The benefit of IOP reduction in the treatment of glau-coma has been confirmed in large controlled, prospective clinical trials. Medical treatment is predominantly used as first line therapy and therefore the majority of patients receive several decades of treatment. The main topical hypotensive medications include timolol (non-selective adrenergic beta-blocker), carteolol (non-selective adrenergic beta-blocker with intrinsic sympathomimetic activity), carbonic anhydrase inhibitors as dorzolamide or brinzol-amide, brimonidine (adrenergic agonist alpha-2 selective), prostaglandin analogues (latanoprost, travoprost, bimato-prost, tafluprost). Prostaglandin analogues and beta-blockers are the most commonly prescribed first-line therapies [1, 2]. Timolol, introduced in 1978, in a few years [23] became the most widely used drug in the treatment of glaucoma since it was approved for general use. Despite the major contraindications (asthma, history of obstructive pulmonary disease, sinus bradycardia, heart block or cardiac failure) today it is one of the most used drugs and is also available in fixed-dose combinations. Already in 1979 Nielsen and Eriksen [24] underlined how in 64 patients treated with timolol eye drops, seven patients developed transitory sensation of dry eyes. Two of these subjects also had xerostomia. In all these seven patients a reduction of Schirmer test and break-up time was noted. Fasina et al. [25] evaluated the effect of Timolol maleate on tear film break-up time in a Nigerian population. 192 eyes of 96 subjects were examined in a hospital based case-control study after being administered pre-coded questionnaires.There was significant difference in the mean break-up time of cases (10.45 secs) and controls (30.18 secs).

Preservative-free timolol solution has a favourable effect on the tear turnover of patients with glaucoma and ocular hypertension in comparison with timolol containing BAK [26], even if the integrity of the precorneal tear film persisted to be affected when using timolol without BAK [26, 27]. Timolol without preservative can be recommended in those patients who have keratoconjunctivitis sicca or a borderline tear production in order not to increase the severity of the dry eye.

In 1996 Latanoprost, the first prostaglandin agent, was introduced in medical therapy, followed by Travoprost and Bimatoprost in 2001 and Tafluprost in 2008. Unoprostone was introduced in some countries in 2000. Prostaglandins and their combination drugs have become the top-line choices for treatment in the past decade because of the once-daily dosing and great efficacy in lowering IOP. Different papers in literature demonstrates how switching the therapy from a preserved to a preservative-free prostaglandin preparation in topical glaucoma medication can lead to a great improve of OSD.

Generally there is no sufficient evidence to demonstrate that the active glaucoma agent can lead to OSD. Blondin et al. [28] demonstrated that carteolol, timolol, betaxolol or latanoprost did not activate complement system, an early mediator of the inflammatory response. Furthermore timolol and betaxolol exerted an anti-inflammatory effect by preventing complement activation.

Pisella et al. [29] even suggested a potential protective effect of the prostaglandin analogue and, to a lesser extent, timolol against the toxicity of BAK in conjunctival cells. A recent multicenter, international, non-interventional study enrolled 448 patients with glaucoma or ocular hypertension in therapy with one ore more topical IOP-lowering medications (on average 1.9 IOP-lowering medications). OSD was highly prevalent in the population studied. The worsening of OSD symptoms and score (measured with Ocular Surface Disease Index, OSDI) was significantly related to longer duration since diagnosis and higher number of medications [30]. Valente et al. [31] published a recent study to evaluate the presence of symptoms of tear film dysfunction by using the Ocular Surface Disease Index (OSDI) questionnaire in fifty glaucomatous patients and to examine whether they have ocular surface signs. Fifty-two percent of patients in therapy with preserved antiglaucomatous drops showed symptoms of tear film dysfunction. Signs of ocular surface diseases seemed to be greater in patients under > 2 medications. Symptoms correlated to signs only in patients in monotherapy with β-blockers drops. Considering the actual state of literature we can conclude that the active glaucoma agent can have a role in OSD, but the main cause seems to be the preservative agent [19].

The need for sterility in multidose eyedrops has led to the inclusion of an antimicrobial preservative in these solutions. Commonly actually used preservatives in antiglaucomatous therapy are BAK, Purite^® (Allergan Inc., Irvine, California, USA), Polyquaternum 1 as Polyquad^® (Alcon Research, Fort Worth, Texas, USA), SofZia^® (Novartis AG, Basel, Switzerland).

Other preservatives include organo-mercurial derivatives (such as thimerosal) that precipitate bacterial proteins and are active against Gram-positive bacteria and non-sporulating organisms; amidines (such as chlorhexidine) that damage the cytoplasmatic membrane and are active particularly on Gram-positive bacteria and cocci; alcohols (such as chlorobutanol) that can cross the bacterial lipid layer (bacteriostatic and antimicotic activity); parabens (the ester of parahydroxybenzoic acid) that target fungi and finally the oxychlorinated complexes which disrupt cells function.

