A C T A O P H T H A L M O L O G I C A V O L . 4 8 1 9 7 0

T h e Department of Ophthalmology (Head: Prof. dr. med. Viggo A . /ensen)

Kommunehospitalet, University of Arhus, 8000 Arhus, Denmark

ON CORNEAL THICKNESS AND INTRAOCULAR PRESSURE. I I

A clinical study on the thickness of the corneal stroma in glaucomatous eyes

BY

NIELS EHLERS

In a previous study (Ehlers & Riise 1967) it was demonstrated, in a group of patients with unilateral retinal detachment, that when the intraocular pressure (IOP) was low the corneal thickness was greater than in the contralateral nor- motensive eye. This observation was in agreement with in vitro experiments on monkey eyes (Ehlers 1966, 1967) showing that the IOP exerts a reducing effect on the swelling of the cornea. The in vitro experiments further suggested that a reduced thickness of the corneal stroma should be found in eyes with elevated IOP.

Only few reports on the correlation between corneal thickness and increased IOP exist (Sbordone 1953, Y t t e b o r g & Dohlman 1965, Zuglio 1967) and the results are contradictory.

The purpose of the present study has been to compare values for the thickness of the corneal stroma in eyes with high IOP with 1) the thickness of the stroma in the contralateral normotensive eyes, and 2) the thickness of the stroma in the same eyes after the IOP has been normalized by medical treatment.

Methods

Patient material. All patients included in this study suffered from non-operated glaucoma, had good central vision, and clinically normal corneas, especially no epithelial oedema. Patients admitted to the Eye Department for glaucoma were

Received March 25, 1970.

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informed not to treat the glaucoma on the day of admission and the day before. As a result a number of patients showed elevated IOP. The thickness of the corneal stroma was measured and compared with the thickness in the contra- lateral eye if this showed normal IQP (Table I), and with the thickness in the same eye after the IOP had been reduced by treatment with pilocarpine, ace- tazolamide, and in some cases adrenaline, if the contralateral eye could not be used for comparison (Table 11). In order to rule out an effect of the given medi- cal treatment on the thickness of the corneal stroma a control group of persons with normal IOP was included (Table 111).

Measuring technique. The corneal thickness was measured according to the principle of Jaeger, using the attachment I to the Haag-Streit slit lamp. This apparatus appears to be the most reliable at the present time (Honegger & Genie 1968). The procedure followed has previously been reported (EhZers & Riise 1967). Although cases showing epithelial oedema have been excluded a subclinical oedema might increase epithelial thickness. For this reason only the stromal thickness was measured, as illustrated in Fig. 1.

The results have been statistically evaluted by paired comparison, using the t-test, as corneal thickness was found not to differ statistically significant from the normal distribution (Kruse Hansen, to be published). In tables 11 and I11 only one eye from each patient have been included, in order to have statistically independent data.

Glaucomatous eye

IOP T (mm Hg)

Table I . Corneal thickness in cases of unilateral glaucoma.

Contralateral eye AT

T (PI (mm HgI

KH f EL m MJ f EM f KF m CM f AT f TJ f

60 32 38 70 38 32 62 50

488 478 480 426 440 410 458 460

21 17 20 17 16 10 10 16

484 478 492 444 436 398 464 476

- 4 0

12 18

- 4 -1 2

6 16

mean 48 4 5 5 k 10 16 459 * 11 4 + 4

The table show corresponding and simultaneous values of IOP and thickness of corneal stroma in patients with unilateral glaucoma. Lower line mean k standard error of mean.

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Table I I . Effect of reduction of elevated IOP upon thicknes of corneal stroma.

ND m 0s 36 0 s m 0 s 48 MJ f 0 s 48 AT f 0 s 62 PB m od 44 HK m od 38 AV m od 32 MN f od 37 AP f 0 s 32 HS m od 35 AM m od 32 EJ f 0 s 34 TP m od 35 MC f 0s 30

446 404 426 458 432 448 452 448 464 454 438 458 426 434

22 19 18 12 15 18 17 19 16 20 17 20 12 20

482 434 460 500 438 480 452 478 468 466 472 468 436 438

36 30 34 42

6 32 0

30 4

12 34 10 10 4

mean 42 442 f 4 18 462 k 5 20 f 4

The table shows corresponding values of IOP and thickness of corneal stroma in eyes at high (IOP 2 308 mmHg) and normal (20 2 IOP 2 10 mmHg) intraocular pressure. The pressure was reduced by treatment with pilocarpine, acetazolamide and adrenaline. Lower line mean f standard error of mean.

Fig. 1. In the corneal microscope the endothelium appears single-contoured. The epithelium, however, shows a definite thickness and it is therefore possible to measure total thick-

ness (a) or, as in the present study, only stromal thickness (b).

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Table III. Effect of antiglaucomatous treatment on the thickness of the corneal stroma

in normal eyes.

EI?I m 0s 14 HB f 0s 17 FK m 0s 12 1s m 0s 15 LB m od 13 MB f 0s 16 GK f 0 s 18 IK m 0 s 18 IS f od 16

466 466 472 434 470 450 430 490 408

12 12 10 13 10 12 14 14 13

470 464 478 432 480 446 426 490 408

4 - 2

6 - 2

10 - 4 - 4

0 0

mean 15 454 * 9 12 454 * 9 1 + 2

The table shows corresponding values of IOP and thickness of corneal stroma in normal eyes before and after treatment for 2 days with pilocarpine, acetazolamide and adrenaline. Lower line mean _+ standard error of mean.

