ORIGINAL PAPER

Evaluation of intraocular pressure according to cornealthickness before and after excimer laser corneal ablationfor myopia

Shirin Hamed-Azzam • Daniel Briscoe •

Oren Tomkins • Raneen Shehedeh-Mashor •

Hanna Garzozi

Received: 18 November 2012 / Accepted: 14 December 2012 / Published online: 29 December 2012

� Springer Science+Business Media Dordrecht 2012

Abstract Intraocular pressure is affected by corneal

thickness and biomechanics. Following ablative cor-

neal refractive surgery, corneal structural changes

occur. The purpose of the study is to determine the

relationship between the mean central corneal thick-

ness (CCT) and the change in intraocular pressure

measurements following various corneal ablation

techniques, using different measurement methods.

Two hundred myopic eyes undergoing laser in situ

keratomileusis (LASIK) or photorefractive keratec-

tomy (PRK) were enrolled into a prospective, non-

randomized study. Corneal parameters examined

included full ocular examination, measurement of

CCT, corneal topography, corneal curvature and

ocular refractivity. Intraocular pressure measurements

were obtained using three different instruments—non-

contact tonometer, Goldmann applanation tonometer

and TonoPen XL (TonoPen-Central and TonoPen-

Peripheral). All measurements were performed

pre-operatively and 4 months post-operatively.

Post-operative intraocular pressure was significantly

lower than pre-operative values, with all instruments

(p value\0.001, Student’s t-test). The post-operative

intraocular pressure decrease was smallest using the

Tonopen-XL compared to the Goldmann applanation

tonometer and non-contact tonometer (p value \0.001, ANOVA). Intraocular pressure readings are

significantly reduced following corneal ablation sur-

gery. We determined in our myopic patient cohort that

the TonoPen XL intraocular pressure measurement

method is the least affected following PRK and

LASIK as compared to other techniques.

Keywords Intraocular pressure � Corneal thickness �Laser in situ keratomileusis � Photorefractive

keratectomy

Introduction

Assessment of intraocular pressure (IOP) is of clinical

importance for detecting early glaucoma, especially

when the patient is myopic, a risk factor for primary

open-angle glaucoma [1]. IOP is known to be affected

by corneal biomechanics, thickness and curvature [2].

Alterations to these properties may lead to erroneous

IOP readings measured by applanation tonometry [2].

Ablative corneal refractive surgery such as laser in situ

keratomileusis (LASIK) and photorefractive keratec-

tomy (PRK) have gained popularity in recent years for

S. Hamed-Azzam � D. Briscoe

Department of Ophthalmology, Haemek Medical Center,

Afula, Israel

S. Hamed-Azzam (&)

Erez 2, 17820 Nazareth Illit, Israel

e-mail: shireenhamed@yahoo.com

O. Tomkins � R. Shehedeh-Mashor � H. Garzozi

Department of Ophthalmology, Bnai-Zion Medical

Center, Haifa, Israel

123

Int Ophthalmol (2013) 33:349–354

DOI 10.1007/s10792-012-9701-7

correcting myopia. Following these procedures, corneal

structural changes occur, i.e., corneal thinning, altera-

tions to corneal curvature and development of irregular

astigmatism [3–5]. These may lead to lowering of

measured IOP and ultimately delay the diagnosis of

future glaucoma, under-estimate the actual IOP during

the follow-up of glaucoma patients, and possibly

contribute to deterioration in visual function [6].

Intraocular pressure can be measured using different

instruments—the Goldmann applanation tonometer

(GAT) is a static measurement device and the gold

standard for measuring IOP and the non-contact

tonometer (NCT, air jet tonometer) deforms the corneal

apex by means of a jet of air. Both techniques are

accurate if the IOP is near normal, but their accuracy

diminishes with extreme range pressures and on eyes

with abnormal corneas; a thin cornea results in falsely

low readings and a thick cornea results in falsely high

readings [7–9]. The TonoPen XL is a portable variable

force applanation tonometer that uses a tip which has a

strain gauge that is activated when it touches the cornea.

There is a good correlation between IOP measurements

by Tonopen and manometric readings in human autopsy

eyes [10]. It has been suggested for use in measuring

IOP when corneal abnormalities exist.

In this prospective, non-randomized clinical study

we aimed to find the most valid method for evaluating

IOP following corneal ablation surgery. Measure-

ments were obtained using different IOP measuring

instruments—NCT, GAT and TonoPen XL. We

further studied the relationship between changes in

mean central corneal thickness (CCT) and the change

in IOP measurements following PRK and LASIK, and

explored possible differences between results obtained

after either procedure.

