Tear Film Stability in Sjögren Syndrome Patients Treated with Hyaluronic Acid Versus Crosslinked Hyaluronic Acid-Based Eye Drops

Introduction

Dry eye disease (DED) is a multifactorial disease of the tear film and of the ocular surface and its prevalence may vary between 5% and 33%, in different reports and with different methods of ocular evaluation. ¹ DED is characterized by tear film instability, visual disturbance, and potential damage to the ocular surface^2–4 and is accompanied by increased tear osmolarity that stimulates the production of inflammatory mediators on the ocular surface.^5–9 Inflammation can also be due to stress of chronic irritation and to systemic inflammatory/autoimmune disease. DED is one of the main clinical manifestations of Sjögren syndrome (SS), a chronic inflammatory disease with an autoimmune basis. DED may be classified as Sjögren syndrome-related dry eye (SSDE) or as non-Sjögren syndrome-related dry eye (NSSDE). Regardless of the initiating cause, a vicious circle can develop on the ocular surface wherein DED initiates inflammation and inflammation in turn leads to further dysfunction of cells related to tear secretion. Traditional therapies for DED are artificial tears that increase the volume of the tear film by either supplementing the tear film or increasing the stability of the tear film. ¹⁰

The formulations based on hyaluronic acid (HA) are among those having greater efficacy for both its rheological properties and biocompatibility, being a natural polymer. HA is a linear polysaccharide formed from disaccharide units containing N-acetylglucosamine and glucuronic acid, the presence of all the fully ionized carboxyl groups gives the molecule a high polarity and, therefore, the ability to complex with many water molecules reaching a high degree of hydration. The HA is metabolized by the organism hyaluronidase, in fact reducing the usability. To slow down the physiological process of degradation, the HA has been crosslinked with various chemical compounds, generating the so-called crosslinked hyaluronic acid (CLHA), which is subsequently purified by crosslinking compounds. ¹¹ These formulations, therefore, could extend the contact time between the HA and the ocular surface, and previous studies in dogs showed that the use of CLHA significantly improves ocular surface health and is more highly effective than the alternative tear supplement.^9–14

This aspect could be very interesting because in DED an essential criterion for assessing the severity of the disease is the stability of the tear film. ¹⁵

To our knowledge, a study has never been conducted on the use of CLHA in human. The aim of our study was to compare the stability of the tear film after instillation of an artificial tear containing CLHA and HA in healthy volunteers and in patients with SSDE.

Methods

Ethics committee approval was obtained (CEAS 2668/15) and the principles outlined in the Declaration of Helsinki were followed in this study. This prospective, randomized study involved patients aged ≥18 years with a diagnosis of SSDE who were referred to the Section of Ophthalmology of the Department of Biomedical and Surgical Sciences of the University of Perugia. SS was diagnosed by a rheumatologist following the American–European Consensus Group SS classification criteria considering blood test results and organ-specific signs. Inclusion criteria for this study were the absence of other eye conditions in addition to dry eye, lack of topical therapy with the exception of tear substitutes, fluorescein breakup time (BUT) under 10" (slit lamp observation), and Schirmer test without anesthesia under 10 mm/5 min.

The exclusion criteria were any relevant eye disease, such as meibomian gland dysfunction, eyelid and conjunctival infection in the previous 3 months, corneal pathologies, hypertension or glaucoma, uveitis and previous ocular surgery, hypersensitivity to the active substance or excipients, acute or chronic systemic diseases incompatible with the study, pregnancy or breastfeeding, and inability to understand the study procedures. Patients included in the study had not instilled any ocular topical therapy in the previous 48 h. Twenty patients suffering from SSDE were included in this study and 20 healthy volunteers constituted the control group. After inclusion, all patients underwent a corneal topography (Sirius, CSO, Italy) in both eyes (time 0) by an expert executor (G.T.). The patients were told to blink several times and corneal images were captured as quickly as possible just after blinking. Corneal topography was performed 3 consecutive times and the average value was taken for analysis. Subsequently a second operator instilled in 1 eye 1 drop of HA crosslinked 0.20% (ICROSS Italdevice.srl) and in the other eye 1 drop of HA 0.20% (Hyalistil BIO–SIFI, Italy). The eye was randomly chosen with a roll of dice and neither the patient nor the executor of corneal topography was aware of which drops were instilled in both eyes. Corneal topography was performed again 5 min (time 1), 30 min (time 2), and 60 min (time 3) after the instillation of the eye drops. Topography surface asymmetry index (SAI) and surface regular index (SRI) were evaluated automatically by Sirius topography. SAI reflects the asymmetry of the corneal surface, as on a hemisphere–ideal cylindrical surface it would be equal to 0. SAI values greater than 1 reflect a significant reduction in the quality of vision, by virtue of a greater asymmetry.^16–18 SRI is the result of the comparison of changes between several adjacent points of the 10 central rings. This area is equivalent to a pupil of about 4.5 cm in diameter. Values greater than 1.5 D express irregularities in the optical zone^19–21

Results

Demographic data are shown in Table 1. We did not observe statistically significant differences in baseline characteristics between 2 eyes (P < 0.05). In Tables 2 and 3 are reported results in the control group using HA and CLHA for SAI and SRI, respectively. Data analysis shows that for SAI there is statistically significant difference at time 0 and at time 5, whereas there is no statistically significant difference after 30 and 60 min from instillation. For SRI, there is no statistically significant difference at any time.

