RESEARCH PAPER

Influence of corneal parameters in keratoconus on IOP readingsobtained with different tonometers

Clin Exp Optom 2013; 96: 233–237 DOI:10.1111/cxo.12016

Penpe Gul Firat MDGozde Orman MDSelim Doganay MDSoner Demirel MDDepartment of Ophthalmology, Inonu UniversitySchool of Medicine, Malatya, TurkeyE-mail: pfiratmd@gmail.com

Background: Accurate intraocular pressure (IOP) measurement is important but of unsurereliability in patients with keratoconus. Different types and models of tonometers areavailable. This study investigated the influence of corneal parameters on IOP readingsobtained by a Goldmann applanation tonometer, a non-contact tonometer and a dynamiccontour tonometer.Methods: IOP readings with the Goldmann applanation, non-contact and dynamic contourtonometers were obtained from 52 patients with keratoconus and from 50 normal subjectsand their corneal parameters were measured using a Pentacam.Results: The mean IOP measurements in keratoconus obtained with the Goldmann appla-nation, non-contact and dynamic contour tonometers were statistically significantly differentfrom the mean IOP measurements in the normal subjects (p < 0.0001; p < 0.005; p < 0.0001,respectively). In the keratoconus group, the thinnest corneal thickness (TCT), steepestkeratometry, the corneal curvature (CC), central corneal thickness (CCT) and the posteriorcorneal curvature (PCC) had a significant effect on the Goldmann applanation and non-contact tonometers but not on the dynamic contour tonometer. In the control group,thinnest and central corneal thicknesses had a significant effect on findings with theGoldmann and non-contact tonometers but not on the dynamic contour tonometer. Thecorneal volume (CV) had no significant effect on the three tonometers in both groups.Conclusions: The corneal parameters affecting the IOP readings of the Goldmann appla-nation tonometers, non-contact tonometers and the dynamic contour tonometers are notthe same. While the Goldmann applanation and non-contact tonometers were significantlyaffected by the corneal parameters that were measured, the dynamic contour tonometer wasnot affected by any of these corneal parameters.

Submitted: 31 July 2012Revised: 23 August 2012Accepted for publication: 15 September2012

Key words: cornea, dynamic contour tonometer, Goldmann applanation tonometer, intraocular pressure, non-contact tonometer

Keratoconus is a non-inflammatory diseaseof the cornea characterised by steepening,distortion and corneal ectasia. It is usuallyseen bilaterally and has a progressive nature.A decrease in corneal thickness and cornealvolume (CV) and an increase in cornealcurvature (CC) have been demonstrated.1

These corneal alterations not only cause areduction in visual acuity (VA) but also makeaccurate measurement of intraocular pres-sure (IOP) problematic. In addition to beinga risk factor for glaucoma, IOP is an impor-tant parameter in the diagnosis and manage-ment of glaucoma.

The Goldmann applanation tonometerhas been the world standard for IOP meas-urement since the first intervention with thedevice by Goldmann. Unfortunately, under-and over-estimations occur with this device

when the corneal thickness is outsidenormal limits.2

Non-contact tonometers are basicallyapplanation tonometers but they measurethe IOP without a topical anaesthetic. Theapplanation force is achieved by a columnof air that flattens the cornea in a veryshort time. The force for flattening dividedby the area of applanation is the IOP. LikeGoldmann tonometry, many studies haveshown that corneal abnormalities influ-ence the IOP measured by non-contacttonometers.3–5

The Pascal dynamic contour tonometer isa recently introduced tonometer, whichdoes not rely on the principle of flatteningor indenting the cornea. Thus, IOP meas-ured with the dynamic contour tonometer isexpected to be unaffected by corneal prop-

erties. The Pascal tonometer measures IOPwith a hypothetical corneal contour that isachieved when the pressures on the anteriorand posterior of the cornea are equal. Theapplied force gently fits the corneal surfaceto that hypothetical contour that counter-balances the force distribution generatedby the IOP. The tonometer’s curved tiptakes the measurements by duplicating thecorneal curvature with a sensor located in itscentre. The dynamic contour tonometer canalso measure ocular pulse amplitude by dif-ferentiating between diastolic and systolicIOP.6

