Filtering surgery throughout its development has mainly focused on the ocular surface, most particularly the conjunctiva. Similarly, all doctors treating patients with glaucoma should also concentrate on the status of the patient’s ocular surface, not only at the time of surgery but especially before. The glaucomatous patient is the primary category of patients who may have side effects from long term topical treatment.

Currently, it is well known that the chronic use of eye drops has a cumulative toxic effect on the conjunctival epithelium. Broadway et al^1,2 have shown the role of medical treatment as a risk factor for filtering surgery failure. They clearly showed that the success rate was considerably lower (55%) in patients treated with one or more drugs for glaucoma for at least 3 years than in other patients (94%, p<0.001). Later, Baudouin et al³ showed the major component of conjunctival toxicity inside a bottle of eyedrops for glaucoma—preservatives, with few differences between the various types, but particularly well studied in benzalkonium chloride (BAK), still the most frequently found preservative in eye drops. …