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Applanation tonometry andcorrection according to cornealthicknessRichard Stodtmeister

Private practice, Pirmasens, Germany and Department of Ophthalmology,St. Elisabeth Hospital, Rodalben (Palatinate), Germany

ABSTRACT.Purpose: In previous studies, it has been shown that the central corneal thick-ness influences the measurement values of applanation tonometry. The aim ofthis study is to answer the question as to which values and with what frequencythe tonometer readings as based on corneal thickness have to be corrected inthe patients attending an ophthalmic practice.Subjects and methods: The corneal thickness was measured in 579 patientsusing ultrasound pachymetry under the conditions prevailing in a medical prac-tice; from the thickness obtained, the correction values for intraocular pressurewere calculated.Results: Correction values of ∫2 mm Hg and above were found in half of thepatients examined, and correction values of ∫3 mm Hg and above in a goodquarter of the patients. The correction value was ∫4 mm Hg and more in everyfifth patient.Conclusions: Corneal thickness does, in fact, influence the results of applanationtonometry to a clinically relevant degree.

Key words: ultrasound pachymetry – intraocular pressure – glaucoma – ocular hypertension –corneal thickness – applanation tonometry.

Acta Ophthalmol. Scand. 1998: 76: 319–324Copyright c Acta Ophthalmol Scand 1998. ISSN 1395-3907

In developing their applanation tonom-eter, Goldmann and Schmidt started

from the hypothesis that the corneamight be considered as a sheath coveredby 2 membranes between which almostindisplacable water is located, so thatcorneal thickness consequently has no in-fluence on the measurement values ob-tained in applanation tonometry (Gold-mann 1957; Goldmann and Schmidt1957; 1961). Ehlers et al. (1975) and Whi-tacre et al. (1993) then demonstrated ex-perimentally that the corneal thickness inhumans shows a clear scattering, andthat, in corneal thicknesses below themean value, lower tonometer readingswere obtained than corresponding to thehydrostatically determined intraocularpressure. Within the natural scattering ofthe central corneal thickness, measure-

ment errors in applanation tonometry ofover ∫4 mm Hg were found.

Using ultrasound pachymetry, cornealthickness can nowadays be measuredwith justifiable outlay. The study herepresented was performed to find out,under practice conditions, how frequentlyand by what value intraocular pressurehas to be corrected according to cornealthickness. From the data here obtained,the application of corneal thicknessmeasurements in ophthalmological diag-nosis is to be assessed.

MaterialExaminations were carried out in pa-tients attending the author’s practiceover a period of 3 months. The ex-

clusion criteria were: age below 40years, unilateral anophthalmia, dilatedpupils at the time of examination, badcompliance, scarred cornea, a biomicro-scopically visible thickening or thinningof the cornea, acute inflammation in theanterior section of the eye, pathologicalconditions of the optic nerve other thanglaucomatous changes to the opticnerve head. Evaluation included the re-sults from 579 patients [age 66∫11years (arithmetic mean∫standard devi-ation), 323 females, 256 males]. Table 1lists patients’ data from the total sampleand the subgroups.

From data covering a period reachingas far back as 18 years obtained in theauthor’s practice, the patients included inthe study were classified as follows:

379 patients without glaucoma therapy:conditions for inclusion in this group(those with healthy eyes) were: the opticnerve head was to show no signs of glau-coma, no visual field defects were to beknown, and no intraocular pressureabove 21 mm Hg had been noted in anyprevious records. For simplicity’s sake,these patients were designated healthy.

200 patients with glaucoma therapy:this group was once more subdividedinto:

179 patients without certain signs ofglaucoma (visual field defects and/or oph-thalmoscopically visible damage to thehead of the optic nerve). These patientswere designated as having ocular hyper-tension.

18 patients with certain signs of glau-coma: excavation of the optic nerve head±0.8, reproducible visual field defects of±6db at more than 6 test locations inmore than 2 examinations.

3 patients with low-pressure glaucoma:certain signs of glaucoma with intraocul-

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ar pressure values below 22 mm Hg in allcases.

MethodsVisual fields were examined using a Rod-enstock Peristat 433, double thresholdcrossing, 81 or 50 test locations, dynamicmethod.

