Limbal Pneumatonometry

LIMBAL PNEUMATONOMETRY JULIANNA M. BREITFELLER, M.A., AND DAVID L. KROHN, M.D. New York, New York

Tonometry on deformed or edematous corneas is inaccurate with commonly used methods including Schietz and Goldmann applanation. In such diseased eyes, accurate pressure measurements may be especially critical for effective management. However, distortion from scarring or edema may leave the juxtalimbal area and corneoscleral limbus relatively normal. Even if only a single quadrant remains relatively disease-free, limbal or paralimbal corneal tonometry will have clinical value, provided the method is sufficiently accurate. Additionally, application of an instrument to an epithelial surface weakened by disease is avoided. This study was designed to test the reliability of measurements made at the corneoscleral limbus of normal corneas with the Langham pneumatonograph--" relative to conventional Goldmann tonometry at various pressures. MATERIAL AND METHODS Intraocular pressures in Bellevue Hospital Eye Clinic patients were tested on the same visit by Goldmann applanation and by the pneumatonograph applied to the central cornea and the limbal area with the patient in both sitting and supine positions. Topical proparacaine anesthesia was used throughout the testing peri-

od. Limbal pneumatonograph measurements were made with the 5-mm diameter sensor tip bisected by the apparent corneoscleral limbus. In older eyes, the anatomic transition at the corneoscleral limbus was sometimes obscured by senility changes, so that some variation in positioning was inevitable. In such cases, the most apical edge of the sensor tip rested on clear cornea. Placement variation was estimated to be no more than 1.5 mm from one eye to the other. In order to expand the range of pressures, both glaucomatous and nonglaucomatous eyes were measured. Of 131 eyes examined, 58 (44%) were known to have glaucoma in various phases of treatment, but none had corneal disease, edema, or previous surgery. The patients ranged in age from 15 to 84 years, the average age was 50 ± 18 (S.D.) years, and 37 (55%) were female. The order of measurement was as follows: Goldmann applanation; pneumatonograph, sitting-center; pneumatonograph, sitting-limbal. The patient then assumed a supine position. After a minimum of two minutes, pneumatonograph supine-center was followed by pneumatonograph supine-limbal. All

TABLE 1 MEANS AND STANDARD DEVIATIONS FOR VARIOUS MODES OF lOP MEASUREMENTS

From the Glaucoma Service, Department of Ophthalmology, New York University Medical Center, New York, New York. This study was supported in part by Grants CA 18705 and EY 02428 (Dr. Krohn), from the National Institutes of Health, U.S. Public Health Service. Presented in part at the Annual Meeting of the Association for Research in Vision and Ophthalmology, Sarasota, Florida, April 30, 1979. Reprint requests to D. L. Krohn, M.D., Department of Ophthalmology, New York University Medical Center, 550 First Ave., New York, NY 10016. 344

Mode Goldmann applanation Pneumatonograph Sitting-center Sitting-limbal Supine-center Supine-limbal

Mean Measurement (rnm Hg)

(rnm Hg)

17.1

4.7

18.4 19.2 20.0 20.1

4.5 4.9 4.4 4.8

S.D.

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TABLE 2 TWO-TAILED, PAIRED SAMPLE t-TEST FOR DIFFERENCES IN MEANS*

Comparison Between Modes of lOP Measurement

Mean Difference (mm Hg)

S. D. (mm Hg)

P Value

Goldmann minus PTG, sitting, center Goldmann minus PTG, sitting, limbal Goldmann minus PTG, supine, center Goldmann minus PTG, supine, limbal PTG, sitting (center minus limbal) PTG, supine (center minus limbaI)

-1.3 -2.1 -2.9 -3.0 -0.8 -0.1

1.8 2.6 2.2 2.8 2.5 2.9

<.001 <.001 <.001 <.001 <.001 >.50

*PTG designates pneumatonograph.

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Fig. 1 (BreitfelIer and Krohn). Relationship between applanation and pneumatonometric (center, sitting) lOP measurements. Plot of paired measurements on the same eye: the number of dots within a circle indicates the number of values with the same coordinates; the solid straight line is the regression; the dotted lines are the 95% confidence limits for the prediction of Goldmann from pneumatonograph (PTG) (inverse prediction); the dashed line has a slope of 1 and a unit Y-intercept, representing values predicted by use of the Clinical Correlation Estimator for the relationship depicted. Histogram: the frequency distribution of the difference between the pneumatonograph and the applanation values in the same eye. Note the correspondence between the value of maximum frequency and the Clinical Correlation Estimator (Y-intercept of the dashed line).

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Fig. 2 (Breitfeller and Krohn). Relationship between applanation and pneumatonometric (limbal, sitting) lOP measurements (See Figure 1 for explanation).

