THE C O R N E A L T H I C K N E S S D U R I N G P E R F U S I O N OF

THE A N T E R I O R C H A M B E R WITH S O L U T I O N S OF

V A R I O U S T O N I C I T Y

by

GUNNAR VON BAHR

( Uppsala)

With 3 Figures

It is a well known fact that the cornea has a great swelling capacity so that it will swell considerably even when placed in blood serum, and that the cornea in vivo is kept dehydrated to some extent. The mech- anism for this dehydration, however, is still not very well known. The epithelial and endothelial layers can act as semipermeable membranes, so that solutions of various concentrations in contact with the surface can make the cornea thinner if they are hypertonic and thicker if they are hypotonic as shown by COGAN • KINSEY (1942) on excised eyes and by VON BAHR (1948, 1956 a, b) on living eyes. As the tears as well as the aqueous humour are hypertonic to blood plasma the theory has been developed that the deturgescence of the cornea is caused by osmotic forces. This theory has met severe criticism and experiments with radio- active isotopes (POTTS et al., 1950) on living eyes and studies on the metabolism of the cornea in various conditions (LANGHAM, 1954; SCHWARTZ, DANES tfr LEINFELDER, 1954; DAVSON 1955; HARRIS & NORD- QUIST, 1955) have given good reasons for the assumption of some vital activity of the cellular constituents of the cornea for the dehydration of the cornea.

In measurements of the thickness of the human cornea in the evening and just after the opening of the eyes in the morning I have found no difference. As there is no evaporation from the tear film during sleep the tear fluid should be isosmotic with the tissues and should not have any osmotic effect on the cornea during sleep. The thickness of the cornea does not increase, however, and this shows that osmotic action through the epithelial surface cannot be essential for the corneal deturgescence.

Similar measurements have now been performed when the anterior chamber was perfused with an artificial aqueous humour the tonicity of which was varied by variations in the concentration of sodium chloride.

Methods

The artificial aqueous humour was an aqueous solution containing the most important ions of the natural aqueous and glucose to about the normal amount with the exception for sodium chloride the content of which was varied. The solution was saturated by a mixture of 5 % CO2 and 95 % 02 which together with the sodium bicarbonate of the solution buffered it to the normal level of pH. The tonicity of the solutions was measured by determinations of the freezing point depression (A).

For the peffusion of the anterior chamber the cornea was run through by a cannula with two side openings separated by a stopper in the lumen, one for the inflow and one for the ouflow of the fluid. If the flow was kept rapid enough there was no disturbing coagulation of secondary aqueous in the anterior chamber.

Figure 1.

The experiments were performed on rabbits which were kept narcotized

with intlavenous Numal.

The corneal thickness was measured by the optical method of yon BArIR.

R e s u l t s

When the artificial aqueous humour had a freezing point depression of -0~ being of about normal tonicity, the cornea generally kept its normal thickness for at least a couple o f hours.

I f the solution was a little less concentrated with A = -0~ which is

that of blood plasma, a small increase was most often - but not always -

i I~ ~:i

Figure 2.

observed, In a series of 9 experiments (Fig. 1) where the eye was at first perfused with the solution with A = -0~ the change to the solution with A = -0~ resulted in an increase in thickness of about 7 ~ as an average. This increase occurred in the first few minutes after which the thickness remained failly constant at the higher level. It was observed that the thickness could remain constant for at least a couple of hours during perfusion with the solution isotonic with blood. (Fig. 2).

Perfusion with more concentrated solutions decreased the corneal thickness, and perfusion with a less concentrated solution increased it. (Fig. 3). It is remarkable that the change in thickness occurred very rapidly, within less than 10 minutes, and that the thickness then remained constant as long as the solution was the same.

30

2t

10

~=-0.7~-

I

*.=-o.~'o

Figure 3.

r

o ,

A = --0.74.

Conclusions

These experiments confirm that the inner surface layer of the cornea to some extent acts as a semipermeable membrane, and that the osmotic pressure of the content of the anterior chamber has some influence on the thickness of the cornea. They also show, however, that this effect is very limited. There will very soon be a state of equilibrium even if the tonicity is considerably higher or lower than that of the normal aqueous. The difference in colneal thickness when the fluid in the anterior chamber has the tonicity of normal aqueous or that of normal blood plasma is rather small.

It is evident, therefore, t ha t though the hydration of the cornea is influenced by osmotic forces acting across its surfaces, the normal deturgescence cannot depend essentially on the hypertonicity of the aqueous or the tear film. For this some vital activity in the corneal tissue must be present.

Summary

The experiments confirm that the inner surface of the cornea acts as a semipermeable membrane. The osmotic pressure of the aqueous humour has a certain influence on the thickness of the cornea, but the effect is limited.

Thus, it is proved that the hypertonicity of the aqueous or the tears cannot alone explain the normal deturgescent state of the cornea. Some vital activity of the corneal tissue is also required.

R~sum6

Les experiences confirment que la couche interne de la corn6e agit comme une membrane semi-perm6able. La pression osmotique de l 'humeur aqueuse a une certaine influence sur l'6paisseur de la corn~e, mais l'effet est limitS.

I1 est ainsi d6montr6 que l'hypertonicit6 de l 'humeur aqueuse ou des larmes ne peut / t elle seule expliquer l'6tat de d~turgescense dans lequel la corn6e se trouve normalement. I1 faut, au contraire, faire appel une certaine activit6 vitale dans le tissu corn6e.

Zusammenfassung

Die Versuche best~tigen, dass die innere Hornhautschicht als eine semi-permeable Membran angesehen werden kann. Der osmotische

D t u c k des Kammerwassers i ibt einen deut l ichen Einfluss auf die Dicke der

H o r n h a u t aus, aber die Ver~inderungen sind nicht ausgiebig.

Es ist somit bewiesen, dass die Hypertonizit~it des Kammerwassers

oder der Tr~inen nicht geniigt, den Zus tand der Entquel lung der normalen

H o r n h a u t zu erkliiren. Es muss eine vitale Aktivi t i i t im Hornhau tgewebe

angenommen werden.

R E F E R E N C E S

BAHR VON, G. (1948) Measurement of the Effect of Solutions of Different Osmotic Pressure on the Thickness of the Living Cornea. Trans. ophthal. Soc. U. K. 68, 515-524.

, (1956a) Corneal Thickness. Amer. J. Ophthal. 42, 251-263. , (1956b) Corneal Deturgescence. Modern ProbL OphthaL 1, 20-34.

COGAN, D. G. & KINSEY, V. E. (1942) The Cornea. Physiological Aspects. Arch. Ophthal. 28, 661-669.

DAVSON, H. (1955) The Hydration of the Cornea. Biochern. J. 59, 24-28. HARRIS, J. E. & NORDQUIST, L. T. (1955) The Hydration of the Cornea. Transport of

Water From the Cornea. Amer. J. Ophthal. 40, 100-110. LANGHAM, M. E. (1954) Relation of Corneal Respiration to Hydration and Opaci-

fication. Acta XFII Concil. ophthal. Canada, U. S. A. (1955) 1,500-505. POTTS, A. M. & JOHNSON, L. W. (1950) The Nutritional Supply of Corneal Regions in

Experimental Animals; the Supply of Some Inorganic Ions. Amer. J. Ophthal. 33, 405-419.

SCHWARZ, B., DAWES, B. & LEINFELDER, P. J. (1954) The Role of Metabolism in the Hydration of the Isolated Lens and Cornea. Amer. J. Ophthal. 38, 182-194.

Ophthalmological Clinic of the University.

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