The corneal parameters are the mainsource of error for all three devices;2,7,8

however, there are limited data on whetherIOP measurements obtained with theGoldmann applanation, non-contact and

C L I N I C A L A N D E X P E R I M E N T A L

OPTOMETRY

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Clinical and Experimental Optometry © 2013 Optometrists Association Australia 233

dynamic contour tonometers are influencedby the same corneal parameters in normaland keratoconic eyes. The purpose of thisstudy was to determine the agreementbetween IOP readings obtained by thesethree tonometers and to determine theinfluence of corneal parameters such ascentral corneal thickness (CCT), thinnestcorneal thickness (TCT), steepest keratom-etry (K), corneal curvature, corneal volumeand posterior corneal curvature (PCC) onthe IOP readings in keratoconic and normaleyes.

METHODS

Data were obtained on 52 eyes of 52 patientswith keratoconus and 50 eyes of 50 healthysubjects. In each of the subjects, one eye wasrandomly selected for the study. Informedconsent was obtained from each individualbefore the study, which was conducted inaccordance with the ethical standards of theDeclaration of Helsinki and approved by ourinstitutional ethics board.

Each subject underwent a full ophthalmicexamination, including VA, slitlamp biomi-croscopy and stereoscopic fundus evaluationon the slit-lamp using a 90 D lens. The diag-nosis of keratoconus was made as describedpreviously.9 The diagnosis was also con-firmed topographically using a Pentacam.

Patients with glaucoma, those suspectedas having glaucoma and those with anydisease that can affect IOP were excluded.

The order of the examination waskeratometry and refractive error measure-ments using an ophthalmic unit (TopconDelta Ophthalmic Unit IS-775, Tokyo,Japan) and IOP measurements with thethree tonometers. The interval between theIOP measurements was approximately 15

minutes. Three consecutive IOP readingswere obtained with each of the three tonom-eters and the means of the readings wererecorded. First, the IOP of the subjects wasmeasured using the non-contact tonometer(Topcon CT-80A Computerized Tonom-eter, Tokyo, Japan) and a Pascal dynamiccontour tonometer (SMT Swiss Microtech-nology AG, Port, Switzerland) and the meanof the three measurements was taken. Threereadings were taken using the Pascal tonom-eter. Finally, the IOP was measured with theGoldmann applanation tonometer (Haag-Streit, Bern, Switzerland).

The Pascal tonometer uses a digital scorein the quality assessment. IOP readings witha score of 1, 2, and 3 were recorded.

Following the IOP measurements, thecorneal parameters were measured using aPentacam rotating Scheimpflug camera(Oculus, Wetzlar, Germany).

All the statistical analyses were performedusing SPSS for windows, version 15 (SPSS,Inc, Chicago, IL, USA). The Shapiro–Wilktest was used to test the distribution of thenumerical data. Non-normally distributeddata of the groups were compared by theKruskal–Wallis H test. Multiple comparisonswere carried out using the Conover test. TheChi-square test was used to compare cat-egorical data. The relationships among theCCT, TCT, K, CC, CV, PCC and IOP mea-surements were assessed by Spearman’scorrelation analysis for each group. Bland–Altman plots were used to evaluate theagreement between the three tonometers.In all the statistical analyses, p < 0.05 wasconsidered statistically significant.

RESULTS

The mean age and standard deviation of thepatients in the keratoconus group (21

women and 31 men) was 25.5 � 1.36 yearsand in the normal group (25 women and 25men) was 26.7 � 1.13 years. The mean valuesof the corneal parameters are given inTable 1.

The mean IOP readings of the Goldmann,non-contact and dynamic contour tonom-eters are shown in Figure 1. The mean IOPmeasurements in the keratoconus groupusing the three tonometers were statisticallysignificantly different from the mean IOPmeasurements in the normal subjects (p <0.0001, p < 0.005, p < 0.0001, respectively).The median (minimum to maximum) IOPmeasurements using the Goldmann, non-contact and dynamic contour tonometers inthe keratoconus group were 10 (4–16), 11(5–22) and 15.4 (10–21.4) mmHg, respec-tively. The median (min-max) IOP measure-ments using the Goldmann, non-contactand dynamic contour tonometers in thenormal subjects were 14 (9–17), 15 (8–21)and 16.8 (6.2–26.3) mmHg, respectively.