Goldmann and pneurnatonograph measurements were made by the same practiced examiner, but the pneumatonograph tracings were evaluated by a different investigator. A value was given for each tracing and none were discarded so that the worst case potential for variation was deliberately maximized. Each measurement was made without repetition. Additionally, in a separate series of 65 eyes, pressure values for pneumatonograph measurements over the conjunctiva-sclera were recorded with the patient in the sitting position directly after

Goldmann applanation in the same eye. In these examinations, the corneal extremity of the pneumatonograph sensor tip was 2 mm or more from the apparent corneosclerallimbus. Statistical treatment: 1. For the determination of the relation between the various pneumatonograph and the Goldmann measurements, the following statistical methods were applied to the data: A. two-tailed, paired sample ttest; B. regression analysis;

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Fig. 3 (Breitfeller and Krohn). Relationship between applanation and pneumatonometric (central, supine) lOP measurements (See Figure I for explanation).

C. frequency distribution for the difference between measurements; D. calculation of inverse prediction limits. For this statistical treatment, the Goldmann values were taken as the independent variable and so assumed to be the true intraocular pressure. The inherent variation of the Goldmann measurements (as contrasted with direct manometry) is therefore artifactually attributed to the pneumatonometric method in this analysis.

2. Limbal pneumatonograph measurements were compared to central corneal pneumatonograph measurements, both in sitting and supine positions, by application of the same statistical methods as outlined. RESULTS

In the separate series of 65 eyes pneumatonograph conjunctival-scleral intraocular pressure measurements were, in all cases, higher than those made with the Goldmann instrument in the same eye. For this series, the mean Goldmann value was 20.1 mm Hg (5.8 S.D.), whereas that

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Fig. 4 (Breitfeller and Krohn). Relationship between applanation and pneurnatonometric (Iimbal, supine) lOP measurements (See Figure 1 for explanation).

for pneumatonograph scleral measurements was 35.5 mm Hg (10.2 S.D.). Paired sample analysis showed no reliable correlation. For the major series of 131 eyes, Table 1 shows the mean and standard deviation for each mode of intraocular pressure measurement. The mean pneumatonograph values are all higher than those found for Goldmann applanation. The relatively high standard deviations obscures the significance of these differences. However, a two-tailed, paired sample t-test for the difference between means (Table 2) shows the differences

between each of the pneumatonograph modes and the Goldmann determinations to be highly significant. Also, the relatively small differences between the pneumatonograph sitting-center and pneumatonograph sitting-limbal measurements was statistically significant. In contrast, the supine pneumatonograph measurements at the center were not distinguishable from those at the corneoscleral limbus. (The frequency histograms inserted in Figures 1 to 6 show the relatively normal distribution of the differences between paired samples.) The regression analysis for each of the relationships between the

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Fig. 5 (Breitfeller and Krohn). Pneumatonograph measurements on sitting subjects. Relationship between limbal and central (apical) values. Plot of paired measurements on the same eye: the number of dots within a circle indicates the number of values with the same coordinates; the solid straight line is the regression; the dotted lines are the 95% confidence limits for the prediction of central from limbal (inverse prediction). Histogram: the frequency distribution for the difference between limbal and central lOP values for the same eye.

various modes of measurement is shown in Table 3, which includes the correlation coefficients (R) indicating the linearity of the relationships in all cases. The relationships are depicted graphically in Figures 1 through 6. Inverse prediction limits to the 95% confidence level were calculated for the estimation of a Goldmann value corresponding to any single pneumatonograph measurement based on this data, which had been collected so as to imitate worst case clinical situations. For

all comparisons with Goldmann values, a line having a slope of 1 and a positive unit Y-intercept was drawn that approximates the regression and falls within the inverse prediction limits. The Y-intercept of this line is designated as the Clinical Correlation Estimator. DISCUSSION

These findings indicate a clinically useful relationship between limbal pneumatonometry and Goldmann applanation to-

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Fig. 6 (Breitfeller and Krohn). Pneumatonograph measurements on supine subjects. Relationship between limbal and central (apical) values (See Figure 5 for explanation).

nometry, despite a deliberate attempt in this study to maximize possible sources of variation, including the stipulation that a value was assigned to every pneumatonograph tracing, and the exclusion of opportunity to confirm readings by repetition. Under actual clinical conditions, much less scatter would be expected because only tracings of good quality would be acceptable and poor tracings would naturally be repeated. The difference found between pneumatonograph center-sitting and conventional Goldmann applanation was com-

parable to that found by others.v" The difference found in this study between central-sitting and central-supine by pneumatonograph was comparable to that found by Jain and Marmion." but somewhat lower than that reported by Krieglstein and Langham." To the best of our knowledge, there are no published reports of systematic use of the corneoscleral limbus as a tonometry site except for that of Wilke," who found good correlation between central corneal and limbal or paralimbal measurements with use of a vibration tonometer. The difference be-

