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Solution-induced corneal staining is a relatively common finding in contact lens wearers that must be taken seriously because it is a sign of corneal compromise that may lead to infiltrative keratitis.
"Findings from laboratory studies measuring fluorescein dye diffusion to evaluate corneal barrier function provide objective evidence that SICS has physiological consequences," said Dr. Paugh, professor and associate dean of research, Southern California College of Optometry, Fullerton, CA.
"With repeated insult to the cornea, certain sensitive individuals may develop inflammation that can lead to infiltrative keratitis," he continued.
Induced hapten mechanism
Reported rates of SICS, based on data from randomized, controlled clinical trials, range from about 2% to 7%. There is evidence that SICS increases the risk of infiltrative keratitis threefold. The process whereby SICS progresses to infiltrative keratitis has not been definitively established, but Dr. Paugh proposed that it may develop over the longer term, via a hapten mechanism, in patients who are sensitive to repeated toxic or noxious insults.
According to this theory, a chemical entity from the care solution acts as a hapten (a partial antigen), where it may combine with a tissue protein on the ocular surface to become a full antigen that induces an immune response, that is, a type IV delayed hypersensitivity reaction.
"Research establishes this mechanism occurs with thimerosal, although further study is needed to demonstrate its relevance with newer preservatives," Dr. Paugh said.
The clinical features evolve with the progression from short-term solution toxicity to an immune response. Initially, patients present with diffuse, punctate staining, symptoms of irritation (primarily burning), and bright red bulbar injection. Infiltrates are usually absent. However, if an immune response develops, itching becomes the major symptom. The redness of the eye becomes paler, staining is minimal, and infiltrates are always present.
"SICS is important because it can lead to infiltrative keratitis, but even in the short-term, its related discomfort causes unhappy patients," he said.