What do your patients talk about after they leave your consulting room? One way to find out is to lurk on the many internet chat sites devoted to health and health related issues. Several of these have contact lens threads, the content of which can be either interesting or frightening, depending on your disposition. A lively topic recently has been that of silicone allergy, with many contributors apparently convinced that this is the reason for their red eyes, discomfort, itchiness, etc., etc. Interestingly, some appear to have had this suspicion confirmed by their eye care practitioner, which of course, lends legitimacy to the condition in the eyes of many (pun intended), and belief in silicone allergy does exist among contact lens professionals. Given current prescribing trends , together with the news that silicone hydrogel lenses have recently breached the bastion of daily disposability, it looks probable that these lenses will effectively replace their conventional counterparts in the not too distant future. It would be a matter of major public health concern if a significant numbers of allergic responses were precipitated as a consequence.
Are there reasonable grounds for such anxiety? Searching the academic literature via Medline using combinations of the keywords silicone, allergy, sensitivity and contact lenses actually returned no relevant references. Relaxing the search parameters to include responses from the entire eye produced the same result. Based on these outcomes, it seems clear that either, researchers are not interested in this area, or they have insufficient evidence to merit a publication.
Perhaps there is more noise on the subject outside the circle of formal academic media? Both the Food and Drugs Administration in the United States and the Medicines and Healthcare Products Regulatory Agency in the UK maintain reporting systems for medical device problems. These are open to the general public as well as practitioners, but again, neither contains any notification linking contact lenses with silicone allergy or sensitivity.
It seems then, that clinicians are not encountering this problem or, at least, are not identifying silicone as being associated with distinct, observable signs. While references to reactions in other body sites such the ear  or throat , can be found, they are few in number and it must be concluded that true silicone reactions are exceedingly rare phenomena. Given that silicon is one of the most abundant substances on earth and silicone products are, and have been for some time, extremely widely used in medicine and many other areas, perhaps this should not be so surprising.
What then, is upsetting the eyes of our wearers engaged in their internet chat? While silicone sensitivity is, perhaps, not totally impossible, the available evidence is that other causes are overwhelmingly more likely. A wide range of entities encounter the lens surface during wear, handling, storage or manufacture. These might include tear film proteins, cosmetics, care products, bacteria, pollen, chemicals etc. As silicone hydrogel lenses interact with their surroundings in a much more complex way than their conventional forbears, the scope for one or more of these factors to become associated with the surface and create a contaminant having reactive potential is evident.
From the comments in the chat room it seems that people who encounter this problem generally do so when they switch from a conventional, into a silicone hydrogel, lens. Perhaps bells will be ringing at this point, particularly for those experienced enough to recall upgrading rigid lenses from PMMA to RGP. During that process, wearers were often encountered who, despite the clinical advantages of the newer materials, were extremely unhappy and often displayed symptoms like discomfort, redness, discharge etc. Were these allergic reactions to the new polymers? While such suggestions were certainly made at the time they do not seem to have persisted over the years. Both rigid and soft wearers become accustomed to a certain feel and when made to change to another lens type, some will experience, and complain of, symptoms. Variations in surface rheology, protein/lipid deposition, edge shape, geometry, conjunctival interaction etc., are more likely culprits than true silicone allergy however. New shoes often hurt but no-one blames the leather.
To sum up, formal evidence for silicone allergy during contact lens wear is essentially non existent. Should there be a body of clinical opinion with a different view, there are, as mentioned earlier, a number of notification systems in place that will enable the profile of the issue to be raised, given the availability of factual detail. As with all adverse event reporting, the onus is on the practitioner to use them when appropriate cases present.
- http://www.nzoptics.co.nz/Incontact/2007/Jun07.htm accessed September 19th 2008.
- Morgan PB, Woods CA, Knajian R et al., International Contact Lens Prescribing in 2007, Contact Lens Spectrum, January 2008.
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