The optic nerve head was examinedusing direct ophthalmoscopy as a matterof routine; where there was the slightestsuspicion of an irregularity in the find-ings, the optic nerve head was observedin mydriasis using the 78 dptr magnifyingglass at the slit lamp. Intraocular pressurewas measured using a calibrated Gold-mann tonometer (manuf. Haag-Streit),with patients in a seated position.

Corneal thickness was measured usingan ultrasound pachymeter (Pachette,DGH Technology/Technomed). Accord-ing to the internationally accepted stan-dards for ultrasound measurement, theaccuracy was ∫5 mm. The speed of sound

Table 1. Descriptive data of corneal thickness, intraocular pressure and age in the examined groups.

Corneal thickness, intraocular pressure and age in groups examined

All subjects Healthy subjects Oc. Hypertension Glaucoma

Corneal thickness (mm)

Right Left Right Left Right Left Right Links

Number 579 579 379 379 179 179 18 18Arithmetic mean 585 583 580 577 596 596 593 595Standarddeviation 41 41 39 39 41 41 43 48Minimum 475 440 475 440 509 485 510 5191. Quartile 555 553 553 551 566 566 569 567Median 584 583 580 577 594 596 589 5873. Quartile 608 607 604 604 631 624 611 609Maximum 721 732 715 716 721 702 698 732

Intraocular pressure (mm Hg)

Arithmetic mean 17 17 16 16 20 19 19 19Standarddeviation 4 4 4 3 5 4 4 4Minimum 7 7 7 7 7 8 7 121. Quartile 14 15 14 14 17 17 16 17Median 17 17 16 16 19 19 20 183. Quartile 20 20 18 18 22 21 22 22Maximum 43 36 27 28 43 36 25 27

Age (years)

Arithmetic mean 66 64 68 70Standarddeviation 11 11 9 11Minimum 41 41 41 421. Quartile 58 56 61 64Median 67 64 68 703. Quartile 74 73 75 76Maximum 91 91 88 88

was adjusted at 1640 m/sec. The probewas applied perpendicularly to the cor-nea and 1.5 mm temporally to the reflec-tion of the fixation lamp (Argus 1995).

In all methods, first the right eye andthen the left eye was examined. The cen-tral tendency of the measured values isgiven with the arithmetic mean, thesimple standard deviation from the meanwas taken as scattering factor.

The type of distribution was assessedfrom the angle of slant and curtosis.

In testing for differences of par-ameters, the two-sided t test was used. Aswe are here concerned with a probatoryform of testing, no correction formultiple testing was applied (Sokal &Rohlf 1987).

The corneal thicknesses found arelisted in Table 1. The differences betweenthe values for right and left was slight inall those examined (pΩ0.01724). The cor-relation coefficient between right and leftis 0.90. Fig. 1 shows the scatter diagram.

From the measurement values of theright and left eye, a mean corneal thick-

ness of 578 mm was established as a ‘‘nor-mal value’’. In the healthy group, cornealthickness was, at 580∫39 mm on the rightand 577∫39 mm on the left, noticeablysmaller than in the patients with ocularhypertension, in whom corneal thicknesswas 596∫41 mm on both sides. The dif-ference is very clear (p∞0.00001). Fromthe frequency distribution of cornealthickness (Fig. 2), it is possible to seethat, in ocular hypertension, greater cor-neal thicknesses are more frequentlyfound than in healthy persons.

According to the results of Ehlers et al.(1975), the correction value for the intra-ocular pressure read off at the applan-ation tonometer is calculated using thefollowing formula (Argus 1995):

Correction value [mmHg]Ωª(meas-ured corneal thickness [mm] – 578 [mm])¿(5/70).

In this formula, the number 578 is themean central corneal thickness foundwith the instrument used in the healthypersons in this measurement series. Thefactor 5/70 is obtained as follows: If the

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Fig. 1. Scattergram of the paracentral corneal thickness in all 579 subjects examined. Abscissa:Right eyes. Ordinate: Left eyes.