Resu I ts

Table I shows data for 8 patients in whom both eyes fulfil the criteria outlined above. In 3 cases the corneal stroma was found to be thicker in the eye with elevated IOP, in 4 cases it was thinner, and in one case no difference was found. The mean value of the differences (0.004 mm) is found not to differ statistically significant from zero (P < 0.4).

Table I1 shows data for 14 patients with initial IOP in one eye above 30 mm Hg, and a fall to between 10 and 20 mm Hg after treatment. The mean value of the thickness differences is found to differ statistically significant from zero (t = 5.15, P < 0.001).

The effect of the given treatment upon the thickness of the stroma has been studied by giving the same treatment to a group of volonteers (Table 111). A small reduction in IOP was seen and no effect upon thickness. In 4 cases the thickness was smaller after the treatment, in 3 cases it was greater, and in 2 cases identical values were found before and after treatment for 48 hours. The mean of the differences is found not to differ statistically significant from zero (P < 0.6).

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Discussion

A direct parellel to the previous study on reduced IOP (Ehlers & Riise 1967) would be a report on a greater number of patients with unilateral non-irritative (simple) glaucoma in one eye, and a normal contralateral eye. These patients are, unfortunately, seen only seldom. Table I includes but 8 patients. No signifi- cant difference in thickness has been found. Sbordone (1953) studied the corneal thickness in acute and chronic glaucoma. I n two cases of acute glaucoma the thickness was increased considerably, an observation made several times during the present study. In 12 cases of long-lasting presssure difference (chronic glau- coma) the cornea of the eye with the higher pressure was thinner in 9 cases and thicker in 3 cases. On an average the cornea was thinner in the eye with the higher pressure. Ytteborg & Dohlman (1965) presented a rather impressive table including 15 cases of IOP of 31 to 68 mm Hg in one eye and normal IOP in the other eye. No epithelial oedema was present. A comparison of the two mean values showed no significant difference in thickness (P < 0.25), however, by paired comparison a value of P < 0.02 is found, suggesting an increased thickness of the total cornea at increased IOP. From studies on the physiology of the cornea (Maurice & Giardini 1951, Honegger 1962) it is evident that when the endothelial barrier is removed or broken down an increase in stromal thickness occurs. This is probably what happens in acute glaucomas and after operations, where considerably increased thickness is found. However, it says nothing about the effect of IOP when an intact endothelium is present. In the above mentioned table of Ytteborg & Dohlman it is not mentioned whether all cases were non-irritative glaucomas, neither is it mentioned whether the pres- sure in any of the contralateral eyes had been reduced by operation. luglio (1967) studied 5 cases, at high, normal and low IOP. The cases are not further characterized and for the reasons mentioned above several interpretations are possible.

By comparing the thickness of the stroma of the same cornea at normal and high IOP the data in Table 11 were found. The thickness increased significantly (P < 0.001) when the IOP was reduced from above 30 mm Hg to the normal range (10 s IOP 4 20). The treatment employed to reduce the IOP had no measurable effect upon the thickness (Table 111). It therefore appears that the changes in thickness are caused by the changes in IOP. The IOP had probably been elevated for a prolonged period of time, as no treatment were given the two foregoing days. The values at normal IOP were not measured until the pressure had been normalized for 1 or 2 days. The uncertainty of not being in a steady state condition when measuring pressure and thickness is therefore probably eliminated. I t may be concluded that the IOP exerts a reducing effect upon the thickness. This agrees with the mentioned in nitro experiments on monkey eyes, and supports the existence, within the physiologic range, of me-

1 1 1 1 74’’

chanical factors in the maintenance of normal corneal thickness and state of hydration.

Summary

T h e thickness of the corneal stroma has been studied in cases of glaucoma. In 8 cases of unilateral glaucoma no difference in thickness could be observed. In 14 cases of glaucoma a statistically significant increase in thickness was ob- served when the intraocular pressure was reduced by medical treatment f rom above 30 m m Hg to the range 10-20 m m Hg. In a control series the treatment had n o effect upon the thickness. It is concluded that within the physiologic range increased pressure reduces the thickness of the cornea.

References

Ehlers, N . (1966): The fibrillary texture and the hydration of the cornea. - Acta

Ehlers, N . (1967): Mechanical factors in the maintenance of normal corneal detur-

Ehlers, N . & Riise, D. (1967): On corneal thickness and intraocular pressure. - Acta

Honegger, H . (1962) : Quantitative Untersuchungen iiber die Hornhautendothelregene-

Honegger, H . & Genke, E. (196s) : Hornhautdickenmessung. Ein Vergleich verschiede-

Zuglio, N . (1967): Influenza della pressione intraoculare sullo spessore della cornea

Maurice, D. M. & Giardini, A. A . (1951): Swelling of the cornea in vivo after destruc-

Sbordone, G (1953): Richerche sullo spessore della cornea in occhi glaucomatosi. - Ram.

Ytteborg, /. & Dohlman, C. H . (1965): Corneal edem and intraocular presure, I1 Cli-

ophthal., Kbh. 44: 620-630.

gescence. - Acta ophthal., Kbh. 45: 658-672.

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nical results. - Arch. Ophthal., Chicago 74: 477-484.

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