Patients and methods

This prospective non-randomized clinical trial

included 200 myopic eyes of 100 patients that

underwent corneal ablation surgery for correcting

myopia. Patients underwent the different ablation

surgery techniques according to clinical assessment.

Pre-operative assessment included full ocular

examination, measurement of CCT, corneal topogra-

phy, corneal curvature and ocular refractivity. IOP

was also measured before the operation by three

different methods—GAT (Dr. Goldmann Hans, 1954),

TonoPen XL (Mentor, Norwell, MA, USA), and NCT

(Reichert, Xpert NCT plus, Buffalo, NY, USA). Using

the TonoPen XL, IOP was measured both in the central

cornea (TonoPen-C) and the temporal peripheral

region 1.5 mm from the limbus (TonoPen-P). All the

measurements were performed between 4:00 and

7:00 pm and in primary straight gaze. All ablation

surgeries were performed by a single surgeon (H.J.G.).

The influence of either type of myopic laser surgery on

IOP was assessed, as were the other study variables

except corneal curvature. All patients were examined

4 months following the operation and results were

compared to the baseline values.

Statistics analysis

Statistical analysis was performed using the SPSS

software (version 12.0). Differences in intra-device

measurements before and following surgery were

performed using a paired sample Student’s t test.

Analyzing the differences in IOP results obtained by

the various devices was performed using the analysis

of variance (ANOVA) test. The relationship between

IOP and CCT was described by Pearson’s correlation

coefficient and a linear regression model. Differences

or relationships were considered as statistically sig-

nificant if p \ 0.01. Continuous variable results are

presented as mean ± standard error of mean.

Results

Two hundred eyes of 100 myopic patients were included

in the study. One hundred and twenty eyes were treated

using LASIK and 80 eyes underwent PRK.

The total mean age was 31.9 ± 9.8 years (range

20–58 years). The mean age of the patients in the

LASIK group was 31.1 ± 9.64 years and in the PRK

group 33.1 ± 9.94. The LASIK group included 28

women and 32 men, and the PRK group included 25

women and 15 men. Age and sex distributions were

similar in both groups.

The mean pre-operative and post-operative IOP and

CCT measurements are presented in Table 1. Pre-

operative IOP measurements did not differ between

the various methods.

At the post-operative check-up, IOP was signifi-

cantly lower using all methods (Table 1; Fig. 1a).

350 Int Ophthalmol (2013) 33:349–354

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The delta change in IOP was the greatest using the

GATand the smallest with the TonoPen-P (Fig. 1b).

The delta IOP measured with the TonoPen XL was

significantly lower than that measured by the Gold-

mann applanation (p \ 0.001, ANOVA) or the NCT

(p \ 0.001, ANOVA, Fig. 1b). However, no signifi-

cant differences were found between the Goldman and

NCT (p = 0.75) or between the two TonoPen XL

measurements (p = 1.0). Following refractive surgery

CCT was significantly reduced (Table 1). The rela-

tionship between the change in CCT and delta IOP

demonstrates a linear correlation using all methods of

IOP measurements. This correlation was strongest for

GAT and weakest for TonoPen-C (Fig. 2).

Although the pre- and post-operative IOP measure-

ments recorded in the LASIK group were higher than

the PRK group, no significant differences were found

when examining the changes in CCT and IOP

(Table 2).

Discussion

The aim of this study was to evaluate the influence of

CCT on IOP measurements before and following

corneal refractive surgery and to examine which of the

three IOP measuring instruments was least affected by

the operation. In addition, we wanted to explore

different effects of CCT on IOP in the PRK and

LASIK subgroups.

We found (1) IOP measurement using the Ton-

oPen-P technique was the least affected by corneal

refractive surgery; (2) there was a linear relationship

between the change in CCT and the delta IOP; and (3)

post-operative IOP measurements may not be influ-

enced by the different techniques for corneal ablative

surgery.

