At present most tear replacement products are carboxymethyl cellulose and poly vinyl alcohol based which are artificially produced, need to be given very frequently and are less than perfect substitutes for natural tears. Hyaluronic acid however, is generated using bacterial fermentation and as such retains the natural biological functions which allow it to bind more closely with the cell surfaces of the corneal surface. It is for this reason that hyaluronate eye drops have been shown to be superior to those containing carbomer molecules.
They have a high residence time on the cornea allowing for less frequent application than other tear replacing formulations and also in human studies give less of a blurring action on vision, suggesting a closer appropriation to the corneal surface.
- Spreads evenly over the corneal surface
- Preservative-free tear replacement
- Reduced detrimental effects on the corneal epithelium
- Lower frequency of application
- Binds more closely to the corneal surface
In vitro studies have shown that hyaluronate improves epithelial cell migration such as is required in corneal ulcer healing. Ultrastructural studies show fewer deleterious changes to epithelial cells of the corneal surface using sodium hyaluronic acid compared to other tear replacements, which is what one might expect given the greater protective effects of a molecule that stays more tightly bound to the ocular surface and has a longer half life in the fluid covering the ocular surface.
A significant problem with most topical ocular medications is the need to include preservative and stabilisers and in particular a compound known as benzalkonium chloride. This quaternary ammonium compound is basically a bacteriostatic detergent and thus it is not surprising that it has deleterious effects on the corneal surface and especially on migrating corneal epithelial cells. Therefore from the perspective of lower frequency of application and lack of detrimental effects on the corneal epithelium, a tear replacement containing sodium hyaluronate in a preservative-free formulation offers an ideal tear replacement for the many animals affected by keratoconjunctivitis sicca.