O B S E R V A T I O N S O N T H E CLINICAL D E T E R M I N A T I O N O F SCLERAL RIGIDITY*

D A V I D A. R O S E N , M.D. , A N D

Kingston

The term scleral rigidity refers to the resistance which the eye offers to a change of intraocular volume. Such a change is produced when the footplate of a tonometer flattens the cornea and its plunger indents it. To a limited extent this change is cush-ioned by compression of the intraocular vascular bed. The remainder of the change of volume is translated into an increase in intraocular pressure since the sclera, through its rigidity, resists expansion. The elevation of intraocular pressure thus produced ( P t ) will be higher when the scleral rigidity is high than when it is low. The indentation produced by the tonometer plunger, in addi-tion to other factors such as corneal curva-ture, is dependent both on the intraocular pressure and on the resistance of the ocular coats to the ocular volume changes produced by the tonometer weight. The conversion scales used with indentation tonometers as-sume that the eyes to be examined will have an average and normal scleral rigidity.

The ocular pressure-volume relationships have intrigued investigators for a great many years. One finds the studies on this subject in a variety of species dating back to 1 8 8 4 . 1 β W e owe to FriedenwakT the appreciation of the importance of the scleral rigidity factor in human tonometry and the development of the first clinical method for its quantitative determination. He recog-nized that when the tonometer plunger in-dents the cornea, fluid is displaced beneath it and the intraocular pressure rises. Part of the displaced fluid may be accounted for by an expulsion of blood from the intraocular

* From the Department of Ophthalmology, Faculty of Medicine, Queen's University . T h i s work was supported by grant M.T . 519 of the N a -tional Research Council of Canada. Presented at the meeting of the Canadian Ophthalmological S o -ciety, Montreal, Canada, June, 1961.

A L B E R T O G. W A R M A N , M.D.

Ontario

vascular bed and by outflow of aqueous. The remainder must be accommodated by a dis-tortion of the eyeball as a whole. The first factor he considered to be insignificant as the pressure-volume relationships were found to be unchanged in enucleated eyes of exsan-guinated animals.

The determination of scleral rigidity has been achieved by a variety of techniques. In a number of species, including the human, the effects of measured changes of intra-ocular volume on the intraocular pressure have been studied by direct cannulation and manometry. A number of refinements of technique, including devices for accurately delivering microquantities of fluid into the eye and strain gauge manometry for ac-curately measuring the intraocular pressure, have been applied to this problem.

The results of these studies have been variable and, in many instances, inconsist-ent. In the rabbit eye the coefficient of scleral rigidity has been found to increase with increasing intraocular pressure.8"1 0 In the cat, the rigidity constant has been found to increase with intraocular pressures up to a level of 30 mm. H g and to diminish there-after . 1 1 ' 1 2 Human eye studies have, in gen-eral, indicated that the rigidity coefficient falls as the intraocular pressure rises"' 1 4

but results have not, in all studies, been con-sistent."-1 7

Studies have been made on the extensi-bility of strips of sclera and cornea.1 8 These studies have shown that with increasing pressure the rigidity factor rises.

The clinical methods for assessment of scleral rigidity are based on the contribu-tions of Friedenwald and Goldmann. Frie-denwald's method7 consists of the determina-tion of the Schijzitz scale reading for two tonometer weights in the human eye (dif-ferential tonometry). By relating the loga-

376 D A V I D A. R O S E N A N D A L B E R T O G. W A R M A N

rithm of the P t with each of the two tonom-eter weights and the volume changes pro-duced by these weights (both derived from Friedenwald tables which assume an aver-age coefficient of scleral rigidity which is independent of the intraocular pressure) an expression for scleral rigidity can be de-rived and proves to average 0.0215.

Kr = b g P + 1 ° - l 0 g F+6i= 0.0215 V10 - V5.5

It is recognized that the clinical reading error with a mechanical tonometer is plus or minus 0.5 scale units. Hence the error in recording a single pair of scale readings is plus or minus 1.0 scale units. One pair of readings may, therefore, give large errors in the estimation of the coefficient of scleral rigidity and the true intraocular pressure. The error is reduced if several pairs of readings are made and these are averaged. The taking of several readings, however, will produce a massage effect and an up-ward drift of the tonometer scale. Hence, in the clinical application of this technique one considers that if corresponding results are obtained with two weights, the rigidity co-efficient is probably normal. If the readings are discordant, several pairs of readings should be obtained.

This source of error is eliminated in using the electronic tonometer coupled to a recorder which obviates the visual assess-ment of the scale reading.