Table 1 Mean pre-operative and post-operative IOP and CCT measurements

Pre-operative Post-operative Delta p value

CCT 560.37 ± 26.33 479.85 ± 3.42 54.53 ± 2.24 0.002

GAT 12.18 ± 0.16 10.41 ± 0.17 1.78 ± 0.09 \0.0001

NCT 11.85 ± 0.12 10.27 ± 0.17 1.58 ± 0.1 \0.0001

TonoPen-C 11.97 ± 0.15 10.98 ± 0.17 0.99 ± 0.09 \0.0001

TonoPen-P 12.51 ± 0.16 11.6 ± 0.16 0.92 ± 0.08 \0.0001

Mean refraction –0.19 ± 4.76 0.11 ± 0.03 4.65 ± 0.18 \0.0001

CCT central corneal thickness, GAT Goldmann applanation tonometer, NCT non-contact tonometer, TonoPen C TonoPen Central,

TonoPen P TonoPen Peripheral

Fig. 1 a Pre-operative and post-operative IOP using the

different measurement methods. Post-operative IOP was sig-

nificantly lower using all methods. b Delta IOP (pre-operative

IOP–post-operative IOP) by the different measurement meth-

ods. The greatest delta IOP was using the Goldmann tonometer

and the lowest was using the TonoPen-P. NCT non-contact

tonometer, TonoPen-C TonoPen-Central, TonoPen-P TonoPen-

Peripheral, DIOP delta IOP. ** Statistically significant

(p \ 0.001)

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Fig. 2 Relationship between the change in CCT and delta IOP.

A linear correlation between the change in central corneal

thickness and the delta IOP using all the measurement methods.

NCT non-contact tonometer, TonoPen-C TonoPen-Central,

TonoPen-P TonoPen-Peripheral, delta IOP (pre-operative

IOP–post-operative IOP). Delta central corneal thickness (pre-

operative central corneal thickness–post-operative central cor-

neal thickness)

Table 2 Mean pre-operative and post-operative IOP, CCT and refraction measurements in the PRK and the LASIK group

PRK LASIK

Pre-operative Post-operative Delta Pre-operative Post-operative Delta p value

CCT 514.83 ± 4.1 455.9 ± 5.31 51.59 ± 2.96 590.73 ± 43.6 495.81 ± 3.84 58.93 ± 3.38 0.11

GAT 11.41 ± 0.26 9.71 ± 0.27 1.83 ± 0.11 12.69 ± 0.19 10.87 ± 0.2 1.7 ± 0.15 0.493

NCT 11.15 ± 0.3 9.75 ± 0.28 1.7 ± 0.14 12.32 ± 0.21 10.62 ± 0.22 1.4 ± 0.15 0.154

TonoPen-C 11.19 ± 0.25 10.33 ± 0.28 1.08 ± 0.12 12.48 ± 0.18 11.41 ± 0.2 0.86 ± 0.11 0.228

TonoPen-P 11.75 ± 0.25 10.91 ± 0.27 0.97 ± 0.1 13.02 ± 0.18 12.05 ± 0.19 0.84 ± 0.13 0.42

Mean refraction -5.11 ± 0.31 -0.09 ± 0.04 5.02 ± 0.3 -4.52 ± 0.24 -0.12 ± 0.03 4.40 ± 0.22 0.97

CCT central corneal thickness, GAT Goldmann applanation tonometer, NCT non-contact tonometer, TonoPen C TonoPen Central,

TonoPen P TonoPen Peripheral, delta (pre-operative–post-operative)

352 Int Ophthalmol (2013) 33:349–354

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We found lower post-operative IOP measurements

using all techniques. The least affected technique was

TonoPen-P, followed by TonoPen-C, NCT and finally

GAT, respectively. The finding of lower IOP readings

in thinner corneas was previously established in

normal eyes (unpublished data) as well as following

PRK [3, 6] and LASIK surgery [5, 11]. Schipper et al.

[3] found that after PRK the central pressure readings

with the GAT and the TonoPen were between 1.8 and

2.3 mmHg lower than those measured temporally

(statistically significant p \ 0.0001). Garzozi et al. [6]