There were correlations among the IOPmeasurements of the three tonometers(Table 2) both in the keratoconus groupand in the normal group.

Figures 2, 3 and 4 show Bland–Altmanplots of the agreement among the Gold-mann, non-contact and dynamic contourtonometers. Table 3 shows the mean andstandard deviation differences and 95 percent limits of agreement between the threetonometers.

The corneal parameters affecting the IOPmeasurements were not the same in the

Keratoconus Normal p

TCT, mm 455 (139–556) 530 (238–592) 0.000K, D 49.5 (42.1–67) 43.8 (40.9–46.5) 0.000CC, D 48.3 (40.3–64) 43.1 (40.8–46) 0.000CCT, mm 475.5 (384–589) 549 (477–605) 0.000CV, mm3 57.2 (48–70) 60.5 (47.4–71.5) 0.001PCC, D -7.0 (-10.3–5.3) -6.2 (-6.7–4.5) 0.000

TCT: thinnest corneal thickness, K: steepest corneal keratometry, CC: corneal curvature, CCT: centralcorneal thickness, CV: corneal volume, PCC: posterior corneal curvature, mm: micron, D: dioptre.

Table 1. Corneal parameters of the groups Keratoconus Normal

0

2

4

6

8

10

12

14

16

18

IOP

(m

mH

g)

GAT

NCT

DCT

Figure 1. Mean intraocular pressurereadings of the groups obtained usingthe Goldmann applanation (GAT), non-contact (NCT) and dynamic contour(DCT) tonometers

IOP readings in keratoconus Firat, Orman, Doganay and Demirel

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234 Clinical and Experimental Optometry © 2013 Optometrists Association Australia

keratoconus and the control groups. In thekeratoconus group, the steepest keratom-etry, anterior and posterior corneal curva-tures and thinnest and central cornealthicknesses and had a significant effect onboth the Goldmann and non-contact tonom-etry but not on the dynamic contour tonom-etry. In the control group, the thinnest andcentral corneal thicknesses had a significanteffect on both the Goldmann and non-contact tonometric readings but not on dy-namic contour tonometry. Corneal volumehad no significant effect on the three tonom-eters in both groups (Table 4).

Finally, the keratoconus patients weredivided into three groups based on mean Kreadings obtained from the Pentacam: mild(up to 47 D), moderate (between 47 and52 D), and severe (52 D or greater), asdescribed previously.10 While there werestatistically significant differences among

Keratoconus Normal

GAT NCT DCT GAT NCT DCT

GAT 1 0.694* 0.766* 1 0.507* 0.456*NCT 0.694* 1 0.621* 0.507* 1 0.376*DCT 0.766* 0.621* 1 0.456* 0.376* 1

* Correlation is significant at the 0.01 level

Table 2. The correlation coefficients for the Goldmann applanation tonometer(GAT), non-contact tonometer (NCT) and dynamic contour tonometer (DCT)

5 10 15 20 25

Mean of NCT and DCT (mmHg)

15

10

5

0

–5

–10

–15

NC

T m

inu

s D

CT

(m

mH

g)

+1.96 SD

4.6

Mean

–3.0

–1.96 SD

–10.6

Figure 2. Bland–Altman plot showing the agreement between thenon-contact (NCT) and dynamic contour (DCT) tonometers

5 10 15 20

–6

–4

–2

0

2

4

6

8

10

Mean of NCT and GAT (mmHg)

NC

T m

inu

s G

AT

(m

mH

g)

+1.96 SD

7.6

Mean

1.9

–1.96 SD

–3.8

Figure 3. Bland–Altman plot showing the agreementbetween the non-contact (NCT) and Goldmann applanation(GAT) tonometers

8 10 12 14 16 18 20 22 24

–10

–5

0

5

10

15

Mean of DCT and GAT (mmHg)

DC

T m

inus G

AT

(m

mH

g)

+1.96 SD

11.0

Mean

4.9

–1.96 SD

–1.2

Figure 4. Bland–Altman plot showing the agreement betweenthe dynamic contour (DCT) and Goldmann applanation(GAT) tonometers

Mean � SD(mmHg)

95% LoA(mmHg)

NCT-DCT -3.01 � 3.86 -10.59 to 4.55NCT-GAT 1.88 � 2.90 -3.81 to 7.57DCT-GAT 4.89 � 3.12 -1.22 to 11.02

NCT: non-contact tonometer, DCT: dynamiccontour tonometer, GAT: Goldmann applanationtonometer, LoA: limits of agreement.

