In summary, existing studies about the presence and function of HA in the ocular surface and its role as a therapeutic tool show:
1. HA is found as a component of the normal ocular surface. Its presence was described in tears, lacrimal tissue, conjunctiva and cornea.
2. In the normal ocular surface, it has been demonstrated that the presence of the HA receptor CD44 is always closely associated with HA. HA plays a role in the regulation of corneal epithelial regeneration, promoting epithelial migration and wound healing through an interaction with the receptor CD44.
3. Evident immuno-staining for HA has also been observed on the surface of the superficial corneal cells. This HA can originate from tears and may play a role in stabilizing the preocular tear film. Furthermore, the great hydrophilic properties of HA suggest a possible role in the regulation of corneal hydration.
4. The structure of HA is that of a multi-charged molecule with non-Newtonian characteristics. In this respect, it is similar to tears so that it has been suggested as a tear substitute.
5. Where there is ocular surface disease, HA and its receptor CD44 modify their pattern of distribution within the ocular surface structures.
HA is involved in the reparative processes of the ocular surface following corneal surgery. In fact, it appears that HA and its receptor CD44 are over-expressed by corneal epithelium in the course of wound healing. HA has been shown in strict association with subepithelial water accumulation in the area of corneal wound healing.
6. A topical application of HA has been shown to confer both subjective and objective improvement in patients with dry eye syndrome or keratoconjunc-tivitis sicca.
7. The treatment of severe aqueous deficient dry eye and dry eye states related to increased tear osmolarity benefit from a hypotonic solution of HA.
HA was used with success acting as a mask in assisting excimer laser surgery for the correction of irregular astigmatism resulting from previous corneal refractive surgery.
8. The mucoadhesive properties of HA can be used with the purpose of increasing the residence time of drugs on the ocular surface.
HA is an essential component of tear film and the ocular surface. Its physical and chemical properties render it suitable for tear film replacement since it has been demonstrated that its administration proved to ameliorate the ocular surface in the course of dry eye disorders. Further studies are needed in order to achieve a better understanding about which physico-chemical characteristics, such as molecular weight and dilution, are more suitable to treat the various forms of ocular surface diseases.
HA's mucoadhesive characteristics also show a potential for a more diffuse use of HA as a vehicle to improve the residence time and the bioavailability of topical drugs used for the treatment of ocular disorders.
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