Physical Properties of Ophthalmic Solutions are Crucial for Successful Treatment
The topical administration of ophthalmic solutions increases the volume of tears in the conjunctival sac. The conjunctival sac has the anatomical limitation that it can hold only around 30 µl under normal conditions in an upright position without blinking. When this maximum capacity is reached, the tears flow out of the conjunctival sac. As a result, this overflow leads to a lower bioavailability of exogenous compounds due to the shorter period the artificial tears remain on the surface of the eye.
Ophthalmic solutions with a higher viscosity are more effective thanks to a longer retention time, and in the meantime increase the adhesion of macromolecules to the mucin layer.
In addition to viscosity, pH is another factor that can cause adverse symptoms in the eyes. The formulations for ophthalmic solutions should therefore be within the normal comfort range for eyes (pH from 6.6 to 7.8). pH levels that are outside the normal comfort range for eyes lead to increased tear production and to burning and stinging, which can indirectly affect the patient's compliance.
For the treatment of dry eye, ophthalmologists and patients are now faced with a large number of ophthalmic solutions to choose from.
In order to guide daily practice, it is important to know in details the physical properties of ophthalmic solutions. This information is not always mentioned in the patient leaflet or in promotional brochures, but it would help to ensure the long-term success of treatment.
A recent study by Che Arif et al.1 evaluated 18 ophthalmic solutions based on their physical properties.
For this study, the ophthalmic solutions were classified according to their viscosity as follows: low viscosity 0.55 - 7.7 cP; medium viscosity 7.8 - 20 cP; and high viscosity, 21 - 305 cP.
The statistical examination of the median of the 3 groups resulted in a significant difference in the viscosity between the low (n = 7), medium (n = 8) and high (n = 3) viscosity group (p= 0.001).
Ophthalmic solutions with a high viscosity extend the retention time on the surface of the eye, thus allowing intensive lubrication and sustained relief from the symptoms of dry eye.
CONCLUSION: The viscosity and pH of ophthalmic solutions are important factors for determining the success of treatment of dry eye patients. Ophthalmic solutions with a high viscosity that are within the natural pH range such as VISMED® GEL and VISMED® have a longer retention time and are more comfortable in the eyes.