Fig. 2. Relative frequency distribution of the paracentral corneal thickness in 379 healthy subjects(black colums) and in 179 patients with ocular hypertension (dotted colums). Abscissa: Cornealthickness. Ordinate: Relative frequency given in percent. Right eyes.

corneal thickness deviates from the meanthickness by 70 mm, the intraocular press-ure is determined either 5 mm Hg toohigh or too low. The preceding plus/minus sign was selected in accordancewith standard practice in navigation(Conrad & Steppes 1943), so that oneproceeds from the incorrect (i.e. the un-corrected) to the correct intraocularpressure (i.e. that actually present in theeye) with the plus/minus sign accordingly

correct: the correction value must beadded to the intraocular pressure valueread off at the applanation tonometer insuch a way that, in the case of markedcorneal thickness, the corrected intraocu-lar pressure (the intraocular pressure ac-tually present) is lower than the valueread off. The correction values calculatedin this way are given in Table 2. They arenothing else than a linear function of thecorneal thickness.

ResultsThe correction values found both inhealthy cases and in ocular hypertensionare shown in Fig. 3 as a cumulated fre-quency. The orientation of the X axis wasselected so that the negative correctionvalues are on the right in the case of pro-nounced corneal thickness. It can be seenquite clearly that, in ocular hypertension,the correction values are positionedfurther in the negative range than inhealthy persons. Or, to express it in a dif-ferent way: the cornea is thicker whereocular hypertension is involved, thevalues read off at the tonometer have tobe corrected downwards more frequently.

The quantile ranks of the correctionvalues both in healthy persons and in pa-tients with ocular hypertension are shownin Table 3. Using these values as a basis,the frequency of the correction valueswas calculated from corneal thickness(Table 4). As regards the figure found andthe magnitude, the correction values areidentical with the measurement error).Therefore, we are able to see that, in pa-tients with ocular hypertension for ex-ample, the measurement error and thusthe correction value is at 3 mm Hg andabove in 33 % of the eyes measured.

DiscussionIf the central corneal thickness is deter-mined by optical means, we find meanvalues between 400 mm and 573 mm (Dra-eger et al. 1995). In measuring with ultra-sound, values of 512 mm and 558 mm(Wheeler et al. 1992) and 567 mm (Argus1995) are given. The ultrasound pachy-meter here used was selected because theprototype was found to be the most suit-able after comparing units by differentmanufacturers (Bores 1993, p. 121); han-dling and accuracy had also been im-proved in the new model.

At the same ultrasonic velocity andwith the same measurement object, ultra-sound pachymeters by different manufac-turers yield values differing by 50–150 mm(Haigis 1995). The different coupling ofthe measurement probe of the units fromdifferent manufacturers to the corneamay be considered to be a cause for thisdifference. In a direct comparison with aunit by another manufacturer, a proto-type of the DGH 1000 unit used by theauthor yielded higher measurementvalues (Wheeler et al. 1992). This ex-plains why, in healthy persons, the mean

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Table 2. Correction values for corneal thickness calculated according to the data of Ehlers et al. (1975) by the formula given by Argus (1995) for amean corneal thickness of 578 mm.

Applanation tonometryCorrection values for tonometer readings dependent from corneal thickness

The values listed are to be added to the tonometer readings

Corneal thickness (mm)410 420 430 440 450 460 470 480 490 500 510 520 530 540 550 560 570 580 590 600 610 620 630 640 650 660 670 680 690 700 710 720Correction value (mm Hg)12 11 11 10 9 8 8 7 6 6 5 4 3 3 2 1 1 0 ª1 ª2 ª2 ª3 ª4 ª4 ª5 ª6 ª7 ª7 ª8 ª9 ª9 ª10

corneal thickness found here is 578 mm.The measurement units function on a lin-ear basis due to the standards accordingto which they are constructed.

In the data presented from this investi-gation, the scattering assessed in accord-ance with the standard deviation is withinthe same range of 40 mm as in the studiescited above. This means that the measure-ments here presented can be assessed asbeing just as reliable as those in the pre-viously published series of measurements.These differences in mean corneal thick-ness found with different pieces of equip-ment are not relevant for correction ofthe intraocular pressure, as long as themean corneal thickness – as obtainedwith the unit used – is used as a referencevalue for correction. In fact, this is howthe author proceeded in this study.