In Tables 4 and 5 are reported results in SSDE patients using HA and CLHA for SAI and SRI, respectively. Data analysis shows that there is no statistically significant difference at time 0 and 5, whereas there is a statistically significant difference after 60 min for SAI, and after 30 and 60 min for SRI from instillation.

We observed that both SAI and SRI in dry eyes were significantly greater than in control eyes at time 0 (P < 0.05 for both SAI and SRI). No patients reported stinging or any other adverse ocular event.

Discussion

Tear substitutes are constituted by a buffer solution that contains lubricating agents, among which most popular are HA, polyacrylic acid, and polyvinyl alcohol. ¹² The purposes of these formulations are to make the ocular surface hydrophilic, reduce the surface tension, and provide a density which allows tear film to remain on the corneal surface for a suitable time. To improve the efficacy of tears substitute, there is an increased attention to formulations with non-Newtonian behavior. They have high viscosity, when subjected to moderate cutting forces (eye open) and low viscosity when the shear forces become more intense (blink). This allows adequate distribution and optimum lubrication of the ocular surface. In vitro HA demonstrated the capacity to promote cell migration and to stabilize the ocular epithelial barrier, suggesting that it is probably directly involved in epithelial repair process through the activation of CD44 (−). ¹⁴

HA in patients with moderate to severe dry eye syndrome can improve significantly symptoms, such as burning, redness, photophobia, and foreign body sensation, and it can improve clinical signs as significant reduction of corneal fluorescein staining.^10,12,13 Moreover, a better stability of lacrimal film means minor disorders of visual acuity. Therefore, to extend the contact time between the HA artificial tears and the ocular surface could improve properties of the molecule.

The CLHA has previously been used to compare the ocular surface effects in animals and they demonstrate the efficacy of the CLHA in reducing the clinical signs associated with dry eye. ²²

Viscoelasticity of tear supplements containing HA is an important feature because it influences tear stability and consequently many of the symptoms of dry eyes.^23–25 The viscoelasticity of HA-based dry eye formulation can vary significantly, depending on some factors, including the molecular weight and concentration of the HA^23,26,27: they are often a solution of high molecular weight and low concentration HA.

Our results show that CLHA has a better efficacy compared with HA in maintaining the stability of the tear film. In healthy subjects the instillation of the 2 different tears substitute did not show statistically significant differences between SAI and SRI values, except at 5 min for SAI. On the contrary, patients with DED showed a marked improvement in topographic index after CLHA use with respect to patients treated with HA eye drops after 30 and 60 min from eye drop instillation. The greater stability on the ocular surface is probably due to the high viscoelasticity of CLHA. This means that in patients with major tear film instability and more suffering ocular surface, the stability of the tear film at a distance of time from instillation is much higher after the use of CLHA than that with HA.

Furthermore, this means that CLHA remains on the ocular surface longer than HA and this aspect could be explained with crosslinked formulation of CLHA eye drop. A better tear film stability for a prolonged time after eye drop instillation could be very important and improves the quality of life in DED patients. Future studies could test how the SAI and SRI relate to visual function or preoperative assessment for cataract and refractive surgery.

We included the healthy control group to verify the validity of CLHA in normal and in dry eyes. Marked differences emerged in the topographic indices in the 2 groups and before treatment we observed a significant difference in the 2 groups. A possible explanation of these differences could be that the use of artificial tears in normal eyes does not modify tear film stability because it is already good. Differently, in dry eyes, tear film stability gets better because initially it is not so good. Dry eye patients often need multiple daily administration of artificial tears, and better tear film stability after CLHA instillation could permit a less frequent application. All this would be a great advantage, as well as a general improvement in the quality of life, especially for those patients who, by virtue of this debilitating disease, are now forced to give up their work, with obvious repercussions on the social and psychological.

Tear film stability is important to be taken into account when diagnosing dry eyes. There are multiple systems to measure tear film stability.^18–21 Differently, corneal topography automatically analyzes SAI and SRI. To our knowledge, this is the first study using corneal topography that confirms SAI and SRI are increased in dry eyes compared with control eyes. Therefore, topographic analysis might be an adjunctive tool in the evaluation of dry eyes and the effects of treatment.

Our experience has some limitations. We evaluated tear film stability only by SRI and SAI, without fluorescein or the BUT test, because, according to the literature, SRI and SAI are valid indices of tear film stability. ²⁰ Moreover, this study has a contralateral eye study design, which we preferred, because by eliminating any kind of interindividual differences, the reliability of the results was increased.

In conclusions, our study seems to indicate that CLHA tears substitute improve ocular surface tear stability. Larger studies about CLHA artificial tears properties are needed with attention about its properties that effectively decrease DED symptomatology and improve ocular surface health.