In an ongoing effort to ameliorate the discomfort and blurred vision that stems from dry eye, tear substitutes have undergone numerous improvements to enhance their efficacy and safety to the ocular surface.
As we learn more about the changes of the ocular surface in disease, so have we improved the effectiveness of tear replacements.
Generations of tear substitutes
First generation tears were simply a saline-based, isotonic or hypotonic solution with preservatives, notably benzalkonium chloride (BAK). Although these tear substitutes did not blur vision, they were short acting and needed frequent re-application.
Second generation tears added natural and synthetic polymers (methylcellulose derivatives) and synthetic polymers (e.g., polyvinyl alcohol, povidone, polysorbate-80, HP guar) with gentler preservatives or preservative-free products. These agents are of thicker viscosity or a gel-forming nature and afford higher retention times. Often, the trade-off for this improvement can be short-term blur.
Third generation tears are considered those that incorporate hyaluronic acid (HA). Hyaluronic acid is found naturally in the human body, mainly in connective tissue, but also in vitreous body and synovial fluid and in the tear fluid of the eye. Sodium hyaluronate (sodium salt of hyaluronic acid) has water-retaining properties and reduces the shear forces of the blink. It is highly effective at entrapping water and preventing evaporation prolonging beneficial effects. Hyaluronic acid, with its natural bio-adhesive properties, provides longer hydration. Sodium hyaluronate also seems to have protective effects on the corneal epithelium.1
Considered fourth generations tears because of the addition of lipid oil-in-water nanoemulsions, these most advanced tear replacements address the evaporation concern of other tear replacements. Artificial tears with a lipid component are presumed to replenish the lipid layer. They have been shown to have a long residence time in the tear film, reduce the tear evaporation rate, improve the signs and symptoms of dry eye, improve the structure of the lipid layer, and to improve diagnostic test results, particularly the Schirmer score and tear break-up time.2