Table 3. Results of Bland–Altman analyses

IOP readings in keratoconus Firat, Orman, Doganay and Demirel

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the three keratoconus groups in the IOPreadings obtained with the Goldmann andnon-contact tonometers, there were no sta-tistically significant differences among theIOP readings for the three keratoconusgroups obtained with the dynamic contourtonometer.

DISCUSSION

A major factor in the diagnosis of glaucomaand management is an accurate determina-tion of IOP. Due to corneal changes in kera-toconus, exact IOP measurement has beena challenge. Currently, two applanationtonometers, the Goldmann and non-contact, are the most widely used andaccepted tonometers around the world;however, many previous studies have dem-onstrated errors with applanation tonom-eters in altered corneas.11,12 These errors ledto a search for a new tonometer, which canmeasure IOP independently of the cornealparameters. The recently developed Pascaltonometer uses contour matching instead ofapplanation to measure IOP. Several studieswith this dynamic contour tonometer inpatients with keratoconus have demon-strated that IOP readings are not affected bycorneal parameters.13,14 Conversely, otherstudies have suggested that corneal changesdo affect IOP readings with the dynamiccontour tonometer.2,9,15

In the current study, we investigated theeffect of several corneal parameters on IOPreadings with the Goldmann, non-contactand dynamic contour tonometers in patientswith keratoconus and also in normal

subjects, to identify difference in cornealfactors affecting IOP measurements with thetwo conditions. We demonstrated that thecorneal parameters that affect IOP measure-ment with these three tonometers in kerato-conic and normal eyes are not the same. Inkeratoconic eyes, the thinnest corneal thick-ness, keratometric reading, corneal curva-ture, central corneal thickness and posteriorcorneal curvature significantly affected IOPmeasurements with the Goldmann and non-contact tonometers. In a similar study withkeratoconic eyes, Read and Collins14 alsofound that the average anterior axial curva-ture, the steepest anterior curvature and theaverage posterior axial curvature signifi-cantly influence IOP readings with thenon-contact tonometer in patients with kera-toconus. These findings are in agreementwith ours.

An interesting finding in both studies isthat the posterior corneal curvature affectsthe IOP readings. Anterior and posteriorcurvatures are higher in patients with kera-toconus and a weaker correlation has beendemonstrated between the anterior and theposterior curvature in clinical keratoconiceyes than in normal ones.16 This finding canhelp explain why the posterior corneal cur-vature affects IOP readings in keratoconiceyes but not in normal ones. In contrast toRead and Collins’ study,14 we also investi-gated the effect of corneal parameters onnormal eyes with the Goldmann, non-contact and dynamic contour tonometers.We found that the thinnest and centralcorneal thicknesses significantly influenceIOP measurement in normal eyes using the

Goldmann and non-contact tonometers.Three corneal factors—the steepest kerato-metric reading, corneal curvature and pos-terior corneal curvature—influenced theIOP readings of the Goldmann and non-contact tonometers in keratoconic eyes butnot in normal eyes. In light of these results, itseems that a normal range of keratometryand the anterior and posterior curvaturevalues do not influence IOP readings withapplanation tonometers.

A number of recent studies have investi-gated the effect of central corneal thicknesson IOP readings in normal and in kerato-conic eyes. These studies have reported asignificant association between centralcorneal thickness and IOP readings usingthe Goldmann and non-contact tono-meters;17–19 however, the effect of the thin-nest corneal thickness on IOP readings wasnot previously studied. The thinnest cornealand central corneal thicknesses were thecommon corneal parameters in both ofthe groups.