The most frequently used preservative, benzalkonium chloride (BAK), has consistently demonstrated its toxic effects in laboratory, experimental, and clinical studies. As a quaternary ammoniums act mainly with a detergent activity, which dissolve bacterial walls and membranes and damage the cytoplasmatic contents. Nenciu et al. [32] carried out a 40 week, prospective, randomized trial enrolling 18 patients divided in four groups according to their therapy: beta-blockers, prostaglandin analogues, carbonic anhydrase inhibitors. The morphometric analysis of histological sections of conjunctiva revealed squamous metaplasia, inflammation and fibrosis. Also the immunohistochemistry confirmed the tendency to chronic inflammations. These findings pointed to BAK as a major causal factor of conjunctival metaplasia. The group of Baudouin confirmed histopathologic effects of antiglaucomatous drugs on the conjunctiva and showed similar effects in the trabecular meshwork with human and animal studies [33]. This experimental studies showed that BAK is for a large part responsible for these toxic or immunoinflammatory effects on the ocular structures. As a quaternary ammonium BAK has been shown to cause tear film instability, loss of goblet cells, conjunctival squamous metaplasia and apoptosis, disruption of the corneal epithelium barrier, and damage to deeper ocular tissues. The mechanisms causing these effects have not been fully elucidated, although the involvement of immunoinflammatory reactions with the release of proinflammatory cytokines, apoptosis, oxidative stress, as well as direct interactions with the lipid components of the tear film and cell membranes have been well established [34]. Lot of researches have been conducted to discover and develop less toxic preservative than BAK and quaternary ammoniums, but only a few have been proposed and are commercially available, since a preservative must be a potent antimicrobial agent while not being cytotoxic [34].

Noecker et al. [35] investigated in rabbit eyes the effect of Purite^®, a stabilized oxychloro complex. Although the adverse effects of glaucoma medications on the ocular surface are likely multifactorial, 1-month treatment with glaucoma medications containing higher levels of benzalkonium chloride (BAK) resulted in greater corneal damage and conjunctival cell infiltration than medications preserved with Purite ^® or with lower levels of BAK.

Polyquad^®is a polyquaternum, a family of polycationic polymers that are used in personal care industries. Polyquaternum 1 (as Polyquad^®, PQ) is commonly used in ophthalmology. Liang H et al. [36] evaluated in rabbits a new formulation of travoprost 0.004% ophthalmic solution preserved with PQ, with commercially available formulations of BAK-preserved travoprost 0.004% ophthalmic solution and BAK-preserved latanoprost 0.005% ophthalmic solution, suggesting a greater safety advantage for the ocular surface of patients receiving chronic glaucoma treatment with PQ-preserved drugs. Considering Labbé et al. [37] PQ, except in high concentration of 0.5%, did not create important changes in ocular surface compared to saline solution, whereas BAK induced major toxic effects at mild to high concentrations with destruction of globet cells.

SofZia^® is a new preservative generation that functions as a microbicidal agent through oxidative properties. This ionic buffer solution, comprised of borate, propylene glycol, sorbitol, and zinc chloride, converts to nontoxic byproducts after contact with ocular surface cations [38]. A prospective randomized multicentre single-masked comparative study [39] enrolled 220 patients with open-angle glaucoma or ocular hypertension who had been treated with BAK-preserved latanoprost 0.005% monotherapy for at least 3 months. After a switch to SofZia-preserved travoprost 0.004% the study indicated that frequency of keratoconjunctival epitheliopathy significantly decreased in the travoprost SofZia-preserved group (p = 0.036) and the intergroup difference was also significant (p = 0.001). SPK scores and TBUT were significantly improved in the travoprost group (p = 0.034, 0.049), also with significant intergroup differences in the cornea excluding the inferior area and TBUT. There were no significant intergroup differences in changes of the hyperaemia scores and the IOP reduction.

Ammar [40] investigated the potential cytotoxicity of various topical ophthalmic glaucoma formulations containing different preservatives in cultured human trabecular meshwork (TM) and non-pigmented ciliary epithelial (NPCE) cell lines. They demonstrated that substitution of BAK from topical ophthalmic drugs (compared to SofZia^® and Polyquad^®) results in greater viability of cultured TM cells, the cells involved in the conventional outflow pathway. Cultured NPCE, responsible for aqueous inflow, appear more resilient to BAK. Anyway BAK enhances all characteristics of TM degeneration typical of glaucoma, i.e. trabecular apoptosis, oxidative stress, induction of inflammatory chemokines. This causes degeneration in acute experimental conditions, potentially mimicking long-term accumulation. BAK was also shown to access the TM after repeated instillations. These findings support the hypothesis that preservatives may cause further long-term trabecular degeneration and therefore enhance outflow resistance, reducing the impact of IOP-lowering agents [41].

We can conclude that BAK is the most commonly used preservative in ophthalmology and is more toxic than other or newer preservatives [42]. Substitution of BAK with Polyquad^®, Purite^® or sofZia^® resulted in significantly higher percentages of live conjunctival and corneal cells. Further studies are needed to understand the role upon TM cells and the clinical implications of these findings [43].

Totally preservative-free topical medication can be suggested for patients with ocular surface disease, frequent eye drop administration, proven allergy to preservatives and contact lens wear [43]. Improvements in OSD and quality of life have been demonstrated in several studies switching to preservative free drugs, i.e. timolol, prostaglandin agents, timolol/dorzolamide fixed combination, prostaglandin/ timolol fixed combination [44-48].