had similar results. TonoPen temporal periphery post-

PRK IOP measurements had the best correlation with

pre-operative GAT IOP (p \ 0.0001). Similar results

were obtained after LASIK surgery. The post-opera-

tive GAT-measured IOP decrease was strongly related

with a decrease in CCT and change in keratometry

[12]. Shemesh et al. [12] found a difference of

1.71 ± 1.43 mmHg in GAT measurements between

pre- and post-operative periods. In a prospective study

by El Danasoury et al. [13], they reported a mean

decrease in IOP of 4.3 ± 2.1 mmHg with GAT and

6.1 ± 2.3 mmHg with air puff tonometry post LASIK

surgery. The IOP change was significantly correlated

with the baseline IOP and the ablation depth. Our

results support these findings; however, we found that

the effect of surgery on IOP measurement in our study

was not uniform to all instruments. The TonoPen

readings were less affected than the GAT or the NCT

in both the PRK and LASIK subgroups. A possible

explanation is based on the Mackay-Marg tonometer

in which the TonoPen has a 1.02 mm diameter micro

strain gauge surrounded by a 3.22 mm annulus. When

the cornea is applanated, the outer annular ring flattens

the cornea and bears the restoring force of the flattened

cornea. Therefore, the region of corneal tissue that

contacts the central plunger is flat, with no structural

forces acting to depress the plunger. Consequently, the

only force that presses the cornea against the central

plunger is the IOP. This design minimizes the effects

of corneal irregularities, including variations in thick-

ness, rigidity, and astigmatism on the measured IOP

[14–17]. Therefore, although GAT is the gold standard

for IOP measurements it may be affected by such

changes because it is a static measurement and

therefore may be unreliable for eyes after refractive

surgery.

Recently new instruments have been presented

such as the Reichert Ocular Response Analyzer (ORA;

Reichert, Buffalo, NY, USA) which is claimed to

measure IOP independent of central corneal thickness.

Kirwen and O’Keefe [18] found that IOP measure-

ments were similar using both the ORA Goldmann

correlated IOP and GAT following LASIK surgery.

However, there is no data comparing ORA and

TonoPen post-refractive surgery.

The thickness of the cornea has been shown to

influence the pressure measurement. A thin cornea

results in falsely low readings and a thick cornea

results in falsely high readings [7, 9]. In our study, the

relationship between the change in CCT and delta IOP

demonstrates a significant linear correlation using all

methods of IOP measurement. This correlation was

strongest for Goldmann, which could be explained by

the fact that the GAT is a static measurement, and thus

affected by static metrics, such as corneal rigidity and

thickness. Contrary to our findings, Liu and Roberts

[2] found by using a predictive model that there is a

nonlinear relationship between corneal thickness and

IOP readings. Their predictive model included corneal

radius, curvature and Young’s modulus of elasticity.

They concluded that biomechanical properties could

substantially influence the measurement error intro-

duced by the same deviation in corneal thickness.

Their study was in good agreement with a review

article on CCT and IOP measurement, which did not

recommend a single value for correcting the effect of

CCT in applanation tonometry. Instead, they recom-

mended a 1.1 mmHg correction for a 10 % variation

in thickness for normal human eyes and a 2.5 mmHg

correction for eyes with chronic disease such as ocular

hypertension and normal-tension glaucoma [19]. The

differences from our results may be attributed to

corneal biomechanical properties that we did not

include in our study, such as viscosity, elasticity,

hydration and physiologic properties such as rate of

aqueous formation and angle structure. The linear

relationship we found may reflect only a localized

change in the cornea and not general ocular structure

and physiology differences related to CCT. Therefore,

this finding may only apply to changes in IOP

measurement following refractive surgery.

Following PRK and LASIK surgery, the cornea

undergoes distinct biomechanical changes as well as

an IOP measurement error. While previous studies

concentrated only on IOP differences following one

surgical technique, in our study we analyzed and

compared the IOP values after two different surgical

Int Ophthalmol (2013) 33:349–354 353

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technique subgroups. The effect of cutting the conti-

nuity of the superficial corneal collagen fibers in PRK

may affect the IOP readings differently to LASIK

where the Bowman’s layer is left intact. Such a

comparison may help us understand the role of the

Bowman’s layer in the measurement of IOP. We found

that the decrease in post-operative IOP was similar

following PRK or LASIK. Some authors have sug-

gested that IOP measurements after LASIK are

affected by corneal flap stability [13, 14, 20]. Kohlhaas

et al. [20] found an absolute decrease of 0.75 mmHg

in IOP readings due to the influence of flap stability.

A greater decrease of 1.36 mmHg was observed by

Chang and Stulting [14]. Changes in corneal stability

may also have influenced our IOP readings in addition

to the change in corneal thickness in the LASIK group;

however, a significant decrease in IOP reading was

still observed in the PRK group that cannot be

explained by flap stability. We must note that subjects

from both groups were not matched for depth of

ablation. Patients undergoing LASIK had thicker

corneal measurements pre-operatively and deeper

ablations than those in the group undergoing PRK.

