Central and peripheral pressuremeasurements with the Goldmanntonometer and Tono-Pen afterphotorefractive keratectomy for myopia

Isac Schipper, MD, Peter Senn, MD, Karin Oyo-Szerenyi, MD,Roland Peter, MD

ABSTRACT

Purpose: To compare the accuracy of Goldmann tonometry with that of the Tono-PenT inmeasuring intraocular pressure (IOP) after photorefractive keratectomy (PRK).

Setting: Eye Clinic, Cantonal Hospital, Lucerne, Switzerland.

Methods: Thirty-five eyes (25 patients) had PRK for a mean myopia of 26.2 diopters 6 2.6(SD). Intraocular pressure (IOP) measurements were performed first with the Goldmanntonometer and then with the Tono-Pen in 2 corneal locations: centrally in the usualmanner and temporally. For the temporal measurements, the patient was directed togaze nasally as the tonometer was placed perpendicular to the temporal portion of thecornea and with the rim of the tonometer positioned just inside the limbus of the cornea.Subsequently, similar measurements were made with the Tono-Pen. Measurementswere performed before and 1 and 3 months after PRK.

Results: Preoperative IOPs measured centrally and temporally were similar. After PRK, thecentral pressure readings with the Goldmann tonometer and the Tono-Pen were be-tween 1.8 and 2.3 mm Hg lower than those measured temporally. The differences werestatistically significant (P , .0001).

Conclusion: Central corneal pressure readings obtained with either the Goldmanntonometer or the Tono-Pen after myopic PRK are inaccurate. Measurements over thetemporal part of the cornea are likely more reliable. J Cataract Refract Surg 2000; 26:929–933 © 2000 ASCRS and ESCRS

The Goldmann tonometer enables accurate mea-surement of intraocular pressure (IOP). However,

such measurement may be subject to significant errorunder certain conditions, including corneal epithelial

edema,1,2 stromal thickness variations,3–6 and the pres-ence of high or irregular astigmatism.7,8

After photorefractive keratectomy (PRK) for myo-pia, corneal changes occur that may cause inaccuracies inIOP measurements. These changes include corneal thin-ning, alterations in corneal curvature, development ofirregular astigmatism, and disappearance of Bowman’slayer.

We found in a previous study9 that after excimerlaser PRK for myopia, IOP in different corneal locations

Accepted for publication December 15, 1999.

Reprint requests to Isac Schipper, MD, Eye Clinic, Cantonal Hospital,6000 Lucerne 16, Switzerland.

© 2000 ASCRS and ESCRS 0886-3350/00/$–see front matterPublished by Elsevier Science Inc. PII S0886-3350(00)00461-7

in the same eye varied. Pressure measurements werelower at the center of the cornea than at the periphery.

It has been suggested that the Tono-Pent (Xomed)be used to measure IOP when corneal abnormalitiesexist, as they do for example after penetrating kerato-plasty (PKP) and epikeratophakia.10 Measurementswith the Tono-Pen are more objective because the in-strument yields digital values.

In this second study, analogous to our first, we com-pare IOP measurements with the Goldmann tonometerto those with the Tono-Pen before and after PRK formyopia.

Patients and MethodsIntraocular pressure was measured in the central

and temporal cornea in 35 eyes before excimer laser PRKfor myopia. The measurements were repeated 1 and 3months postoperatively. Distance and near visual acuity,glare sensitivity, corneal topography, keratometry withthe Javal keratometer (Haag Streit), and pachymetry(Storz) were assessed at the same intervals.

Photorefractive keratectomy was performed tocorrect myopia of –6.2 diopters (D) 6 2.6 (SD) (range2.0 to 9.0 D). After the epithelium was abraded, theablation was made with the Meditec MEL 60 laser (Aes-culap). A mask with an ablation zone of 5.0 mm and anadditional tapered transition zone of 1.0 to 2.0 mm wasused.

The IOP measurements were done 3 times and themedian of the readings recorded as the IOP. Measure-ments were made first with a Goldmann tonometer. Thepressure was measured in the center of the cornea ini-tially and then peripherally (temporally). During tem-poral measurement, the patient was directed to gazenasally while the tonometer was placed perpendicular tothe temporal portion of the cornea with the rim of thetonometer positioned just inside the limbus of the cor-nea. Subsequently, similar measurements were madewith the Tono-Pen.