We also have to underline that there is a large evidence from clinical and experimental studies that long-term use of topical drugs may induce, as a consequence of chronic ocular surface changes, the potential risk of failure for further glaucoma surgery. Subclinical inflammation has also been widely described in patients receiving antiglaucoma treatments for long periods of time, with inflammatory cell infiltration and fibroblast activation in the conjunctiva and subconjunctival space. The preservative, especially BAK, could induce or enhance such inflammatory changes. Postoperatively, the ocular surface also plays an important role, as the conjunctiva interacts with aqueous humor and subconjunctival fibrosis may block aqueous outflow and cause surgical failure [49].

Martone et al. [50] ruled an in vivo confocal microscopy analysis of effects of topical antiglaucoma therapy with preservative on corneal innervation and morphology. The density of superficial epithelial cells was reduced in all glaucomatous patients, except for the preservative-free group. On the contrary, the density of basal epithelial cells of glaucomatous preservative therapy groups was higher than control and preservative-free groups. Stromal keratocyte activation and the number of beads were higher in all glaucoma preservative groups. The number of sub-basal nerves was lower in all glaucoma groups than in the control group and tortuosity was significantly higher in glaucoma than control groups. Confocal microscopy can be an important diagnostic tool in the evaluation of dry eye disease, with possible therapeutical implications [51, 52] as switching topical therapy.

In clinical practice the ideal medical therapy for glaucoma to prevent OSD (and to improve adherence) follows some basic statements.

1. The introduction of BAK-free or even preservative free agents in common use is a great advancement in glaucoma therapy, considering that this therapy is a continuous one.
2. The severity of OSD symptoms is positively correlated to the number of IOP-lowering medication used [53]. It’s important to underline that medical therapy has improved over the past decade through the introduction of more powerful topical drugs with long-lasting effects. These drugs make it possible to control IOP in many glaucoma patients with a once daily dose of a single agent. However, in some patients in whom IOP is not sufficiently controlled with monotherapy, adjunctive therapy is required. In such patients it is possible to use a fixed combination of glaucoma drugs, two IOP lowering agents combined in a single formulation. That’s why the introduction of combination therapies can be considered an important advancement in glaucoma therapy [54].
3. A complete and good clinical evaluation of OSD should be periodically performed. In the clinical evaluation of dry eye patients there is no actually established gold standard: several tests are used in the clinical practice which analyze patient’s symptoms, tear production, tear quality, ocular surface damage. All of these diagnostic test have low sensibility and little reproducibility, in particulary in patients with severe dry eye. Tear film hyperosmolarity is recognized as an important pathogenetic factor in dry eye syndrome, but its use is currently limited due to the complexity in its measurement [12, 55]. The same is true for confocal microscopy, a promising very good diagnostic tool with poor clinical practice yet [50, 55]. Arita R. et al. evaluated also how lid margin abnormality, superficial punctate keratopathy and meibum scores were higher in glaucoma patients under therapy, concluding how long-term use of antiglaucoma eye drops was associated with alterations in meibomian gland morphology and function [56].

Considering these statements it becomes fundamental to evaluate periodically:

-   Schirmer test score

-   tear breakup time score with with fluorescein 1%

-   corneal staining with fluorescein 1% and a validate grading scale (i.e. National Eye Institute (N.E.I.) / Industry Workshop Scale; Oxford Grading Scale; SICCA ocular staining score) [57-59]

-   conjunctival staining preferably with lissamine green and a validate grading scale (i.e. National Eye Institute (N.E.I.) / Industry Workshop Scale; Oxford Grading Scale; SICCA ocular staining score) [57-59]

-   the quality of life with a validate questionnaire (i.e. OSDI questionnaires; Ocular Comfort Index (OCI)/ Ocular Symptom Scale (OSS) questionnaires) [60, 61]

A recent paper underlines the importance of the assessment of Quality of Life (QoL). [62]. This cross-sectional study has been ruled with OSDI score and with Glaucoma Quality of Life-15 /GQL15 [62] and demonstrated that OSD is more common in patients with increasing glaucoma severity and is associated with poorer glaucoma-related QoL and higher exposure to BAK.

It is fundamental to evaluate QoL with a validate method, i.e. the National Eye Institute Visual Function Questionnaire (NEI-VFQ) or the Ocular Surface Disease Index (OSDI).

Abnormal OSDI was found in a large number of glaucomatous and ocular hypertensive patients, even if large proportion of patients with severe OSDI has got normal or a mild to moderate alteration of clinical test. OSDI increases with the greater the number of glaucoma drugs prescribed. A large proportion of patients with severe OSDI had normal or a mild to moderate alteration of clinical tests. Also psychological and environmental factors or the severity and awareness of the disease can influence OSDI [63].

Li M et al. [64] underlined in a recent paper how vision-related QoL in dry eye patients was impaired and was correlated with anxiety and depression, further implicating this condition as an important public health problem deserving increased attention and resources.