The rigidity factor may be determined more simply by reference to the Frieden-wald nomograms or tables. Those now in use are based on the 1955 tonometer calibration figures.19

With the introduction of a practical ap-planation tonometer by Goldmann, an alter-native method of clinically assessing the scleral rigidity has become available.2 0 This consists of the determination of the intra-ocular pressure by applanation tonometry and by a single Schätz tonometer reading. The formula for calculation of scleral rigid-

ity is identical to that used in the Frieden-wald method. The P„ and P t readings with the applanation tonometer may be assumed to be identical and the volume displacement produced by the applanation tonometer has been calculated by Goldmann to be 0.45 cu. mm. Using this method, Goldnann found the average scleral rigidity value for the human eye to be 0.0203.

log P + Schätz — log P + Appl. Kr = •

V Schätz - 0.45 = 0.0203

Friedenwald7 found the range of varia-tion of scleral rigidity to be from 0.002 to 0.55. The scleral rigidity was found to in-crease with age. It was found to decrease with increasing axial length of the eye and was also found to decrease with increasing corneal curvature. He emphasized that both the compressibility of the intraocular vas-cular bed and the distensibility of the sclera enter into the rigidity determination and he was unable to determine which of these factors was responsible for the variations found with age and high myopia.

Schmidt 2 1 - 2 2 and Drance2 3 have found that significant variation from the average scleral rigidity is unusual in normal eyes while in the glaucoma population variations both above and below the average are quite common.

Goldmann has confirmed the relationship of refractive error to scleral rigidity by his method. Others have pointed to other clini-cal situations in which alteration of scleral rigidity is to be expected. Notably, it has been demonstrated that a fall in scleral rigidity occurs in patients with progres-sive endocrine exophthalmos.2 4 Others have pointed to a reduction in scleral rigidity in the water provocative test . 2 3 ' 2 5 The medical and surgical reduction of intraocular pres-s é e in the treatment of glaucoma have like-wise been found to be associated with altera-tion of scleral r ig id i ty . 2 2 ' 2 8 , 2 6

The importance of the scleral rigidity

D E T E R M I N A T I O N O F S C L E R A L R I G I D I T Y 377

T A B L E 1

E F F E C T OF SCLERAL RIGIDITY O N INTRAOCULAR

P R E S S U R E L E V E L S D E T E R M I N E D I N D E N T A T I O N TONOMETRY

BY

Scale reading 5 / 5 . S Ε = 0 . 0 2 Ρ = 18 Scale reading 5 / 5 . S Ε = 0 . 0 4 Ρ = 9 Ε = 0 . 0 1 Ρ = 23 Ε = 0 . 0 0 5 Ρ = 28

Scale reading 2 / 7 . 5 Ε = 0 . 0 2 Ρ = 4 4 Scale reading 2 / 7 . 5 Ε = 0 . 0 4 Ρ = 3 3 Ε = 0 . 0 1 Ρ = 4 9 Ε = 0 . 0 0 5 Ρ = 54

factor in the clinical determination of intra-ocular pressure is apparent when one com-pares the true intraocular pressures corre-sponding to a given tonometer scale reading in eyes with varying scleral rigidity (table 1) . The most desirable clinical method for circumventing this potential source of error is the use of applanation tonometry where the scleral rigidity factor does not enter into the reading as there are minimal ocular volume changes.

A second clinical area in which scleral rigidity can be of great importance is in the assessment of aqueous outflow facility by tonography. Here one finds that at high levels of scleral rigidity one obtains falsely high facilities of aqueous outflow unless one introduces a correction for scleral rigidity. A reverse relationship is found in instances of reduced rigidity (table 2 ) . A practical method of correcting for this factor in tonog-raphy has been proposed by Moses and Becker.2 7

We have studied a number of aspects of the clinical determination of scleral rigidity in human subjects. For this purpose, use

T A B L E 3

S C L E R A L RIGIDITY A S D E T E R M I N E D BY

D I F F E R E N T METHODS

(38 normal eyes )

Applanat ion 5.5 g m . 7.5 g m . 7.5 g m . 10 g m . 15.0 g m .