The sequence of measurements, centrally followedby temporally, was chosen for convenience. In somepatients not included in this study, the sequence of mea-surements was reversed, and the results were similar.

Statistical analysis of the results was done using apaired t test. A P value of 0.05 or less was consideredsignificant.

ResultsPreoperative and 1 month postoperative IOP mea-

surements were done in 35 eyes of 25 patients with amean age of 37.2 6 9.5 years (range 22 to 59 years).Three month postoperative measurements were ob-tained in 27 eyes. The results of the Goldmann tonom-eter and Tono-Pen measurements are shown in Tables 1and 2.

Goldmann TonometryPreoperatively, the mean IOP measured with the

Goldmann tonometer over the temporal cornea was0.46 mm Hg higher than that measured centrally (P 5.26) (Figure 1). One and 3 months postoperatively, themean temporal pressure was significantly higher thanthe central. After 1 month, the mean temporal readingexceeded the central reading by 2.27 mm Hg(P , .0001). After 3 months, the mean temporal pres-sure exceeded the mean central pressure by 1.6 mm Hg(P , .0001).

Tono-Pen TonometryWith the Tono-Pen, there was no significant differ-

ence between central and temporal corneal IOP mea-surements preoperatively (Figure 2). The differencebetween the mean readings was 0.3 mm Hg (P 5 .22),

Table 1. Comparison of pressure readings in the center of thecornea: Goldmann versus Tono-Pen.

ExaminationNumberof Eyes

Mean 6 SDP

ValueGoldmann Tono-Pen

Preoperative 35 13.0 6 3.1 13.4 6 2.3 .298

Postoperative

1 month 35 14.1 6 4.5 15.3 6 4.4 .009

3 months 27 15.5 6 4.5 15.6 6 4.1 .732

Table 2. Comparison of pressure readings in the periphery of thecornea: Goldmann versus Tono-Pen.

ExaminationNumberof Eyes

Mean 6 SDP

ValueGoldmann Tono-Pen

Preoperative 35 13.2 6 3.3 13.7 6 2.4 .567

Postoperative

1 month 35 16.4 6 4.5 17.0 6 4.3 .151

3 months 27 17.5 6 4.1 17.5 6 4.2 .688

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with the mean temporal IOP exceeding the mean centralIOP. Postoperative results were similar to those withGoldmann tonometry. Mean IOP measured centrallywas lower than the mean measured temporally. The dif-ferences in the mean readings were 1.7 mm Hg after 1month and 1.9 mm Hg after 3 months (P , .0001).

Goldmann Versus Tono-PenIn general, the Goldmann tonometer measurements

were lower than those of the Tono-Pen. However, thedifference was statistically significant for the centralmeasurements only at 1 month (mean of 1.2 mm Hglower values with the Goldmann tonometer; P 5.0094).

PachymetryCentral corneal thickness (CCT) decreased by a

mean of 45.63 6 5.20 mm after 1 month and by39.21 6 4.01 mm after 3 months. There was no corre-lation between changes in CCT and changes in IOPreadings.

KeratometryJaval readings showed a reduction in mean keratom-

etry readings of 4.20 6 0.39 D after 1 month and3.87 6 0.28 D after 3 months. There was no correla-tion between changes in keratometry and changes inIOP readings.

Intraocular PressureAn IOP increase was noted 1 and 3 months post-

operatively. The pressure readings and thus the in-creases were more obvious in the temporal cornealmeasurements.

DiscussionWe have shown that IOP measured with the Gold-

mann tonometer can be inaccurate.9 After PRK, thecornea thins, with marked change in its topography andthe disappearance of the Bowman’s layer. Therefore, theinaccuracy is not surprising.

