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  • The Influence of Corneal Thickness and Corneal Curvature on Tonometry Readings after Corneal

    Refractive Surgery M. KOHLHAAS*, R-CH. LERCHE*, J. DRAEGER*, M. KLEMM*, N. EHLERSt, J. HJORDTALt, H. OLSENt,

    C. BARRAQUER:j::, J. I. BARRAQUER:j::, D. FLiCKER:j::, F. RIVERA:j:: and C. CARRIAZO:j::

    Department of Ophthalmology, Hamburg, Germany, tDepartment of Ophthalmology, Aarhus, Denmark and tlnstituto Barraquer, Bogota, Colombia

    M. Kohlhaas, R.-Ch. Lerche, J. Draeger, M. Klemm, N. Ehlers, J.lljordtal, H. Olsen, C. Barraquer, J. I. Barraquer, D. Flicker, F. Rivera and C. Carriazo. The Influence of Corneal Thickness and Corneal Curvature on Tonometry Readings after Corneal Refractive Surgery. Eur. J. Implant Ref. Surg., 1995; 7: 84-88.

    INTRODUCTION

    The intraocular pressure (lOP) is a fundamental parameter of ocular health and disease. Not only is lOP important in the diagnosis and management of glaucomatous conditions, but its assessment is important in the postoperative management of corneal, lenticular and vitreoretinal diseases. In recent decades the most common way to assess lOP is with Goldmann-style applanation tonometry.

    Hans Goldmann has given an equation correlat-ing the defined mechanical stability of the intact cornea as a membrane of finite thickness and stiffness, and the liquid between cornea and the applanation surface developing capillary adhesion forces. This is valid for a diameter of the flattened area between 3 and 3.5 mm. In cases of such a defined applanation, the increase of lOP caused by the compressed volume of less than 0.5 III can be neglected [1, 2].

    It has been reported that measurements of lOP using the Goldmann applanation tonometer are influenced by corneal thickness and probably cor-neal curvature. Normally hydrated, thicker corneas cause elevated readings, and thinner corneas cause lower readings. The clinical significance of these observations is demonstrated in studies which have shown decreased corneal thickness in some cases of apparent low-tension glaucoma, and increased

    Correspondence to: Dr. Markus Kohlhaas, Department of Ophthalmology, MartinstraBe 52, 20246 Hamburg, Germany.

    0955-3681/95/020084 + 05 $08.0010 84

    corneal thickness in some refractory cases of glau-coma and ocular hypertension [3-5].

    Ehlers et ai. [3, 4] reported that an oedematous cornea would result in applanation tonometer read-ings which are much too low. This is significant in that an oedematous cornea would be measured as thick by ultrasonic or specular pachometry, and could lead the clinician to perceive lOP as being lower than its true value. The obvious danger is the under diagnosis of ocular hypertension and glaucoma.

    Mark [6] reported that many of the uncertainties in tonometry arise from variable physical parame-ters of the individual globe. One of these factors is the curvature of the cornea to which the tonometer is applied. Theoretically, one would expect that greater force would be needed to flatten a structure with greater curvature. In applanation tonometric terms, this theory suggests that, all else being equal, tonometric readings might be lower in flatter cor-neas, and vice versa.

    Draeger investigated the effect of contact lenses of 32 and 80% water content on the accuracy of appla-nation tonometry. He found that a 0 D 80% water content lens produced a reduction in the measured lOP varying between 2 and 2.6 mmHg within the lOP range 12-39.6 mmHg. A + 12 D lens produced an overestimation varying between 3.8 and 8.65 mmHg within the lOP range of 6.2-28.3 mmHg [4].

    This must be taken into consideration even more when the intraocular pressure is taken on corneas after corneal refractive microsurgery.

    1995 W.B. Saunders Company Limited

  • The Influence of Corneal Thickness and Corneal Curvature on Tonometry Readings 85

    Table 1

    Eyes Men

    Aarhus Group 1 (40 normal probands) 60 26 Group 2 (32 excimer patients) 44 16 Group 3 (36 RK patients) 71 26

    Bogota Group 4 (20 eryo-keratomileusis patients) 31 8 Group 5 (50 keratomileusis in situ patients) 91 14

    Table 2

    Group 2 Group 1 Preoperative Postoperative

    Visus (ee) 1.12 (0.3) 0.94 (0.2) 0.92 (0.27) Sphere (D) -2.4 (3.26) --6.77 (2.32) -1.44 (1.87) Cylinder (D) -0.43 (0.49) -0.79 (2.9) -0.84 (0.69) Paehymetry (mm) 0.53 (0.025) 0.52 (0.03) 0.5 (0.034) Axial length (mm) 24.4 (1.6) 25.8 (0.86)

    Corneal curvature K 1 (mm) 7.9 (0.23) 7.79 (0.25) 8.57 (0.37) K2(mm) 7.68 (0.22) 7.6 (0.2) 8.29 (0.32)

    Tonometry (mmHg) Goldmann 14.0 (2.19) 14.0 (3.1) 14.1 (2.1) Draeger 14.0 (2.02) 14.09 (1.94)

    However, after refractive surgery for the correc-tion of myopia, the corneal stiffness might be altered, the superficial corneal curvature is flat-tened and corneal thickness decreases or increases depending on the surgical technique.