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LIMBAL PNEUMATONOMETRY TABLE 3 REGRESSION ANALYSIS·

Modes of lOP Measurement

Slope

Sb

Y-intercept

R

Goldmann vs PTG, sitting, center Goldmann vs PTG, sitting, limbal Goldmann vs PTG, supine, center Goldmann vs PTG, supine, limbal PTG, sitting (center vs limbal) PTG, supine (center vs limbal)

0.883 0.886 0.830 0.846 0.943 0.877

0.032 0.048 0.038 0.051 0.049 0.057

3.26 4.02 5.80 5.67 1.87 2.60

0.927 0.850 0.886 0.827 0.862 0.804

·PTG designates pneurnatonograph.

tween measurements at the two sites was similar to that found in this study. The results of our study show that intraocular pressure measurements on normal corneas by pneumatonometry performed with the sensor tip on the corneosclerallimbus or slightly corneal to this location are uniformly higher than standard Goldmann applanation pressures. This is true for a range of pressures extending from low normal to glaucomatous values. Of clinical significance is that Goldmann pressures may be predicted from both central and limbal pneumatonograph measurements within clinically useful limits at the 95% confidence level by use of the Clinical Correlation Estimator. To estimate corresponding Goldmann applanation lOP values, the Clinical Correlation Coefficient is subtracted from the lOP value measured by using the following pneumatonograph modes: sitting, center, 1; sitting, limbal, 2; supine, center, 3; and supine, limbal, 3. If the patient is supine, there appears to be no statistically significant difference between the central and the limbal readings. This suggests that application of the pneumatonograph sensor tip to any disease-free area of the cornea, including the limbal and paralimbal areas, affords a reliable estimate of the intraocular pressure as measured by the familiar Goldmann applan-

ation instrument, and that the supine position for pneumatonograph for measurements is optimal for this purpose. The problem of tonometry in the presence of corneal disease or after keratoplasty has been approached with the use of various tonometers placed centrally. 7,8 For this application, the Mackay-Marg tonometer appears to date to have the best correlation with direct manometry," Limbal pneumatonometry has not, as yet, been subjected to such testing. SUMMARY

A series of comparisons in normal and glaucomatous eyes, all with normal corneas, was made between Goldmann applanation tonometry and Langham pneumatonometry, central and limbal, and with the patient in sitting and supine positions. All measurements were made at one visit with a standardized sequence. All mean pneumatonograph measurements were significantly higher than the corresponding applanation value. However, no significant difference was found between limbal and apical pneumatonograph values in the supine subject. Conversion values relating pneumatonograph measurements and applanation values have been derived and designated as Clinical Correlation Estimators. This allows prediction of Goldmann applanation values by extrapolation of limbal

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pneumatonograph measurements at a 95% confidence level. The method has potential use in the determination of intraocular pressure in the presence of corneal disease. ACKNOWLEDGMENT

Edward Y. T. Shen, M.D., assisted in the collection of clinical data. REFERENCES 1. Langham, M. K, Leydhecker, W., Krieglstein, G., and Waller, W.: Pneumatonographic studies on normal and glaucomatous eyes. Adv. OphthalmoI. 32:108, 1976. 2. Jain, M. R., and Marmion, V. J.: A clinical evaluation of the applanation pneumatonograph. Br. J. OphthalmoI. 60:107, 1976. 3. Quigley, H. A., and Langham, M. K: Comparative intraocular pressure measurements with the

MARCH, 1980

pneumatonograph and Goldmann tonometer. Am. J. OphthalmoI. 80:226, 1975. 4. Jain, M. R., and Marmion, V. J.: Rapid pneumatic and Mackay-Marg applanation tonometry to evaluate the postural effect on intraocular pressure. Br. J. Ophthalmol, 60:687, 1976. 5. Krieglstein, G. K., and Langham, M. E.: Influence of body position on the intraocular pressure of normal and glaucomatous eyes. Ophthalmologica 171:132, 1975. 6. Wilke, K.: Intraocular pressure measurement on various parts of the cornea. Acta Ophthalmol, 49:545, 1971. 7. McMillan, F., and Forster, R. K.: Comparison of Mackay-Marg, Goldmann and Perkins tonometers in abnormal corneas. Arch. Ophthalmol. 93:420, 1975. 8. West, C. E., Capella, J. A., and Kaufman, H. E.: Measurement of intraocular pressure with a pneumatic applanation tonometer. Am. J. Ophthalmol, 74:505, 1972. 9. Kaufman, H. E., Wind, C. A., and Waltman, S. R.: Validity of Mackay-Marg electronic applanation tonometer in patients with scarred irregular corneas. Am. J. Ophthalmol, 69:1003, 1970.