Topical medications and the ocular surface

There is a large body of evidence from experimental and clinical studies showing that the long-term use of preservatives, especially benzalkonium chloride (BAK), may induce severe ocular surface changes, causing ocular discomfort, tear film instability, conjunctival inflammation, subconjunctival fibrosis, epithelial apoptosis, or corneal surface alterations. Hence, the prevalence of dry eye is much higher in this population than expected in the general population. A low-grade subclinical inflammation has also been clearly demonstrated in patients receiving antiglaucoma treatments for long periods of time. The mechanisms involved are complex and multifactorial. They may include allergic, toxic, inflammatory reactions, as well as interactions with the active compound and preexisting or concomitant ocular surface disorders. BAK, the most frequently used preservative, has consistently demonstrated its toxic effects in laboratory, experimental, and clinical studies. Avoiding this compound by preservative-free strategies or through the development of nontoxic BAK-free eyedrops could become in the near future the gold standard of chronic ophthalmic therapies.