    Several methods of measuring lOP are accurate in healthy eyes with a normal corneal surface. These include the Schi6tz indentation tonometer, the Gold-mann applanation tonometer, the McKay-Marg tonometer, the Tonopen tonometer, and the non-contact and contact pneumotonometers, all of which have some limitations [8-12]. This study demon-strates the effects of corneal thickness and corneal curvature on Goldmann applanation tonometry in patients after refractive corneal surgery (radial keratotomy, superficial photorefractive keratectomy and keratomileusis) for correcting myopia.

    PATIENTS AND METHODS

    In combined studies with N. Ehler, Department of Ophthalmology in Aarhus and C. Barraquer, lnsti-tuto Barraquer in Bogota, the lOP was measured with different tonometers (Goldmann applanation tonometer, I)raeger handheld applanation tonom-eter) to investigate if there is any influence of the above mentioned parameters (Table 1).

    Eur J Implant Ref Surg, Vol 7, April 1995

    Women Age (mean ( S. D.)) Examination (postop)(months)

    14 33.27 (8.5) 16 28.73 (6.94) 12.32 (9.94) 10 30.88 (7.02) 32.9 (15.33)

    12 28 (10) 8.55 (5.46) 36 28 (7.4) 8.1 (4.7)

    Group 3 Groups 4 and 5 Preoperative Postoperative Preoperative Postoperative

    1.03 (0.16) 1.08 (0.18) 0.69 (0.53) 0.54 (0.19) -3.13 (0.55) -0.84 (0.99) -10.85 (5.4) 0.05 (3.66) -0.63 (0.55) -0.61 (0.68) -1.69 (1.16) -2.07 (1.22)

    0.53 (0.03) 0.53 (0.026) 0.55 (0.034) 0.49 (0.039) 24.75 (0.79)

    7.9 (0.25) 8.45 (0.66) 7.92 (0.34) 9.45 (0.92) 7.74 (0.24) 8.23 (0.35) 7.63 (0.31) 9.15 (0.95)

    14.0 (2.5) 13.8 (2.56) 14.19 (3.01) 13.8 (2.73) 9.67 (3.16)

    Forty probands (group 1) with intact corneas as a control group, 32 patients after excimer ablation (group 2), 36 patients after radial keratotomy (group 3) and 70 patients after keratomileusis in situ (group 4) or cryo-keratomileusis (group 5) were examined (Table 1).

    The best-corrected visual acuity was recorded, and the cornea was examined for any abnormalities of the epithelium, stroma, or endothelium. Patients with abnormalities of any layer of the cornea were excluded. Keratometry was then performed using the Zeiss keratometer, the calibration of which was checked before beginning the study. The central corneal thickness was obtained by using the Haag-Streit Depth Measuring Attachment for the Haag-Streit slit lamp. The axial length was measured by A-scan ultrasonography. Before measuring the lOP with the Goldmann applanation tonometer or with the Draeger handheld applanation tonometer the precorneal film was stained with fluorescein solution.

    RESULTS

    Table 2 demonstrates the results obtained from the above-mentioned 5 groups.

    In our control group of normal and intact corneas the lOP readings measured with the Goldmann

  • 86

    applanation tonometer and the Draeger handheld tonometer are identical in mean value and S.D. This is well known, but a handheld tonometer is in general much more practical for laboratory work than a slit lamp.

    The linear regression line demonstrates a slight correlation of an increasing corneal thickness and corneal curvature on the tonometry readings (Fig. 1).

    The corneal thickness decreases after photore-fractive keratectomy by about 0.02 mm, the corneal curvature flattens by about 1. 7 mm radius. The lOP readings of the 2 different tonometers are identical compared with the control group.

    The corneal thickness in the RK group increases in the centre after surgery by about 0.01 mm. This increased corneal thickness could be explained with a loss of endothelial cells after surgery or with a disturbed or decreased barrier function of Bowmans membrane. The corneal curvature flattens about 1.5 mm radius.

    Although the corneal thickness is increased the mean value of the obtained lOP readings measured with applanation tonometry are statistically 0.2 mmHg lower than before the operation. The linear regression lines of the RK group demonstrate only a slight influence of the increased corneal thickness for the applanation readings (Fig. 2).

    Only a slight influence of the flattened corneal curvature on applanation tonometry readings could be noticed after excimer ablation (Fig. 3).

    20 b:D 18

    ::c:~ 16 . . . S 1:1 14 . . ; -S@ . 12 0 . ~S 10 1:1'1:1

    0- 8 .... 0 000 6 Regression line (n = 58) nonnal probands I:I~ 4 ~ 2

    0 0.46 0.48 0.5 0.52 0.54 0.56 0.58 0.6

    Corneal curvature (mm) Fig. 1 Regression line concerning corneal thickness and actual measured lOP of healthy subjects.

    0:; 25 '" IlD '" '" 20 ... 8

    b:D 15 ::c: j 10 1:1 5 .S rfJ 1:1 0 ~ 7.5

    Regression line (n = 45) RK . . :. .... .. ... " ..

    . . .. . . .

    u. o

    .... ..

    o

    - u

    Corneal curvature (mm) 9.5

    Fig. 2 Regression line concerning corneal curvature and actual measured lOP of RK-patients.

    M. Kohlhaas et 81

    With

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