CORNEAL CURVATURE IN APPLANATION TONOMETRY

HARRY H. MARK, M.D.

New Haven, Connecticut

Many of the uncertainties in tonometry arise from the variable physical parameters of the individual globe.1 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 tonomet-ric terms, this theory suggests that, all else being equal, tonometric readings might be lower in flatter corneas, and vice versa.

I attempted to ascertain in a purely empir­ical manner whether variations in the corneal curvature of different eyes affected the read­ing of the tonometer, and if so, to what de­gree.

MATERIAL AND METHODS

Corneal curvature was measured, and to­nometry was performed in a series of 200 patients coming in for routine eye examina­tion. There were 136 women and 64 men and their ages ranged from 40 to 93 years (mean 59.36). Mean intraocular pressure (15.35 mm Hg) and other statistical data showed this group to be a representative sample of the general population.2

The Bausch and Lomb keratometer was used to measure horizontal corneal curva­tures along the visual axis. Tonometry was then performed as nearly as possible along the same axis, with the tonometer mounted on a slit lamp at the horizontal meridian. The data were then analyzed by computer.

RESULTS

The results of the coupled measurements of the 400 eyes are shown in Figure 1. A positive correlation between corneal curva-

Reprint requests to Harry H. Mark, M.D., 2 Church Street South, New Haven, Connecticut 06519.

ture and tonometric reading emerged, with an F ratio of 13.15 indicating a definite slope ( P < .0005). The correlation coeffi­cient is 0.1788 (r2 = 0.03), meaning statisti­cally that 3 % of the tonometric values could be explained by variation in corneal curva­ture.

For these data, an increase of 1 D in cur­vature corresponds to an increase of 0.34 mm Hg in the tonometric value. Neverthe­less, because of the wide scatter, the chi square test showed no significant interdepen­dence (0.150, with one degree of freedom).

DISCUSSION

Based on certain theoretical premises, some investigators3'4 believe that variation in corneal curvature should have no effect on the reading of the applanation tonometer. Others5·6 have challenged this view.

The data in the present study indicate that variation in corneal curvature between dif­ferent eyes has a small but definite effect on the tonometric reading. A difference of three diopters in the corneal curvature of two eyes may affect a tonometric variation of 1 mm Hg. Within the extreme range of 40 to 49 D, the range of affect on tonometer readings amounts to 3 mm Hg, a degree that could be significant in certain borderline patients.

Attention to corneal curvature would therefore appear helpful in the interpretation of the tonometric record.

SUMMARY

Coupled recordings of corneal curvature and applanation tonometry (Goldmann) in 400 eyes revealed a positive correlation be­tween these values as analyzed by computer. An increase of three diopters in corneal power amounted statistically to an increase of about 1 mm Hg in the recording of the tonometer.

223

224 AMERICAN JOURNAL OF OPHTHALMOLOGY AUGUST, 1973

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ACKNOWLEDGMENT

Statistical computations were done under the di­rection of Emanuel Lerner at the Eastern Research Support Center, Veterans Administration Hospital, West Haven, Connecticut.

R E F E R E N C E S

1. Smith, P. : Limitations of the tonometer. Ophth. Rev. 34:65, 1915.

2. Mark, H. H. : Emmetropization. Ann. Ophth. 4:393,1972.

3. Schmidt, T. : Zur Applanationstonometrie an der Spaltlampe. Ophthalmologica 133 :337, 1957.

4. Goldmann, H. : Glaucoma. New York, Macy Foundation, 1957, p. 167.

5. Gloster, J., and Perkins, E. S. : The validity of the Imbert-Fick law as applied to applanation to-nometry. Exp. Eye Res. 2:274, 1963.

6. Markiewitz, H. H. : The so-called Imbert-Fick law. Arch. Ophth. 64:159, 1960.