In our study, the dynamic contour tonom-eter did not influence any of the cornealparameters that were measured in both thekeratoconic and normal eyes. This findingsuggests that the dynamic contour tonomet-ric IOP readings are more reliable thanthose of the Goldmann and non-contacttonometers. In a clinical study of kerato-conus, pellucid marginal degeneration andpenetrating keratoplasty patients, Ozbekand colleagues7 compared the IOP readingsof the Goldmann, Tono-Pen and dynamiccontour tonometers. They demonstratedthat unlike the Goldmann and tono-pen,the dynamic contour tonometer was notaffected by central corneal thickness. Papas-tergiou, Kozobolis and Siganos20 showedthat the dynamic contour tonometer was notsignificantly affected by central cornealthickness in normal and keratoconic eyes.The results of our study are consistent withthese studies. In our study, the dynamiccontour tonometer seemed to be unaffectedby the corneal parameters that were meas-ured but the IOP measurements in the kera-toconus group were significantly differentfrom those in normal eyes. The biomechani-cal parameters of the cornea provide a pos-sible explanation for this difference. In arecent study, Bayer and colleagues9 showedthat the dynamic contour tonometer wassignificantly affected by corneal hysteresisand the corneal resistance factor in kerato-conic eyes. They suggested that this resultmay be attributable to the design of the

Keratoconus Normal

GAT NCT DCT GAT NCT DCT

TCT 0.282* 0.330* 0.106 0.274* 0.287* 0.076K -0.413* -0.519** -0.212 0.103 -0.172 -0.071CC -0.304* -0.477** -0.174 0.027 -0.021 -0.042CCT 0.448** 0.568** 0.272 0.285* 0.318* -0.175CV -0.008 0.092 0.076 0.074 0.224 -0.087PCC 0.290* 0.422** 0.144 -0.56 0.047 -0.060

TCT: thinnest corneal thickness, K: steepest corneal keratometry, CC: corneal curvature, CCT: centralcorneal thickness, CV: corneal volume, PCC: posterior corneal curvature. * Correlations significantat 0.05 levels; ** Correlations significant at 0.01 levels

Table 4. The correlation coefficients for the Goldmann applanation tonometer(GAT), non-contact tonometer (NCT) and dynamic contour tonometer (DCT) andcorneal parameters

IOP readings in keratoconus Firat, Orman, Doganay and Demirel

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dynamic contour tonometer, which is not anapplanation tonometer. The conformabledesign of the device is influenced bythe changes in the cornea of keratoconiceyes, such as corneal geometrics andviscoelasticity.

We also divided the patients with kerato-conus into three groups according to theircorneal curvature. We found that more kera-toconic changes lead to an underestimationof IOP with the Goldmann and non-contacttonometers but not with the dynamiccontour tonometer. In contrast to someprevious studies,2,20 we found a significantcorrelation of IOP measurement with theGoldmann and non-contact tonometers andcorneal curvature. In a study with 30 eyes,Mark21 demonstrated that IOP increases asthe corneal curvature increases. Many otherclinical studies support this finding.22,23 Ourfindings are also in general agreement withthese previous studies; however, insteadof a positive correlation between cornealcurvature and IOP, we found a negativecorrelation.

Measurements of IOP using the Gold-mann, non-contact and dynamic contourtonometers appear to be unaffected bycorneal volume. Studies have demonstratedthat corneal volume is significantly reduceddue to corneal tissue loss during the progres-sion of keratoconus.10,24 This finding is con-sistent with our results; however, in ourstudy, corneal volume did not influence IOPreadings with the three tonometers, perhapsbecause of more dominant variables, such ascorneal hysteresis and the curvature havinga more significant effect than the cornealvolume.

In conclusion, we demonstrated thatcorneal parameters affecting IOP readingsof the Goldmann applanation tonometers,non-contact tonometers and the dynamiccontour tonometers are not the same.Although the Pascal dynamic contour tono-meter measured the highest IOP both inkeratoconic and in normal eyes, it was notaffected by any of the corneal parametersin this study. The results of our studysuggest that using a tonometer, whichwas not affected by corneal parameters isnecessary equipment, especially in corneadepartments.

ACKNOWLEDGEMENTAuthors have no financial or proprietaryinterest in any instrument or product used inthis study.

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