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, Situ P, Chalmers RL. (1999). Sympathetic swelling response of the control 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 change in corneal swelling in the other! That is the intriguing possibility suggested by the work reported in this paper by Fonn et al.

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

The results are partially reproduced in Table 1 and, as expected, indicate that both no lens wear, and the highly oxygen permeable polymer induced considerably less corneal swelling that did the low Dk lens. A more interesting finding emerged because of the way the experiment was arranged. No matter which lens was being worn in the test eye, the other eye always remained empty to serve as a control. This enabled a comparison to be made between the respective control eyes when the Dk 140 and Dk 18 lenses were being worn in the other eye. One might anticipate that the control eye swelling would be about the same in both these situations, but surprisingly, more swelling occurred in those that were paired with 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 partially communicated to the cornea in the other.

The story does not end there. In a further study, Fonn et al re-measured overnight corneal swelling in a subset (n=13) of their original sample (n=20) while they wore no lenses in either eye. On this occasion 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, greater than that seen in the high Dk control eyes by a small but significant amount. So, the implication seems to be 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 underlying mechanism can only be guessed at. The conjecture of Fonn et al is that a component of the high Dk lens, perhaps silicone, acts to limit swelling behaviour and that this effect is capable of being exerted not only directly but also contralateraly.

In assessing the impact of this study the possibility must be considered that the result is simply artifactual. A previous attempt to observe contralateral swelling was unsuccessful¹ and little or no other direct corroborating evidence exists. Nevertheless the 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 behaviour of the group, to one degree or another all the individual subjects showed evidence of the same response. These are compelling details, but alternative data are clearly needed to fully resolve the issue.

Irrespective of whether such studies prove to be confirmatory or otherwise, the consequences for researchers and practitioners will be substantial. Sympathetic responses are important phenomena as 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 two different contact lenses, we cannot use both eyes of the same person, at the same time. Twice the number of trials then become necessary, inevitably involving extra time and expense.

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

Establishing the physiological processes that might be responsible for such a mechanism would pose an intriguing challenge, and one that would enrich our understanding of the complexities of ocular function.