Measuring Corneal Sensitivity in Contact Lens Wearers
BY ERIC PAPAS, PHD, MCOPTOM, DIPCL
Back in optometry school, what I was taught about the influence of contact lenses on corneal sensitivity seemed pretty straightforward. Based largely on work such as that of Knoll and Williams as well as that of Millodot, the general understanding was that hard lenses substantially reduced touch sensitivity, whereas soft lenses had a lesser (Millodot, 1976) or even minimal (Knoll and Williams, 1970) effect. Of course these experiments were conducted some 35 years or so ago, when lenses were predominantly made from either PMMA or pHEMA, so we might expect that there would have been some revisions to this view in the interim. In reviewing the literature, however, information on how modern lens types affect ocular surface sensitivity is hard to find.
So far as GP lenses are concerned, the most recent available data come from studies using materials that would be considered to have low oxygen permeability by present day standards (~10 barrers). Nevertheless, their effects on corneal touch sensitivity are certainly less severe compared to PMMA (Douthwaite and Connelly, 1986) and may even be negligible based on the observation that subjects refitted to GP lenses after initially wearing PMMA regained normal sensitivity after a few weeks (Bergenske and Polse, 1987).
More contemporary information is on hand for soft silicone hydrogel lenses, daily wear of which was found to have essentially no effect (Liu et al, 2009). Though it would be helpful to have more data, particularly for GPs, these studies tend to support the view that wearers of modern, higher-Dk lenses have little reason to be concerned about the impact of their lens wear on corneal sensitivity.
Measuring Ocular Surface Sensation
Before settling on this conclusion though, it is important to appreciate that there is another side to the story; one that has been illuminated by a radical shift in our understanding of the way ocular surface sensations are mediated and measured. The majority of sensitivity studies, including the ones mentioned previously, have been conducted using the Cochet-Bonet aesthesiometer. This instrument works by repeatedly bringing a nylon filament into contact with the surface of the eye. The filament is progressively shortened between trials to increase the stimulus intensity until the subject indicates that a touch has been felt. This sensation is registered by receptors that are sensitive to pressure or mechanical force—but it has become evident that the eye also possesses receptors that are capable of responding to other kinds of energy (Belmonte et al, 2004).
In fact, pure mechano-receptors make up only about 15 to 20 percent of the total number of sensory elements on the ocular surface. About four-times more abundant are a group that can respond to the multiple stimuli of heat, chemical irritants of various kinds, and mechanical force. These polymodal nociceptors account for around 70 percent of the total receptor number. The remaining 10 to 15 percent are also thermally sensitive, but respond to falling temperature, i.e. cooling. None of these receptors are limited to the cornea alone, but can be found distributed across the entire ocular surface.
Faced with this diverse array of sensory elements, it is apparent that the range of responses possible from the eye is considerably richer than that which can be elicited with the simple nylon filament of a Cochet-Bonet aesthesiometer. This realization, together with other problems associated with filament aesthesiometry—such as the need for good alignment, humidity effects on filament behavior, and the aversion response experienced by subjects as the filament approaches their cornea (Brennan and Bruce, 1991)—has fostered the development of devices utilizing alternative means of stimulation. To this end, carbon dioxide lasers and jets of either saline or air have all been used, each with some measure of success.
Air jet aesthesiometers have gained popularity in recent years. In their most sophisticated form, these instruments offer users the ability to vary not only the flow rate, and therefore the force, of the air pulse, but also its temperature and carbon dioxide content (Belmonte et al, 1999). The value of controllable carbon dioxide is that the polymodal nociceptors can detect its presence, so it therefore provides a convenient means of delivering an irritant chemical stimulus.
In addition, warming the air is a useful experimental device because the resultant pulse is at the same temperature as the ocular surface. This eliminates induced cooling effects and prevents activation of the cold receptor fibers. What results is essentially a purely mechanical stimulus, as distinct from the more complex one that is presented by cooler—or indeed warmer—air.
Combined with an appropriate psychophysical routine to access subjective responses, an apparatus of this kind allows the multimodal sensory characteristics of the ocular surface to be directly studied in humans and opens up a potentially fruitful area of research enquiry. Based on such experiments, both the cornea and conjunctiva appear able to interpret different qualities of sensation depending on the nature of the stimulus. Further, the quality of that sensation can differ between the two tissues (Acosta et al, 2001; Feng and Simpson, 2004). In other words, the ocular surface does not just signal when a stimulus is potentially damaging, it can also communicate less intense sensory information such as feelings of coolness or itching. Moreover, the cornea and conjunctiva have an interactive sensory relationship, meaning that the response from the former can influence that of the latter (Feng and Simpson, 2005).
Against this background, we can view the known behavior of mechanical sensitivity as only a part of a much bigger picture. A substantial amount of work is required to complete this view, but a number of interesting findings have emerged. For example, while there is some variation in sensitivity between the central and the peripheral cornea along the central meridian, the differences are substantially less than previously thought. This is irrespective of whether the stimulus was carbon dioxide, warmed air or cooling air (Situ, 2007). The absence of great variation in this regard suggests a reasonably even distribution of sensory elements across the cornea, which is consistent with the results from recent confocal microscopy studies (Patel and McGhee, 2005).
A warmed air stimulus has shown that male corneas and conjunctivas are less sensitive than their female counterparts are and that, intriguingly, while women get more sensitive as they age, men may not (Golebiowski et al, 2008). Because these findings are not replicated with a cooling stimulus (Murphy et al, 2004; Situ et al, 2008), it is possible that some sensory elements are more affected by age-related changes than others are and that the quality of ocular sensory experience may also subtly alter during life.
Equally interesting findings have emerged from the limited amount of work so far conducted with contact lenses. While neither traditional nor silicone hydrogel soft lenses appear to change the sensitivity of the central cornea to a warmed air stimulus in short-term wear (Stapleton et al, 2004), there is evidence that sensitivity may actually increase in some circumstances. For example, in this same study, conjunctival sensitivity rose in association with silicone hydrogel wear, perhaps suggesting a mechanical effect. In the much longer term, suspending wear in a group who had worn traditional hydrogel lenses on an extended wear basis for an average of more than 13 years was accompanied by a reduction in corneal sensitivity to a warmed air stimulus. This reduced level of response was maintained even after refitting with silicone hydrogel lenses (Golebiowski et al, 2003). The significance of this finding is yet to be established, but it demonstrates the potential for further study in this field. CLS
For references, please visit www.clspectrum.com/references.asp and click on document #172.
Associate Professor Papas is executive director of Research & Development and director of Post Graduate Studies, Institute for Eye Research and Vision Cooperative Research Centre, and senior visiting fellow, School of Optometry & Vision Science, University of New South Wales, Sydney, Australia. He has received research funds from Ciba Vision, Alcon, AMO, and Allergan.