Dry eye disease causes changes in ocular surface that are not fully understood and its treatment is a significant clinical challenge.
However topical lubricants that are specifically designed for the treatment of dry eye have been shown to have a positive impact on the condition.In a recent study published in the journal Eye, the performance of four optical lubricants was compared. (1)
The eye drop samples were sodium hyaluronate 0.4% (Clinitas/Clinitas Soothe), hypromellose 0.3% (that is also the commercial name) carboxymethylcellulose sodium 0.5%/ glycerin 0.9% (Optive), and carmellose sodium 0.5% (Celluvisc 0.5%).
Previously it has been established that solutions with high viscosity have longer precorneal residence times and therefore maintain bioavailability of the active ingredients. This study is believed to be the first to investigate the effects of pH, temperature and dilution on eye-drop viscosity.
The study suggests that carmellose sodium 0.5% and hypromellose 0.3% do not have sufficient kinematic viscosity to maintain precorneal residence and this would compromise their performance. Indeed other studies also suggest that hypromellose performed poorly when compared to carbomer gels and hyaluronic acid products, so low viscosity might explain this.
Clinitas Soothe and Optiva had the highest absolute kinematic viscosities. Although both showed declines in viscosity with increasing temperatures, this remained above the critical level of 10mm2/s and there was no effect with changes in pH.
The study concluded that the best performing medications were sodium hyaluronate 0.4% (Clinitas/Clinitas Soothe) and carboxy methylcellulose sodium 0.5%/ glycerin 0.9% (Optive).
This in vitro study is supported by other in vivo work that found that sodium hyaluronate does indeed have the highest precorneal residence times when compared with other ocular lubricants such as polyvinyl alcohol or hydroxypropylmethylcellulose.
The researchers suggested that lubricants with stable viscosities maximise therapeutic efficacy.
1. Rahman MQ, Chuah KS, Macdonald EC, Trusler JP, Ramaesh K. The effect of pH, dilution, and temperature on the viscosity of ocular lubricants—shift in rheological parameters and potential clinical significance. Eye (Lond). 2012;26(12):1579-84 [PubMed]