Commercial (unported, v3.0) License. By accessing the work you hereby accept the Terms. Non-commercial uses of the work are permitted without any further permission from Dove Medical Press Limited, provided the work is properly attributed. For permission for commercial use of this work, please see paragraphs 4.2 and 5 of our Terms.