Mean 0 . 0 2 4 7 0 . 0 2 3 2 0 . 0 2 6 6 S . D . ± 0 . 0 0 6 7 ± 0 . 0 0 9 0 ± 0 . 0 0 9 5 S .E .M. 0 . 0 0 0 9 0 . 0 0 1 2 0 . 0 0 1 2

has been made of a Mueller electronic to-nometer recording on a Leeds and Northrup Speedomax Β recording potentiometer or on an Esterline-Angus recorder, thus elimi-nating the errors inherent in visual assess-ment of tonometer scale units. Applanation tonometry has been carried out by two ex-perienced persons using the Goldmann to-nometer mounted on a Haag-Streit slitlamp microscope. Scleral rigidity was calculated using the Friedenwald formula and the 1955 calibration tables. All the eyes studied were organically normal and had no refractive error greater than ± 3 . 0 diopters. The fol-lowing observations have been made:

1. Assessment of scleral rigidity by vari-ous methods (tables 3, 4 ) . In this study repeated measurements were made on the eyes of exceptionally reliable patients by a variety of methods. The standard deviation was found to be lowest when a 10 gm. Schi^tz recording was coupled with an ap-planation pressure reading for calculation purposes. The mean coefficient of scleral rigidity in this group was found to be 0.0247. This method was used in all subse-quent studies.

T A B L E 2

E F F E C T OF SCLERAL RIGIDITY ON V A L U E S FOR OUTFLOW FACILITY CALCULATED FROM

TONOGRAPHIC TRACINGS

Scale Reading Fall ing from 5.0 to 7.0 wi th 5.5 g m . weight

F. = 0 . 0 2 1 5 E = 0 . 0 1 3 7 E = 0 . 0 3 4 2

C = 0 . 1 7 C = 0 . 3 4 C = 0 . 0 9 3

T A B L E 4

S C L E R A L RIGIDITY A S D E T E R M I N E D BY

D I F F E R E N T METHODS

(40 normal eyes )

Aoolanat ion ADolanation Aoplanation (5.5 g m . ) (7.5 g m . ) (10 g m . )

Mean S . D . S .E .M.

0 . 0 2 8 7 ± 0 . 0 1 0 5

0 . 0 0 1 6

0 . 0 2 5 4 ± 0 . 0 1 4 2

0 .0022

0 . 0 2 4 7 ± 0 . 0 0 3 4

0 . 0 0 0 5

378 D A V I D A. R O S E N A N D A L B E R T O G. W A R M A N

T A B L E 5

DIURNAL VARIATIONS OF INTRAOCULAR PRESSURE AND RIGIDITY

8 : 0 0 A.M. 11:00 A . M . 2 : 0 0 P.M. 5 : 0 0 P.M. 8 : 0 0 P.M. 11:00 P.M.

Ρ Ε Ρ Ε Ρ Ε Ρ Ε Ρ Ε Ρ Ε

N o . 28 28 28 28 28 28 28 28 28 28 28 28

Mean 1 9 . 2 0 . 0 1 8 8 20 0 . 0 1 8 3 1 9 . 6 0 . 0 1 6 8 1 6 . 9 0 . 0 2 2 9 18 0 . 0 1 9 0 1 7 . 8 0 . 0 1 7 3

S . D . ± 5 . 1 0 . 0 0 9 2 4 . 9 0 . 0 0 7 9 4 . 6 0 . 0 0 3 1 6 . 3 0 . 0 1 1 1 5 . 1 0 . 0 0 6 5 3 . 5 0 . 0 0 5 9

S .E .M. 0 . 9 7 0 . 0 0 1 7 0 . 9 3 0 . 0 0 1 5 0 . 8 7 0 . 0 0 0 6 1 .19 0 . 0 0 2 1 0 . 9 6 0 . 0 0 1 2 0 . 6 5 0 . 0 0 1 1

2. Diurnal variations of scleral rigidity (table 5 ) . Two hourly diurnal curves of intraocular pressure and coefficient of scleral rigidity were carried out in 38 normal eyes. The rigidity coefficient was not found to correlate with fluctuations of intraocular pressure and the variations noted were, on analysis, judged to be due to chance. These results are at variance with those of Mac-Donald 2 8 who observed in the eyes of glau-comatous patients that the rigidity coefficient falls as the intraocular pressure rises.

3. Results in repeated determinations of scleral rigidity at regular intervals. Nine

young adult males were examined weekly at the same time of day. The values obtained were not found to vary significantly and the standard deviation of the determinations was 0.009.

4. Effect of menstrual cycle on scleral rigidity (fig. 1) . Twelve student nurses were examined during the menstrual cycle at seven-day intervals after the onset of menstruation. The study was limited to subjects having a regular 28-day cycle and was repeated in two to six cycles. The re-sults are plotted graphically in Figure 1 and indicate no significant variation in ocu-

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Fig . 1 (Rosen and W a r m a n ) . T h e effect of the menstrual cycle on ocular rigidity.