According to the results obtained byEhlers et al. (1975), a correction value

Fig. 3. Cumulated relative frequency of the correction values of the intraocular pressurecalculated according to the data of Ehlers et al. (1975) by the formula given by Argus (1995)in 379 healty subjects (broad dark line) and in 179 patients with ocular hypertension (thin linewith squares). Abscissa: Correction values given in mm Hg. The abscissa is inverted in order topoint in comparison with fig. 2 to the fact that high corneal thickness is associated with negativecorrection values and vice versa. Ordinate: Cumulated relative frequency. Right eyes.

can be calculated from corneal thickness.For calculation, the simplified linearmodel according to Argus (1995) wasused in this study, i.e.:

correction value [mmHg]Ω–(measuredcorneal thickness [mm] – 578 [mm]) ¿(5/70). According to the usual way of as-sessing biological systems, this linearitycan only be assumed within a limitedrange although it may be regarded as suf-ficient (Ehlers et al. 1975) for a correctassessment of the overwhelming majorityof cases.

That non-linearities must exist is dem-onstrated by one case (the only one in thepatients examined), in whom the cor-rected intraocular pressure assumed anegative value (ª1.6 mm Hg) This was,however, in the patient who had, at 698mm, the highest corneal thickness meas-ured.

As measuring location for corneal

thickness, a point 1.5 mm temporally tothe reflection of the fixing lamp waschosen, such as Argus (1995) rec-ommends, corresponding to a methodapplied in excimer laser surgery. Thismethod has the advantage that the ‘‘land-mark’’ of the fixing lamp reflex is not ob-scured. In pilot studies, no differencecould be found between the cornealthickness at the centre and that found atthe site of measuring.

The results here presented confirm theobservation by Argus (1995) to the effectthat, in patients with ocular hyperten-sion, the cornea is thicker than in thosewithout this condition.

A very wide range was covered by the‘‘ocular hypertension’’ group, as it in-cluded all patients having received glau-coma therapy and who had had an intra-ocular pressure of over 21 mm Hg meas-ured at least once during the preceding18 years as found in their records. Thus,this group includes persons with in-creased intraocular pressure but also un-treated and without damage to the opticnerve, as well as persons in whom dam-age to the optic nerve could be preventedthanks to therapy. As only 18 patientswith primary open-angle glaucoma con-firmed from diagnoses based on visualfield defects and changes in the opticnerve head were found among the 579persons examined, it can be assumed thatnot a few patients with primary open-angle glaucoma, in whom damage to theoptic nerve has not yet been discovered,are concealed within the ‘‘ocular hyper-tension’’ group. This fact is, however, un-important as concerns the results pre-sented here, as these also constitute per-sons with increased intraocular pressure.The 18 glaucoma patients presented acorneal thickness which is practicallyequal to that found in the ‘‘ocular hyper-tension’’ group (see Table 1).

In the course of this discussion, thequestion must be asked as to whether amoderately increased intraocular press-

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Table 3. Relative frequencies and quantiles of correction values of tonometer readings according to corneal thickness.

Correction values according to the corneal thickness of the right eyes

Healthy subjects Ocular hypertension Healthy subjects Ocular hypertension

Correction value (mm Hg) Relative frequency (%) Relative frequency (%) Quantile Quantile

7 1 0 0.996 1.0006 2 0 0.986 1.0005 3 2 0.970 1.0004 6 4 0.928 0.9843 9 6 0.873 0.9522 11 10 0.777 0.8871 15 13 0.656 0.7790 11 13 0.513 0.638

ª1 16 14 0.396 0.508ª2 12 9 0.227 0.378ª3 3 8 0.113 0.286ª4 4 10 0.082 0.192ª5 2 4 0.044 0.101ª6 1 3 0.027 0.059ª7 1 1 0.013 0.028ª8 0 1 0.003 0.019ª9 0 1 0.001 0.011

ª10 0 1 0.000 0.002

ure may not induce an increased cornealthickness. In the data presented here, thecorrelation coefficient between the centralcorneal thickness and the intraocularpressure is rΩ0.17. This means that, inagreement with the results of Ytterborgand Dohlmann (1965), the answer to thisquestion is in the negative.

The distribution of corneal thicknessboth in the healthy persons and the ‘‘ocu-lar hypertension’’ group does not deviateconsiderably from a normal distribution.The arithmetic mean of the correctionvalues in the ‘‘ocular hypertension’’group is positioned by a good 1 mm Hgwith the negative values, which is ex-plained by the greater corneal thicknessin this group. The measurement error ofapplanation tonometry is within thesame order of magnitude. This is why acorrection value of 1 mm Hg is of littlerelevance for practical purposes.

