Contact Lens Design & Materials
Contact Lens Dry Eye—What Triggers the Sensation?
By Neil Pence, OD, FAAO
Over the past decade or so, a great deal of attention has been paid to contact lens-related dryness. Despite an increasing volume of investigations, however, we still do not really know the where, how, or what of the trigger for the lens-related dryness sensation.
Patient dryness symptoms differ for contact lens wearers versus non-wearers. Contact lens dry eye is more commonly reported as “dryness” or “scratchiness,” while non-wearers might be more likely to describe a “burning” sensation (Chalmers, 2006; Begley, 2000). For non-wearers, the loss of surface hydration might somehow trigger a sensation described as “burning.” At least several studies with contact lens wearers have found that the dryness discomfort ratings do not relate well to the amount of contact lens dehydration (Fonn, 1999; Pritchard, 1995), perhaps suggesting different mechanisms for the differing symptoms for these two groups.
Looking Beyond the Eye
Perhaps we can gain some insight from a fairly large amount of work relating to “dryness” in the mouth. The oral mucosal surfaces have the ability to detect friction (DeWijk, 2006). The oral sensation of “dryness” arises from increased friction between mucosal surfaces. The increased friction is reported by patients as “dryness and/or roughness.” (Green, 1993)
Interestingly, attempts to study the effect of tongue roughness on friction have used silicone surfaces to simulate the tongue (Ranc, 2005). The results of these investigations may have implications for the friction between a silicone hydrogel lens and the lid. Findings of note (followed by possible implications) are:
1. The coefficient of friction (CoF) is temperature dependent. (Should testing for the CoF of contact lens materials be routinely performed at the temperature of the ocular surface?)
2. The amount of friction generated increases as the speed that the surfaces pass each other increases. (Does a slow, full blink create less symptoms compared to several quick, partial blinks?)
3. The CoF was four to five times less when an oil-based lubricant was employed as opposed to an aqueous solution. (How critical is the function of the meibomian glands in reducing lens/ lid friction, which in turn may be the origin of the “dryness” sensation, and how much more important does that make the MGD Workshop findings in possibly reducing symptoms in contact lens wearers?)
A Need to Dig Deeper
To better understand the symptoms of dryness in lens wearers, it seems that we need to more thoroughly understand what triggers this sensation. Is it a different type of sensation or receptor than in non-wearers? Is the eyelid/lens surface relationship similar to the tongue/oral mucosa interaction? Is the sensation of dryness related to friction, and if so, where exactly do the receptors that detect this friction reside—in Korb's lid wiper area? Across the surface of the upper palpebral conjunctiva? Both? Neither? Until more is known about the actual trigger mechanism and location, it would seem difficult to truly target materials or design care solutions to help lessen the discomfort symptoms associated with lens wear.
Until such a time, it might seem prudent to learn more about how we can reduce the friction between the lid and the lens surface and to employ strategies that can lessen this friction whenever possible. CLS
For references, please visit www.clspectrum.com/references.asp and click on document #188.
Dr. Pence is the associate dean for Clinical and Patient Care Services, Indiana University School of Optometry in Bloomington, Indiana. He is a consultant or advisor to B+L, Ciba Vision, and Vistakon, and has received research funding from AMO. You can reach him at email@example.com.