Corneal Swelling

Eric Papas- PhD, MCOptom, DipCL
Clinical Discipline Leader
Cornea and Contact Lens Research Unit (CCLRU)
Cooperative Research Centre for Eye Research and Technology (CRCERT)

Reviewed Article: Fonn D, du Toit R, Simpson TL, Vega JA, SituP, Chalmers RL. (1999). Sympathetic swelling response of thecontrol eye to soft lenses in the other eye. Invest. Opthalmol.Vis. Sci., 40, 3116- 3122.

Imagine fitting a contact lens in one eye and getting a changein corneal swelling in the other! That is the intriguing possibilitysuggested by the work reported in this paper by Fonnet al.

The experiments conducted by this group involved a series of studiesduring which subjects’ corneal thicknesses were measured,by optical pachometry, before and after overnight wear of two typesof soft contact lenses. One of these was a highly oxygen permeablesilicone hydrogel (Dk 140) and the other a conventional hydrogelwith relatively low oxygen permeability (Dk 18). The corneal swellingproduced by these lenses was compared, both with one another andwith the changes measured in control eyes, which wore no lensesduring the experiment.

The results are partially reproduced in Table 1 and, as expected,indicate that both no lens wear, and the highly oxygen permeablepolymer induced considerably less corneal swelling that did thelow Dk lens. A more interesting finding emerged because of theway the experiment was arranged. No matter which lens was beingworn in the test eye, the other eye always remained empty to serveas a control. This enabled a comparison to be made between therespective control eyes when the Dk 140 and Dk 18 lenses were beingworn in the other eye. One might anticipate that the control eyeswelling would be about the same in both these situations, butsurprisingly, more swelling occurred in those that were pairedwith low Dk lenses than in those twinned with the high Dk material.This seems to imply that a sympathetic response had taken place,i.e. increased corneal thickness in one eye was being partiallycommunicated to the cornea in the other.

The story does not end there. In a further study, Fonnet alre-measuredovernight corneal swelling in a subset (n=13) of their originalsample (n=20) while they wore no lenses in either eye. On thisoccasion the amount of swelling recorded was the same in both eyes,and comparable to that originally seen in the low Dk control corneas.It was however,greaterthan that seen in the high Dk control eyesby a small but significant amount. So, the implication seems tobe that if a contralateral effect does exist in these experiments,it is a thinning, rather than swelling phenomenon.
Such an event would be a remarkable occurrence whose underlyingmechanism can only be guessed at. The conjecture of Fonnetalis that a component of the high Dk lens, perhaps silicone, actsto limit swelling behaviour and that this effect is capable ofbeing exerted not only directly but also contralateraly.

In assessing the impact of this study the possibility must beconsidered that the result is simply artifactual. A previousattempt to observe contralateral swelling was unsuccessful¹andlittle or no other direct corroborating evidence exists. Neverthelessthe reported data appear accurate and are well supported by other,concurrent measurements, such as corneal light scatter. Furthermore,in addition to the effect being evident from the average behaviourof the group, to one degree or another all the individual subjectsshowed evidence of the same response. These are compelling details,but alternative data are clearly needed to fully resolve theissue.

Irrespective of whether such studies prove to be confirmatoryor otherwise, the consequences for researchers and practitionerswill be substantial. Sympathetic responses are important phenomenaas they indicate that the two eyes do not behave independently.Experimentally, that is usually something of a nuisance. It means,for example, that if we wish to compare the behaviour of twodifferent contact lenses, we cannot use both eyes of the sameperson, at the same time. Twice the number of trials then becomenecessary, inevitably involving extra time and expense.

On the physiological side, inter-ocular linkages are by no meansrare. Phenomena such as accommodation and the pupillary lightreflex are familiar examples, and there are others like tearfilm osmolality changes that are less so. While the neural andbiochemical bases for these kinds of systems are reasonably evident,the same cannot be said for inter-corneal communication.

Establishingthe physiological processes that might be responsible for sucha mechanism would pose an intriguing challenge, and one thatwould enrich our understanding of the complexities of ocularfunction.