Rationale for use of topical hyaluronan in dry eyes
Dry eyes are associated with symptoms of ocular discomfort, dryness, scratchiness, burning, soreness and grittiness. (Other clinical signs are often present, including superficial keratitis, tear film instability, decreased tearing and tear hyperosmolarity.) The primary aim of tear supplements is relief of these uncomfortable sensations.
For greatest efficacy, the tear supplements used should have a long retention time on the ocular surface. Accordingly, a solution should be used that has a greater viscosity than saline solutions. This has led to the use of viscosity-enhancing compounds as the main ingredient in tear supplements.
However, some viscous preparations currently available for the treatment of dry eyes are not ideal. Viscous agents such as cellulose derivatives, polyvinyl alcohol and carbomer can exert resistance in response to the shear forces that occur between blinks or during rapid eye movement, often leading to reduced comfort and tolerance in the eye. In addition, viscous solutions are cleared from the eye during the blinking process and may cause discomfort if excess solution is allowed to dry on the lids. Products that are too viscous may cause blurred vision. Another perceived problem is that the agents available are synthetic molecules.
These concerns have led to interest in natural viscoelastic solutions such as hyaluronan, a substance that is commonly used as an active ingredient in many medicinal products and medical devices. It is especially used in ocular surgery involving the anterior or posterior segment of the eye, where it is used to maintain the shape of the structure, cover the surgical instruments and protect the sensitive corneal endothelium from further damage. Hyaluronan displays non-Newtonian (pseudoplastic) properties, increasing its residence time in the eye and optimising comfort.
Benefits of hyaluronan solutions in the eye
The characteristics of hyaluronan enable it to provide significant benefits in the management of dry eyes. Many of the properties of hyaluronan in solution (i.e. rheology) are either dependent on or enhanced by the correct balance of its concentration and molecular weight:
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