D E T E R M I N A T I O N O F S C L E R A L R I G I D I T Y 3 7 9

T A B L E 6

E F F E C T OF MYDRIATICS O N SCLERAL RIGIDITY

Cyclopentolate 1 % Phenylphrine 1 0 % ( 5 8 eyes ) ( 4 8 eyes )

Before After Before After

Mean 0 . 0 2 4 9 0 . 0 2 7 7 0 . 0 2 3 5 0 . 0 2 3 2

S .D . ± 0 . 0 0 4 5 ± 0 . 0 0 7 1 ± 0 . 0 0 4 5 ± 0 . 0 0 4 5

S .E .M. 0 . 0 0 0 5 0 . 0 0 0 9 0 . 0 0 0 6 0 . 0 0 0 6

lar rigidity in different phases of the men-strual cycle.

5. Effect of mydriatics on scleral rigidity (table 6 ) . Scleral rigidity values were de-termined before and 45 minutes after the instillation of 10-percent phenylephrine or 1.0-percent cyclopentolate. No significant alteration of the rigidity coefficient was ob-tained.

6. Effect of pilocarpine on scleral rigidity (table 7 ) . The coefficient of scleral rigidity was assessed both before and 45 minutes after the application of 2.0-percent pilocar-pine in 38 normal eyes. The small decline of scleral rigidity observed was not considered to be statistically significant.

7. Effect of intraocular surgery on scleral rigidity (table 8 ) . The rigidity of 55 eyes was assessed repeatedly up to 120 days after surgery for senile cataract. As noted in the table, the scleral rigidity was not observed to vary significantly. This finding conflicts with those of Roberts2 6 and Schmidt2 2 who commonly observed a striking decline of scleral rigidity after filtering and nonfilter-ing ophthalmic surgery. W e have made similar observations on occasion but our overall impression is that cataract surgery does not significantly modify the ocular

T A B L E 7

E F F E C T OF P I L O C A R P I N E O N SCLERAL RIGIDITY

( 3 8 eyes )

Before After

Mean 0 . 0 3 4 5 0 . 0 3 1 2

S . D . ± 0 . 0 1 1 5 ± 0 . 0 1 0 1 S .E .M. 0 . 0 0 1 8 0 . 0 0 1 6

T A B L E 8

E F F E C T OF CATARACT EXTRACTION O N SCLERAL RIGIDITY

( 5 5 eyes )

Preoperative Under 5 0

D a y s 5 0 - 7 5 D a y s

Over 7 5 D a y s

0 . 0 2 4 7 0 . 0 2 5 7 0 . 0 2 3 8 0 . 0 2 4 8

rigidity. The fall of scleral rigidity can, at times, be very impressive, and in such in-stances there would obtain a false low esti-mation of intraocular pressure by indenta-tion tonometry.

8. Effect of medical treatment of glau-coma on scleral rigidity. Others 2 8 have ob-served that in the instillation of miotics, especially the more powerful anticholin-esterase agents, into the eyes of glaucomatous patients may result in an important decline of scleral rigidity with resultant erroneous interpretation of intraocular pressure by Schi^tz tonometry. W e have observed such a decline in only six of 62 patients treated with demecarium bromide or echothiophate iodide and this reduction was not consist-ently noted in repeated observations on these patients. The possibility of such an effect of miotic drugs should keep one alert to the potential for alteration of scleral rigidity in glaucomatous patients and the resultant dis-crepancies in ocular pressure assessment by indentation tonometric techniques. W e have, at no time, noted a significant elevation of ocular rigidity in patients under treatment for glaucoma.

S U M M A R Y

1. The nature and measurement of scleral rigidity has been discussed.

2. Scleral rigidity may be assessed most reliably by calculation from a 10 gm. Schätz tonometer reading and a Goldmann applana-tion reading.

3. Scleral rigidity of normal eyes does not vary significantly in diurnal pressure variations, in repeated determinations and in the menstrual cycle.

380 D A V I D A. R O S E N A N D A L B E R T O G. W A R M A N

4. Phenylephrine, cyclopentolate, and pi-locarpine do not significantly alter the scleral rigidity of the normal eye.

5. Cataract surgery has no significant effect on scleral rigidity of otherwise normal eyes within 120 days.

6. The lowering of intraocular pressure by strong miotics only occasionally lowers the scleral rigidity.

Faculty of Medicine.

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