Even if viewing the mean values quanti-tatively is not useful for practical pur-poses, we do in fact find, when we viewthe scattering of the correction valuesquantitatively, aspects which are very rel-evant to practice:

in Table 3 we can see the quantileranks, i.e. the cumulated frequency,added onto the correction values. Inocular hypertension, the quantile rankof the correction value of π2 mm Hg isat 0.887, and for the correction value ofª2 mm Hg at 0.378. All values higherin number tend towards the ends of the

distribution curve, and their frequencymay be calculated thus: (1–0.887)π0.378Ω0.491. This means that, in pa-tients with ocular hypertension, the in-traocular pressure is read off with anerror of at least ∫2 mm Hg in 49.1%of the measurements made with an ap-planation tonometer. The frequencies ofthe calculated correction values are sep-arated in Table 4 according to their oc-currence in healthy eyes and in ocularhypertension. As corneal thickness isgreater on average in patients with ocu-lar hypertension than in healthy per-sons, as mentioned above, we thus finddifferent relative frequencies for the cor-rection values in each group.

Consequently, in ocular hypertension,correction values of 3 mm Hg and aboveare found in a third of the cases, this cor-rection value occurring in a quarter ofthe cases where healthy persons are in-

Table 4. Relative frequency of correction values for applanation tonometry readings in healthysubjects and in ocular hypertension (values in the tails of the distribution included).

Relative frequency of correction values and measurementerrors in Goldmann applanation tonometry

Correction value Healthy OcularMeasurement error subjects hypertension

∫2 mm Hg 45% 49%∫3 mm Hg 24% 33%∫4 mm Hg 15% 21%∫5 mm Hg 7% 10%

volved. For the clinician, this means that,as a result of the physiological deviationin corneal thickness, an intraocular press-ure read off at 21 mm Hg is actually posi-tioned at 18 mm Hg and below or at 24mm Hg and above in one third of all pa-tients with ocular hypertension. Intraocu-lar pressure is assessed too high or toolow by as much as 4 mm Hg in every fifthpatient with ocular hypertension; inhealthy persons, intraocular pressure isassessed too high or too low by 4 mm Hgin 15% of all cases.

Over the last twenty years, the state-ment value of intraocular pressure forprognosis of the disease’s further develop-ment in the individual patient has beenincreasingly called into question (Som-mer 1989; Smith 1990). The frequencyand magnitude of the measurementerrors presented in this study can lead tothe hypothesis that this low statement

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value observed up to now could be pro-duced by the measurement error whichhas crept in unnoticeably because cornealthickness was not taken into account. Inan initial rough survey in approximately400 of the author’s own patients withocular hypertension, primary open angleglaucoma and low tension glaucoma, acorrection value was found in more thana quarter of the cases according to whichprevious development of the diseaseagrees to a considerably better extentwith the pressure theory of glaucoma.This frequency is within the same orderof magnitude as the frequency which canbe expected from the data presented hereindependently of the individual case.

If the correction value is applied on apurely calculatory basis to the maximumintraocular pressure measured at history,the value for the maximum pressure cor-rected here is below 22 mm Hg in 40 ad-ditional patients.

In the population here considered, alarge number of similar cases were foundsuch as were observed in ocular hyperten-sion by Kruse Hansen and Ehlers (1971).In the 3 cases of low tension glaucoma, thecentral corneal thickness was clearly be-low the mean value, which had also beennoted by Ehlers and Kruse Hansen (1974).

As based on the experimental investi-gations made up to date on the influenceof corneal thickness on Goldmann ap-planation tonometer readings (Ehlers etal. 1975; Whitacre et al. 1993), and basedon the results presented here, we are ableto conclude that applanation tonometryis improved to a clinically relevant degreeby correcting the measurement values ac-

cording to corneal thickness. As cornealthickness cannot be estimated at thenecessary accuracy via biomicroscopy, ithas to be measured. At present, ultra-sound pachymetry seems to be the mostsuitable method, as it is the least incon-venient to the patient and has a justifiabletechnical outlay.

AcknowledgementAn abstract of a poster has been presented atthe meeting of the Deutsche Ophthalmochi-rurgen, Nuremberg, June 13–15, 1997.

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Received on May 12th, 1997.Accepted on November 11th, 1997.

Corresponding author:

Prof. Dr. med. Richard StodtmeisterAugenarztTurnstrasse 24D-66953 PirmasensPhone: 06331-64207Fax: 06331-64288