Other studies have confirmed our findings that IOPis lower after PRK or laser in situ keratomileusis(LASIK).11–14 Chatterjee et al.,11 in a study of 1320patients, found that a decrease in IOP correlated withthe degree of myopic correction. In contrast, Faucherand coauthors13 found no correlation among corneal oroperative parameters. Munger et al.14 found that CCTcorrelated well with IOP before PRK and 6 monthspostoperatively but not during other examinations.They also observed that there was no correlation be-tween changes in CCT and changes in IOP; for a givenchange in CCT, variations of 65 mm Hg weremeasured in some cases. The results of the correla-tion between IOP and CCT after LASIK are alsocontroversial.15,16

Figure 1. (Schipper) Goldmann tonometry pressure readings inthe center and in the periphery of the cornea preoperatively and 1month after PRK (center 5 central cornea; temp 5 temporal cornea).

Figure 2. (Schipper) Pressure readings with the Tono-Pen in thecenter and in the periphery of the cornea preoperatively and 1 monthafter PRK (center 5 central cornea; temp 5 temporal cornea).

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In our study, IOP measured in the center of thecornea 1 and 3 months postoperatively was higher thanpreoperatively. This can be explained as a response totreatment with corticosteroids. After 1 year, when thecorticosteroids were discontinued, the centrally mea-sured pressure fell to values lower than those preopera-tively, as reported in our previous study.9

The Goldmann tonometer is very accurate. Its ac-curacy, however, depends on normal configuration ofthe cornea, with an ideal thickness of 0.5 mm.3–6 Theaccuracy of IOP measurements is affected by change incorneal shape, thickness, or radius.

The MacKay–Marg has been reported to be an “ex-act” tonometer, able to accurately measure IOP on anabnormal cornea (e.g., after PKP).17 The Tono-Pen,which was developed based on this tonometer, ap-planates a much smaller area of the cornea than theGoldmann tonometer, enabling accurate measurementsin the presence of corneal abnormalities. Thus, morereliable IOP measurements can be expected after PRKusing the Tono-Pen.

Not all authors agree that the Tono-Pen is moreaccurate when measuring abnormally high or low pres-sures. Kao et al.18 found inaccuracies when measuringIOP less than 10 mm Hg or higher than 30 mm Hg.Frenkel and coauthors19 reported good results withpressures between 10 and 30 mm Hg but inaccurateresults with pressures of 30 to 45 mm Hg. Hessemer andcoauthors20 found good correlations in cadaver eyes.Boothe and coauthors21 found a good correlation toapplanation tonometry when measuring pressures be-tween 10 and 35 mm Hg.

The Tono-Pen yields more objective readings. Theinstrument displays a result only after 5 similar pressuredeterminations are made. The results are averaged by theTono-Pen and displayed digitally. Tono-Pen readingsare therefore less subject to technician bias.

The IOP measured in the temporal part of the cor-nea was similar to that in the central corneal preopera-tively. It can be assumed, therefore, that our temporalmeasurements were accurate and reliable. This assump-tion is limited after PRK because the topography andstructure of the cornea can be abnormal; however, it islikely that IOP measured in the temporal area moreaccurately reflects the true IOP. Temporal measure-ments with the Goldmann tonometer and with theTono-Pen should be more accurate than those measured

in the central cornea because this part of the cornea isnot significantly altered by PRK. The slight horizontaldeviations of the eyes required for temporal measure-ments do not cause a significant variation in pres-sure.22–24 In addition, the Tono-Pen has provedaccurate when used on different locations of thecornea.25

Abbasog#lu and coauthors26 found differences be-tween central and peripheral measurements similar toours when they used the Goldmann tonometer afterPRK for myopia. However, when they used the pneu-motonometer, readings in the center of the cornea weresimilar to those obtained with the Goldmann tonometerperipherally.

It is important for ophthalmologists to recognizethat IOP measured in the central corneal region afterPRK may be falsely low. Myopic patients are more proneto develop glaucoma, and many of them could be steroidresponders.

In conclusion, excimer laser PRK for myopiachanges corneal shape and thickness, inducing inaccu-racies in IOP measurements, especially in the centralcornea. Applanation in the temporal cornea with theGoldmann tonometer and the Tono-Pen yielded moreaccurate readings.

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From the Eye Clinic, Cantonal Hospital, Lucerne, Switzerland.

None of the authors has a financial or proprietary interest in any materialor method mentioned.

Charles Hyams, MD, reviewed this manuscript.

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