The indications used for choosing between PRK and

LASIK are different but the overall result was that

neither the flap creation in LASIK nor the Bowman’s

layer ablation in PRK made any difference to the post-

operative IOP measurement.

In conclusion, following corneal refractive surgery,

lower readings are obtained by all available IOP

measurement instruments. The device least affected in

our study was the Tonopen. There was no difference

in IOP measurement following PRK or LASIK.

Future studies are needed to specifically examine the

influence of either operation technique on IOP

measurement.

Conflict of interest The authors declare that we have no

conflict of interest.

References

1. Perkins ES, Phelps CD (1982) Open angle glaucoma, ocular

hypertension, low tension glaucoma and refraction. Arch

Ophthalmol 100:1464–1467

2. Liu J, Roberts C (2005) Influence of corneal biomechanical

properties on intraocular pressure measurement. J Cataract

Refract Surg 31:146–155

3. Schipper I, Senn P, Thomann U, Suppiger M (1995) Intra-

ocular pressure after excimer laser photorefractive kera-

tectomy for myopia. J Refract Surg 11:366–370

4. Kf Damji, Munger R (2000) Influence of central corneal

thickness on applanation intraocular pressure. J Glaucoma

9:205–207

5. Cheng AC, Fan D, Tang E, Lam DS (2006) Effect of corneal

curvature and corneal thickness on the assessment of

intraocular pressure using noncontact tonometry in patients

after myopic LASIK surgery. Cornea 25:26–28

6. Garzozi HJ, Chung HS, Lang Y, Kagemann L, Harris A

(2001) Intraocular pressure and photorefractive keratec-

tomy: a comparison of three different tonometers. Cornea

20:33–36

7. Hansen FK, Ehlers N (1971) Elevated tonometer readings

caused by a thick cornea. Acta Ophthalmol 49:775–778

8. Ehlers N, Hansen FK (1974) Central corneal thickness in

low-tension glaucoma. Acta Ophthalmol 52:740–746

9. Argus WA (1995) Ocular hypertension and central corneal

thickness. Ophthalmology 102:1810–1812

10. Boothe WA, Lee DA, Panek WC, Pettit TH (1988) The

Tono-Pen: a manometric and clinical study. Arch Oph-

thalmol 106:1214–1217

11. Duch S, Serra A, Gastanera J, Abos R, Quintana M (2001)

Tonometry after laser in situ keratomileusis treatment.

J Glaucoma 10:261–265

12. Shemesh G, Man O, Michaeli A, Varssano D, Lazar M

(2007) Pressure phosphene tonometry versus goldmann

applanation tonometry for measuring intraocular pressure

before and after LASIK. J Refract Surg 23:405–409

13. El Danasoury MA, El Maghraby A, Coorpender SJ (2001)

Change in intraocular pressure in myopic eyes measured

with contact and non-contact tonometers after LASIK.

J Refract Surg 17:97–104

14. Chang DH, Stulting RD (2005) Change in intraocular

pressure measurements after LASIK: the effect of the

refractive correction and the lamellar flap. Ophthalmology

112:1009–1016

15. Levy Y, Zadok D, Glovinsky Y, Krakowski D, Nemet P

(1999) TonoPen versus Goldmann tonometer after excimer

laser photorefractive keratectomy. J Cataract Refract Surg

25:486–491

16. Minckler DS, Baerveldt G, Heuer DK, Quillen-Thomas B,

Walonker AF, Weiner J (1987) Clinical evaluation of the

oculab Tono-Pen. Am J Ophthalmol 104:168–173

17. Moses RA, Marg E, Oechsli R (1962) Evaluation of the

basic validity and clinical usefulness of the Mackay-Marg

tonometer. Invest Ophthalmol 1:78–85

18. Kirwan C, O’keefe M (2008) Measurement of intraocular

pressure in LASIK and LASEK patients using the reichert

ocular response analyzer and goldmann applanation to-

nometry. J Refract Surg 24:366–370

19. Dought MJ, Zaman ML (2000) Human corneal thickness

and its impact on intraocular pressure measures: a review

and meta-analysis approach. Surv Ophthalmol 44:367–408

20. Kohlhaas M, Spoerl E, Boehm AG, Pollack K (2006) A

correction formula for the real intraocular pressure after

LASIK for the correction of myopic astigmatism. J Refract

Surg 22:263–267

354 Int Ophthalmol (2013) 33:349–354

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